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Overcoming Nanoscale Inhomogeneities in Thin-Film Perovskites via Exceptional Post-annealing Grain Growth for Enhanced Photodetection

  • Tian Du*
    Tian Du
    School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom
    Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
    *Email: [email protected]; [email protected]
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  • , 
  • Filipe Richheimer
    Filipe Richheimer
    National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom
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    Orcidhttps://orcid.org/0000-0002-5360-8381
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  • Kyle Frohna
    Kyle Frohna
    Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
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  • Nicola Gasparini
    Nicola Gasparini
    Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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    Orcidhttps://orcid.org/0000-0002-3226-8234
  • , 
  • Lokeshwari Mohan
    Lokeshwari Mohan
    School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom
    Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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  • Ganghong Min
    Ganghong Min
    Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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  • Weidong Xu
    Weidong Xu
    Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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  • Thomas J. Macdonald
    Thomas J. Macdonald
    School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom
    Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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    Orcidhttps://orcid.org/0000-0002-7520-6893
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  • Haozhen Yuan
    Haozhen Yuan
    School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom
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  • Sinclair R. Ratnasingham
    Sinclair R. Ratnasingham
    School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom
    Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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  • Saif Haque
    Saif Haque
    Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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    Orcidhttps://orcid.org/0000-0001-5483-3321
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  • Fernando A. Castro
    Fernando A. Castro
    National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom
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  • James R. Durrant
    James R. Durrant
    Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
    SPECIFIC IKC, College of Engineering, Swansea University, Swansea SA2 7AX, United Kingdom
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    Orcidhttps://orcid.org/0000-0001-8353-7345
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  • Samuel D. Stranks
    Samuel D. Stranks
    Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
    Department of Chemical Engineering & Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, United Kingdom
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    Orcidhttps://orcid.org/0000-0002-8303-7292
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  • Sebastian Wood
    Sebastian Wood
    National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom
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  • Martyn A. McLachlan
    Martyn A. McLachlan
    Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
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    Orcidhttps://orcid.org/0000-0003-3136-1661
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  • Joe Briscoe*
    Joe Briscoe
    School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-5925-860X
Cite this: Nano Lett. 2022, 22, 3, 979–988
Publication Date (Web):January 21, 2022
https://doi.org/10.1021/acs.nanolett.1c03839

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Abstract

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Antisolvent-assisted spin coating has been widely used for fabricating metal halide perovskite films with smooth and compact morphology. However, localized nanoscale inhomogeneities exist in these films owing to rapid crystallization, undermining their overall optoelectronic performance. Here, we show that by relaxing the requirement for film smoothness, outstanding film quality can be obtained simply through a post-annealing grain growth process without passivation agents. The morphological changes, driven by a vaporized methylammonium chloride (MACl)–dimethylformamide (DMF) solution, lead to comprehensive defect elimination. Our nanoscale characterization visualizes the local defective clusters in the as-deposited film and their elimination following treatment, which couples with the observation of emissive grain boundaries and excellent inter- and intragrain optoelectronic uniformity in the polycrystalline film. Overcoming these performance-limiting inhomogeneities results in the enhancement of the photoresponse to low-light (<0.1 mW cm–2) illumination by up to 40-fold, yielding high-performance photodiodes with superior low-light detection.

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Solution-processed metal halide perovskites (MHPs) are inherently polycrystalline, consisting of crystallographic (1) and electronic defects. (2) Although high-performance solar cells can be achieved from these solution-processed films, (3) as the trap states in MHPs are reported to be less detrimental under solar irradiance, (4) the impact of trap-mediated recombination is non-negligible at low carrier densities. (5,6) Under weak illumination, for example, trap-mediated recombination may completely switch off charge transfer to contact layers. (4,7) Hence, the control of crystallographic defects and thus electronic trap states in thin-film MHPs is of great importance to devices working in the dark or under low-light conditions, such as transistors, (8) photodetectors, (9) or indoor solar cells. (10)
Since first being introduced, (11) the “antisolvent washing” for spin coating of perovskite films has become the dominant technique used for high-performing devices. The overriding factor driving the uptake of this method is the need for smooth, compact, and planar perovskite films for subsequent deposition of charge transport layers by solution-processed routes. However, the rapid perovskite crystallization induced by antisolvent washing and subsequent thermal annealing gives rise to relatively small grains (∼150–200 nm in diameter). These small grains typically demonstrate a strongly heterogeneous shape owing to limited space for postcrystallization growth. As such, local inhomogeneity exists in these films in terms of material composition, (12) crystallographic defects, (13) trap density, (14) and lattice strains (15) with the local inhomogeneity being observed not only intergrain (16,17) but also intragrain. (18,19) In addition, the prevalent grain boundaries (GBs) in polycrystalline films are heterogeneous to the grain interior in nature, (20,21) and they typically host a variety of crystallographic defects. (22) Therefore, overcoming the local inhomogeneities, particularly at the GBs, is a critical route toward improvement of the overall quality of polycrystalline films.
Post-deposition crystal growth is widely used to improve the grain sizes of thin-film polycrystalline semiconductors above the film thickness. (23) For MHP films, this has been realized through elevated temperature, (24) high-energy laser pulses, (25) or solution-assisted Ostwald ripening. (26,27) However, the poor stability of perovskites (22) poses a fundamental limit to the time and temperature windows for post-deposition treatments. Herein, we report a solid-state secondary grain growth method enabling a nearly 10-fold increase of lateral grain size at a typical thermal annealing temperature for MAPbI3 perovskites (100 °C) within a short period of time (5 min). The treatment drives a comprehensive morphological transition toward minimization of surface energy, enabling not only the novel photophysical observation of emissive GBs but also exceptional nanoscale photoconductive uniformity and superior low-light photoresponse, leading to high-performance photodiodes.

Results and Discussion

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The perovskite (MAPbI3) films are deposited through an established “antisolvent washing” method followed by thermal annealing. (28) Post-deposition treatment of the MAPbI3 films was carried out in an aerosol-assisted chemical vapor deposition setup (29) by passing through aerosolized dimethylformamide (DMF) containing methylammonium chloride (MACl; 0.1 mol L–1) carried by continuous nitrogen flow over the film heated at 100 °C. The laminar flow of aerosol vaporizes near the film surface, and the vapor ingresses into the film. Figure 1a–c shows the scanning electron microscope (SEM) images of the MAPbI3 films before and after the treatment. The as-deposited MAPbI3 film comprises small grains (Figure 1a), while after 5 min of treatment, closely packed, monolithic grains are formed (Figure 1b). Overtreatment for 10 min results in the grains dewetting from the substrates, which leaves voids in the film (Figure 1c), strongly indicating the grain growth is driven by the minimization of surface energy.

Figure 1

Figure 1. Grain growth of MAPbI3 films induced by treatment with DMF–MACl aerosol. (a–c) Tilt-angle cross-section SEM images of untreated MAPbI3 film after antisolvent-assisted spin coating (a), treated MAPbI3 film for 5 min, (b) and overtreated MAPbI3 film for 10 min (c). Scale bars in all images are 500 nm. (d) Impact of MACl concentration in DMF on the statistical data of lateral grain size. (e) Impact of different halide compound additives in DMF (all with concentrations of 0.1 M) on the statistical data of lateral grain size. (f) Schematic drawing of MAPbI3 film morphological transition during MACl treatment and the process of aerosol ingression to the MAPbI3 films; growth of larger MAPbI3 grains at the expense of surrounding smaller grains.

