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Ultralong Charge Carrier Recombination Time in Methylammonium Lead Halide Perovskites
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  • András Bojtor
    András Bojtor
    Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
    Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary
    Semilab Co. Ltd., Prielle Kornélia u. 4/a, 1117 Budapest, Hungary
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    Orcidhttps://orcid.org/0000-0003-1028-9584
  • Sándor Kollarics
    Sándor Kollarics
    Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
    Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary
    Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest H-1525, Hungary
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  • Bence Gábor Márkus
    Bence Gábor Márkus
    Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary
    Stavropoulos Center for Complex Quantum Matter, Department of Physics and Astronomy, University of Notre Dame, Notre Dame, Indiana 46556, United States
    Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest H-1525, Hungary
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  • Andrzej Sienkiewicz
    Andrzej Sienkiewicz
    Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
    ADSresonances SARL, Route de Genève 60B, CH-1028 Prèverenges, Switzerland
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    Orcidhttps://orcid.org/0000-0003-3527-7379
  • Márton Kollár
    Márton Kollár
    Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
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    Orcidhttps://orcid.org/0000-0001-9335-2715
  • László Forró
    László Forró
    Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
    Stavropoulos Center for Complex Quantum Matter, Department of Physics and Astronomy, University of Notre Dame, Notre Dame, Indiana 46556, United States
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  • Ferenc Simon*
    Ferenc Simon
    Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
    Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary
    Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest H-1525, Hungary
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-9822-4309
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ACS Photonics

Cite this: ACS Photonics 2022, 9, 10, 3341–3350
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https://doi.org/10.1021/acsphotonics.2c00687
Published August 3, 2022

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Due to their exceptional photovoltaic properties, metal halide perovskites (MHPs) are extensively studied for their potential applications in solar cells. In recent years, the power conversion efficiencies of MHPs-based solar cells rapidly increased from the initial few % toward more than 25% for single-junction devices. Therefore, also taking into account their low costs and ease of manufacturing, MHPs-based solar cells have become the fastest-advancing photovoltaic technology. In this regard, much of the recent work has been dominated by absorber materials based on methylammonium MHPs, such as MAPbX3, where MA = CH3NH3 and X = Cl, Br, and I. Here, we present the results of contactless time-resolved photoconductivity measurements in an exceptionally wide range of temperatures of 4 to 290 K that were performed for the various crystalline forms of the three parent MAPbX3, that is, MAPbCl3, MAPbBr3, and MAPbI3. This approach was made possible by the use of a high quality factor (Q) microwave resonator, which cooperated with a commercially available microwave bridge equipped with an automatic frequency control (AFC) and a helium gas-flow cryostat. The structural phase transitions from orthorhombic to tetragonal are found to drastically affect the transient photoconductivity signal, and we also observe ultralong charge carrier recombination times approaching 70 μs at low temperatures. The difference caused by morphology on the photophysical properties is supported by a marked difference between rapidly cooled (quenched) and slowly cooled samples. The sensitive technique also allowed to observe differences between samples with different morphologies and crystallite sizes.

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Introduction

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Methylammonium lead halide perovskites (MHPs) emerged as potential candidates to replace conventional silicon-based solar cells due to their impressive photovoltaic properties, (1) relative ease of the crystallization process, (2) and relative insensitivity to defects. (3) Their additional applications include gas sensorics, (4) light-emitting diodes, (5) and gamma and neutron radiation detectors, (6) as well as their use in space, which has recently been proposed. (7)
The efficiency of MHP-based solar cells is already over 25% (8) and is increasing constantly toward the theoretical maximum of single junction solar cells at 33%. (9−11) Despite the enormous potential of MHPs, several difficulties have to be overcome before their widespread applications, including degradation issues caused by temperature, (12) water or humidity, (13) and oxygen. (14) The lead content of the material poses challenges for the environmentally friendly disposal of the material.
MHPs crystallize in the perovskite structure and are orthorhombic at low temperatures (typically below 170 K), followed by a tetragonal and a cubic structure as the temperature increases. (15,16) The degrees of freedom of the methylammonium (MA) group are gradually reduced upon lowering the symmetry, induced by the phase transitions at low temperatures. In the orthorhombic phase, the MA cations align in one of two directions, which results in localized domains with a built-in potential that can separate charge carrier pairs. (17−20) The phase transitions significantly affect the mobility of charge carriers; however, their role on the photovoltaic properties is yet unclear. The dominant factors that influence the photovoltaic properties of MHPs are a matter of debate, as both the methylammonium cation orientation and ion migration are affected by the temperature. (21−25)
In principle, temperature-dependent photoconductivity studies can resolve these open issues. In particular, the time-resolved microwave-detected photoconductivity decay (26,27) (TRMCD) measurement is a contactless, nondestructive method, which is widely used in the semiconductor industry to gain information about the wafer quality and purity as well about the charge carrier mobility and photoexcited charge carrier lifetime, τc. (28−30) The latter information is particularly important for photovoltaic applications as the magnitude of τc greatly influences the photovoltaic (PV) cell efficiency: it determines whether the lifetime of light-generated charge carriers is long enough to reach the device electrodes (31) before recombination. A study of τc as a function of temperature can provide information about the charge recombination mechanism and the influence of structural changes. In addition, the amplitude of the TRMCD signal is proportional to the photoconductivity of the material, which in turn characterizes the PV efficiency of the sample.
We studied the light-induced charge carrier lifetime τc and the transient photoconductivity ΔG in methylammonium lead halide perovskites (CH3NH3PbX3, X = I, Br, and Cl; denoted as MAPbX3 in the following) in the 4–270 K temperature range. We find clear evidence that the orthorhombic to tetragonal phase transition affects both τc and ΔG, while the influence of the tetragonal to cubic transition on the photoconductivity is less pronounced. The longest charge carrier lifetime of 68 μs was observed in MAPbBr3 at 10 K. We also find evidence that the sample crystallization procedure, as well as the cooling protocol, have observable roles. These results shed light on the interplay of photovoltaic-relevant properties and the structure in these materials, whose understanding can lead to further improvements in the PV efficiency.

Results and Discussion

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The result of the temperature-dependent microwave photoconductivity decay measurements for crystals of all three parent MHPs down to 4 K are shown in Figure 1. We show the temperature dependence of two properties: the charge carrier lifetime, τc, and the photoconductivity, ΔG. The former is determined from the time constant of the photoconductivity decay curves and the latter from its initial amplitude, as described in Methods and Tools. All photoconductivity decay curves can be described with a single exponent, indicating the presence of a dominant recombination channel.

Figure 1

Figure 1. Temperature-evolution of charge carrier lifetime τc (top) and photoconductance ΔG (bottom) for crystals of three parent MHPs, i.e., MAPbCl3, MAPbBr3, and MAPbI3. A marked change in both the recombination time and photoconductivity can be observed at the orthorhombic-to-tetragonal phase transition, which is indicated by vertical lines. The materials show ultralong charge carrier recombination times at low temperatures with the maximal value exceeding 68 μs for MAPbBr3. The measurements were made with a Q-switched nanosecond pulsed laser operating at 450 nm. The data points were recorded while raising the temperature with the TRMCD method.

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The orthorhombic to tetragonal phase transition is found to have a relatively smaller effect on τc in the case of MAPbBr3 and more prominent in the case of MAPbCl3 and MAPbI3, as shown in Figure 1. More interestingly, the orthorhombic-to-tetragonal phase transition has a well-pronounced impact on the photoconductivity, which was observed to increase by a factor of ∼10, ∼6, and ∼2.5, for MAPbCl3, MAPbBr3, and MAPbI3, respectively. The first order nature of the phase transition also manifests in a hysteretic behavior on the photoconductivity, which is presented in detail in Methods and Tools. This hysteretic behavior caused by the temporal coexistence of both phases around the phase transition was previously observed by Osherov et al. (32) There is also a generic trend that the photoconductivity reaches a maximum in the orthorhomic phase, which is followed by a decrease toward lower temperatures. The temperature for the maximum in the photoconductivity also shows a halide dependence; between the MAPbCl3 and the MAPbBr3, the observed narrow peak with the maxima shifted toward lower temperatures. There might be a similar peak for the MAPbI3 sample below our measurement range if there is a trend of decreasing peak temperature as the atomic size of halogens increases. The temperature of the peaks and the observed lifetimes are summarized in Table 1.
Table 1. Maximal Values of the Charge Carrier Lifetime, τc, for the Measured Methylammonium Lead Halide Perovskites, the Temperature Where the Value Is Attained, Tc, and the Temperature of the Maximal Value of Photoconductivity, TΔG
 Tc (K)τc (μs)TΔG (K)
MAPbCl31315.244
MAPbBr31068.312
MAPbI32628.248
The influence of the structure on the photoconducting properties was observed in ref (33) for MAPbI3 and MAPbBr3, which was associated with the ferroelectric ordering of the MA groups that results in a built-in electric field that assists the charge separation (thus, increasing the photoconductivity) and also increases the charge carrier lifetime. (21,22) The appearance of the maximum in ΔG at low temperatures, which is followed by a decreasing photoconductivity toward lower temperatures, is tentatively assigned to a yet unobserved but predicted antiferroelectric ordering of the methylammonium groups. (34) Such an ordering is expected to cancel the positive effect of the ferroelectric ordering, thus reducing the observed photoconductivity.
The tetragonal to cubic transition has no visible effect on either the charge carrier lifetime or the photoconductivity. This supports the earlier assignment that the MA ordering has a pronounced effect on the photovoltaic properties, which only occurs in the orthorhombic state. For crystals of MAPbBr3 and MAPbI3, we observe a gradual increase in τc with decreasing temperature, which can be attributed to gradually increasing constraints on orientational dynamics of MA cations upon transition to the orthorhombic phase. It is assumed that MA groups lose their orientational degree of freedom and only two possible orientations remain allowed. (20) This loss of movement is drastically different from the one observed at the cubic-tetragonal phase transition thus the enormous difference in the effect the change has on the recombination process. Polarized domains of unidirectional organic cations emerge inside the material resulting in a built-in local electric field that propels the generated charge carriers away from each other this way elevating the recombination time. (33)
We observe ultralong charge carrier lifetimes in all methylammonium lead halide perovskites at low temperatures, with the values summarized in Table 1. The observed values are all larger than those found in earlier TRMCD studies, (35−41) which may be related to an improved sample quality in the present study.
We also address the slight drop in the charge carrier lifetime at the phase transition from tetragonal to orthorhombic phase for the MAPbBr3 and MAPbI3 samples. We believe that it is due to the coexistence of orthorhombic and tetragonal domains right below the phase transition. Charge carriers are scattered at the domain boundaries thus lowering their lifetime. By further decreasing the temperature the whole crystal becomes orthorhombic and the domains disappear. The coexistence of a small number of domains in the tetragonal phase in the regime of the orthorhombic phase was observed previously. (42,43) Interestingly the MAPbCl3 sample shows the opposite change for the lifetime, as τc increases in the orthorhombic phase compared to the value observed in the tetragonal one.
The interplay between crystal structure and electronic properties can be suitably studied with varying the cooling speed. An extreme case of this investigation is when the sample is rapidly cooled or quenched from room temperature to 4 K. The construction of our cryostat (Oxford Instr. Sci.Div., Model ESR900) allows this by precooling it to 4 K and then immersing the sample holder, which contains the sample into it. Given the small sample volume, we estimate that it thermalizes within less than a few seconds. A similar protocol helped previously to stabilize otherwise thermodynamically unstable alkali fulleride structures. (44)
In Figure 2, the effect of quenching on a MAPbBr3 sample is presented. First, the sample was quenched from room temperature to 4 K and then subjected to a slow gradual heating, during which the TRMCD measurements were performed to get insight into the above-mentioned properties, that is, τc and ΔG. Then, the sample was cooled down once more, while it remained in place in the cryostat and warmed up again slowly. We compare the data for the warming up protocols. Consecutive slowly cooled and warmed up runs reproduce the previous slow cooling results.

Figure 2

Figure 2. Charge carrier lifetime, τc, and the photoconductivity, ΔG, in quenched and nonquenched MAPbBr3 cubic crystals as measured by TRMCD during slow gradual heating. In the orthorhombic phase, τc is significantly shorter for the quenched crystal than for its nonquenched counterpart (left panel). In contrast, ΔG shows the opposite trend, reaching much higher values for the quenched crystal (right panel). Note also a marked jump in both measured properties, τc and ΔG, at the orthorhombic-to-tetragonal phase transition. In particular, upon this phase transition, the initial large overshoot in τc for the quenched crystal quickly falls down to the level observed for its nonquenched counterpart (left panel).

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The photoconductivity increases below the tetragonal to orthorhombic phase transition for both types of experiments, however, the increase of ΔG is significantly larger for the quenched sample. It is important to note that the two set of ΔG data were not normalized as the sample was not removed from the cavity between the two measurements. While ΔG is larger for the quenched sample, the corresponding charge carrier lifetime is smaller and the low temperature peak is less pronounced. We believe that both effects can be assigned to a lower structural order in the orthorhombic phase, probably in the form of smaller ferroelectrically oriented structural domains. Small domain sizes give rise to increased scattering thus reducing τc. As we observed in the case of samples with varying morphology, the lowered symmetry increases the photoconductivity, which explains the behavior in the quenched sample.
Another compelling effect is the sudden upright jump in τc for the quenched sample immediately when it is warmed above the orthorhombic-to-tetragonal phase transition. Again, this effect is robust, and it was observed for several quenching runs. Although we do not have a consistent explanation for this effect, we propose that a possible coexistence of tetragonal and orthorhombic domains may play a role, which merits further studies, for example, by means of the crystallographic structural investigation in the quenched sample.
In Figure 3 we show the difference in the charge carrier lifetime and photoconductivity for three different morphologies of MAPbBr3 samples. The top panel in the figure shows photographs for these morphologies: a cube-shaped crystal, a less crystalline sample made using a different, acid–based growth process, and a sample consisting of needle-shaped crystallites. The synthesis routes for these sample types are detailed in the Methods section.

Figure 3

Figure 3. Impact of the crystalline morphology of MAPbBr3 on the charge carrier lifetime, τc, and the photoconductivity, ΔG, as probed by TRMCD. Top panel, from left to right: photographs of cubic, amorf (acid-based crystal growth), and needle-shaped samples (the scale is indicated by the raster grid lines on corresponding paper sheets or by a scale bar). The cubic samples reveal the longest charge carrier lifetimes, whereas the photoconductivity is largest for the acid-based samples. The ΔG data is normalized by the respective values above the orthorhombic-to-tetragonal phase transition.

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The sample properties display characteristic differences for the measured parameters: the charge carrier lifetime has a tendency to be longest for the sample with the best crystallinity (the cubic sample), whereas the photoconductivity is largest below the tetragonal-orthorhombic transition for the acid-based sample. We note that the latter statement relies on a normalization of the ΔG data in the tetragonal phase (as shown in Figure 3) since the measured photoconductivity in our method does not provide absolute values and depend on the sample amount. This normalization also attempts to eliminate artifacts such as a higher absorption cross section of the needle-shaped samples as compared to the cubic ones (e.g., due to the index of refraction discontinuity on the sample surface) as we expect that these effects would be the same irrespective when the sample undergoes the structural phase transition. We verified that the observed properties are reproducible for several different samples from various batches, produced with the same synthesis methods.
The impact of the crystalline morphology on the temperature dependencies of τc and ΔG is also worth discussing. The cube-shaped and the acid-based samples have similar temperature dependencies, since both show the longest charge carrier lifetimes at low temperatures. Likewise, for these both crystalline versions of MAPbBr3, ΔG reveals a pronounced jump at the orthorhombic-to-tetragonal phase transition (∼148–154 K), while for the needle-shaped sample slight step changes in τc and ΔG values are shifted toward lower temperatures by ∼15 K (to ∼135 K). Moreover, compared to cube-shaped and the acid-based samples, the needle-shaped sample shows significantly different temperature dependencies of τc and ΔG. In particular, the temperature-dependence of τc for the needle-shaped sample is almost featureless as compared to the other two samples.
These observations support the previous generic findings from the quenching experiments: lowering the crystal symmetry (such as in the orthorhombic phase) and inducing disorder either by quenching or a sample witch consists of multiple crystallites results in a shorter charge carrier lifetime, probably due to the presence of grain boundaries which act as recombination centers. However, at the same time, the induced disorder caused by quenching has a positive effect on the photoconductivity. On the other hand, a fully disordered sample, such as that observed for the needle-shaped ones, again result in reduced photoconductivity. We note that our result does not provide information about the recombination mechanism. It is known for standard photovoltaic materials (such as, e.g., Si) that three competing mechanisms contribute to this: impurity-assisted recombination (also known as Shockley–Read–Hall), radiative recombination, and Auger process.
The efficiency of a photovoltaic cell depends on both of these factors: the charge carrier lifetime, which determines whether the electron–hole pair can reach the respective electrodes, and the photoconductivity, which is directly proportional to the number of generated charge carrier pairs. Our observations suggest that the optimal sample morphology has to be determined with this trade-off in mind, for example, for a thin film photovoltaic cell, where a short charge carrier lifetime limits little the performance, a sample type with low crystallinity is preferred, whereas for a cell where a large current capability or mechanical robustness is required, the cubic sample type is the preferable.