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In Figure 1d, we demonstrate the critical role of MACl as an additive in driving exceptional grain growth. An increase in the concentration of MACl in the DMF aerosol consistently increases the average lateral grain size of the MAPbI3 films (Figure S1) within the fixed time of 5 min. An analogous effect can be seen by incorporating similar AX compounds, where A = MA, formamidine (FA), or Cs and X = I or Br, Figures 1e and S2. Clearly, it is the A cation, instead of the halide, that drives exceptional grain growth, as potassium iodide (KI) shows no such effect since it cannot form a stable 3D ABX3 (here B = Pb) perovskite. MACl is employed in this study as we observed that its lower mass assists the formation of a consistent laminar aerosol flow, and it remains most stable in the DMF against continuous aerosolization.
The morphological change of MAPbI3 films and mechanism of grain growth are schematically illustrated in Figure 1f. The ingression of the DMF–MACl vapor is insufficient to fully dissolve the MAPbI3 films but enables the selective consumption of the smaller grains, allowing mass transfer of material from the smaller grains to the larger ones and eventually leading to an increase of overall grain size as time elapses, a process described by Ostwald ripening theory. (23) Contrary to large perovskite grains grown from solution, (13,26,30) the films remain in the solid state throughout our treatment (Figure S3). The absence of a liquid phase prevents additional perovskite nucleation on the surface; thus, the overriding process is the growth of the existing grains, leading to minimization of both surface energy and surface crystallographic defects.
While DMF vapor is known to break apart perovskite crystals, (29) the role of MACl, as well as other AX (A = Cs, FA, MA, X = Br, I) additives, is to increase the solvating power of DMF, thereby driving continuous dissolution of the smaller grains among the existing perovskite grains. We think this is because the ingression of MA+, Cs+, or FA+ can distort the [PbI6]4– octahedra in the MAPbI3 lattice and therefore facilitates the collapse of the 3D perovskite structures, analogous to the dissolution of MAPbI3 films by MA gas. (31) It is worth noting that the additives are mostly expelled from the film when larger MAPbI3 grains are formed, as we observed a negligible band gap change that would be expected from incorporation of FA, Cs, Br, or Cl. (Figure S4).
The above results show that, by controlling the additives in the aerosol and the time of aerosol ingression, a remarkable grain size increase can be achieved while the film compactness is maintained. Hereon, we focus on the films before (denoted as “untreated”) and after 5 min of treatment with 0.1 M MACl in DMF (denoted as “MACl treated”). The surface and cross-sectional SEM images in Figure 2a–d further highlight the morphological difference. The large number of crystallographic terraces observed on the grains of the untreated film (Figure 2a), giving a “wrinkled” appearance, indicate the heterogeneous nature of the grain morphology due to rapid crystallization, as such a variety of crystallographic facets are exposed on the surface. (32) The cross-sectional image further elucidates the heterogeneous morphology of the grains (Figure 2b). In contrast, the terraces are completely removed from the surface of the MACl treated film (Figure 2c). The cross-sectional image demonstrates the morphology of the monolithic grains that all have flattened side facets and smooth convex surfaces (Figure 2d), indicating comprehensive recrystallization at the surfaces and the grain boundaries.

Figure 2

Figure 2. Characterization of MAPbI3 films. (a–d) Surface and cross-sectional SEM images of untreated MAPbI3 film (a, b) and MACl treated MAPbI3 film (c, d). Scale bars are 500 nm on all images. (e) X-ray diffraction patterns of untreated and MACl treated MAPbI3 films. (f) Zoom-in of the (110) diffraction peak. (g, h) Time-resolved PL spectra of untreated MAPbI3 (g) and MACl treated MAPbI3 (h) films, measured with a 435 nm pulsed laser under varied excitation density. (i) Steady-state PL spectra of MAPbI3 films measured with a 635 nm continuous-wavelength laser with an excitation density of 1.5 mW cm–2. The inset figure shows the spectra with normalized PL intensity, and the arrow indicates peak broadening of the MACl treated film.

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X-ray diffraction (XRD) patterns, Figure 2e, show that the MACl treated film has a large increase of diffraction intensity, indicating improved crystallinity, and an approximate 4-fold increase of the relative intensity between the (110) and (310) peak, suggesting a strong preferential (110) crystallographic orientation. Closer inspection of the (110) XRD peak, Figure 2f, highlights a peak shift to lower 2θ angles after MACl treatment, indicating increased lattice spacing in the out-of-plane directions. This is typically ascribed to relaxation of the residual tensile strains in the in-plane direction, (33) which is achieved through grain growth and morphological transition toward minimized surface energy.
To elucidate the change of charge recombination mechanisms, time-resolved photoluminescence (PL) spectroscopy was measured under varied excitation densities, Figure 2g–h. The PL decay dynamics of the untreated MAPbI3 film (Figure 2g) show typical biexponential behavior comprising a fast decay in the first few nanoseconds and a slow decay in tens of nanoseconds. (4) Under the lowest excitation density, 0.028 μJ cm–2, the fast decay quenches over 99.5% of the initial PL, whereas increasing the excitation density consistently reduces the fast-decay component. Such excitation-dependent behavior is assigned to the filling of trap states by photogenerated carriers, and thus, the fast-decay component is ascribed to charge trapping. (34,35) In contrast, the fast-decay quenches only around 80% of the initial PL in the MACl treated film under 0.028 μJ cm–2 (Figure 2h). As excitation density increases, the fast-decay magnitude shows much less change while the lifetime of slow decay becomes shorter, ascribed to the acceleration of band-to-band recombination as the density of photogenerated charge carriers increases. These results show the dominance of trap-assisted recombination in the untreated MAPbI3 film and that the density of trap states is substantially reduced by the MACl treatment.
Consistent with time-resolved PL data, the steady-state PL spectra show an approximate 12-fold increase of PL intensity by MACl treatment, Figure 2i. At the same time, the enhancement of PL emission is accompanied by a change of spectral shape, in that the peak becomes asymmetric and exhibits a “shoulder” on the low-energy side, shown in the inset figure. This indicates that the emissive film exhibits additional radiative recombination pathways with slightly reduced transition energy. We note that we observe a similar increase of grain size and spectral broadening of PL in a Cs0.1FA0.9Pb(I0.95Br0.05)3 film after MACl treatment (Figure S5), indicating MA is not a critical component for this effect to occur.
To understand the spatial origin of enhanced PL and the spectral “shoulder”, we turn to hyperspectral PL maps to probe local spectra. Figure 3 shows broadband emission maps and wavelength-specific emission maps at 775 and 800 nm from the untreated (a–c) and MACl treated films (d–f). The untreated film exhibits local variations of emission intensity in the broadband map (Figure 3a), but no clear trend in their spatial distribution is observed in wavelength-specific maps (Figure 3b,c), typical for perovskite films prepared with antisolvent washing. (36) In the MACl treated film, we are readily able to resolve individual grains, as there is notably intensified PL along the GBs (Figure 3d). When one looks at the wavelength-specific maps, the brightening of the GB is less prominent in the 775 nm map (Figure 3e) but is the dominant origin of emission in the 800 nm map (Figure 3f), which coincides with the low-energy “shoulder” in the PL peak. We note that no such GB brightening is exhibited in films treated with DMF-only vapors (Figure S6), indicating the critical role that MACl plays in facilitating this behavior.

Figure 3

Figure 3. Photoluminescence mapping of MAPbI3 films. (a–f) Hyperspectral PL maps on an untreated MAPbI3 film (a–c) and a MACl treated MAPbI3 film (d–f) probed with broadband detector (a, d) and wavelength-selective detector at 775 nm (b, e) and at 800 nm (c, f). Maps were taken with 405 nm laser excitation with an intensity of 240 mW cm–2 in an ambient atmosphere. (g) Localized PL spectra from three different grains labeled “I”, “II”, and “III” in (d). On all images, a.u. refers to arbitrary units.