Conclusions

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To summarize, we studied the photoconductivity and the charge carrier lifetime of photoinduced charge carriers using a microwave reflectometric method down to liquid helium temperatures for the three methylammonium lead halide perovskites (CH3NH3PbX3, X = I, Br, and Cl). An ultralong charge carrier recombination time in the orthorhombic phase for each methylammonium lead halide perovskite was observed. The structural changes have a clear effect on both of these parameters, namely the low temperature orthorhombic phase, where a ferroelectric ordering of the methylammonium cation groups is present, shows improved properties, which may be associated with improved photovoltaic efficiencies. These findings are corroborated by rapidly cooled studies which indicates structural disorder and also on samples with varying morphologies. Our findings shed light on the delicate interplay between structure and PV efficiencies in these materials, which may help to construct a variant where the improved properties are retained at room temperature.

Methods and Tools

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Sample Preparation

Acid growth: Crystals of the methylammonium lead trihalide (CH3NH3PbX3, X = Br, Cl, I) were synthesized by solution growth. The 3.3 mmol lead(II) acetate trihidrate (Pb(ac)2 × 3H2O, >99.9%) was reacted with 6 mL of saturated HX solution (saturated HX solution in H2O). The formed PbX2 precipitate is stable in the acidic solution. The respective amount (3.30 mmol) of methylamine (CH3NH2) solution (40 wt % in H2O) was pipetted into the 5 °C ice-cooled solutions of PbX2. The cold solution avoids the evaporation of methylamine during the exothermic reaction. The microcrystallites of CH3NH3PbX3 were formed immediately and settled down at the bottom of the vessel. In a temperature gradient of 15 °C in the acidic media, large (1–8 mm) perovskite single crystals grew on the cold side of the vessel. Inverse temperature growth: MAPbX3 single crystals were synthesized using the inverse temperature crystallization method from its saturated solution in dimethylformamide (DMF). MAPbBr3 was dissolved in DMF at room temperature. Crystal growth was initialized by increasing the temperature of the solution from room temperature to 50 °C with a heating rate of 0.5 °C per hour, as depicted. Nice cubic-like, centimeter-sized single crystals can be harvested after only a couple of hours of crystallogenesis. Besides the bromide version, MAPbI3 and MAPbCl3 can be grown using the identical method, with γ-butyrolactone (GBL) and 50 v/v% DMF/50 v/v% dimethyl sulfoxide (DMSO) in the case of perovskites containing iodide and chloride, respectively.
Needle-shaped MAPbBr3 crystals: Needle-shaped perovskite crystals were grown by cooling down the room temperature saturated perovskite solution in DMF to −15 °C, where needle shape crystals were formed. The harvest of these crystals was done at low temperature and the DMF solvent was removed by freeze-drying.

Microwave-Detected Photoconductivity Measurement

In this study we present a simple and so far not implemented approach to the contactless probing of photoconductivity with microwaves for the crystalline forms of MHPs compounds, namely MAPbI3, MAPbBr3 and MAPbCl3. In particular, our experimental setup for contactless measurement of light induced changes in conductivity is based on commercially available building blocks, which can be found in typical electron spin resonance spectrometers. Thanks to this, we were able to take advantage of two key elements, which have been optimized to carry out sensitive measurements of the paramagnetic susceptibility. The sensitivity of the measurement is ensured by placing the sample in a microwave resonator with high Q-factor (loaded Q-factor value, QL ≈ 6550) and by employing a frequency-stabilized microwave source, which, thanks to automatic frequency control, locks the resonant frequency of the microwave generator to that of the microwave resonator. This solution allows for the sensitive measurement of small changes in the magnetic susceptibility occurring in the sample under the influence of a changing magnetic field. Furthermore, the automatic frequency control takes care if the resonator frequency shift this way eliminating the dispersive component resulting in the detection of the absorptive component only. We utilized this characteristic property of the microwave bridge and cavity of a standard electron spin resonance spectrometer to separate the dispersive and absorptive components of photoconductance. The novelty of the experimental setup lies in the combination of the type of microwave cavity, the placement of the sample in relation to the microwave field and the method of bringing the exciting laser light to the samples.
The diagram of the experimental system that was used in this work for the microwave measurement of photoconductivity in parent MHPs is shown in Figure 4. The sensitive detection is based on using the microwave bridge of a commercial electron spin resonance spectrometer (Bruker Elexsys E500). The use of a microwave cavity amplifies the effect of light induced changes in the photoconductivity, as shown in refs (30and45−47), whereas its finite bandwidth only slightly limits the detectable transient signals to τtr = 2Q/ω0. Here, Q ≈ 6550 is the quality factor of the loaded cavity and ω0 ≈ 2π × 9.5 GHz, which gives τtr ≈ 220 ns. This also means that we inevitably miss any faster dynamic processes. However, the known typical recombination times are usually above 1 μs. Given that the targeted decay lifetimes are beyond the μs time scale, this does not significantly affect the measurement. We note that the microwave output of the Bruker broadband signal preamplifier (20 Hz to 6.5 MHz, model ER 047-PH) follows a low-pass filter characteristic with a bandwidth of 6 MHz, which also gives a limiting transient time of τtr = 1/2π × 6 MHz = 25 ns, again not affecting our measurements.

Figure 4

Figure 4. Experimental setup used in this work for microwave photoconductivity measurements. (a) Block diagram of the system including its modifications corresponding to TRMCD and quasi-steady state measurements. (b) Schematic representation of the laser light delivery to a sample placed in the center of a cylindrical resonator operating in TE011 mode. (c) Illustration of the light delivery to the incission in the 4 mm diameter quartz rod where the sample is mounted. Photos of the bottom part of the quartz rod taken: without illumination, under illumination, and under illumination and with the MAPbBr3 crystal attached are shown in the left, middle, and right panels, respectively. For the purpose of visualization of the light delivery to the sample, the photos were taken under illumination with a fiber-coupled CW LED light source (λ = 470 nm, ADSresonances Sàrl).

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The sample is positioned at the center of the cylindrical TE011 microwave cavity (BioSpin high-Q cavity, Model ER 4122SHQE) inside a double-walled quartz insert, which sustains the helium gas flow, thus allowing to control the sample temperature between 4 and 270 K. The samples are in the center of the TE011 cavity where the microwave electric field has a node, and the magnetic field has its maximum. However, the exact center is surrounded by a circularly rotating electric field which induces currents in the sample whose loss is detected. (48) The automatic frequency control (AFC) of the commercial CW X-band microwave bridge (Model ER049X from Bruker BioSpin) locks the frequency of the microwave source (a Gunn diode) on the actual resonance frequency of the cavity. Thanks to this, it is possible to relatively easily isolate the real (dispersive) and the imaginary (absorptive) parts of the changes in the overall sample susceptibility. (49,50) Even when the temperature affects the latter. The automatic frequency control of the microwave bridge has a frequency of 76.8 kHz. Such an experimental setup significantly facilitates the direct measurement of the changes in the Q-factor of the microwave cavity, which are related to the light-induced changes in the sample conductivity.
In order to be able to bring light to the sample positioned at the center of a high-Q resonant cavity (otherwise devoid of illumination slots), we used a 25 cm long section of a clear fused quartz rod with diameter of 4.0 mm (Spectrosil 2000 from Heraeus Quarzglas GmbH & Co., Kleinostheim, Germany), which, due to its high refraction index (of ∼1.46 at 470 nm), served as an efficient light-guide. The laser beam light was focused on the top edge of the quartz rod, while the sample was attached (glued) at its bottom notch. This method of excitation is similar to the one presented by Mantulnikovs et al. (51)
We used two different measurement setups, which served for quasi-steady state (QSS) and time-resolved (TR) microwave photoconductivity measurements. In the case of the QSS approach, the beam of the laser light (473 nm, 50 mW) from a CW fiber-coupled laser (Model MBL-III-473, from CNI Lasers Ltd.) was modulated with a mechanical optical chopper (Model SR540 from Stanford Research Systems, Inc.). To perform TR measurements, we used a Q-switched nanosecond pulsed laser-diode system (Model NPL45C from Thorlabs Inc.) which operated at 450 nm, with a pulse duration of 6–129 ns and a pulse energy of 200 nJ. The wavelength of the Q-switched laser is close to the optical absorption edge in the case of MAPbCl3, (52,53) while according to our results, the excitation of the sample was achieved sufficiently nonetheless.
The spectrometer output is fed to a lock-in amplifier (Stanford Research Systems SR830) for the QSS and to an oscilloscope for the TR measurement. A representative time-trace measurement is also shown in Figure 5. The clear advantage of the lock-in measurement is its higher sensitivity due to the well-controlled measurement bandwidth, however its disadvantage is that the measured signal is the product of the photoconductivity and the charge carrier lifetime thus the two quantities cannot be separated.

Figure 5

Figure 5. Example time-traces of the measurement fitted with a single exponent. The two traces were recorded during the same temperature sweep and with the same excitation. The time-traces were collected during the measurement presented in Figure 2.

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The differences and similarities between the two types of measurements are demonstrated in Figure 6, where the photoconductivity is presented near the tetragonal-orthorhombic phase transition for CH3NH3PbBr3. The hysteresis in the photoconductivity is much better observed for the QSS technique as more points can be obtained for the same measurement time and warming protocol due to the higher sensitivity of the lock-in detection technique.

Figure 6

Figure 6. Hysteresis of the microwave photoconductivity signal at the tetragonal-orthorhombic phase transition for the time-resolved (left) and the quasi-steady-state (right) measurements for CH3NH3PbBr3. Note the larger number of points for the QSS measurement as a result of the noise-free lock-in technique.

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In the following, we show that the relationship between the spectrometer output and the sample photoconductivity is linear. In high frequency measurements, the properties of materials are conveniently described by the concept of surface impedance at a finite electromagnetic frequency of ω: (54,55)
(1)
which contains the effects of magnetism (through the permeability, μ), dielectrics (through the permittivity, ϵ), and conductivity (through σ).
A sample with surface impedance Zs affects both the resonance frequency, f0 and the quality factor, Q of the cavity through: (56,57)
(2)
where Δf is the resonance frequency shift of the empty cavity and ν is the so-called filling factor, roughly equal to the sample volume per the cavity volume ratio. This expression shows that both the resonance frequency shift and the Q are directly connected to the surface impedance of the sample. The microwave bridge of our spectrometer was set up to detect the change in the Q factor only, thus changes in the imaginary part of Zs is measured. (58) The instrument output is linear in .
The illumination causes an extra conductivity of the sample, Δσ, which also affects the sample surface impedance as
(3)
Given that the MHP materials are poor conductors without illumination, ωϵ ≫ σ thus a series expansion with the small parameter Δσ yields
(4)
Therefore, the excess spectrometer output due to the illumination is linear in Δσ, as expected.

Charge Carrier Generation

The change in the number of charge carrier pairs in a given sample is represented by the so-called rate equations:
(5)
where τ is the trap-limited charge carrier lifetime, Gphoto is the carrier generation rate, and n is the carrier concentration. The differential equation can be solved in different regimes, including the start of a laser pulse, a quasi steady state, and the relaxation of the generated charge carriers following a laser pulse. The general solution for this equation gives the following time-dependent charge carrier concentration:
(6)
where c is constant determined by the initial conditions. In the case where the laser pulse is just beginning, increasing the charge carrier concentration the n(0) = 0 assumption can be made, resulting in the following solution:
(7)
Since the carrier lifetime is orders of magnitude longer than the full width at half-maximum of the laser pulse the recombination process will not be significant during the time frame. This can be shown by calculating the Taylor series around t = 0, which gives . This leads to the charge carrier concentration during the laser pulse to be proportional only to the carrier generation rate:
(8)
By assuming a quasi steady state (n′ = 0) for the same differential equation yields
(9)
If one is looking at the relaxation of the charge carriers the charge carrier generation should be assumed to be zero (Gphoto = 0). This leads to a solution that consists of only an exponential component:
(10)
where n0 is the initial population.

Author Information

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  • Corresponding Author
    • Ferenc Simon - Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;  Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary;  Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest H-1525, Hungary;  Orcidhttps://orcid.org/0000-0001-9822-4309;  Email: [email protected]
  • Authors
    • András Bojtor - Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;  Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary;  Semilab Co. Ltd., Prielle Kornélia u. 4/a, 1117 Budapest, Hungary;  Orcidhttps://orcid.org/0000-0003-1028-9584
    • Sándor Kollarics - Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;  Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary;  Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest H-1525, Hungary
    • Bence Gábor Márkus - Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary;  Stavropoulos Center for Complex Quantum Matter, Department of Physics and Astronomy, University of Notre Dame, Notre Dame, Indiana 46556, United States;  Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest H-1525, Hungary
    • Andrzej Sienkiewicz - Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;  ADSresonances SARL, Route de Genève 60B, CH-1028 Prèverenges, Switzerland;  Orcidhttps://orcid.org/0000-0003-3527-7379
    • Márton Kollár - Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;  Orcidhttps://orcid.org/0000-0001-9335-2715
    • László Forró - Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;  Stavropoulos Center for Complex Quantum Matter, Department of Physics and Astronomy, University of Notre Dame, Notre Dame, Indiana 46556, United States
  • Author Contributions

    Samples were prepared by M.K. under the guidance of L.F. Magnetic resonance and photoluminescence characterization was provided by A.S. A.B., S.K., B.G.M., and F.S. performed the photoconductivity measurements, and the data were analyzed by A.B. All authors contributed to writing the paper.

  • Funding

    This work was supported by the Hungarian National Research, Development and Innovation Office (NKFIH), Grants K137852, TKP2021-EGA-02, TKP2021-NVA-02, and the V4-Japan program, and the Quantum Information National Laboratory sponsored by the Ministry of Innovation and Technology via the NKFIH. This work was supported by the KDP-2021 Program of the Ministry for Innovation and Technology from the source of the National Research, Development and Innovation Fund.

  • Notes
    The authors declare no competing financial interest.

References

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This article references 58 other publications.

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    Aristidou, Nicholas; Sanchez-Molina, Irene; Chotchuangchutchaval, Thana; Brown, Michael; Martinez, Luis; Rath, Thomas; Haque, Saif A.
    Angewandte Chemie, International Edition (2015), 54 (28), 8208-8212CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In this paper we report on the influence of light and oxygen on the stability of CH3NH3PbI3 perovskite-based photoactive layers. When exposed to both light and dry air the mp-Al2O3/CH3NH3PbI3 photoactive layers rapidly decomp. yielding methylamine, PbI2, and I2 as products. We show that this degrdn. is initiated by the reaction of superoxide (O2-) with the methylammonium moiety of the perovskite absorber. Fluorescent mol. probe studies indicate that the O2- species is generated by the reaction of photoexcited electrons in the perovskite and mol. oxygen. We show that the yield of O2- generation is significantly reduced when the mp-Al2O3 film is replaced with an mp-TiO2 electron extn. and transport layer. The present findings suggest that replacing the methylammonium component in CH3NH3PbI3 to a species without acid protons could improve tolerance to oxygen and enhance stability.
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  15. 15
    Poglitsch, A.; Weber, D. Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy. J. Chem. Phys. 1987, 87, 6373– 6378,  DOI: 10.1063/1.453467
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    15
    Dynamic disorder in methylammonium trihaloplumbates(II) observed by millimeter-wave spectroscopy
    Poglitsch, A.; Weber, D.
    Journal of Chemical Physics (1987), 87 (11), 6373-8CODEN: JCPSA6; ISSN:0021-9606.
    The temp.-dependent structure of cryst. methylammonium trihaloplumbates(II) CH3NH3+PbX3- (X = Cl, Br, I) as detd. by x-ray diffraction, was compared with measurements of the temp.-dependent complex permittivity at frequencies of 50-150 GHz. The dielec. measurements reveal a ps relaxation process which corresponds to a dynamic disorder of the methylammonium group in the high-temp. phases of the trihaloplumbates.
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  16. 16
    Ava, T. T.; Al Mamun, A.; Marsillac, S.; Namkoong, G. A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells. Appl. Sci. 2019, 9, 188,  DOI: 10.3390/app9010188
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    16
    A review: thermal stability of methylammonium lead halide based perovskite solar cells
    Ava, Tanzila Tasnim; Mamun, Abdullah Al; Marsillac, Sylvain; Namkoong, Gon
    Applied Sciences (2019), 9 (1), 188CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
    Perovskite solar cells have achieved photo-conversion efficiencies greater than 20 %, making them a promising candidate as an emerging solar cell technol. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degrdn. can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temps. can be over 45 °C higher than ambient temps. when operating under direct sunlight. Herein, recent advances in resolving thermal stability problems are highlighted through literature review. Moreover, the most recent and promising strategies for overcoming thermal degrdn. are also summarized.
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  17. 17
    Chen, T.; Foley, B. J.; Ipek, B.; Tyagi, M.; Copley, J. R. D.; Brown, C. M.; Choi, J. J.; Lee, S.-H. Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite. Phys. Chem. Chem. Phys. 2015, 17, 31278– 31286,  DOI: 10.1039/C5CP05348J
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    17
    Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite
    Chen, Tianran; Foley, Benjamin J.; Ipek, Bahar; Tyagi, Madhusudan; Copley, John R. D.; Brown, Craig M.; Choi, Joshua J.; Lee, Seung-Hun
    Physical Chemistry Chemical Physics (2015), 17 (46), 31278-31286CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown impressive power conversion efficiencies of above 20%. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH3NH3+ cations and their impact on relevant processes such as charge recombination and exciton dissocn. are still poorly understood. Here, using elastic and quasi-elastic neutron scattering techniques and group theor. anal., we studied rotational modes of the CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327 K) and tetragonal (165 K < T < 327 K) phases, the CH3NH3+ ions exhibit four-fold rotational symmetry of the C-N axis (C4) along with three-fold rotation around the C-N axis (C3), while in the orthorhombic phase (T < 165 K) only C3 rotation is present. At around room temp., the characteristic relaxation times for the C4 rotation are found to be τC4 ≈ 5 ps while for the C3 rotation τC3 ≈ 1 ps. The T-dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C4 rotational mode and the temp. dependent dielec. permittivity. Our findings on the rotational dynamics of CH3NH3+ and the assocd. dipole have important implications for understanding the low exciton binding energy and a slow charge recombination rate in CH3NH3PbI3 which are directly relevant for the high solar cell performance.
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  18. 18
    Bernard, G. M.; Wasylishen, R. E.; Ratcliffe, C. I.; Terskikh, V.; Wu, Q.; Buriak, J. M.; Hauger, T. Methylammonium Cation Dynamics in Methylammonium Lead Halide Perovskites: A Solid-State NMR Perspective. J. Phys. Chem. A 2018, 122, 1560– 1573,  DOI: 10.1021/acs.jpca.7b11558
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    18
    Methylammonium Cation Dynamics in Methylammonium Lead Halide Perovskites: A Solid-State NMR Perspective
    Bernard, Guy M.; Wasylishen, Roderick E.; Ratcliffe, Christopher I.; Terskikh, Victor; Wu, Qichao; Buriak, Jillian M.; Hauger, Tate
    Journal of Physical Chemistry A (2018), 122 (6), 1560-1573CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    In light of the intense recent interest in the methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temp. via 2H, 14N, and 207Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ2, for the C-N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temp. dependence of the cation ordering is rationalized using a six-site model, with two equiv. sites along the c-axis and four equiv. sites either perpendicular or approx. perpendicular to this axis. As the cubic ↔ tetragonal phase transition temp. is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, 2H NMR line shapes indicate that the C-N axis is essentially frozen.
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  19. 19
    Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goñi, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide. Phys. Rev. B 2015, 92, 144308,  DOI: 10.1103/PhysRevB.92.144308
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    19
    Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide
    Brivio, Federico; Frost, Jarvist M.; Skelton, Jonathan M.; Jackson, Adam J.; Weber, Oliver J.; Weller, Mark T.; Goni, Alejandro R.; Leguy, Aurelien M. A.; Barnes, Piers R. F.; Walsh, Aron
    Physical Review B: Condensed Matter and Materials Physics (2015), 92 (14), 144308/1-144308/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behavior, with successive transitions between orthorhombic, tetragonal, and cubic polymorphs around 165 and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equil. structures compare well to solns. of temp.-dependent powder neutron diffraction. By following the normal modes, we calc. IR and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degrdn. of the sample. Despite a clear sepn. in energy between low-frequency modes assocd. with the inorg. (PbI3-)n network and high-frequency modes of the org. CH3NH3+ cation, significant coupling between them is found, which emphasizes the interplay between mol. orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temp.
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  20. 20
    Weller, M. T.; Weber, O. J.; Henry, P. F.; Di Pumpo, A. M.; Hansen, T. C. Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K. Chem. Commun. 2015, 51, 4180– 4183,  DOI: 10.1039/C4CC09944C
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    20
    Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K
    Weller, Mark T.; Weber, Oliver J.; Henry, Paul F.; Di Pumpo, Antonietta M.; Hansen, Thomas C.
    Chemical Communications (Cambridge, United Kingdom) (2015), 51 (20), 4180-4183CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The methylammonium cation in [CH3NH3]PbI3 demonstrates increasing positional disorder on heating from 100 K to 352 K. In the tetragonal phase, stable between 165 K and 327 K, the cation is disordered over four sites directed toward the faces of the distorted cubic [PbI3]- framework and migrates towards the cavity center with increasing temp. Crystallog. data and at. coordinates are given.
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  21. 21
    Quarti, C.; Mosconi, E.; De Angelis, F. Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell Operation. Chem. Mater. 2014, 26, 6557– 6569,  DOI: 10.1021/cm5032046
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    21
    Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell
    Quarti, Claudio; Mosconi, Edoardo; De Angelis, Filippo
    Chemistry of Materials (2014), 26 (22), 6557-6569CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The ab initio electronic structure and Car-Parrinello mol. dynamics simulations are reported for several structural models of the prototype MAPbI3 perovskite for solar cells applications. Both configurations having a preferred orientation are considered of the MA cations, giving rise to a net dipole alignment, and configurations with an isotropic distribution of the MA cations, resp. representative of polar (ferroelec.) and apolar (antiferroelec.) structures. The calcns. demonstrate the preferred stability of a set of polar structures over apolar ones, with an energy difference within 0.1 eV and a conversion barrier within 0.2 eV per unit cell (four MAPbI3), thus possibly accessible at room temp. Ferroelec.-like orientations lead to a quasi I4cm structure for the inorg. component, characterized by lack of inversion symmetry, while the antiferroelec.-like orientations are assocd. to a quasi I4/mcm structure. Ab initio mol. dynamics simulations on the polar structures show no mol. rotations in the investigated time-scale, while several MA rotations are obsd. in the same time scale for the considered apolar structure, which is thus characterized by a higher disorder. The I4cm and I4/mcm types of structure have markedly different band structures, despite showing a relatively small band gap variation. Simulations carried out on finite surface slabs demonstrate that a net orientation of the MA cations gives rise to a strong bending in the valence and conduction bands, which could definitely assist charge sepn. and reduce carrier recombination, provided one is able to effectively stabilize polar compared to apolar structures. The results could contribute an important step toward an in-depth comprehension of the basic properties of organohalide perovskites, assisting a further optimization of their photovoltaic response.
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  22. 22
    Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584– 2590,  DOI: 10.1021/nl500390f
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    22
    Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells
    Frost, Jarvist M.; Butler, Keith T.; Brivio, Federico; Hendon, Christopher H.; van Schilfgaarde, Mark; Walsh, Aron
    Nano Letters (2014), 14 (5), 2584-2590CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitized and org. photovoltaics. High-power conversion efficiency can be realized in both mesoporous and thin-film device architectures. The authors address the origin of this success in the context of the materials chem. and physics of the bulk perovskite as described by electronic structure calcns. In addn. to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As the authors show, hybrid perovskites exhibit spontaneous elec. polarization; the authors also suggest ways in which this can be tuned through judicious choice of the org. cation. The presence of ferroelec. domains will result in internal junctions that may aid sepn. of photoexcited electron and hole pairs, and redn. of recombination through segregation of charge carriers. The combination of high dielec. const. and low effective mass promotes both Wannier-Mott exciton sepn. and effective ionization of donor and acceptor defects. The photoferroic effect could be exploited in nanostructured films to generate a higher open circuit voltage and may contribute to the current-voltage hysteresis obsd. in perovskite solar cells.
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  23. 23
    Kim, H.-S.; Kim, S. K.; Kim, B. J.; Shin, K.-S.; Gupta, M. K.; Jung, H. S.; Kim, S.-W.; Park, N.-G. Ferroelectric Polarization in CH3NH3PbI3 Perovskite. J. Phys. Chem. Lett. 2015, 6, 1729– 1735,  DOI: 10.1021/acs.jpclett.5b00695
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    23
    Ferroelectric Polarization in CH3NH3PbI3 Perovskite
    Kim, Hui-Seon; Kim, Sung Kyun; Kim, Byeong Jo; Shin, Kyung-Sik; Gupta, Manoj Kumar; Jung, Hyun Suk; Kim, Sang-Woo; Park, Nam-Gyu
    Journal of Physical Chemistry Letters (2015), 6 (9), 1729-1735CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    We report on ferroelec. polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination. Perovskite crystals with three different sizes of 700, 400, and 100 nm were prepd. for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of elec. field, where the size dependency to polarization was not significant. Whereas the photoinduced stimulation was not significant without an external elec. field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelec. polarization was also obsd. after removal of the elec. field, in which larger crystals showed longer retention behavior compared to the smaller sized one. Addnl., we suggest the effect of perovskite crystal size (morphol.) on charge collection at the interface of the ferroelec. material even though insignificant size dependency in elec. polarization was obsd.
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  24. 24
    Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P. Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films. J. Phys. Chem. Lett. 2014, 5, 3335– 3339,  DOI: 10.1021/jz501697b
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    24
    Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films
    Kutes, Yasemin; Ye, Linghan; Zhou, Yuanyuan; Pang, Shuping; Huey, Bryan D.; Padture, Nitin P.
    Journal of Physical Chemistry Letters (2014), 5 (19), 3335-3339CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelec., almost nothing is known about their ferroelec. properties exptl. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelec. domains in high-quality β-CH3NH3PbI3 perovskite thin films that have been synthesized using a new soln.-processing method. The size of the ferroelec. domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelec. domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelec. behavior of hybrid perovskites, in particular in situ photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics.
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  25. 25
    Beilsten-Edmands, J.; Eperon, G. E.; Johnson, R. D.; Snaith, H. J.; Radaelli, P. G. Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices. Appl. Phys. Lett. 2015, 106, 173502,  DOI: 10.1063/1.4919109
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    25
    Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices
    Beilsten-Edmands, J.; Eperon, G. E.; Johnson, R. D.; Snaith, H. J.; Radaelli, P. G.
    Applied Physics Letters (2015), 106 (17), 173502/1-173502/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelec. response. A strong frequency dependence of the hysteresis is obsd. in the range of 0.1-150 Hz, with a hysteretic charge d. in excess of 1000 μC cm-2 at frequencies below 0.4 Hz, a behavior uncharacteristic of a ferroelec. response. We show that the obsd. hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of ≈ 1 μC cm-2 on any intrinsic frequency-independent polarization, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices. (c) 2015 American Institute of Physics.
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    Kunst, M.; Beck, G. The study of charge carrier kinetics in semiconductors by microwave conductivity measurements. J. Appl. Phys. 1986, 60, 3558– 3566,  DOI: 10.1063/1.337612
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    The study of charge carrier kinetics in semiconductors by microwave conductivity measurements
    Kunst, M.; Beck, G.
    Journal of Applied Physics (1986), 60 (10), 3558-66CODEN: JAPIAU; ISSN:0021-8979.
    The reliability and the possibility of microwave cond. measurements were investigated. The general background for excess cond. measurements in the microwave range, is first presented, and then a quant. relation between the reflected microwave signal and the change in cond. for a wafer of single-crystal Si is derived. For this sample, the theory of excess current-carrier kinetics is also developed. After a short description of the app., kinetic measurements on a ns and μs timescale are compared to theory.
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    The study of charge carrier kinetics in semiconductors by microwave conductivity measurements. II
    Kunst, M.; Beck, G.
    Journal of Applied Physics (1988), 63 (4), 1093-8CODEN: JAPIAU; ISSN:0021-8979.
    In previous work, an introduction was given to the study of charge carrier kinetics in semiconductors by microwave cond. measurements. This paper compares quant. the exptl. results to theor. calcns. for single-crystal Si wafers, taking into account the dependence on the microwave frequency and the dark cond. of the sample, which ranged from σ = 0.2 to 400 Ω-1 m-1. In particular, difficulties arising from exptl. conditions that cannot easily be treated by theory are discussed. Quant. measurements of samples with low dark cond. can be performed even in a very simple configuration, which permits detn. of the sum of charge-carrier mobilities.
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    Sinton, R. A.; Cuevas, A.; Stuckings, M. Quasi-steady-state photoconductance, a new method for solar cell material and device characterization Conference Record of the Twenty Fifth IEEE Photovoltaic Specialists Conference - 1996, IEEE, 1996; pp 457– 460.
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    Goodarzi, M.; Sinton, R.; Macdonald, D. Quasi-steady-state photoconductance bulk lifetime measurements on silicon ingots with deeper photogeneration. AIP Adv. 2019, 9, 015128,  DOI: 10.1063/1.5086378
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    Quasi-steady-state photoconductance bulk lifetime measurements on silicon ingots with deeper photogeneration
    Goodarzi, Mohsen; Sinton, Ronald; MacDonald, Daniel
    AIP Advances (2019), 9 (1), 015128/1-015128/6CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
    Quasi-steady-state photoconductance measurements on silicon ingots and blocks with different photo-generation profiles are simulated in this work. The results show that deeper generation profiles, achieved by using a long-pass optical filter with a longer cut-off wavelength, can reduce the impact of the high surface recombination velocity of the ingot surface. This results in higher measured effective lifetimes and reduces the reliance on transfer functions to convert the measured lifetimes into bulk lifetimes. However, there exists a trade-off between generating carriers further from the surface to reduce surface recombination and ensuring that the generated carriers are within the sensitivity range of the photoconductance sensing coil. The simulations are compared with exptl. results measured on a monocryst. silicon block using both quasi-steady-state and transient photoconductance decay, as the transient method is relatively less prone to the impact of surface recombination, and provides a lower bound on the bulk lifetime. The results confirm an increased accuracy in bulk lifetimes extd. from quasi-steady-state measurements when measured using deeper photo-generation profiles. (c) 2019 American Institute of Physics.
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    Gyüre-Garami, B.; Blum, B.; Sági, O.; Bojtor, A.; Kollarics, S.; Csősz, G.; Márkus, B. G.; Volk, J.; Simon, F. Ultrafast sensing of photoconductivity decay using microwave resonators. J. Appl. Phys. 2019, 126, 235702,  DOI: 10.1063/1.5129597
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    Osherov, A.; Hutter, E. M.; Galkowski, K.; Brenes, R.; Maude, D. K.; Nicholas, R. J.; Plochocka, P.; Bulović, V.; Savenije, T. J.; Stranks, S. D. The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide Perovskites. Adv. Mater. 2016, 28, 10757– 10763,  DOI: 10.1002/adma.201604019
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    The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide Perovskites
    Osherov, Anna; Hutter, Eline M.; Galkowski, Krzysztof; Brenes, Roberto; Maude, Duncan K.; Nicholas, Robin J.; Plochocka, Paulina; Bulovic, Vladimir; Savenije, Tom J.; Stranks, Samuel D.
    Advanced Materials (Weinheim, Germany) (2016), 28 (48), 10757-10763CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The extent to which the soft structural properties of metal halide perovskites affect their optoelectronic properties is unclear. X-ray diffraction and micro-photoluminescence measurements are used to show that there is a coexistence of both tetragonal and orthorhombic phases through the low-temp. phase transition, and that cycling through this transition can lead to structural changes and enhanced optoelectronic properties.
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  33. 33
    Gélvez-Rueda, M. C.; Cao, D. H.; Patwardhan, S.; Renaud, N.; Stoumpos, C. C.; Schatz, G. C.; Hupp, J. T.; Farha, O. K.; Savenije, T. J.; Kanatzidis, M. G.; Grozema, F. C. Effect of Cation Rotation on Charge Dynamics in Hybrid Lead Halide Perovskites. J. Phys. Chem. C 2016, 120, 16577– 16585,  DOI: 10.1021/acs.jpcc.6b06722