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To gain further spectral insight, we plot in Figure 3g the local spectra at both the GB and grain interior from three representative grains (labeled “I” to “III” in Figure 3d), where the spectral difference between the GB and grain interior can immediately be seen. All these spectra show a redshift of peak position at the GB regions with the peak area comparable or even greater than the grain interiors, highlighting the emissive nature of GBs in MACl treated films. Therefore, we can conclude that the overall spectral asymmetry observed in Figure 2i associated with the emergence a low-energy subpeak stems directly from local brightening at the GBs in the MACl treated MAPbI3 film.
The observation of the red-shifted PL emission in the perovskite films has been ascribed to compression of the perovskite lattice and is usually accompanied by increased charge carrier lifetime, (33,37,38) hereby signaling an improvement of film quality. The emergence of GB emission herein indicates a substantial defect elimination in these regions, while the redshift is likely correlated to a relaxation of tensile strains (39) on the side facets of the monolithic grain. The latter is supported by the XRD data where relaxation of in-plane tensile strain leads to increased out-of-plane lattice spacing (Figure 2f). The grain edges show the largest improvement in optoelectronic properties as the GBs have maximum exposure to the DMF–MACl aerosol during the post-treatment. Hence, a complete recrystallization preferentially occurs at the edges of the existing grains, thereby eliminating the conventionally defective regions. We note that Cl passivation (40) is not likely to play an overriding role in the formation of emissive GBs, as Cl-doping of MAPbI3 leads to an increase of the band gap whereas we observe a redshift. We also exclude the contribution of PL scattering at the grain edges to the spectral redshift/broadening by measuring probe-angle-dependent PL spectra, Figure S7, in which the change in spectral shape by varying the probing angle is negligible compared to the change induced by MACl treatment.
To further examine how the improved quality in the GB regions affects the local optoelectronic properties, we turn to photoconductive atomic force microscopy (pc-AFM) measurements. Figure 4a–h shows the maps for the untreated film (a–d) and the MACl treated film (e–h), including height (a, e), dark current (Idark) (b, f), photocurrent (Iph) (c, g), and photoresponse (d, h). Here, the photoresponse [nA cm2 mW–1] is defined as (41)
(1)
and is used as an assay of photodetecting properties under certain incident light intensity (Pin). We note that the direct estimation of the responsivity [A W–1] remains challenging due to the difficulty in determining the effective tip–sample emission area and thus the current density. (42)Iph and photoresponse are mapped under varied Pin values between 0.03 and 160 mW cm–2 (Figures S8 and S9). Representative line-scan data taken from an untreated film (line “I”) and MACl treated film (lines “II”, “III”, and “IV”) are plotted in Figure 4i,j, respectively. Different grains along the line scans are indicated to highlight the intra- and intergrain variation of these quantities.

Figure 4

Figure 4. pc-AFM measurement of perovskite films. (a–h) pc-AFM maps of untreated MAPbI3 film (a–d) and MACl treated MAPbI3 film (e–h): surface height (a, e), dark current Idark (b, f), and the photocurrent Iph (c, g) and photoresponse (d, h) under 0.03 mW cm–2. Both films are deposited on ITO substrate. All Idark and Iph data are measured with an applied bias of −1.5 V on ITO. Illumination is provided by a 633 nm laser. (i, j) Line-scan data of height, Idark, Iph, and photoresponse taken from untreated film (marked “I”) and from three different positions in MACl treated film (marked “II”, “III”, and “IV”, respectively) under varied light intensities from 0.03 to 160 mW cm–2. (k) Schematic drawing of the energy diagram of the ITO/perovskite/Au heterojunction under bias. (l) Area averaged photoresponse as a function of light intensity. The error bars indicate spatial variation of the photoresponse within the measured area.

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The map of Idark highlights a substantial spatial inhomogeneity in untreated MAPbI3 (Figure 4b). The line-scan data (Figure 4i) further elucidate that the variation of Idark is both intragrain and intergrain, but in general, Idark tends to be greater near the GBs. When an illumination of 0.03 mW cm–2 is turned on, Iph remains spatially inhomogeneous and does not fully track the distribution of Idark (Figure 4c,i). We thus highlight a set of regions in Figure 4b–d that exhibit exceptionally high Idark but do not show the proportionality of Iph and thereby have a negligible photoresponse. Importantly, these regions showing a negligible photoresponse under these low-light levels serve as performance-limiting clusters in the untreated MAPbI3 in terms of photodetection. It can also be observed that these clusters tend to overlap certain crystallographic facets that are particularly defective. Note that these clusters all have a stronger photoresponse when the light intensity is increased, suggesting that the performance-limiting factor is trap-mediated charge recombination that prevails under weak illumination levels. This in itself is noteworthy, as it allows us to directly visualize trap-rich, performance-limiting regions of the perovskite films. In contrast, the MACl treated film exhibits remarkably improved spatial homogeneity of both Idark and Iph, particularly under weak illumination levels as shown by the maps (Figure 4f,g) and line-scan data (Figure 4j). Importantly, the spatial variations of Idark and Iph are minimized both intergrain and intragrain in spite of the surface height change being greater than in the untreated film (Figure 4j). Therefore, the GBs in the MACl treated film become “invisible” in the electrical measurement under dark or weak illumination. The line-scan data also indicate that Iph is much higher than Idark as the illumination is applied, yielding stronger photoresponse in the MACl treated film.
We then turn to a quantitative analysis of the photoresponse of what is essentially an ITO/perovskite/Au photoconductor under bias, as indicated in Figure 4k. As Figure 4l shows, the photoresponse of both films decreases as light intensity increases, but the MACl treated films exhibit a substantially enhanced photoresponse; such an enhancement is much greater under lower light intensities, showing an increase by a factor of 20–40 in the range of 0.03–3.2 mW cm–2. We can also observe a much greater variation of both Iph and photoresponse in the untreated film under lower light intensities, which tracks their large spatial inhomogeneity, indicating that the low Iph and poor photoresponse are correlated to the large local variations of these quantities. These results highlight an exceptional improvement in low-light detecting capability using MACl treated films.
Finally, to investigate how overcoming local inhomogeneities in MAPbI3 films translates to the performance of full optoelectronic devices, we fabricated and tested perovskite photodiodes (PPDs) with the architecture shown in Figure 5a. Figure 5b plots the current density–voltage (J–V) curves in the dark and under illumination (100 mW cm–2). As a key parameter governing the sensitivity of a photodiode, the dark current (Jd) reported for PPDs varies in a wide range depending largely on the engineering of the contact layers and/or defects in the perovskite layer. (43) We herein employ relative thick contact layers that are reported to suppress leakage current and thus reduce Jd (see Table S1 and Figure S10). (44) Even for the PPD with an otherwise optimized layer thickness, MACl treatment brings about a remarkable reduction of Jd. At −2 V, for example, Jd is 2.2 × 10–8 A cm–2 for the MACl treated PPD, about two orders lower than the Jd of 3.5 × 10–6 A cm–2 for the reference PPD. This is among the lowest Jd value reported for a PPD at this bias. (45)

Figure 5

Figure 5. Characterization of the perovskite photodiodes (PPDs). (a) Schematic drawing of the architecture of the PPDs where poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) is used as the electron blocking layer, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is used as the hole blocking layer, poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide (PFN-Br) is used as the surface modifier of PTAA to reduce its hydrophobicity, and bathocuproine (BCP) is used as the interfacial dipole layer for the cathode. (b) Current density–voltage (J–V) scans of the PPDs under the dark and illumination of 100 mW cm–2 provided by an AM1.5G solar simulator. Scan rate is 25 mV s–1 in the forward direction from −2 to 1.5 V. (c) Responsivity spectra of the PPDs measured under an applied bias of −0.5 V. (d) Specific detectivity (D*) of the PPDs measured at −0.5 V, calculated from the responsivity and noise spectra. (e) Photocurrent measured at −0.5 V bias under varied light intensities provided by an AM1.5G solar simulator with neutral density filters. The solid line is a linear guideline, and the dashed lines indicate the Jdark of the respective PPDs.