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    Effect of Cation Rotation on Charge Dynamics in Hybrid Lead Halide Perovskites
    Gelvez-Rueda, Maria C.; Cao, Duyen H.; Patwardhan, Sameer; Renaud, Nicolas; Stoumpos, Constantinos C.; Schatz, George C.; Hupp, Joseph T.; Farha, Omar K.; Savenije, Tom J.; Kanatzidis, Mercouri G.; Grozema, Ferdinand C.
    Journal of Physical Chemistry C (2016), 120 (30), 16577-16585CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Org.-inorg. hybrid halide perovskites are a promising class of materials for photovoltaic application with reported power efficiencies over ∼22%. However, not much is known about the influence of the org. dipole rotation and phase transitions on charge carrier dynamics. Here, the authors report substantial changes in mobility and lifetime of charge carriers in CH3NH3PbI3 after the low-temp. tetragonal (β) to orthorhombic (γ) phase transition. By using microwave cond. measurements, the mobility and lifetime of ionized charge carriers increase as the temp. decreases and a sudden increment is seen after the β-γ phase transition. For CH3NH3PbI3, the mobility and the half-lifetime increase by a factor of 3-6 compared with the values before the β-γ phase transition. The authors attribute the considerable change in the dynamics at low temp. to the decrease of the inherent dynamic disorder of the org. cation (CH3NH3+) inside the perovskite crystal structure.
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  34. 34
    Leguy, A. M. A.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kockelmann, W.; Law, C.; Li, X.; Foglia, F.; Walsh, A.; O’Regan, B. C.; Nelson, J.; Cabral, J. T.; Barnes, P. R. F. The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells. Nat. Commun. 2015, 6, 7124,  DOI: 10.1038/ncomms8124
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    The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells
    Leguy Aurelien M A; McMahon Andrew P; Nelson Jenny; Barnes Piers R F; Frost Jarvist Moore; Walsh Aron; Sakai Victoria Garcia; Kochelmann W; Law ChunHung; Li Xiaoe; O'Regan Brian C; Foglia Fabrizia; Cabral Joao T
    Nature communications (2015), 6 (), 7124 ISSN:.
    Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3(+) ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1-200-ps time window. Monte Carlo simulations of interacting CH3NH3(+) dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3(+) in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3(+) to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1-1 ms, faster than most observed hysteresis.
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  35. 35
    Semonin, O. E.; Elbaz, G. A.; Straus, D. B.; Hull, T. D.; Paley, D. W.; van der Zande, A. M.; Hone, J. C.; Kymissis, I.; Kagan, C. R.; Roy, X.; Owen, J. S. Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals. J. Phys. Chem. Lett. 2016, 7, 3510– 3518,  DOI: 10.1021/acs.jpclett.6b01308
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    Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals
    Semonin, Octavi E.; Elbaz, Giselle A.; Straus, Daniel B.; Hull, Trevor D.; Paley, Daniel W.; van der Zande, Arend M.; Hone, James C.; Kymissis, Ioannis; Kagan, Cherie R.; Roy, Xavier; Owen, Jonathan S.
    Journal of Physical Chemistry Letters (2016), 7 (17), 3510-3518CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Using a combination of scanning photocurrent microscopy (SPCM) and time-resolved microwave cond. (TRMC) measurements, we monitor the diffusion and recombination of photoexcited charges in CH3NH3PbI3 perovskite single crystals. The majority carrier type was controlled by growing crystals in the presence or absence of air, allowing the diffusion lengths of electrons (LDe-) and holes (LDh+) to be directly imaged with SPCM (LDe- = 10-28 μm, LDh+ = 27-65 μm). TRMC measurements reveal a photogenerated carrier mobility (μh + μe) of 115 ± 15 cm2V-1s-1 and recombination that depends on the excitation intensity. From the intensity dependence of the recombination kinetics and by accounting for carrier diffusion away from the point of photogeneration, we ext. a second-order recombination rate const. (krad = 5 ± 3 × 10-10 cm3/s) that is consistent with the predicted radiative rate. First-order recombination at low photoexcited carrier d. (knrp-type = 1.0 ± 0.3 × 105 s-1, knrn-type = 1.5 ± 0.3 × 105 s-1) is slower than that obsd. in CH3NH3PbI3 thin films or in GaAs single crystals with AlGaAs passivation layers. By accounting for the diln. of photogenerated carriers upon diffusion, and by combining SPCM and TRMC measurements, we resolve disagreement between previous reports of carrier diffusion length.
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  36. 36
    Labram, J. G.; Perry, E. E.; Venkatesan, N. R.; Chabinyc, M. L. Steady-state microwave conductivity reveals mobility-lifetime product in methylammonium lead iodide. Appl. Phys. Lett. 2018, 113, 153902,  DOI: 10.1063/1.5041959
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    Steady-state microwave conductivity reveals mobility-lifetime product in methylammonium lead iodide
    Labram, John G.; Perry, Erin E.; Venkatesan, Naveen R.; Chabinyc, Michael L.
    Applied Physics Letters (2018), 113 (15), 153902/1-153902/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Many time-resolved techniques to study charge carrier recombination involve pulsed high-power optical excitation and photo-generated carrier densities many orders of magnitude higher than present under typical solar cell operating conditions. In this report, we demonstrate a steady-state contactless microwave cond. technique to evaluate the photocond. of carriers in semiconductors at low illumination intensity, as a function of optical power d. We studied characteristics of both thin films and single crystals of a hybrid halide perovskite compd., methylammonium lead iodide (MAPbI3). The aggregate mobility-lifetime product of majority and minority carriers in thin films of MAPbI3 was detd. and found to be highly-dependent on incident optical power d., even at sub-1-sun illumination intensities, and attributed to trap states within the films. (c) 2018 American Institute of Physics.
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    Hutter, E. M.; Eperon, G. E.; Stranks, S. D.; Savenije, T. J. Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes, and Concentrations of Trap States. J. Phys. Chem. Lett. 2015, 6, 3082– 3090,  DOI: 10.1021/acs.jpclett.5b01361
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    Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes, and Concentrations of Trap States
    Hutter, Eline M.; Eperon, Giles E.; Stranks, Samuel D.; Savenije, Tom J.
    Journal of Physical Chemistry Letters (2015), 6 (15), 3082-3090CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Efficient solar cells were obtained using thin films of soln.-processed org.-inorg. perovskites. However, there remains limited knowledge about the relation between prepn. route and optoelectronic properties. The authors use complementary time-resolved microwave cond. (TRMC) and photoluminescence (PL) measurements to study the charge carrier dynamics in thin planar films of CH3NH3PbI3-xClx, CH3NH3PbI3, and their meso-structured analogs. High mobilities close to 30 cm2/(V s) and microsecond-long lifetimes are found in thin films of CH3NH3PbI3-xClx, compared to lifetimes of only a few hundred nanoseconds in CH3NH3PbI3 and meso-structured perovskites. The authors describe the TRMC and PL expts. with a global kinetic model, using one set of kinetic parameters characteristic for each sample. The trap d. is <5 × 1014 cm-3 in CH3NH3PbI3-xClx, 6 × 1016 cm-3 in the CH3NH3PbI3 thin film and ∼1015 cm-3 in both meso-structured perovskites. Also, the authors' results imply that band-to-band recombination is enhanced by the presence of dark carriers resulting from unintentional doping of the perovskites. Finally, the authors' general approach to det. concns. of trap states and dark carriers is also highly relevant to other semiconductor materials.
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    Chattopadhyay, S.; Kokenyesi, R. S.; Hong, M. J.; Watts, C. L.; Labram, J. G. Resolving in-plane and out-of-plane mobility using time resolved microwave conductivity. J. Mater. Chem. C 2020, 8, 10761– 10766,  DOI: 10.1039/D0TC00328J
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    Resolving in-plane and out-of-plane mobility using time resolved microwave conductivity
    Chattopadhyay, Shirsopratim; Kokenyesi, Robert S.; Hong, Min Ji; Watts, C. Lowell; Labram, John G.
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2020), 8 (31), 10761-10766CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    The contactless characterization technique time resolved microwave cond. (TRMC) provides a means to rapidly and unambiguously approx. carrier mobilities and lifetimes for a variety of semiconducting materials. When using a cavity-based approach however, the technique can conventionally only resolve carrier mobilities in the plane of the substrate. In solar cells, charge carriers are extd. in the direction perpendicular to the substrate, therefore it would be beneficial if one were able to evaluate the mobility in this direction also. Here we present a novel approach for resolving charge carrier mobilities in different planes within a sample. Using a range of 3D-printed sample holders, where the sample is held at various angles relative to the incident light, we are able to simultaneously resolve the mobility in the plane of the sample and out of the plane of the sample. As examples, we have studied the 3-dimensional corner-connected metal halide perovskite methylammonium lead iodide and the 2-dimensional perovskite precusor, lead iodide.
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  39. 39
    Hutter, E. M.; Gélvez-Rueda, M. C.; Osherov, A.; Bulović, V.; Grozema, F. C.; Stranks, S. D.; Savenije, T. J. Direct-indirect character of the bandgap in methylammonium lead iodide perovskite. Nat. Mater. 2017, 16, 115– 120,  DOI: 10.1038/nmat4765
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    Direct-indirect character of the bandgap in methylammonium lead iodide perovskite
    Hutter, Eline M.; Gelvez-Rueda, Maria C.; Osherov, Anna; Bulovic, Vladimir; Grozema, Ferdinand C.; Stranks, Samuel D.; Savenije, Tom J.
    Nature Materials (2017), 16 (1), 115-120CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temp. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temp. orthorhombic phase, fast quenching of mobile charges occurs independent of the temp. and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyze spectroscopic data.
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    Hong, M. J.; Svadlenak, S. R.; Goulas, K. A.; Labram, J. G. Thermal stability of mobility in methylammonium lead iodide. J. Phys. Mater. 2020, 3, 014003,  DOI: 10.1088/2515-7639/ab442e
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    Bi, Y.; Hutter, E. M.; Fang, Y.; Dong, Q.; Huang, J.; Savenije, T. J. Charge Carrier Lifetimes Exceeding 15 μs in Methylammonium Lead Iodide Single Crystals. J. Phys. Chem. Lett. 2016, 7, 923– 928,  DOI: 10.1021/acs.jpclett.6b00269
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    Charge Carrier Lifetimes Exceeding 15 μs in Methylammonium Lead Iodide Single Crystals
    Bi, Yu; Hutter, Eline M.; Fang, Yanjun; Dong, Qingfeng; Huang, Jinsong; Savenije, Tom J.
    Journal of Physical Chemistry Letters (2016), 7 (5), 923-928CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The charge carrier lifetime in org.-inorg. perovskites is one of the most important parameters for modeling and design of solar cells and other types of devices. In this work, we use CH3NH3PbI3 single crystal as a model system to study optical absorption, charge carrier generation, and recombination lifetimes. We show that commonly applied photoluminescence lifetime measurements may dramatically underestimate the intrinsic carrier lifetime in CH3NH3PbI3, which could be due to severe charge recombination at the crystal surface and/or fast electron-hole recombination close to the surface. By using the time-resolved microwave cond. technique, we investigated the lifetime of free mobile charges inside the crystals. Most importantly, we find that for homogeneous excitation throughout the crystal, the charge carrier lifetime exceeds 15 μs. This means that the diffusion length in CH3NH3PbI3 can be as large as 50 μm if it is no longer limited by the dimensions of the crystallites.
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    Li, D.; Wang, G.; Cheng, H.-C.; Chen, C.-Y.; Wu, H.; Liu, Y.; Huang, Y.; Duan, X. Size-dependent phase transition in methylammonium lead iodide perovskite microplate crystals. Nat. Commun. 2016, 7, 11330,  DOI: 10.1038/ncomms11330
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    Size-dependent phase transition in methylammonium lead iodide perovskite microplate crystals
    Li, Dehui; Wang, Gongming; Cheng, Hung-Chieh; Chen, Chih-Yen; Wu, Hao; Liu, Yuan; Huang, Yu; Duan, Xiangfeng
    Nature Communications (2016), 7 (), 11330CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Methylammonium lead iodide perovskite has attracted considerable recent interest for soln. processable solar cells and other optoelectronic applications. The orthorhombic-to-tetragonal phase transition in perovskite can significantly alter its optical, elec. properties and impact the corresponding applications. Here, we report a systematic investigation of the size-dependent orthorhombic-to-tetragonal phase transition using a combined temp.-dependent optical, elec. transport and transmission electron microscopy study. Our studies of individual perovskite microplates with variable thicknesses demonstrate that the phase transition temp. decreases with reducing microplate thickness. The sudden decrease of mobility around phase transition temp. and the presence of hysteresis loops in the temp.-dependent mobility confirm that the orthorhombic-to-tetragonal phase transition is a first-order phase transition. Our findings offer significant fundamental insight on the temp.- and size-dependent structural, optical and charge transport properties of perovskite materials, and can greatly impact future exploration of novel electronic and optoelectronic devices from these materials.
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    Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films. APL Mater. 2014, 2, 081513,  DOI: 10.1063/1.4891595
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    Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films
    Wehrenfennig, Christian; Liu, Mingzhen; Snaith, Henry J.; Johnston, Michael B.; Herz, Laura M.
    APL Materials (2014), 2 (8), 081513/1-081513/10CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3-xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concs. on the behavior of the perovskite at room temp., the obsd. influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temp. phenomena can give valuable addnl. insights into the underlying physics. Here, we present a temp.-dependent study of optical absorption and photoluminescence (PL) emission of vapor-deposited CH3NH3PbI3-xClx exploring the nature of recombination channels in the room- and the low-temp. phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temp., a second PL emission peak emerges in addn. to the peak from the room-temp. phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temp. above the transition, significantly accelerated recombination is obsd. in the low-temp. phase. Our data suggest that small inclusions of domains adopting the room-temp. phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate consts. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centers that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion. (c) 2014 American Institute of Physics.
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    Lappas, A.; Kosaka, M.; Tanigaki, K.; Prassides, K. An Orientationally-Ordered Primitive-Cubic Form of the Fulleride CsC60. J. Am. Chem. Soc. 1995, 117, 7560– 7561,  DOI: 10.1021/ja00133a036
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    An Orientationally-Ordered Primitive-Cubic Form of the Fulleride CsC60
    Lappas, Alexandros; Kosaka, Mayumi; Tanigaki, Katsumi; Prassides, Kosmas
    Journal of the American Chemical Society (1995), 117 (28), 7560-1CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The crystal structure of the conducting phase of CsC60, formed by rapid cooling to liq. N temp., was detd. from the anal. of neutron powder diffraction data. The structure is primitive cubic (space group Pa‾3, a 13.9671(3) Å at 4.5 K) and results from orientational ordering of the C60- ions in a manner similar to that encountered in pristine C60. At. coordinates are given. The three-dimensional character of the present structure evidently leads to the suppression of the structural and magnetic instabilities which lead to insulating behavior in the low-dimensional forms of CsC60.
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    Janes, R.; Edge, M.; Robinson, J.; Allen, N. S.; Thompson, F. Microwave photodielectric and photoconductivity studies of commercial titanium dioxide pigments: the influence of transition metal dopants. J. Mater. Sci. 1998, 33, 3031– 3036,  DOI: 10.1023/A:1004371117530
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    Microwave photodielectric and photoconductivity studies of commercial titanium dioxide pigments: the influence of transition metal dopants
    Janes, R.; Edge, M.; Robinson, J.; Allen, N. S.; Thompson, F.
    Journal of Materials Science (1998), 33 (12), 3031-3036CODEN: JMTSAS; ISSN:0022-2461. (Kluwer Academic Publishers)
    The effect of transition metal ion doping on the charge carrier dynamics in com. TiO2 pigments has been investigated. The response to extended polychromatic irradn. was monitored by real-time measurements of microwave photocond. and photodielec. effects. In addn. to intrinsic differences between the rutile and anatase polymorphs, shifts in the resonance frequency and quality factor of a tuned microwave cavity as a function of annealing, and foreign-ion incorporation provided a gauge of the influence of extrinsic trapping centers on photoactivity. Parallel luminescence studies revealed an extension in the lifetime of the emission band on doping, the energy assocd. with recombination being independent of the dopant ion.
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    Savenije, T. J.; Ferguson, A. J.; Kopidakis, N.; Rumbles, G. Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity. J. Phys. Chem. C 2013, 117, 24085– 24103,  DOI: 10.1021/jp406706u
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    Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity
    Savenije, Tom J.; Ferguson, Andrew J.; Kopidakis, Nikos; Rumbles, Garry
    Journal of Physical Chemistry C (2013), 117 (46), 24085-24103CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    A review. During the past decade, time-resolved microwave cond. (TRMC) has evolved to an established, powerful technique to study photoactive layers. With this feature paper, the authors aim to fulfill two goals: (1) give a full description of the photoinduced TRMC technique, including exptl. details and data anal., and discuss to what extent the TRMC technique differs from more conventional d.c. techniques and (2) illustrate the potential of this technique for probing charge carrier dynamics in photoactive materials. For these reasons recent studies on conjugated polymer:fullerene blends will be presented and discussed. The findings from these studies have advanced the insight into the mechanism of charge carrier generation and decay in polymer:fullerene blends, which allows the authors to improve the efficiency of org. photovoltaic cells based on this active layer architecture. In short, the TRMC technique can be used as a versatile method to screen the potential of new photovoltaic materials.
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    Savenije, T. J.; Guo, D.; Caselli, V. M.; Hutter, E. M. Quantifying Charge-Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time-Resolved Microwave Photoconductivity Measurements. Adv. Energy Mater. 2020, 10, 1903788,  DOI: 10.1002/aenm.201903788
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    Quantifying Charge-Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time-Resolved Microwave Photoconductivity Measurements
    Savenije, Tom J.; Guo, Dengyang; Caselli, Valentina M.; Hutter, Eline M.
    Advanced Energy Materials (2020), 10 (26), 1903788CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    A review. The unprecedented rise in the power conversion efficiency of solar cells based on metal halide perovskites (MHPs) has led to enormous research effort to understand their photophys. properties. The progress made in understanding the mobility and recombination of photogenerated charge carriers from nanosecond to microsecond time scales, monitored using electrodeless transient photocond. techniques, is reviewed. In addn., a kinetic model to obtain rate consts. from transient data recorded using a wide range of laser intensities is presented. For various MHPs the temp. dependence of the mobilities and recombination rates are evaluated. Furthermore, it is shown how these rate consts. can be used to predict the upper limit for the open-circuit voltage Voc of the corresponding device. Finally, the photophys. properties of MHPs that are not yet fully understood are explored, and recommendations for future research directions are made.
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    Csősz, G.; Márkus, B. G.; Jánossy, A.; Nemes, N. M.; Murányi, F.; Klupp, G.; Kamarás, K.; Kogan, V. G.; Bud’ko, S. L.; Canfield, P. C.; Simon, F. Giant microwave absorption in fine powders of superconductors. Sci. Rep. 2018, 8, 11480– 11488,  DOI: 10.1038/s41598-018-29750-7
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Cited By

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  1. Pradeep R. Nair. Characterization of Recombination Phenomena in Perovskites from Logarithmic and Power-Law Spectroscopy Transients. The Journal of Physical Chemistry C 2023, 127 (43) , 20987-20994. https://doi.org/10.1021/acs.jpcc.3c04172
  2. András Bojtor, Dávid Krisztián, Ferenc Korsós, Sándor Kollarics, Gábor Paráda, Thomas Pinel, Márton Kollár, Endre Horváth, Xavier Mettan, Hidetsugu Shiozawa, Bence G. Márkus, László Forró, Ferenc Simon. Millisecond‐Scale Charge‐Carrier Recombination Dynamics in the CsPbBr 3 Perovskite. Advanced Energy and Sustainability Research 2024, 12 https://doi.org/10.1002/aesr.202400043
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  • Abstract

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    Figure 1

    Figure 1. Temperature-evolution of charge carrier lifetime τc (top) and photoconductance ΔG (bottom) for crystals of three parent MHPs, i.e., MAPbCl3, MAPbBr3, and MAPbI3. A marked change in both the recombination time and photoconductivity can be observed at the orthorhombic-to-tetragonal phase transition, which is indicated by vertical lines. The materials show ultralong charge carrier recombination times at low temperatures with the maximal value exceeding 68 μs for MAPbBr3. The measurements were made with a Q-switched nanosecond pulsed laser operating at 450 nm. The data points were recorded while raising the temperature with the TRMCD method.

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    Figure 2

    Figure 2. Charge carrier lifetime, τc, and the photoconductivity, ΔG, in quenched and nonquenched MAPbBr3 cubic crystals as measured by TRMCD during slow gradual heating. In the orthorhombic phase, τc is significantly shorter for the quenched crystal than for its nonquenched counterpart (left panel). In contrast, ΔG shows the opposite trend, reaching much higher values for the quenched crystal (right panel). Note also a marked jump in both measured properties, τc and ΔG, at the orthorhombic-to-tetragonal phase transition. In particular, upon this phase transition, the initial large overshoot in τc for the quenched crystal quickly falls down to the level observed for its nonquenched counterpart (left panel).

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    Figure 3

    Figure 3. Impact of the crystalline morphology of MAPbBr3 on the charge carrier lifetime, τc, and the photoconductivity, ΔG, as probed by TRMCD. Top panel, from left to right: photographs of cubic, amorf (acid-based crystal growth), and needle-shaped samples (the scale is indicated by the raster grid lines on corresponding paper sheets or by a scale bar). The cubic samples reveal the longest charge carrier lifetimes, whereas the photoconductivity is largest for the acid-based samples. The ΔG data is normalized by the respective values above the orthorhombic-to-tetragonal phase transition.

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    Figure 4

    Figure 4. Experimental setup used in this work for microwave photoconductivity measurements. (a) Block diagram of the system including its modifications corresponding to TRMCD and quasi-steady state measurements. (b) Schematic representation of the laser light delivery to a sample placed in the center of a cylindrical resonator operating in TE011 mode. (c) Illustration of the light delivery to the incission in the 4 mm diameter quartz rod where the sample is mounted. Photos of the bottom part of the quartz rod taken: without illumination, under illumination, and under illumination and with the MAPbBr3 crystal attached are shown in the left, middle, and right panels, respectively. For the purpose of visualization of the light delivery to the sample, the photos were taken under illumination with a fiber-coupled CW LED light source (λ = 470 nm, ADSresonances Sàrl).

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    Figure 5

    Figure 5. Example time-traces of the measurement fitted with a single exponent. The two traces were recorded during the same temperature sweep and with the same excitation. The time-traces were collected during the measurement presented in Figure 2.

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    Figure 6

    Figure 6. Hysteresis of the microwave photoconductivity signal at the tetragonal-orthorhombic phase transition for the time-resolved (left) and the quasi-steady-state (right) measurements for CH3NH3PbBr3. Note the larger number of points for the QSS measurement as a result of the noise-free lock-in technique.