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Figure 5c plots the responsivity (R) of the PPDs calculated from the external quantum efficiency (EQE) as an assay of photon-to-electron conversion
(2)
where λ is the wavelength of incident light, q is the elementary charge, h is Planck’s constant, and c is the speed of light. We can observe a moderate increase of R in the MACl treated PPD, but the difference is less significant than that of Jd. To properly calculate the specific detectivity (D*), we measured the noise power spectra for both PPDs, Figure S11. The noise floor is reached near 0.3 Hz, and the noise current (in) at −0.5 V is approximately 1 order of magnitude lower in the MACl treated PPD, tracking the change of Jd. Here, in comprises not only the shot noise from Jd but also the thermal noise from carrier agitation, but it is clear that the reduction of in can be attributed mostly to the reduction of Jd. Using R and in, we can determine D*, Figure 5d, which describes the sensitivity of PPD,
(3)
where A is the pixel area and Δf is electrical bandwidth. The MACl treated PPD shows higher D* over the whole spectrum with a peak value of 1.24 × 1013 Jones, while the peak value of D* for the reference device is 7.6 × 1012 Jones.
To highlight the improvement of low-light detection, J–V scans are performed under varied light intensities with Jph at −0.5 V plotted in Figure 5e. The MACl treated PPD shows linearity between Jph and light intensity down to approximately 2 × 10–8 W cm–2: 2 orders of magnitude lower than the value of 10–6 W cm–2 for the reference PPD. The improvement of low-light detection can be further confirmed by repeated Jph measurements with increasing and decreasing light intensities (Figure S12). From these data we can calculate the linear dynamic range (LDR),
(4)
where Jmax (Jmin) is the maximum (minimum) value of the measured current. The LDR is 93 dB for the reference PPD and 126 dB for the MACl treated PPD. We can therefore conclude that the impact of our morphological modification on PPD is manifested by a remarkable reduction of Jd and thus in, which enables a greater D* and extended LDR toward lower light levels. These improvements are consistent with the exceptional enhancement of the low-light photoresponse observed in the pc-AFM measurements on the bare perovskite films.

Conclusions

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In summary, we have demonstrated the inherent nanoscale inhomogeneity in antisolvent-assisted spin-coated perovskite films in which local defective clusters are significant and undermine the overall film quality. These performance-limiting clusters can be effectively eliminated using a vapor-mediated, post-annealing grain growth without the need of passivation agents. The facile treatment leads to remarkable photoluminescence observed at the grain boundaries, while the grain boundaries are almost invisible in the local photoconduction measurements, features which are atypical to polycrystalline thin films. The removal of local defective clusters is found to most significantly benefit the low-light photoresponse of perovskite films and perovskite photodiodes, enhancing their low-light detecting capabilities.
In a broader context, our findings highlight an issue that rapid crystallization of the perovskite film may have simultaneously created new challenges surrounding material uniformity and optoelectronic quality. The challenges not only undermine the low-light performance of these perovskites but also may limit their performance potential in photovoltaics (PV) or light-emitting diode (LED) devices. To this end, post-annealing grain growth can be an effective process to resolve this issue toward outstanding material quality and uniformity.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03839.

  • Experimental methods, SEM images, PL spectra, photocurrent and photoresponse maps, optimization of the layer thickness in the PPDs, noise power spectra of the PPDs, cyclic low-light Jph measurements of the PPDs (PDF)

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Author Information

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  • Corresponding Authors
    • Tian Du - School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom;  Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom;  Present Address: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg, Martensstraße 7, 91058 Erlangen, Germany;  Email: [email protected] [email protected]
    • Joe Briscoe - School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom;  Orcidhttps://orcid.org/0000-0002-5925-860X;  Email: [email protected]
  • Authors
    • Filipe Richheimer - National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom;  Orcidhttps://orcid.org/0000-0002-5360-8381
    • Kyle Frohna - Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
    • Nicola Gasparini - Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom;  Orcidhttps://orcid.org/0000-0002-3226-8234
    • Lokeshwari Mohan - School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom;  Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
    • Ganghong Min - Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
    • Weidong Xu - Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
    • Thomas J. Macdonald - School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom;  Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom;  Orcidhttps://orcid.org/0000-0002-7520-6893
    • Haozhen Yuan - School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom
    • Sinclair R. Ratnasingham - School of Engineering and Materials Science and Materials Research Institute, Queen Mary University of London, London E1 4NS, United Kingdom;  Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom
    • Saif Haque - Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom;  Orcidhttps://orcid.org/0000-0001-5483-3321
    • Fernando A. Castro - National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom
    • James R. Durrant - Department of Chemistry and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom;  SPECIFIC IKC, College of Engineering, Swansea University, Swansea SA2 7AX, United Kingdom;  Orcidhttps://orcid.org/0000-0001-8353-7345
    • Samuel D. Stranks - Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom;  Department of Chemical Engineering & Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, United Kingdom;  Orcidhttps://orcid.org/0000-0002-8303-7292
    • Sebastian Wood - National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom
    • Martyn A. McLachlan - Department of Materials and Centre for Processable Electronics, Imperial College, London W12 0BZ, United Kingdom;  Orcidhttps://orcid.org/0000-0003-3136-1661
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank the EPSRC Plastic Electronics CDT (EP/L016702/1) for financial support and provision of equipment resources. J.B. and T.D. acknowledge the QMUL-EPSRC Impact Accelerator Account (EP/R511596/1) for financial support. T.D. gratefully acknowledges the Stephen and Anna Hui Scholarship (Imperial College London) for financially supporting his doctoral studies. F.R., F.A.C., and S.W. acknowledge funding from the European Union’s Horizon 2020 research and Innovation programme under the Marie Skłodowska-Curie grant agreement number 721874 (SPM2.0) and from the UK National Measurement System via the Department for Business, Energy and Industrial Strategy. K.F. acknowledges a George and Lilian Schiff Studentship, Winton Studentship, the Engineering and Physical Sciences Research Council (EPSRC) studentship, Cambridge Trust Scholarship, and Robert Gardiner Scholarship. S.D.S. acknowledges the Royal Society and Tata Group (UF150033). S.D.S. acknowledges EPSRC (EP/R023980/1) for funding.