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  • References

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      5
      Bright light-emitting diodes based on organometal halide perovskite
      Tan, Zhi-Kuang; Moghaddam, Reza Saberi; Lai, May Ling; Docampo, Pablo; Higler, Ruben; Deschler, Felix; Price, Michael; Sadhanala, Aditya; Pazos, Luis M.; Credgington, Dan; Hanusch, Fabian; Bein, Thomas; Snaith, Henry J.; Friend, Richard H.
      Nature Nanotechnology (2014), 9 (9), 687-692CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Solid-state light-emitting devices based on direct-bandgap semiconductors have, over the past two decades, been utilized as energy-efficient sources of lighting. However, fabrication of these devices typically relies on expensive high-temp. and high-vacuum processes, rendering them uneconomical for use in large-area displays. Here, we report high-brightness light-emitting diodes based on soln.-processed organometal halide perovskites. We demonstrate electroluminescence in the near-IR, green and red by tuning the halide compns. in the perovskite. In our IR device, a thin 15 nm layer of CH3NH3PbI3-xClx perovskite emitter is sandwiched between larger-bandgap titanium dioxide (TiO2) and poly(9,9'-dioctylfluorene) (F8) layers, effectively confining electrons and holes in the perovskite layer for radiative recombination. We report an IR radiance of 13.2 W sr-1 m-2 at a c.d. of 363 mA cm-2, with highest external and internal quantum efficiencies of 0.76% and 3.4%, resp. In our green light-emitting device with an ITO/PEDOT:PSS/CH3NH3PbBr3/F8/Ca/Ag structure, we achieved a luminance of 364 cd m-2 at a c.d. of 123 mA cm-2, giving external and internal quantum efficiencies of 0.1% and 0.4%, resp. We show, using photoluminescence studies, that radiative bimol. recombination is dominant at higher excitation densities. Hence, the quantum efficiencies of the perovskite light-emitting diodes increase at higher current densities. This demonstration of effective perovskite electroluminescence offers scope for developing this unique class of materials into efficient and color-tunable light emitters for low-cost display, lighting and optical communication applications.
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    6. 6
      Náfrádi, B.; Náfrádi, G.; Forró, L.; Horváth, E. Methylammonium Lead Iodide for Efficient X-ray Energy Conversion. J. Phys. Chem. C 2015, 119, 25204– 25208,  DOI: 10.1021/acs.jpcc.5b07876
      6
      Methylammonium Lead Iodide for Efficient X-ray Energy Conversion
      Nafradi, Balint; Nafradi, Gabor; Forro, Laszlo; Horvath, Endre
      Journal of Physical Chemistry C (2015), 119 (45), 25204-25208CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We report on photocurrent generation by X-ray irradn. in methylammonium lead iodide CH3NH3PbI3 single crystals. We obsd. 75% of charge collection efficiency in millimeter sized samples. This efficiency is partially due to the high photon absorption coeff. and X-ray beam stopping power of our lead contg. material. The material shows a less than 20% decrease in its performance up to 40 Sv of total X-ray dose, which represents a very good stability. Moreover, our Monte Carlo N-Particle Transport calcns. indicate that energetic particle radiation can also be harvested by CH3NH3PbI3. These properties are compared to CH3NH3SnI3 and PbI2. The possible application of these advantageous properties could be in simultaneous radiation protection and direct electricity prodn. in environments with highly energetic background radiation.
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    7. 7
      Ho-Baillie, A. W. Y.; Sullivan, H. G. J.; Bannerman, T. A.; Talathi, H. P.; Bing, J.; Tang, S.; Xu, A.; Bhattacharyya, D.; Cairns, I. H.; McKenzie, D. R. Deployment Opportunities for Space Photovoltaics and the Prospects for Perovskite Solar Cells. Adv. Mater. Technol. 2022, 7 (3), 2101059,  DOI: 10.1002/admt.202101059
      7
      Deployment Opportunities for Space Photovoltaics and the Prospects for Perovskite Solar Cells
      Ho-Baillie, Anita W. Y.; Sullivan, Hamish G. J.; Bannerman, Thomas A.; Talathi, Harsh. P.; Bing, Jueming; Tang, Shi; Xu, Alan; Bhattacharyya, Dhriti; Cairns, Iver H.; McKenzie, David. R.
      Advanced Materials Technologies (Weinheim, Germany) (2022), 7 (3), 2101059CODEN: AMTDCM; ISSN:2365-709X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The rapid progress in space exploration, mining, and tourism has been fuelled by both public and private sector investments. The latter has led to the need to reduce manufg. and launch cost of space hardware to create a competitive and sustainable space economy. A major step in making space accessible is to develop affordable power systems for "com. space" use. Photovoltaics has in the past and will in the future be a key component. Metal halide perovskite solar cells show the greatest potential of all emerging technologies for low-cost space photovoltaics. They have demonstrated the highest rate of power conversion efficiency improvement. Compared to the triple junction III-V compd. semiconductor cells commonly used for space applications, perovskite cells have a higher power to wt. ratio and are significantly cheaper to be manufd. They have high radiation tolerance and can be fabricated onto flexible substrates for expand-on-demand solar panels. This paper outlines the major space markets for photovoltaics, and research and development opportunities for perovskite space solar cells in the context of their recent progress.
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    8. 8
      Green, M. A.; Dunlop, E. D.; Hohl-Ebinger, J.; Yoshita, M.; Kopidakis, N.; Hao, X. Solar cell efficiency tables (version 59). Progress in Photovoltaics: Research and Applications 2022, 30, 3– 12,  DOI: 10.1002/pip.3506
      There is no corresponding record for this reference.
    9. 9
      Park, N.-G.; Zhu, K. Scalable fabrication and coating methods for perovskite solar cells and solar modules. Nat. Rev. Mater. 2020, 5, 333– 350,  DOI: 10.1038/s41578-019-0176-2
      9
      Scalable fabrication and coating methods for perovskite solar cells and solar modules
      Park, Nam-Gyu; Zhu, Kai
      Nature Reviews Materials (2020), 5 (5), 333-350CODEN: NRMADL; ISSN:2058-8437. (Nature Research)
      A review. Abstr.: Since the report in 2012 of a solid-state perovskite solar cell (PSC) with a power-conversion efficiency (PCE) of 9.7% and a stability of 500 h, intensive efforts have been made to increase the certified PCE, reaching 25.2% in 2019. The PCE of PSCs now exceeds that of conventional thin-film solar-cell technologies, and the rate at which this increase has been achieved is unprecedented in the history of photovoltaics. Moreover, the development of moisture-stable and heat-stable materials has increased the stability of PSCs. Small-area devices (<1 cm2) are typically fabricated using a spin-coating method; however, this approach may not be suitable for the prepn. of the large-area (>100 cm2) substrates required for commercialization. Thus, materials and methods need to be developed for coating large-area PSCs. In this Review, we discuss soln.-based and vapor-phase coating methods for the fabrication of large-area perovskite films, examine the progress in performance and the parameters affecting the properties of large-area coatings, and provide an overview of the methodologies for achieving high-efficiency perovskite solar modules.
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    10. 10
      Bhandari, K. P.; Ellingson, R. J. In A Comprehensive Guide to Solar Energy Systems; Letcher, T. M., Fthenakis, V. M., Eds.; Academic Press, 2018; pp 233– 254.
      There is no corresponding record for this reference.
    11. 11
      Fu, F.; Feurer, T.; Weiss, T.; Pisoni, S.; Avancini, E.; Andres, C.; Buecheler, S.; Tiwari, A. High-efficiency inverted semi-transparent planar perovskite solar cells in substrate configuration. Nat. Energy 2017, 2, 16190,  DOI: 10.1038/nenergy.2016.190
      11
      High-efficiency inverted semi-transparent planar perovskite solar cells in substrate configuration
      Fu, Fan; Feurer, Thomas; Weiss, Thomas Paul; Pisoni, Stefano; Avancini, Enrico; Andres, Christian; Buecheler, Stephan; Tiwari, Ayodhya N.
      Nature Energy (2017), 2 (1), 16190CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
      The ability to grow perovskite solar cells in substrate configuration, where light enters the devices from the film side, allows the use of non-transparent flexible polymer and metal substrates. Furthermore, this configuration could facilitate processing directly on Cu(In,Ga)Se2 solar cells to realize ultrahigh-efficiency polycryst. all-thin-film tandem devices. However, the inversion of conventional superstrate architecture imposes severe constraints on device processing and limits the electronic quality of the absorber and charge selective contacts. Here we report a device architecture that allows inverted semi-transparent planar perovskite solar cells with a high open-circuit voltage of 1.116 V and substantially improved efficiency of 16.1%. The substrate configuration perovskite devices show a temp. coeff. of -0.18% °C-1 and promising thermal and photo-stability. Importantly, the device exhibits a high av. transmittance of 80.4% between 800 and 1,200 nm, which allows us to demonstrate polycryst. all-thin-film tandem devices with efficiencies of 22.1% and 20.9% for Cu(In,Ga)Se2 and CuInSe2 bottom cells, resp.
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    12. 12
      Divitini, G.; Cacovich, S.; Matteocci, F.; Cinà, L.; Di Carlo, A.; Ducati, C. In situ observation of heat-induced degradation of perovskite solar cells. Nat. Energy 2016, 1, 15012,  DOI: 10.1038/nenergy.2015.12
      12
      In situ observation of heat-induced degradation of perovskite solar cells
      Divitini, G.; Cacovich, S.; Matteocci, F.; Cina, L.; Di Carlo, A.; Ducati, C.
      Nature Energy (2016), 1 (2), 15012CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
      The lack of thermal stability of perovskite solar cells is hindering the progress of this technol. towards adoption in the consumer market. Different pathways of thermal degrdn. are activated at different temps. in these complex nanostructured hybrid composites. Thus, it is essential to explore the thermal response of the mesosuperstructured composite device to engineer materials and operating protocols. Here we produce devices according to four well-established recipes, and characterize their photovoltaic performance as they are heated within the operational range. The devices are analyzed using transmission electron microscopy as they are further heated in situ, to monitor changes in morphol. and chem. compn. We identify mechanisms for structural and chem. changes, such as iodine and lead migration, which appear to be correlated to the synthesis conditions. In particular, we identify a correlation between exposure of the perovskite layer to air during processing and elemental diffusion during thermal treatment.
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    13. 13
      Arakcheeva, A.; Chernyshov, D.; Spina, M.; Forró, L.; Horváth, E. CH3NH3PbI3: precise structural consequences of water absorption at ambient conditions. Acta Crystallogr., Sect. B 2016, 72, 716– 722,  DOI: 10.1107/S2052520616010428
      13
      CH3NH3PbI3: precise structural consequences of water absorption at ambient conditions
      Arakcheeva, Alla; Chernyshov, Dmitry; Spina, Massimo; Forro, Laszlo; Horvath, Endre
      Acta Crystallographica, Section B: Structural Science, Crystal Engineering and Materials (2016), 72 (5), 716-722CODEN: ACSBDA; ISSN:2052-5206. (International Union of Crystallography)
      The crystal structure of the pristine (I) and aged (II) crystals of CH3NH3PbI3 (hereafter MAPbI3) hybrid org.-inorg. lead iodide has been studied at 293 K with high-precision single-crystal X-ray diffraction using a synchrotron light source. We show that (I) and (II) are characterized by an identical tetragonal unit cell but different space groups: I422 for (I) and P42212 for (II). Both space groups are subgroups of I4/mcm, which is widely used for MAPbI3. The main difference between (I) and (II) comes from the difference in hydrogen bonds between the MA+ cation and the PbI3 framework which is the direct consequence of H2O insertion in the aged crystal (II).
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    14. 14
      Aristidou, N.; Sanchez-Molina, I.; Chotchuangchutchaval, T.; Brown, M.; Martinez, L.; Rath, T.; Haque, S. A. The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers. Angew. Chem., Int. Ed. 2015, 54, 8208– 8212,  DOI: 10.1002/anie.201503153
      14
      The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers
      Aristidou, Nicholas; Sanchez-Molina, Irene; Chotchuangchutchaval, Thana; Brown, Michael; Martinez, Luis; Rath, Thomas; Haque, Saif A.
      Angewandte Chemie, International Edition (2015), 54 (28), 8208-8212CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this paper we report on the influence of light and oxygen on the stability of CH3NH3PbI3 perovskite-based photoactive layers. When exposed to both light and dry air the mp-Al2O3/CH3NH3PbI3 photoactive layers rapidly decomp. yielding methylamine, PbI2, and I2 as products. We show that this degrdn. is initiated by the reaction of superoxide (O2-) with the methylammonium moiety of the perovskite absorber. Fluorescent mol. probe studies indicate that the O2- species is generated by the reaction of photoexcited electrons in the perovskite and mol. oxygen. We show that the yield of O2- generation is significantly reduced when the mp-Al2O3 film is replaced with an mp-TiO2 electron extn. and transport layer. The present findings suggest that replacing the methylammonium component in CH3NH3PbI3 to a species without acid protons could improve tolerance to oxygen and enhance stability.
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    15. 15
      Poglitsch, A.; Weber, D. Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy. J. Chem. Phys. 1987, 87, 6373– 6378,  DOI: 10.1063/1.453467
      15
      Dynamic disorder in methylammonium trihaloplumbates(II) observed by millimeter-wave spectroscopy
      Poglitsch, A.; Weber, D.
      Journal of Chemical Physics (1987), 87 (11), 6373-8CODEN: JCPSA6; ISSN:0021-9606.
      The temp.-dependent structure of cryst. methylammonium trihaloplumbates(II) CH3NH3+PbX3- (X = Cl, Br, I) as detd. by x-ray diffraction, was compared with measurements of the temp.-dependent complex permittivity at frequencies of 50-150 GHz. The dielec. measurements reveal a ps relaxation process which corresponds to a dynamic disorder of the methylammonium group in the high-temp. phases of the trihaloplumbates.
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    16. 16
      Ava, T. T.; Al Mamun, A.; Marsillac, S.; Namkoong, G. A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells. Appl. Sci. 2019, 9, 188,  DOI: 10.3390/app9010188
      16
      A review: thermal stability of methylammonium lead halide based perovskite solar cells
      Ava, Tanzila Tasnim; Mamun, Abdullah Al; Marsillac, Sylvain; Namkoong, Gon
      Applied Sciences (2019), 9 (1), 188CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
      Perovskite solar cells have achieved photo-conversion efficiencies greater than 20 %, making them a promising candidate as an emerging solar cell technol. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degrdn. can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temps. can be over 45 °C higher than ambient temps. when operating under direct sunlight. Herein, recent advances in resolving thermal stability problems are highlighted through literature review. Moreover, the most recent and promising strategies for overcoming thermal degrdn. are also summarized.
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    17. 17
      Chen, T.; Foley, B. J.; Ipek, B.; Tyagi, M.; Copley, J. R. D.; Brown, C. M.; Choi, J. J.; Lee, S.-H. Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite. Phys. Chem. Chem. Phys. 2015, 17, 31278– 31286,  DOI: 10.1039/C5CP05348J
      17
      Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite
      Chen, Tianran; Foley, Benjamin J.; Ipek, Bahar; Tyagi, Madhusudan; Copley, John R. D.; Brown, Craig M.; Choi, Joshua J.; Lee, Seung-Hun
      Physical Chemistry Chemical Physics (2015), 17 (46), 31278-31286CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown impressive power conversion efficiencies of above 20%. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH3NH3+ cations and their impact on relevant processes such as charge recombination and exciton dissocn. are still poorly understood. Here, using elastic and quasi-elastic neutron scattering techniques and group theor. anal., we studied rotational modes of the CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327 K) and tetragonal (165 K < T < 327 K) phases, the CH3NH3+ ions exhibit four-fold rotational symmetry of the C-N axis (C4) along with three-fold rotation around the C-N axis (C3), while in the orthorhombic phase (T < 165 K) only C3 rotation is present. At around room temp., the characteristic relaxation times for the C4 rotation are found to be τC4 ≈ 5 ps while for the C3 rotation τC3 ≈ 1 ps. The T-dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C4 rotational mode and the temp. dependent dielec. permittivity. Our findings on the rotational dynamics of CH3NH3+ and the assocd. dipole have important implications for understanding the low exciton binding energy and a slow charge recombination rate in CH3NH3PbI3 which are directly relevant for the high solar cell performance.
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    18. 18
      Bernard, G. M.; Wasylishen, R. E.; Ratcliffe, C. I.; Terskikh, V.; Wu, Q.; Buriak, J. M.; Hauger, T. Methylammonium Cation Dynamics in Methylammonium Lead Halide Perovskites: A Solid-State NMR Perspective. J. Phys. Chem. A 2018, 122, 1560– 1573,  DOI: 10.1021/acs.jpca.7b11558
      18
      Methylammonium Cation Dynamics in Methylammonium Lead Halide Perovskites: A Solid-State NMR Perspective
      Bernard, Guy M.; Wasylishen, Roderick E.; Ratcliffe, Christopher I.; Terskikh, Victor; Wu, Qichao; Buriak, Jillian M.; Hauger, Tate
      Journal of Physical Chemistry A (2018), 122 (6), 1560-1573CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      In light of the intense recent interest in the methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temp. via 2H, 14N, and 207Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ2, for the C-N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temp. dependence of the cation ordering is rationalized using a six-site model, with two equiv. sites along the c-axis and four equiv. sites either perpendicular or approx. perpendicular to this axis. As the cubic ↔ tetragonal phase transition temp. is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, 2H NMR line shapes indicate that the C-N axis is essentially frozen.
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    19. 19
      Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goñi, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide. Phys. Rev. B 2015, 92, 144308,  DOI: 10.1103/PhysRevB.92.144308
      19
      Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide
      Brivio, Federico; Frost, Jarvist M.; Skelton, Jonathan M.; Jackson, Adam J.; Weber, Oliver J.; Weller, Mark T.; Goni, Alejandro R.; Leguy, Aurelien M. A.; Barnes, Piers R. F.; Walsh, Aron
      Physical Review B: Condensed Matter and Materials Physics (2015), 92 (14), 144308/1-144308/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behavior, with successive transitions between orthorhombic, tetragonal, and cubic polymorphs around 165 and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equil. structures compare well to solns. of temp.-dependent powder neutron diffraction. By following the normal modes, we calc. IR and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degrdn. of the sample. Despite a clear sepn. in energy between low-frequency modes assocd. with the inorg. (PbI3-)n network and high-frequency modes of the org. CH3NH3+ cation, significant coupling between them is found, which emphasizes the interplay between mol. orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temp.
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    20. 20
      Weller, M. T.; Weber, O. J.; Henry, P. F.; Di Pumpo, A. M.; Hansen, T. C. Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K. Chem. Commun. 2015, 51, 4180– 4183,  DOI: 10.1039/C4CC09944C
      20
      Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K
      Weller, Mark T.; Weber, Oliver J.; Henry, Paul F.; Di Pumpo, Antonietta M.; Hansen, Thomas C.
      Chemical Communications (Cambridge, United Kingdom) (2015), 51 (20), 4180-4183CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The methylammonium cation in [CH3NH3]PbI3 demonstrates increasing positional disorder on heating from 100 K to 352 K. In the tetragonal phase, stable between 165 K and 327 K, the cation is disordered over four sites directed toward the faces of the distorted cubic [PbI3]- framework and migrates towards the cavity center with increasing temp. Crystallog. data and at. coordinates are given.
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    21. 21
      Quarti, C.; Mosconi, E.; De Angelis, F. Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell Operation. Chem. Mater. 2014, 26, 6557– 6569,  DOI: 10.1021/cm5032046
      21
      Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell
      Quarti, Claudio; Mosconi, Edoardo; De Angelis, Filippo
      Chemistry of Materials (2014), 26 (22), 6557-6569CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The ab initio electronic structure and Car-Parrinello mol. dynamics simulations are reported for several structural models of the prototype MAPbI3 perovskite for solar cells applications. Both configurations having a preferred orientation are considered of the MA cations, giving rise to a net dipole alignment, and configurations with an isotropic distribution of the MA cations, resp. representative of polar (ferroelec.) and apolar (antiferroelec.) structures. The calcns. demonstrate the preferred stability of a set of polar structures over apolar ones, with an energy difference within 0.1 eV and a conversion barrier within 0.2 eV per unit cell (four MAPbI3), thus possibly accessible at room temp. Ferroelec.-like orientations lead to a quasi I4cm structure for the inorg. component, characterized by lack of inversion symmetry, while the antiferroelec.-like orientations are assocd. to a quasi I4/mcm structure. Ab initio mol. dynamics simulations on the polar structures show no mol. rotations in the investigated time-scale, while several MA rotations are obsd. in the same time scale for the considered apolar structure, which is thus characterized by a higher disorder. The I4cm and I4/mcm types of structure have markedly different band structures, despite showing a relatively small band gap variation. Simulations carried out on finite surface slabs demonstrate that a net orientation of the MA cations gives rise to a strong bending in the valence and conduction bands, which could definitely assist charge sepn. and reduce carrier recombination, provided one is able to effectively stabilize polar compared to apolar structures. The results could contribute an important step toward an in-depth comprehension of the basic properties of organohalide perovskites, assisting a further optimization of their photovoltaic response.
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    22. 22
      Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584– 2590,  DOI: 10.1021/nl500390f
      22
      Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells
      Frost, Jarvist M.; Butler, Keith T.; Brivio, Federico; Hendon, Christopher H.; van Schilfgaarde, Mark; Walsh, Aron
      Nano Letters (2014), 14 (5), 2584-2590CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitized and org. photovoltaics. High-power conversion efficiency can be realized in both mesoporous and thin-film device architectures. The authors address the origin of this success in the context of the materials chem. and physics of the bulk perovskite as described by electronic structure calcns. In addn. to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As the authors show, hybrid perovskites exhibit spontaneous elec. polarization; the authors also suggest ways in which this can be tuned through judicious choice of the org. cation. The presence of ferroelec. domains will result in internal junctions that may aid sepn. of photoexcited electron and hole pairs, and redn. of recombination through segregation of charge carriers. The combination of high dielec. const. and low effective mass promotes both Wannier-Mott exciton sepn. and effective ionization of donor and acceptor defects. The photoferroic effect could be exploited in nanostructured films to generate a higher open circuit voltage and may contribute to the current-voltage hysteresis obsd. in perovskite solar cells.
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    23. 23
      Kim, H.-S.; Kim, S. K.; Kim, B. J.; Shin, K.-S.; Gupta, M. K.; Jung, H. S.; Kim, S.-W.; Park, N.-G. Ferroelectric Polarization in CH3NH3PbI3 Perovskite. J. Phys. Chem. Lett. 2015, 6, 1729– 1735,  DOI: 10.1021/acs.jpclett.5b00695
      23
      Ferroelectric Polarization in CH3NH3PbI3 Perovskite
      Kim, Hui-Seon; Kim, Sung Kyun; Kim, Byeong Jo; Shin, Kyung-Sik; Gupta, Manoj Kumar; Jung, Hyun Suk; Kim, Sang-Woo; Park, Nam-Gyu
      Journal of Physical Chemistry Letters (2015), 6 (9), 1729-1735CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      We report on ferroelec. polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination. Perovskite crystals with three different sizes of 700, 400, and 100 nm were prepd. for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of elec. field, where the size dependency to polarization was not significant. Whereas the photoinduced stimulation was not significant without an external elec. field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelec. polarization was also obsd. after removal of the elec. field, in which larger crystals showed longer retention behavior compared to the smaller sized one. Addnl., we suggest the effect of perovskite crystal size (morphol.) on charge collection at the interface of the ferroelec. material even though insignificant size dependency in elec. polarization was obsd.
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    24. 24
      Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P. Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films. J. Phys. Chem. Lett. 2014, 5, 3335– 3339,  DOI: 10.1021/jz501697b
      24
      Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films
      Kutes, Yasemin; Ye, Linghan; Zhou, Yuanyuan; Pang, Shuping; Huey, Bryan D.; Padture, Nitin P.
      Journal of Physical Chemistry Letters (2014), 5 (19), 3335-3339CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelec., almost nothing is known about their ferroelec. properties exptl. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelec. domains in high-quality β-CH3NH3PbI3 perovskite thin films that have been synthesized using a new soln.-processing method. The size of the ferroelec. domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelec. domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelec. behavior of hybrid perovskites, in particular in situ photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics.
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    25. 25
      Beilsten-Edmands, J.; Eperon, G. E.; Johnson, R. D.; Snaith, H. J.; Radaelli, P. G. Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices. Appl. Phys. Lett. 2015, 106, 173502,  DOI: 10.1063/1.4919109
      25
      Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices
      Beilsten-Edmands, J.; Eperon, G. E.; Johnson, R. D.; Snaith, H. J.; Radaelli, P. G.
      Applied Physics Letters (2015), 106 (17), 173502/1-173502/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelec. response. A strong frequency dependence of the hysteresis is obsd. in the range of 0.1-150 Hz, with a hysteretic charge d. in excess of 1000 μC cm-2 at frequencies below 0.4 Hz, a behavior uncharacteristic of a ferroelec. response. We show that the obsd. hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of ≈ 1 μC cm-2 on any intrinsic frequency-independent polarization, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices. (c) 2015 American Institute of Physics.
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    26. 26
      Kunst, M.; Beck, G. The study of charge carrier kinetics in semiconductors by microwave conductivity measurements. J. Appl. Phys. 1986, 60, 3558– 3566,  DOI: 10.1063/1.337612
      26
      The study of charge carrier kinetics in semiconductors by microwave conductivity measurements
      Kunst, M.; Beck, G.
      Journal of Applied Physics (1986), 60 (10), 3558-66CODEN: JAPIAU; ISSN:0021-8979.
      The reliability and the possibility of microwave cond. measurements were investigated. The general background for excess cond. measurements in the microwave range, is first presented, and then a quant. relation between the reflected microwave signal and the change in cond. for a wafer of single-crystal Si is derived. For this sample, the theory of excess current-carrier kinetics is also developed. After a short description of the app., kinetic measurements on a ns and μs timescale are compared to theory.
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    27. 27
      Kunst, M.; Beck, G. The study of charge carrier kinetics in semiconductors by microwave conductivity measurements. II. J. Appl. Phys. 1988, 63, 1093– 1098,  DOI: 10.1063/1.340013
      27
      The study of charge carrier kinetics in semiconductors by microwave conductivity measurements. II
      Kunst, M.; Beck, G.
      Journal of Applied Physics (1988), 63 (4), 1093-8CODEN: JAPIAU; ISSN:0021-8979.
      In previous work, an introduction was given to the study of charge carrier kinetics in semiconductors by microwave cond. measurements. This paper compares quant. the exptl. results to theor. calcns. for single-crystal Si wafers, taking into account the dependence on the microwave frequency and the dark cond. of the sample, which ranged from σ = 0.2 to 400 Ω-1 m-1. In particular, difficulties arising from exptl. conditions that cannot easily be treated by theory are discussed. Quant. measurements of samples with low dark cond. can be performed even in a very simple configuration, which permits detn. of the sum of charge-carrier mobilities.
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    28. 28
      Sinton, R. A.; Cuevas, A.; Stuckings, M. Quasi-steady-state photoconductance, a new method for solar cell material and device characterization Conference Record of the Twenty Fifth IEEE Photovoltaic Specialists Conference - 1996, IEEE, 1996; pp 457– 460.
      There is no corresponding record for this reference.
    29. 29
      Goodarzi, M.; Sinton, R.; Macdonald, D. Quasi-steady-state photoconductance bulk lifetime measurements on silicon ingots with deeper photogeneration. AIP Adv. 2019, 9, 015128,  DOI: 10.1063/1.5086378
      29
      Quasi-steady-state photoconductance bulk lifetime measurements on silicon ingots with deeper photogeneration
      Goodarzi, Mohsen; Sinton, Ronald; MacDonald, Daniel
      AIP Advances (2019), 9 (1), 015128/1-015128/6CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
      Quasi-steady-state photoconductance measurements on silicon ingots and blocks with different photo-generation profiles are simulated in this work. The results show that deeper generation profiles, achieved by using a long-pass optical filter with a longer cut-off wavelength, can reduce the impact of the high surface recombination velocity of the ingot surface. This results in higher measured effective lifetimes and reduces the reliance on transfer functions to convert the measured lifetimes into bulk lifetimes. However, there exists a trade-off between generating carriers further from the surface to reduce surface recombination and ensuring that the generated carriers are within the sensitivity range of the photoconductance sensing coil. The simulations are compared with exptl. results measured on a monocryst. silicon block using both quasi-steady-state and transient photoconductance decay, as the transient method is relatively less prone to the impact of surface recombination, and provides a lower bound on the bulk lifetime. The results confirm an increased accuracy in bulk lifetimes extd. from quasi-steady-state measurements when measured using deeper photo-generation profiles. (c) 2019 American Institute of Physics.
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    30. 30
      Gyüre-Garami, B.; Blum, B.; Sági, O.; Bojtor, A.; Kollarics, S.; Csősz, G.; Márkus, B. G.; Volk, J.; Simon, F. Ultrafast sensing of photoconductivity decay using microwave resonators. J. Appl. Phys. 2019, 126, 235702,  DOI: 10.1063/1.5129597
      There is no corresponding record for this reference.
    31. 31
      Sólyom, J. Fundamentals of the Physics of Solids II: Electronic Properties; Springer, 2008.
      There is no corresponding record for this reference.
    32. 32
      Osherov, A.; Hutter, E. M.; Galkowski, K.; Brenes, R.; Maude, D. K.; Nicholas, R. J.; Plochocka, P.; Bulović, V.; Savenije, T. J.; Stranks, S. D. The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide Perovskites. Adv. Mater. 2016, 28, 10757– 10763,  DOI: 10.1002/adma.201604019
      32
      The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide Perovskites
      Osherov, Anna; Hutter, Eline M.; Galkowski, Krzysztof; Brenes, Roberto; Maude, Duncan K.; Nicholas, Robin J.; Plochocka, Paulina; Bulovic, Vladimir; Savenije, Tom J.; Stranks, Samuel D.
      Advanced Materials (Weinheim, Germany) (2016), 28 (48), 10757-10763CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The extent to which the soft structural properties of metal halide perovskites affect their optoelectronic properties is unclear. X-ray diffraction and micro-photoluminescence measurements are used to show that there is a coexistence of both tetragonal and orthorhombic phases through the low-temp. phase transition, and that cycling through this transition can lead to structural changes and enhanced optoelectronic properties.
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    33. 33
      Gélvez-Rueda, M. C.; Cao, D. H.; Patwardhan, S.; Renaud, N.; Stoumpos, C. C.; Schatz, G. C.; Hupp, J. T.; Farha, O. K.; Savenije, T. J.; Kanatzidis, M. G.; Grozema, F. C. Effect of Cation Rotation on Charge Dynamics in Hybrid Lead Halide Perovskites. J. Phys. Chem. C 2016, 120, 16577– 16585,  DOI: 10.1021/acs.jpcc.6b06722
      33
      Effect of Cation Rotation on Charge Dynamics in Hybrid Lead Halide Perovskites
      Gelvez-Rueda, Maria C.; Cao, Duyen H.; Patwardhan, Sameer; Renaud, Nicolas; Stoumpos, Constantinos C.; Schatz, George C.; Hupp, Joseph T.; Farha, Omar K.; Savenije, Tom J.; Kanatzidis, Mercouri G.; Grozema, Ferdinand C.
      Journal of Physical Chemistry C (2016), 120 (30), 16577-16585CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Org.-inorg. hybrid halide perovskites are a promising class of materials for photovoltaic application with reported power efficiencies over ∼22%. However, not much is known about the influence of the org. dipole rotation and phase transitions on charge carrier dynamics. Here, the authors report substantial changes in mobility and lifetime of charge carriers in CH3NH3PbI3 after the low-temp. tetragonal (β) to orthorhombic (γ) phase transition. By using microwave cond. measurements, the mobility and lifetime of ionized charge carriers increase as the temp. decreases and a sudden increment is seen after the β-γ phase transition. For CH3NH3PbI3, the mobility and the half-lifetime increase by a factor of 3-6 compared with the values before the β-γ phase transition. The authors attribute the considerable change in the dynamics at low temp. to the decrease of the inherent dynamic disorder of the org. cation (CH3NH3+) inside the perovskite crystal structure.
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    34. 34
      Leguy, A. M. A.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kockelmann, W.; Law, C.; Li, X.; Foglia, F.; Walsh, A.; O’Regan, B. C.; Nelson, J.; Cabral, J. T.; Barnes, P. R. F. The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells. Nat. Commun. 2015, 6, 7124,  DOI: 10.1038/ncomms8124
      34
      The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells
      Leguy Aurelien M A; McMahon Andrew P; Nelson Jenny; Barnes Piers R F; Frost Jarvist Moore; Walsh Aron; Sakai Victoria Garcia; Kochelmann W; Law ChunHung; Li Xiaoe; O'Regan Brian C; Foglia Fabrizia; Cabral Joao T
      Nature communications (2015), 6 (), 7124 ISSN:.
      Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3(+) ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1-200-ps time window. Monte Carlo simulations of interacting CH3NH3(+) dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3(+) in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3(+) to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1-1 ms, faster than most observed hysteresis.
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    35. 35
      Semonin, O. E.; Elbaz, G. A.; Straus, D. B.; Hull, T. D.; Paley, D. W.; van der Zande, A. M.; Hone, J. C.; Kymissis, I.; Kagan, C. R.; Roy, X.; Owen, J. S. Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals. J. Phys. Chem. Lett. 2016, 7, 3510– 3518,  DOI: 10.1021/acs.jpclett.6b01308
      35
      Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals
      Semonin, Octavi E.; Elbaz, Giselle A.; Straus, Daniel B.; Hull, Trevor D.; Paley, Daniel W.; van der Zande, Arend M.; Hone, James C.; Kymissis, Ioannis; Kagan, Cherie R.; Roy, Xavier; Owen, Jonathan S.
      Journal of Physical Chemistry Letters (2016), 7 (17), 3510-3518CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Using a combination of scanning photocurrent microscopy (SPCM) and time-resolved microwave cond. (TRMC) measurements, we monitor the diffusion and recombination of photoexcited charges in CH3NH3PbI3 perovskite single crystals. The majority carrier type was controlled by growing crystals in the presence or absence of air, allowing the diffusion lengths of electrons (LDe-) and holes (LDh+) to be directly imaged with SPCM (LDe- = 10-28 μm, LDh+ = 27-65 μm). TRMC measurements reveal a photogenerated carrier mobility (μh + μe) of 115 ± 15 cm2V-1s-1 and recombination that depends on the excitation intensity. From the intensity dependence of the recombination kinetics and by accounting for carrier diffusion away from the point of photogeneration, we ext. a second-order recombination rate const. (krad = 5 ± 3 × 10-10 cm3/s) that is consistent with the predicted radiative rate. First-order recombination at low photoexcited carrier d. (knrp-type = 1.0 ± 0.3 × 105 s-1, knrn-type = 1.5 ± 0.3 × 105 s-1) is slower than that obsd. in CH3NH3PbI3 thin films or in GaAs single crystals with AlGaAs passivation layers. By accounting for the diln. of photogenerated carriers upon diffusion, and by combining SPCM and TRMC measurements, we resolve disagreement between previous reports of carrier diffusion length.
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    36. 36
      Labram, J. G.; Perry, E. E.; Venkatesan, N. R.; Chabinyc, M. L. Steady-state microwave conductivity reveals mobility-lifetime product in methylammonium lead iodide. Appl. Phys. Lett. 2018, 113, 153902,  DOI: 10.1063/1.5041959
      36
      Steady-state microwave conductivity reveals mobility-lifetime product in methylammonium lead iodide
      Labram, John G.; Perry, Erin E.; Venkatesan, Naveen R.; Chabinyc, Michael L.
      Applied Physics Letters (2018), 113 (15), 153902/1-153902/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Many time-resolved techniques to study charge carrier recombination involve pulsed high-power optical excitation and photo-generated carrier densities many orders of magnitude higher than present under typical solar cell operating conditions. In this report, we demonstrate a steady-state contactless microwave cond. technique to evaluate the photocond. of carriers in semiconductors at low illumination intensity, as a function of optical power d. We studied characteristics of both thin films and single crystals of a hybrid halide perovskite compd., methylammonium lead iodide (MAPbI3). The aggregate mobility-lifetime product of majority and minority carriers in thin films of MAPbI3 was detd. and found to be highly-dependent on incident optical power d., even at sub-1-sun illumination intensities, and attributed to trap states within the films. (c) 2018 American Institute of Physics.
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    37. 37
      Hutter, E. M.; Eperon, G. E.; Stranks, S. D.; Savenije, T. J. Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes, and Concentrations of Trap States. J. Phys. Chem. Lett. 2015, 6, 3082– 3090,  DOI: 10.1021/acs.jpclett.5b01361
      37
      Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes, and Concentrations of Trap States
      Hutter, Eline M.; Eperon, Giles E.; Stranks, Samuel D.; Savenije, Tom J.
      Journal of Physical Chemistry Letters (2015), 6 (15), 3082-3090CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Efficient solar cells were obtained using thin films of soln.-processed org.-inorg. perovskites. However, there remains limited knowledge about the relation between prepn. route and optoelectronic properties. The authors use complementary time-resolved microwave cond. (TRMC) and photoluminescence (PL) measurements to study the charge carrier dynamics in thin planar films of CH3NH3PbI3-xClx, CH3NH3PbI3, and their meso-structured analogs. High mobilities close to 30 cm2/(V s) and microsecond-long lifetimes are found in thin films of CH3NH3PbI3-xClx, compared to lifetimes of only a few hundred nanoseconds in CH3NH3PbI3 and meso-structured perovskites. The authors describe the TRMC and PL expts. with a global kinetic model, using one set of kinetic parameters characteristic for each sample. The trap d. is <5 × 1014 cm-3 in CH3NH3PbI3-xClx, 6 × 1016 cm-3 in the CH3NH3PbI3 thin film and ∼1015 cm-3 in both meso-structured perovskites. Also, the authors' results imply that band-to-band recombination is enhanced by the presence of dark carriers resulting from unintentional doping of the perovskites. Finally, the authors' general approach to det. concns. of trap states and dark carriers is also highly relevant to other semiconductor materials.
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    38. 38
      Chattopadhyay, S.; Kokenyesi, R. S.; Hong, M. J.; Watts, C. L.; Labram, J. G. Resolving in-plane and out-of-plane mobility using time resolved microwave conductivity. J. Mater. Chem. C 2020, 8, 10761– 10766,  DOI: 10.1039/D0TC00328J
      38
      Resolving in-plane and out-of-plane mobility using time resolved microwave conductivity
      Chattopadhyay, Shirsopratim; Kokenyesi, Robert S.; Hong, Min Ji; Watts, C. Lowell; Labram, John G.
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2020), 8 (31), 10761-10766CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      The contactless characterization technique time resolved microwave cond. (TRMC) provides a means to rapidly and unambiguously approx. carrier mobilities and lifetimes for a variety of semiconducting materials. When using a cavity-based approach however, the technique can conventionally only resolve carrier mobilities in the plane of the substrate. In solar cells, charge carriers are extd. in the direction perpendicular to the substrate, therefore it would be beneficial if one were able to evaluate the mobility in this direction also. Here we present a novel approach for resolving charge carrier mobilities in different planes within a sample. Using a range of 3D-printed sample holders, where the sample is held at various angles relative to the incident light, we are able to simultaneously resolve the mobility in the plane of the sample and out of the plane of the sample. As examples, we have studied the 3-dimensional corner-connected metal halide perovskite methylammonium lead iodide and the 2-dimensional perovskite precusor, lead iodide.
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    39. 39
      Hutter, E. M.; Gélvez-Rueda, M. C.; Osherov, A.; Bulović, V.; Grozema, F. C.; Stranks, S. D.; Savenije, T. J. Direct-indirect character of the bandgap in methylammonium lead iodide perovskite. Nat. Mater. 2017, 16, 115– 120,  DOI: 10.1038/nmat4765
      39
      Direct-indirect character of the bandgap in methylammonium lead iodide perovskite
      Hutter, Eline M.; Gelvez-Rueda, Maria C.; Osherov, Anna; Bulovic, Vladimir; Grozema, Ferdinand C.; Stranks, Samuel D.; Savenije, Tom J.
      Nature Materials (2017), 16 (1), 115-120CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temp. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temp. orthorhombic phase, fast quenching of mobile charges occurs independent of the temp. and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyze spectroscopic data.
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    40. 40
      Hong, M. J.; Svadlenak, S. R.; Goulas, K. A.; Labram, J. G. Thermal stability of mobility in methylammonium lead iodide. J. Phys. Mater. 2020, 3, 014003,  DOI: 10.1088/2515-7639/ab442e
      There is no corresponding record for this reference.
    41. 41
      Bi, Y.; Hutter, E. M.; Fang, Y.; Dong, Q.; Huang, J.; Savenije, T. J. Charge Carrier Lifetimes Exceeding 15 μs in Methylammonium Lead Iodide Single Crystals. J. Phys. Chem. Lett. 2016, 7, 923– 928,  DOI: 10.1021/acs.jpclett.6b00269