References

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    The remarkable performance of hybrid perovskite photovoltaics is attributed to their long carrier lifetimes and high photoluminescence (PL) efficiencies. High-quality films are assocd. with slower PL decays, and it has been claimed that grain boundaries have a negligible impact on performance. We used confocal fluorescence microscopy correlated with SEM to spatially resolve the PL decay dynamics from films of nonstoichiometric org.-inorg. perovskites, CH3NH3PbI3(Cl). The PL intensities and lifetimes varied between different grains in the same film, even for films that exhibited long bulk lifetimes. The grain boundaries were dimmer and exhibited faster nonradiative decay. Energy-dispersive x-ray spectroscopy showed a pos. correlation between chlorine concn. and regions of brighter PL, whereas PL imaging revealed that chem. treatment with pyridine could activate previously dark grains.
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  17. 17
    Jiang, C.; Zhang, P. Crystalline Orientation Dependent Photoresponse and Heterogeneous Behaviors of Grain Boundaries in Perovskite Solar Cells. J. Appl. Phys. 2018, 123 (8), 083105  DOI: 10.1063/1.5007857
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    17
    Crystalline orientation dependent photoresponse and heterogeneous behaviors of grain boundaries in perovskite solar cells
    Jiang, Chuanpeng; Zhang, Pengpeng
    Journal of Applied Physics (Melville, NY, United States) (2018), 123 (8), 083105/1-083105/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Using photoconductive at. force microscopy and Kelvin probe force microscopy, we characterize the local elec. properties of grains and grain boundaries of org.-inorg. hybrid perovskite (CH3NH3PbI3) thin films on top of a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/ITO substrate. Three discrete photocond. levels are identified among perovskite grains, likely corresponding to the crystal orientation of each grain. Local J-V curves recorded on these grains further suggest an anti-correlation behavior between the short circuit current (JSC) and open circuit voltage (VOC). This phenomenon can be attributed to diffusion-limited surface recombination at the non-selective perovskite-tip contact, where a higher carrier mobility established in the perovskite grain results in an enhanced surface recombination and thus a lower VOC. In addn., the photoresponse of perovskite films displays a pronounced heterogeneity across the grain boundaries, with the boundaries formed between grains of the same photocond. level displaying even enhanced photocurrent and open circuit voltage compared to those of the adjacent grain interiors. These observations highlight the significance of controlling the microstructure of perovskite thin films, which will be a necessary route for further improving the efficiency of perovskite solar cells. (c) 2018 American Institute of Physics.
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  18. 18
    Leblebici, S. Y.; Leppert, L.; Li, Y.; Reyes-Lillo, S. E.; Wickenburg, S.; Wong, E.; Lee, J.; Melli, M.; Ziegler, D.; Angell, D. K. Facet-Dependent Photovoltaic Efficiency Variations in Single Grains of Hybrid Halide Perovskite. Nat. Energy 2016, 1 (8), 16093,  DOI: 10.1038/nenergy.2016.93
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    18
    Facet-dependent photovoltaic efficiency variations in single grains of hybrid halide perovskite
    Leblebici, Sibel Y.; Leppert, Linn; Li, Yanbo; Reyes-Lillo, Sebastian E.; Wickenburg, Sebastian; Wong, Ed; Lee, Jiye; Melli, Mauro; Ziegler, Dominik; Angell, Daniel K.; Ogletree, D. Frank; Ashby, Paul D.; Toma, Francesca M.; Neaton, Jeffrey B.; Sharp, Ian D.; Weber-Bargioni, Alexander
    Nature Energy (2016), 1 (8), 16093CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
    Photovoltaic devices based on hybrid perovskite materials have exceeded 22% efficiency due to high charge-carrier mobilities and lifetimes. Properties such as photocurrent generation and open-circuit voltage are influenced by the microscopic structure and orientation of the perovskite crystals, but are difficult to quantify on the intra-grain length scale and are often treated as homogeneous within the active layer. Here, we map the local short-circuit photocurrent, open-circuit photovoltage, and dark drift current in state-of-the-art methylammonium lead iodide solar cells using photoconductive at. force microscopy. We find, within individual grains, spatially correlated heterogeneity in short-circuit current and open-circuit voltage up to 0.6 V. These variations are related to different crystal facets and have a direct impact on the macroscopic power conversion efficiency. We attribute this heterogeneity to a facet-dependent d. of trap states. These results imply that controlling crystal grain and facet orientation will enable a systematic optimization of polycryst. and single-crystal devices for photovoltaic and lighting applications.
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  19. 19
    Du, T.; Burgess, C. H.; Lin, C.-T.; Eisner, F.; Kim, J.; Xu, S.; Kang, H.; Durrant, J. R.; McLachlan, M. A. Probing and Controlling Intragrain Crystallinity for Improved Low Temperature–Processed Perovskite Solar Cells. Adv. Funct. Mater. 2018, 28 (51), 1803943,  DOI: 10.1002/adfm.201803943
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    Li, J. J.; Ma, J. Y.; Ge, Q. Q.; Hu, J. S.; Wang, D.; Wan, L. J. Microscopic Investigation of Grain Boundaries in Organolead Halide Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7 (51), 28518– 28523,  DOI: 10.1021/acsami.5b09801
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    20
    Microscopic Investigation of Grain Boundaries in Organolead Halide Perovskite Solar Cells
    Li, Jiang-Jun; Ma, Jing-Yuan; Ge, Qian-Qing; Hu, Jin-Song; Wang, Dong; Wan, Li-Jun
    ACS Applied Materials & Interfaces (2015), 7 (51), 28518-28523CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Grain boundaries (GBs) play an important role in org.-inorg. halide perovskite solar cells, which have generally been recognized as a new class of materials for photovoltaic applications. To definitely understand the elec. structure and behavior of GBs, here the authors present Kelvin probe force microscopy and conductive at. force microscopy (c-AFM) measurements of both typical and inverted planar organolead halide perovskite solar cells. By comparing the contact p.d. (CPD) of these 2 devices in the dark and under illumination, a downward band bending exists in GBs that predominantly attract photoinduced electrons. The c-AFM measurements obsd. that higher photocurrents flow through GBs when a low bias overcomes the barrier created by the band bending, indicating that GBs act as effective charge dissocn. interfaces and photocurrent transduction pathways rather than recombination sites.
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  21. 21
    Doherty, T. A. S.; Winchester, A. J.; Macpherson, S.; Johnstone, D. N.; Pareek, V.; Tennyson, E. M.; Kosar, S.; Kosasih, F. U.; Anaya, M.; Abdi-jalebi, M. Performance-Limiting Nanoscale Trap Clusters at Grain Junctions in Halide Perovskites. Nature 2020, 580 (4), 360– 366,  DOI: 10.1038/s41586-020-2184-1
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    21
    Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites
    Doherty, Tiarnan A. S.; Winchester, Andrew J.; MacPherson, Stuart; Johnstone, Duncan N.; Pareek, Vivek; Tennyson, Elizabeth M.; Kosar, Sofiia; Kosasih, Felix U.; Anaya, Miguel; Abdi-Jalebi, Mojtaba; Andaji-Garmaroudi, Zahra; Wong, E. Laine; Madeo, Julien; Chiang, Yu-Hsien; Park, Ji-Sang; Jung, Young-Kwang; Petoukhoff, Christopher E.; Divitini, Giorgio; Man, Michael K. L.; Ducati, Caterina; Walsh, Aron; Midgley, Paul A.; Dani, Keshav M.; Stranks, Samuel D.
    Nature (London, United Kingdom) (2020), 580 (7803), 360-366CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Photoemission electron microscopy was used to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor luminescence efficiency, discrete, nanoscale trap clusters were obsd. By correlating microscopy measurements with scanning electron anal. techniques, these trap clusters appear at the interfaces between crystallog. and compositionally distinct entities. By generating time-resolved photoemission sequences of the photoexcited carrier trapping process, a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters was revealed. Managing structure and compn. on the nanoscale will be essential for optimal performance of halide perovskite devices.
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  22. 22
    Du, T.; Burgess, C. H.; Kim, J.; Zhang, J.; Durrant, J. R.; McLachlan, M. A. Formation, Location and Beneficial Role of PbI 2 in Lead Halide Perovskite Solar Cells. Sustain. Energy Fuels 2017, 1 (1), 119– 126,  DOI: 10.1039/C6SE00029K
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    Thompson, C. V. Secondary Grain Growth in Thin Films of Semiconductors: Theoretical Aspects. J. Appl. Phys. 1985, 58 (2), 763– 772,  DOI: 10.1063/1.336194
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    23
    Secondary grain growth in thin films of semiconductors: theoretical aspects
    Thompson, C. V.
    Journal of Applied Physics (1985), 58 (2), 763-72CODEN: JAPIAU; ISSN:0021-8979.