      41
      Charge Carrier Lifetimes Exceeding 15 μs in Methylammonium Lead Iodide Single Crystals
      Bi, Yu; Hutter, Eline M.; Fang, Yanjun; Dong, Qingfeng; Huang, Jinsong; Savenije, Tom J.
      Journal of Physical Chemistry Letters (2016), 7 (5), 923-928CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The charge carrier lifetime in org.-inorg. perovskites is one of the most important parameters for modeling and design of solar cells and other types of devices. In this work, we use CH3NH3PbI3 single crystal as a model system to study optical absorption, charge carrier generation, and recombination lifetimes. We show that commonly applied photoluminescence lifetime measurements may dramatically underestimate the intrinsic carrier lifetime in CH3NH3PbI3, which could be due to severe charge recombination at the crystal surface and/or fast electron-hole recombination close to the surface. By using the time-resolved microwave cond. technique, we investigated the lifetime of free mobile charges inside the crystals. Most importantly, we find that for homogeneous excitation throughout the crystal, the charge carrier lifetime exceeds 15 μs. This means that the diffusion length in CH3NH3PbI3 can be as large as 50 μm if it is no longer limited by the dimensions of the crystallites.
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    42. 42
      Li, D.; Wang, G.; Cheng, H.-C.; Chen, C.-Y.; Wu, H.; Liu, Y.; Huang, Y.; Duan, X. Size-dependent phase transition in methylammonium lead iodide perovskite microplate crystals. Nat. Commun. 2016, 7, 11330,  DOI: 10.1038/ncomms11330
      42
      Size-dependent phase transition in methylammonium lead iodide perovskite microplate crystals
      Li, Dehui; Wang, Gongming; Cheng, Hung-Chieh; Chen, Chih-Yen; Wu, Hao; Liu, Yuan; Huang, Yu; Duan, Xiangfeng
      Nature Communications (2016), 7 (), 11330CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Methylammonium lead iodide perovskite has attracted considerable recent interest for soln. processable solar cells and other optoelectronic applications. The orthorhombic-to-tetragonal phase transition in perovskite can significantly alter its optical, elec. properties and impact the corresponding applications. Here, we report a systematic investigation of the size-dependent orthorhombic-to-tetragonal phase transition using a combined temp.-dependent optical, elec. transport and transmission electron microscopy study. Our studies of individual perovskite microplates with variable thicknesses demonstrate that the phase transition temp. decreases with reducing microplate thickness. The sudden decrease of mobility around phase transition temp. and the presence of hysteresis loops in the temp.-dependent mobility confirm that the orthorhombic-to-tetragonal phase transition is a first-order phase transition. Our findings offer significant fundamental insight on the temp.- and size-dependent structural, optical and charge transport properties of perovskite materials, and can greatly impact future exploration of novel electronic and optoelectronic devices from these materials.
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    43. 43
      Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films. APL Mater. 2014, 2, 081513,  DOI: 10.1063/1.4891595
      43
      Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films
      Wehrenfennig, Christian; Liu, Mingzhen; Snaith, Henry J.; Johnston, Michael B.; Herz, Laura M.
      APL Materials (2014), 2 (8), 081513/1-081513/10CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3-xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concs. on the behavior of the perovskite at room temp., the obsd. influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temp. phenomena can give valuable addnl. insights into the underlying physics. Here, we present a temp.-dependent study of optical absorption and photoluminescence (PL) emission of vapor-deposited CH3NH3PbI3-xClx exploring the nature of recombination channels in the room- and the low-temp. phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temp., a second PL emission peak emerges in addn. to the peak from the room-temp. phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temp. above the transition, significantly accelerated recombination is obsd. in the low-temp. phase. Our data suggest that small inclusions of domains adopting the room-temp. phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate consts. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centers that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion. (c) 2014 American Institute of Physics.
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    44. 44
      Lappas, A.; Kosaka, M.; Tanigaki, K.; Prassides, K. An Orientationally-Ordered Primitive-Cubic Form of the Fulleride CsC60. J. Am. Chem. Soc. 1995, 117, 7560– 7561,  DOI: 10.1021/ja00133a036
      44
      An Orientationally-Ordered Primitive-Cubic Form of the Fulleride CsC60
      Lappas, Alexandros; Kosaka, Mayumi; Tanigaki, Katsumi; Prassides, Kosmas
      Journal of the American Chemical Society (1995), 117 (28), 7560-1CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The crystal structure of the conducting phase of CsC60, formed by rapid cooling to liq. N temp., was detd. from the anal. of neutron powder diffraction data. The structure is primitive cubic (space group Pa‾3, a 13.9671(3) Å at 4.5 K) and results from orientational ordering of the C60- ions in a manner similar to that encountered in pristine C60. At. coordinates are given. The three-dimensional character of the present structure evidently leads to the suppression of the structural and magnetic instabilities which lead to insulating behavior in the low-dimensional forms of CsC60.
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    45. 45
      Janes, R.; Edge, M.; Robinson, J.; Allen, N. S.; Thompson, F. Microwave photodielectric and photoconductivity studies of commercial titanium dioxide pigments: the influence of transition metal dopants. J. Mater. Sci. 1998, 33, 3031– 3036,  DOI: 10.1023/A:1004371117530
      45
      Microwave photodielectric and photoconductivity studies of commercial titanium dioxide pigments: the influence of transition metal dopants
      Janes, R.; Edge, M.; Robinson, J.; Allen, N. S.; Thompson, F.
      Journal of Materials Science (1998), 33 (12), 3031-3036CODEN: JMTSAS; ISSN:0022-2461. (Kluwer Academic Publishers)
      The effect of transition metal ion doping on the charge carrier dynamics in com. TiO2 pigments has been investigated. The response to extended polychromatic irradn. was monitored by real-time measurements of microwave photocond. and photodielec. effects. In addn. to intrinsic differences between the rutile and anatase polymorphs, shifts in the resonance frequency and quality factor of a tuned microwave cavity as a function of annealing, and foreign-ion incorporation provided a gauge of the influence of extrinsic trapping centers on photoactivity. Parallel luminescence studies revealed an extension in the lifetime of the emission band on doping, the energy assocd. with recombination being independent of the dopant ion.
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    46. 46
      Savenije, T. J.; Ferguson, A. J.; Kopidakis, N.; Rumbles, G. Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity. J. Phys. Chem. C 2013, 117, 24085– 24103,  DOI: 10.1021/jp406706u
      46
      Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity
      Savenije, Tom J.; Ferguson, Andrew J.; Kopidakis, Nikos; Rumbles, Garry
      Journal of Physical Chemistry C (2013), 117 (46), 24085-24103CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      A review. During the past decade, time-resolved microwave cond. (TRMC) has evolved to an established, powerful technique to study photoactive layers. With this feature paper, the authors aim to fulfill two goals: (1) give a full description of the photoinduced TRMC technique, including exptl. details and data anal., and discuss to what extent the TRMC technique differs from more conventional d.c. techniques and (2) illustrate the potential of this technique for probing charge carrier dynamics in photoactive materials. For these reasons recent studies on conjugated polymer:fullerene blends will be presented and discussed. The findings from these studies have advanced the insight into the mechanism of charge carrier generation and decay in polymer:fullerene blends, which allows the authors to improve the efficiency of org. photovoltaic cells based on this active layer architecture. In short, the TRMC technique can be used as a versatile method to screen the potential of new photovoltaic materials.
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    47. 47
      Savenije, T. J.; Guo, D.; Caselli, V. M.; Hutter, E. M. Quantifying Charge-Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time-Resolved Microwave Photoconductivity Measurements. Adv. Energy Mater. 2020, 10, 1903788,  DOI: 10.1002/aenm.201903788
      47
      Quantifying Charge-Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time-Resolved Microwave Photoconductivity Measurements
      Savenije, Tom J.; Guo, Dengyang; Caselli, Valentina M.; Hutter, Eline M.
      Advanced Energy Materials (2020), 10 (26), 1903788CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      A review. The unprecedented rise in the power conversion efficiency of solar cells based on metal halide perovskites (MHPs) has led to enormous research effort to understand their photophys. properties. The progress made in understanding the mobility and recombination of photogenerated charge carriers from nanosecond to microsecond time scales, monitored using electrodeless transient photocond. techniques, is reviewed. In addn., a kinetic model to obtain rate consts. from transient data recorded using a wide range of laser intensities is presented. For various MHPs the temp. dependence of the mobilities and recombination rates are evaluated. Furthermore, it is shown how these rate consts. can be used to predict the upper limit for the open-circuit voltage Voc of the corresponding device. Finally, the photophys. properties of MHPs that are not yet fully understood are explored, and recommendations for future research directions are made.
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    48. 48
      Csősz, G.; Márkus, B. G.; Jánossy, A.; Nemes, N. M.; Murányi, F.; Klupp, G.; Kamarás, K.; Kogan, V. G.; Bud’ko, S. L.; Canfield, P. C.; Simon, F. Giant microwave absorption in fine powders of superconductors. Sci. Rep. 2018, 8, 11480– 11488,  DOI: 10.1038/s41598-018-29750-7
      48
      Giant microwave absorption in fine powders of superconductors
      Csosz G; Markus B G; Janossy A; Simon F; Nemes N M; Muranyi F; Klupp G; Kamaras K; Kogan V G; Bud'ko S L; Canfield P C
      Scientific reports (2018), 8 (1), 11480 ISSN:.
      Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB2 and K3C60) for magnetic fields as small as a few % of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K3C60 in the framework of the Coffey-Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant.
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    49. 49
      Novikov, G. F.; Marinin, A. A.; Rabenok, E. V. Microwave measurements of the pulsed photoconductivity and photoelectric effect. Instrum. Exp. Technol. 2010, 53, 233– 239,  DOI: 10.1134/S0020441210020144
      49
      Microwave measurements of the pulsed photoconductivity and photoelectric effect
      Novikov, G. F.; Marinin, A. A.; Rabenok, E. V.
      Instruments and Experimental Techniques (2010), 53 (2), 233-239CODEN: INETAK; ISSN:0020-4412. (Pleiades Publishing, Ltd.)
      A method of pulsed measurements of microwave cond. and photoelec. effect in a range of 36 GHz is described. This method ensures contactless measurements of the elec. properties of semi-conductors and studies of the kinetics of electron-ion processes with the participation of free current carriers and carriers localized in traps, resulting in data on lifetimes and mobilities of carriers in the nano- and micro-second ranges. The time resoln. of the recording system is 5 ns. The method is based on the recording of the kinetics of changes in the resonator quality factor and the shift of the resonance frequency caused by a light-stimulated change in the complex permittivity. The results of measuring the temp. dependences of the microwave photocond. in a p-type CdTe sample doped with 0.25 mol % Ag2Te and a CdSe sample are presented.
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    50. 50
      Joubert, M.-F.; Kazanskii, S. A.; Guyot, Y.; Gâcon, J.-C.; Pédrini, C. Microwave study of photoconductivity induced by laser pulses in rare-earth-doped dielectric crystals. Phys. Rev. B 2004, 69, 165217,  DOI: 10.1103/PhysRevB.69.165217
      50
      Microwave study of photoconductivity induced by laser pulses in rare-earth-doped dielectric crystals
      Joubert, M.-F.; Kazanskii, S. A.; Guyot, Y.; Gacon, J.-C.; Pedrini, C.
      Physical Review B: Condensed Matter and Materials Physics (2004), 69 (16), 165217/1-165217/13CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Transient responses of the dielec. permittivity ε=ε1-iε2 of rare-earth-doped dielec. crystals under pulsed laser excitation were studied by the 8-mm microwave resonator technique at room temp. The fluorite-type crystals (CaF2, SrF2, and BaF2) which contained divalent ions of Sm, Eu, and Tm, as well as Lu2(SiO4)O and Y3Al5O12 doped with trivalent Ce ions were studied. The dielec. response to a laser pulse contains two different types of signals: electronic and heating ones. The electronic peak, which is quite fast (from 40 to 100 ns or more), is the signature of electrons released into the conduction band following an impurity photoionization. The prolonged heating signal has a sawtooth form on which oscillations are imposed. It is due to modulation of the dielec. const. by a sudden temp. rise and subsequent elastic vibrations of the sample caused by the energy absorbed from the laser pulse. In different crystals the electronic peak was caused by the transient response of either ε1, ε2, or a mixt. of the two. The modulation of the dielec. loss factor ε2 corresponds to conventional photocond., i.e., the photoexcitation of mobile electrons. The modulation of the dielec. const. ε1 corresponds to the photoexcitation of bound electrons, probably captured by traps. The threshold energies of photons at which the photoionization of rare-earth ions may occur, were detd. for CaF2:Sm2+ (3.3 eV) and Lu2(SiO4)O:Ce3+ (3.1 eV). In fluorite-type crystals doped with Sm2+ or Tm2+ ions, the significant redn. of a lifetime of electrons in a conduction band was revealed with an increase in energy of laser pulses. In SrF2:Eu crystal the record-high signals of photocond. were obsd. upon excitation by visible light in the optical region of transparency of this crystal. The microwave resonant technique may be used for detail studying the photoionization dynamics of rare-earth ions and finding the location of their energy levels with respect to the host conduction band in doped insulators.
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    51. 51
      Mantulnikovs, K.; Szirmai, P.; Kollár, M.; Stevens, J.; Andričević, P.; Glushkova, A.; Rossi, L.; Bugnon, P.; Horváth, E.; Sienkiewicz, A. Light-induced charge transfer at the CH3NH3PbI3/TiO2 interface-a low-temperature photo-electron paramagnetic resonance assay. JPhys. Photonics 2020, 2, 014007,  DOI: 10.1088/2515-7647/ab6276
      51
      Light-induced charge transfer at the CH3NH3PbI3/TiO2 interface-a low-temperature photo-electron paramagnetic resonance assay
      Mantulnikovs, Konstantins; Szirmai, Peter; Kollar, Marton; Stevens, Jeremy; Andricevic, Pavao; Glushkova, Anastasiia; Rossi, Lidia; Bugnon, Philippe; Horvath, Endre; Sienkiewicz, Andrzej; Forro, Laszlo; Nafradi, Balint
      JPhys Photonics (2020), 2 (1), 014007CODEN: JPPOKR; ISSN:2515-7647. (IOP Publishing Ltd.)
      The performance of org.-inorg. metal halide perovskites-based (MHPs) photovoltaic devices critically depends on the design and material properties of the interface between the light-harvesting MHP layer and the electron transport layer(ETL). Therefore, the detailed insight into the transfer mechanisms of photogenerated carriers at the ETL/MHP interface is of utmost importance. Owing to its high charge mobilities and well-matched band structure withMHPs, titanium dioxide (TiO2) has emerged as the most widely used ETL material in MHPs-based photovoltaic devices. Here, we report a contactless method to directly track the photo-carriers at the ETL/MHP interface using the technique of low-temp. ESR (EPR)in combination with in situ illuminations (Photo-EPR). Specifically, wefocus on a model nanohybrid material consisting of TiO2-based nanowires (TiO2NWs) dispersed in the polycryst. methylammonium lead triiodide (MAPbI3) matrix. Our approach is based on observation of the light-induced decrease in intensity of the EPR signal of paramagnetic Ti3+ (S = 1 2)in non-stoichiometric TiO2NWs.We assoc. the diminishment of the EPR signal with the photo-excited electrons that cross the ETL/MHP interface and contribute to the conversion of Ti3+ states to EPR-silent Ti2+ states. Overall, we infer that the technique of lowtemperature Photo-EPR is an effective strategy to study the transfer mechanisms of photogenerated carriers at the ETL/MHP interface in MAPbI3-based photovoltaic and photoelectronic systems.
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    52. 52
      Mosconi, E.; Umari, P.; De Angelis, F. Electronic and optical properties of MAPbX3 perovskites (X = I, Br, Cl): a unified DFT and GW theoretical analysis. Phys. Chem. Chem. Phys. 2016, 18, 27158– 27164,  DOI: 10.1039/C6CP03969C
      52
      Electronic and optical properties of MAPbX3 perovskites (X = I, Br, Cl): a unified DFT and GW theoretical analysis
      Mosconi, Edoardo; Umari, Paolo; De Angelis, Filippo
      Physical Chemistry Chemical Physics (2016), 18 (39), 27158-27164CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Materials engineering is a key for the enhancement of photovoltaics technol. This is particularly true for the novel class of perovskite solar cells. Accurate theor. modeling can help establish general trends of behavior when addressing structural changes. Here, we consider the effects due to halide substitution in organohalide CH3NH3PbX3 perovskites exploring the halide series with X = Cl, Br, I. For this task, we use accurate DFT and GW methods including spin-orbit coupling. We find the expected band gap increase when moving from X = I to Cl, in line with the exptl. data. Most notably, the calcd. absorption coeffs. for I, Br and Cl are nicely reproducing the behavior reported exptl. A common feature of all the simulated band structures is a significant Rashba effect. This is similar for MAPbI3 and MAPbBr3 while MAPbCl3 shows in general a reduced Rashba interaction coeff. Finally, a monotonic increase of the exciton reduced masses is calcd. when moving from I to Br to Cl, in line with the stronger excitonic character of the lighter perovskite halides.
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    53. 53
      Maculan, G.; Sheikh, A. D.; Abdelhady, A. L.; Saidaminov, M. I.; Haque, M. A.; Murali, B.; Alarousu, E.; Mohammed, O. F.; Wu, T.; Bakr, O. M. CH3NH3PbCl3 single crystals: inverse temperature crystallization and visible-blind UV-photodetector. J. Phys. Chem. Lett. 2015, 6, 3781– 3786,  DOI: 10.1021/acs.jpclett.5b01666
      53
      CH3NH3PbCl3 Single Crystals: Inverse Temperature Crystallization and Visible-Blind UV-Photodetector
      Maculan, Giacomo; Sheikh, Arif D.; Abdelhady, Ahmed L.; Saidaminov, Makhsud I.; Haque, Md Azimul; Murali, Banavoth; Alarousu, Erkki; Mohammed, Omar F.; Wu, Tom; Bakr, Osman M.
      Journal of Physical Chemistry Letters (2015), 6 (19), 3781-3786CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Single crystals of hybrid perovskites showed remarkably improved phys. properties compared to their polycryst. film counterparts, underscoring their importance in the further development of advanced semiconductor devices. A new method of growing sizable MeNH3PbCl3 single crystals based on the retrograde soly. behavior of hybrid perovskites is presented. The energy band structure, charge recombination, and transport properties of MeNH3PbCl3 single crystals are detd. These crystals exhibit trap-state d., charge carrier concn., mobility, and diffusion length comparable with the best quality crystals of methylammonium lead iodide or bromide perovskites reported so far. The high quality of the crystal along with its suitable optical band gap enabled one to build an efficient visible-blind UV-photodetector, demonstrating its potential in optoelectronic applications.
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    54. 54
      Pozar, D. Microwave Engineering - Solutions Manual; Wiley, 2011; Vol. 4.
      There is no corresponding record for this reference.
    55. 55
      Chen, L.; Ong, C.; Neo, C.; Varadan, V.; Varadan, V. Microwave Electronics: Measurement and Materials Characterization; Wiley, 2004.
      There is no corresponding record for this reference.
    56. 56
      Klein, O.; Donovan, S.; Dressel, M.; Grüner, G. Microwave cavity perturbation technique: Part I: Principles. Int. J. Infrared Millimeter Waves 1993, 14, 2423– 2457,  DOI: 10.1007/BF02086216
      56
      Microwave cavity perturbation technique. Part I: Principles
      Klein, Olivier; Donovan, Steve; Dressel, Martin; Gruner, George
      International Journal of Infrared and Millimeter Waves (1993), 14 (12), 2423-57CODEN: IJIWDO; ISSN:0195-9271.
      A review with 22 refs. on the anal. used to ext. the complex cond. of a compd. from a microwave cavity perturbation measurement. The authors intend to present a generalized treatment valid for any spheroidally shaped sample of arbitrary cond. which is placed at either the elec. or magnetic field antinode of the cavity. To begin with, the authors establish the relationship between the measured parameters and the cond. for a spherical sample. Next, the authors extend these results to the case of spheroids; and for the first time, the authors cover all different configurations that one can possibly use to study an arbitrary conducting sample inside a cavity: in particular, all possible orientations of the sample with respect to the applied field are solved.
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    57. 57
      Landau, L.; Bell, J.; Kearsley, M.; Pitaevskii, L.; Lifshitz, E.; Sykes, J. Electrodynamics of Continuous Media; Course Of Theoretical Physics; Elsevier Science, 2013.
      There is no corresponding record for this reference.
    58. 58
      Poole, C. P. Electron Spin Resonance, 1st ed.; John Wiley & Sons: New York, 1983.
      There is no corresponding record for this reference.