    Secondary grain growth in thin films can lead to grain sizes much greater than the film thickness. Surface energy anisotropy often provides an important fraction of the driving force for secondary grain growth, esp. in the early stages of growth. Surface-energy-driven secondary grain growth leads to the development of large grains with restricted crystallog. textures. A model is presented for growth of secondary grains into a uniform matrix of columnar normal grains. The model indicates that secondary grain growth rates should increase with grain boundary energy, surface energy anisotropy, grain boundary mobility, and temp. While final secondary grain sizes will decrease with film thickness, their growth rates will increase. The final secondary grain sizes and orientations will be strongly affected by grain sizes and orientations in the initial film. The models presented here provide anal. tools for exptl. study of secondary grain growth in thin films.
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  24. 24
    Kim, M.; Kim, G. H.; Oh, K. S.; Jo, Y.; Yoon, H.; Kim, K. H.; Lee, H.; Kim, J. Y.; Kim, D. S. High-Temperature-Short-Time Annealing Process for High-Performance Large-Area Perovskite Solar Cells. ACS Nano 2017, 11 (6), 6057– 6064,  DOI: 10.1021/acsnano.7b02015
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    24
    High-Temperature-Short-Time Annealing Process for High-Performance Large-Area Perovskite Solar Cells
    Kim, Minjin; Kim, Gi-Hwan; Oh, Kyoung Suk; Jo, Yimhyun; Yoon, Hyun; Kim, Ka-Hyun; Lee, Heon; Kim, Jin Young; Kim, Dong Suk
    ACS Nano (2017), 11 (6), 6057-6064CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Org.-inorg. hybrid metal halide perovskite solar cells (PSCs) are attracting tremendous research interest due to their high solar-to-elec. power conversion efficiency with a high possibility of cost-effective fabrication and certified power conversion efficiency now exceeding 22%. Although many effective methods for their application have been developed over the past decade, their practical transition to large-size devices has been restricted by difficulties in achieving high performance. Here we report on the development of a simple and cost-effective prodn. method with high-temp. and short-time annealing processing to obtain uniform, smooth, and large-size grain domains of perovskite films over large areas. With high-temp. short-time annealing at 400 °C for 4 s, the perovskite film with an av. domain size of 1 μm was obtained, which resulted in fast solvent evapn. Solar cells fabricated using this processing technique had a max. power conversion efficiency exceeding 20% over a 0.1 cm2 active area and 18% over a 1 cm2 active area. We believe our approach will enable the realization of highly efficient large-area PCSs for practical development with a very simple and short-time procedure. This simple method should lead the field toward the fabrication of uniform large-scale perovskite films, which are necessary for the prodn. of high-efficiency solar cells that may also be applicable to several other material systems for more widespread practical deployment.
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  25. 25
    You, P.; Li, G.; Tang, G.; Cao, J.; Yan, F. Ultrafast Laser-Annealing of Perovskite Films for Efficient Perovskite Solar Cells. Energy Environ. Sci. 2020, 13 (4), 1187– 1196,  DOI: 10.1039/C9EE02324K
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    25
    Ultrafast laser-annealing of perovskite films for efficient perovskite solar cells
    You, Peng; Li, Guijun; Tang, Guanqi; Cao, Jiupeng; Yan, Feng
    Energy & Environmental Science (2020), 13 (4), 1187-1196CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Perovskite solar cells have attracted much attention recently for their high efficiency, ease of prepn. and low cost. Here, we report a novel laser-annealing method for perovskite films at a low substrate temp. by scanning laser spots on the film surfaces. An ultrafast crystn. process within a few seconds is realized under a laser with a high intensity and a fast scanning speed. Because the cryst. perovskite phase has a stronger light absorption than the amorphous phase, the fast laser annealing can induce a higher temp. in the former and lead to the selective growth of large perovskite grains. Under optimum conditions, perovskite films with high crystallinity are successfully fabricated, resulting in perovskite solar cells with high power conversion efficiency and good stability. Moreover, a faster laser-annealing process of perovskite films is achieved by using a linear laser beam, which is expected to be a promising technique for the mass prodn. of large-scale perovskite solar cells.
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  26. 26
    Yang, M.; Zhang, T.; Schulz, P.; Li, Z.; Li, G.; Kim, D. H.; Guo, N.; Berry, J. J.; Zhu, K.; Zhao, Y. Facile Fabrication of Large-Grain CH3 NH3 PbI3-x Brx Films for High-Efficiency Solar Cells via CH3NH3 Br-Selective Ostwald Ripening. Nat. Commun. 2016, 7 (May), 12305,  DOI: 10.1038/ncomms12305
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    26
    Facile fabrication of large-grain CH3NH3PbI3-xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening
    Yang, Mengjin; Zhang, Taiyang; Schulz, Philip; Li, Zhen; Li, Ge; Kim, Dong Hoe; Guo, Nanjie; Berry, Joseph J.; Zhu, Kai; Zhao, Yixin
    Nature Communications (2016), 7 (), 12305CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technol. Structural and electro-optical properties of the perovskite absorber layer are most crit. to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH3NH3PbI3-xBrx (MAPbI3-xBrx) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concn. converts MAPbI3 thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI3-xBrx thin films following an Ostwald ripening process, which is strongly affected by MABr concn. and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concn. enhances I-Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.
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  27. 27
    Pham, N. D.; Tiong, V. T.; Yao, D.; Martens, W.; Guerrero, A.; Bisquert, J.; Wang, H. Guanidinium Thiocyanate Selective Ostwald Ripening Induced Large Grain for High Performance Perovskite Solar Cells. Nano Energy 2017, 41 (8), 476– 487,  DOI: 10.1016/j.nanoen.2017.10.006
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    27
    Guanidinium thiocyanate selective Ostwald ripening induced large grain for high performance perovskite solar cells
    Pham, Ngoc Duy; Tiong, Vincent Tiing; Yao, Disheng; Martens, Wayde; Guerrero, Antonio; Bisquert, Juan; Wang, Hongxia
    Nano Energy (2017), 41 (), 476-487CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    Org.-inorg. lead halide perovskite has become one of the most attractive materials for future low-cost high-efficiency solar technol. However, the polycryst. nature of perovskite thin-film often possesses an exceptional d. of defects, esp. at grain boundaries (GBs) and film surface, limiting further improvement in the power conversion efficiency (PCE) of the perovskite device. Here, we report a simple method to reduce GBs and to passivate the surface of a methylammonium lead tri-iodide (MAPbI3) film by guanidinium thiocyanate (GUTS)-assisted Ostwald ripening post treatment. High-optoelectronic quality MAPbI3 film consisting of micron-sized grains were synthesized by post-treating a MAPbI3 film with GUTS/isopropanol soln. (4 mg/mL, GUTS-4). Anal. of the electrochem. impedance spectra (EIS) of the solar cells showed that interfacial charge recombination resistance of the device based on a GUTS-4 post-treated MAPbI3 absorber film was increased by a factor of 1.15-2.6, depending on light illumination intensity, compared to the control MAPbI3 cell. This is consistent with results of the open-circuit voltage (Voc) decay and the light intensity dependent photovoltage evolution which shows device with GUTS treatment had one order longer charge carrier lifetime and was more ideal (ideality factor = 1.25). Further characterization by Kelvin probe force microscope indicated that GUTS-4 treatment shifted the energetics of the MAPbI3 film by ∼ 100 meV towards better energy level alignment with adjacent SnO2 electron transport layer, leading to a more favorable charge extn. process at the MAPbI3/SnO2 interface. As a result, the PCE of PSCs was enhanced from 14.59% to 16.37% and the hysteresis effect was mitigated.
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  28. 28
    Du, T.; Xu, W.; Daboczi, M.; Kim, J.; Xu, S.; Lin, C. T.; Kang, H.; Lee, K.; Heeney, M. J.; Kim, J. S. P-Doping of Organic Hole Transport Layers in p-i-n Perovskite Solar Cells: Correlating Open-Circuit Voltage and Photoluminescence Quenching. J. Mater. Chem. A 2019, 7 (32), 18971– 18979,  DOI: 10.1039/C9TA03896E
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    28
    p-Doping of organic hole transport layers in p-i-n perovskite solar cells: correlating open-circuit voltage and photoluminescence quenching
    Du, Tian; Xu, Weidong; Daboczi, Matyas; Kim, Jinhyun; Xu, Shengda; Lin, Chieh-Ting; Kang, Hongkyu; Lee, Kwanghee; Heeney, Martin J.; Kim, Ji-Seon; Durrant, James R.; McLachlan, Martyn A.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (32), 18971-18979CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Doping is a widely implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic (PV) devices. Here, in direct contrast to existing understanding, we find that a redn. in p-doping of the org. hole transport layer (HTL) leads to substantial improvements in PV performance in planar p-i-n perovskite solar cells (PSCs), driven by improvements in open circuit voltage (VOC). Employing a range of transient and steady state characterization tools, we find that the improvements of VOC correlate with reduced surface recombination losses in less p-doped HTLs. A simple device model including screening of bulk elec. fields in the perovskite layer is used to explain this observation. In particular, photoluminescence (PL) emission of complete solar cells shows that efficient performance is correlated to a high PL intensity at open circuit and a low PL intensity at short circuit. We conclude that desirable transport layers for p-i-n PSCs should be charge selective contacts with low doping densities.
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  29. 29
    Du, T.; Ratnasingham, S. R.; Kosasih, F. U.; Macdonald, T. J.; Mohan, L.; Augurio, A.; Ahli, H.; Lin, C.-T.; Xu, S.; Xu, W. Aerosol Assisted Solvent Treatment: A Universal Method for Performance and Stability Enhancements in Perovskite Solar Cells. Adv. Energy Mater. 2021, 11, 2101420,  DOI: 10.1002/aenm.202101420
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    29
    Aerosol Assisted Solvent Treatment: A Universal Method for Performance and Stability Enhancements in Perovskite Solar Cells
    Du, Tian; Ratnasingham, Sinclair R.; Kosasih, Felix U.; Macdonald, Thomas J.; Mohan, Lokeshwari; Augurio, Adriana; Ahli, Huda; Lin, Chieh-Ting; Xu, Shengda; Xu, Weidong; Binions, Russell; Ducati, Caterina; Durrant, James R.; Briscoe, Joe; McLachlan, Martyn A.
    Advanced Energy Materials (2021), 11 (33), 2101420CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    Metal-halide perovskite solar cells (PSCs) have had a transformative impact on the renewable energy landscape since they were first demonstrated just over a decade ago. Outstanding improvements in performance have been demonstrated through structural, compositional, and morphol. control of devices, with commercialization now being a reality. Here the authors present an aerosol assisted solvent treatment as a universal method to obtain performance and stability enhancements in PSCs, demonstrating their methodol. as a convenient, scalable, and reproducible post-deposition treatment for PSCs. Their results identify improvements in crystallinity and grain size, accompanied by a narrowing in grain size distribution as the underlying phys. changes that drive redns. of electronic and ionic defects. These changes lead to prolonged charge-carrier lifetimes and ultimately increased device efficiencies. The versatility of the process is demonstrated for PSCs with thick (>1 μm) active layers, large-areas (>1 cm2) and a variety of device architectures and active layer compns. This simple post-deposition process is widely transferable across the field of perovskites, thereby improving the future design principles of these materials to develop large-area, stable, and efficient PSCs.
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  30. 30
    Cao, X.; Zhi, L.; Li, Y.; Fang, F.; Cui, X.; Ci, L.; Ding, K.; Wei, J. Fabrication of Perovskite Films with Large Columnar Grains via Solvent-Mediated Ostwald Ripening for Efficient Inverted Perovskite Solar Cells. ACS Appl. Energy Mater. 2018, 1 (2), 868– 875,  DOI: 10.1021/acsaem.7b00300
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    30
    Fabrication of Perovskite Films with Large Columnar Grains via Solvent-Mediated Ostwald Ripening for Efficient Inverted Perovskite Solar Cells
    Cao, Xiaobing; Zhi, Lili; Li, Yahui; Fang, Fei; Cui, Xian; Ci, Lijie; Ding, Kongxian; Wei, Jinquan
    ACS Applied Energy Materials (2018), 1 (2), 868-875CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    Generally, residual solvent is embedded in perovskite precursor films fabricated from the Lewis adduct method. Most of the research focus on the ligand function of the solvent in forming a solvate complex for fabricating high quality perovskite films. However, little attention has been paid to the latent function of the solvent in the perovskite precursor films during the annealing process due to its fast extravasation at high temp. Here, we develop a sandwich configuration of substrate/perovskite precursor films/PC61BM to retard the extravasation of solvent during annealing. We find that the restrained solvent induces an obvious solvent-mediated dissoln.-recrystn. process, leading to high quality perovskite films with large columnar grains. There is mass transportation from small grains to large grains in the dissoln.-recrystn. process, which follows the Ostwald ripening model. Inverted planar solar cells are fabricated on the basis of this annealing method. The photovoltaic performance of the solar cells is improved significantly due to its high quality perovskite films with large columnar grains.
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  31. 31
    Zhou, Z.; Wang, Z.; Zhou, Y.; Pang, S.; Wang, D.; Xu, H.; Liu, Z.; Padture, N. P.; Cui, G. Methylamine-Gas Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells. Angew. Chemie Int. Ed. 2015, 54 (33), 9705– 9709,  DOI: 10.1002/anie.201504379
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    31
    Methylamine-Gas Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells
    Zhou, Zhongmin; Wang, Zaiwei; Zhou, Yuanyuan; Pang, Shuping; Wang, Dong; Xu, Hongxia; Liu, Zhihong; Padture, Nitin P.; Cui, Guanglei
    Angewandte Chemie, International Edition (2015), 54 (33), 9705-9709CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The discovery is reported of methylamine (CH3NH2) induced defect-healing (MIDH) of CH3NH3PbI3 perovskite thin films based on their ultrafast (seconds), reversible chem. reaction with CH3NH2 gas at room temp. The key to this healing behavior is the formation and spreading of an intermediate CH3NH3PbI3·xCH3NH2 liq. phase during this unusual perovskite-gas interaction. The versatility and scalability is demonstrated of the MIDH process, and dramatic enhancement is shown in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect-free films of hybrid perovskites.
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  32. 32
    Kutes, Y.; Zhou, Y.; Bosse, J. L.; Steffes, J.; Padture, N. P.; Huey, B. D. Mapping the Photoresponse of CH3NH3PbI3 Hybrid Perovskite Thin Films at the Nanoscale. Nano Lett. 2016, 16 (6), 3434– 3441,  DOI: 10.1021/acs.nanolett.5b04157
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    32
    Mapping the Photoresponse of CH3NH3PbI3 Hybrid Perovskite Thin Films at the Nanoscale
    Kutes, Yasemin; Zhou, Yuanyuan; Bosse, James L.; Steffes, James; Padture, Nitin P.; Huey, Bryan D.
    Nano Letters (2016), 16 (6), 3434-3441CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) hold unprecedented promise for low-cost, high-efficiency photovoltaics (PVs) of the future. While PV performance parameters of PSCs, such as short circuit current, open circuit voltage, and max. power, are always measured at the macroscopic scale, it is necessary to probe such photoresponses at the nanoscale to gain key insights into the fundamental PV mechanisms and their localized dependence on the OTP thin-film microstructure. Here we use photoconductive at. force microscopy spectroscopy to map for the first time variations of PV performance at the nanoscale for planar PSCs based on hole-transport-layer free methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) thin films. These results reveal substantial variations in the photoresponse that correlate with thin-film microstructural features such as intragrain planar defects, grains, grain boundaries, and notably also grain-aggregates. The insights gained into such microstructure-localized PV mechanisms are essential for guiding microstructural tailoring of OTP films for improved PV performance in future PSCs.
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  33. 33
    Chen, Y.; Lei, Y.; Li, Y.; Yu, Y.; Cai, J.; Chiu, M. H.; Rao, R.; Gu, Y.; Wang, C.; Choi, W. Strain Engineering and Epitaxial Stabilization of Halide Perovskites. Nature 2020, 577 (7789), 209– 215,  DOI: 10.1038/s41586-019-1868-x
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    Strain engineering and epitaxial stabilization of halide perovskites
    Chen, Yimu; Lei, Yusheng; Li, Yuheng; Yu, Yugang; Cai, Jinze; Chiu, Ming-Hui; Rao, Rahul; Gu, Yue; Wang, Chunfeng; Choi, Woojin; Hu, Hongjie; Wang, Chonghe; Li, Yang; Song, Jiawei; Zhang, Jingxin; Qi, Baiyan; Lin, Muyang; Zhang, Zhuorui; Islam, Ahmad E.; Maruyama, Benji; Dayeh, Shadi; Li, Lain-Jong; Yang, Kesong; Lo, Yu-Hwa; Xu, Sheng
    Nature (London, United Kingdom) (2020), 577 (7789), 209-215CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Strain engineering is a powerful tool with which to enhance semiconductor device performance. Halide perovskites have shown great promise in device applications owing to their remarkable electronic and optoelectronic properties. Although applying strain to halide perovskites has been frequently attempted, including using hydrostatic pressurization, electrostriction, annealing, van der Waals force, thermal expansion mismatch, and heat-induced substrate phase transition, the controllable and device-compatible strain engineering of halide perovskites by chem. epitaxy remains a challenge, owing to the absence of suitable lattice-mismatched epitaxial substrates. Here, the authors report the strained epitaxial growth of halide perovskite single-crystal thin films on lattice-mismatched halide perovskite substrates. They investigated strain engineering of α-formamidinium lead iodide (α-FAPbI3) using both exptl. techniques and theor. calcns. By tailoring the substrate compn.-and therefore its lattice parameter-a compressive strain as high as 2.4 per cent is applied to the epitaxial α-FAPbI3 thin film. The authors demonstrate that this strain effectively changes the crystal structure, reduces the band gap and increases the hole mobility of α-FAPbI3. Strained epitaxy is also shown to have a substantial stabilization effect on the α-FAPbI3 phase owing to the synergistic effects of epitaxial stabilization and strain neutralization. As an example, strain engineering is applied to enhance the performance of an α-FAPbI3-based photodetector.
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    Wen, X.; Feng, Y.; Huang, S.; Huang, F.; Cheng, Y.; Green, M.; Ho-baillie, A. Defect Trapping States and Charge Carrier Recombination in Organic – Inorganic Halide. J. Mater. Chem. C 2016, 4, 793– 800,  DOI: 10.1039/C5TC03109E
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    Defect trapping states and charge carrier recombination in organic-inorganic halide perovskites
    Wen, Xiaoming; Feng, Yu; Huang, Shujuan; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (4), 793-800CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    Org.-inorg. perovskite solar cells have attracted huge research interest due to rapid improvement in device performance showing great potential to be the next generation flexible solar cells. Unique defect properties in perovskite have been considered as the possible mechanism for the superior performance, and closely relevant to the effects of hysteresis and light soaking. To date, the quant. correlation and in-depth understanding of defects in org.-inorg. perovskite are still lacking although extensive investigation have been undertaken. Here we study defect trapping states and carrier recombination dynamics in org.-inorg. halide perovskites. At low excitation the photoluminescence (PL) intensity exhibits a super-linear increase with increasing excitation, due to the slow depopulation rate of the defect states. The steady state and time-resolved photoluminescence (PL) carried out in this work reveal that the carrier recombination dynamics is ultimately correlated with both the defect d. and the relaxation rate of the carriers in defects. A model is established for the relationship between the properties of the defect trapping state and steady state PL intensity. Two key parameters, (i) the ratio of the trap-state d. to the depopulation rate of trapped states and (ii) ratio of the max. d. of covalence band electrons to the trapping rate, can be extd. from the model based on the excitation dependent steady state PL. This work demonstrates that the properties of defect trapping states are closely related to the fabrication technique, and suggests that the org.-inorg. halide perovskite is partly defect-tolerant.
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  35. 35
    Yamada, Y.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Spontaneous Defect Annihilation in CH3NH3PbI3 Thin Films at Room Temperature Revealed by Time-Resolved Photoluminescence Spectroscopy. J. Phys. Chem. Lett. 2015, 6 (3), 482– 486,  DOI: 10.1021/jz5026596
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    35
    Spontaneous Defect Annihilation in CH3NH3PbI3 Thin Films at Room Temperature Revealed by Time-Resolved Photoluminescence Spectroscopy
    Yamada, Yasuhiro; Endo, Masaru; Wakamiya, Atsushi; Kanemitsu, Yoshihiko
    Journal of Physical Chemistry Letters (2015), 6 (3), 482-486CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    We utilized time-resolved photoluminescence (PL) spectroscopy to investigate the photocarrier recombination dynamics in CH3NH3PbI3 thin films as a function of the time elapsed from the film's fabrication. We found that the PL lifetime gradually increased and began to level out once the age of the film reached ∼30 h. Even under weak excitation, the PL dynamics depended on the excitation intensity in the fresh sample, while the mature sample displayed no excitation-intensity dependence assocd. with the PL dynamics. We submit that this can be explained by the fact that a significant no. of defects are initially formed in CH3NH3PbI3 thin films fabricated by the sequential method and are spontaneously reduced by room-temp. annealing. Our results provide important insights for reducing the nonradiative recombination centers, which improves the power conversion efficiency of perovskite solar cells.
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  36. 36
    Andaji-Garmaroudi, Z.; Abdi-Jalebi, M.; Guo, D.; Macpherson, S.; Sadhanala, A.; Tennyson, E. M.; Ruggeri, E.; Anaya, M.; Galkowski, K.; Shivanna, R. A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites. Adv. Mater. 2019, 31 (42), 1902374,  DOI: 10.1002/adma.201902374
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    A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites
    Andaji-Garmaroudi, Zahra; Abdi-Jalebi, Mojtaba; Guo, Dengyang; Macpherson, Stuart; Sadhanala, Aditya; Tennyson, Elizabeth M.; Ruggeri, Edoardo; Anaya, Miguel; Galkowski, Krzysztof; Shivanna, Ravichandran; Lohmann, Kilian; Frohna, Kyle; Mackowski, Sebastian; Savenije, Tom J.; Friend, Richard H.; Stranks, Samuel D.
    Advanced Materials (Weinheim, Germany) (2019), 31 (42), 1902374CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Mixed-halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of soln.-processed triple-cation mixed-halide (Cs0.06MA0.15FA0.79)Pb(Br0.4I0.6)3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar-equiv. illumination. It is found that the illumination leads to localized surface sites of iodide-rich perovskite intermixed with passivating PbI2 material. Time- and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide-rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concn. of charges on the surface layer. This work reveals new insight into phase segregation of mixed-halide mixed-cation perovskites, as well as routes to highly luminescent films by controlling charge d. and transfer in novel device structures.
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    Kong, L.; Liua, G.; Gong, J.; Hu, Q.; Schaller, R. D.; Dera, P.; Zhang, D.; Liu, Z.; Yang, W.; Zhu, K. Simultaneous Band-Gap Narrowing and Carrier-Lifetime Prolongation of Organic-Inorganic Trihalide Perovskites. Proc. Natl. Acad. Sci. U. S. A. 2016, 113 (32), 8910– 8915,  DOI: 10.1073/pnas.1609030113
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    Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic-inorganic trihalide perovskites
    Kong, Lingping; Liu, Gang; Gong, Jue; Hu, Qingyang; Schaller, Richard D.; Dera, Przemyslaw; Zhang, Dongzhou; Liu, Zhenxian; Yang, Wenge; Zhu, Kai; Tang, Yuzhao; Wang, Chuanyi; Wei, Su-Huai; Xu, Tao; Mao, Ho-kwang
    Proceedings of the National Academy of Sciences of the United States of America (2016), 113 (32), 8910-8915CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The org.-inorg. hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a crit. factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chem. or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance.
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    Boopathi, K. M.; Martín-García, B.; Ray, A.; Pina, J. M.; Marras, S.; Saidaminov, M. I.; Bonaccorso, F.; Di Stasio, F.; Sargent, E. H.; Manna, L. Permanent Lattice Compression of Lead-Halide Perovskite for Persistently Enhanced Optoelectronic Properties. ACS Energy Lett. 2020, 5 (2), 642– 649,  DOI: 10.1021/acsenergylett.9b02810
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    Permanent lattice compression of lead-halide perovskite for persistently enhanced optoelectronic properties
    Boopathi, Karunakara Moorthy; Martin-Garcia, Beatriz; Ray, Aniruddha; Pina, Joao M.; Marras, Sergio; Saidaminov, Makhsud I.; Bonaccorso, Francesco; Di Stasio, Francesco; Sargent, Edward H.; Manna, Liberato; Abdelhady, Ahmed L.
    ACS Energy Letters (2020), 5 (2), 642-649CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Under mild mech. pressure, halide perovskites show enhanced optoelectronic properties. However, these improvements are reversible upon decompression, and permanent enhancements have yet to be realized. Here, we report antisolvent-assisted solvent acidolysis crystn. that enables us to prep. methylammonium lead bromide single crystals showing intense emission at all four edges under UV light excitation. We study structural variations (edge-vs-center) in these crystals using micro-X-ray diffraction and find that the enhanced emission at the edges correlates with lattice compression compared to in the central areas. Time-resolved photoluminescence measurements show much longer-lived photogenerated carriers at the compressed edges, with radiative component lifetimes of ∼1.4μs, 10 times longer than at the central regions. The properties of the edges are exploited to fabricate planar photodetectors exhibiting detectivities of 3 × 1013 Jones, compared to 5 × 1012 Jones at the central regions. The enhanced lifetimes and detectivities correlate to the reduced trap state densities and the formation of shallower traps at the edges due to lattice compression.
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    Zhao, J.; Deng, Y.; Wei, H.; Zheng, X.; Yu, Z.; Shao, Y.; Shield, J. E.; Huang, J. Strained Hybrid Perovskite Thin Films and Their Impact on the Intrinsic Stability of Perovskite Solar Cells. Sci. Adv. 2017, 3 (11), eaao5616  DOI: 10.1126/sciadv.aao5616
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    Jamshaid, A.; Guo, Z.; Hieulle, J.; Stecker, C.; Ohmann, R.; Ono, L. K.; Qiu, L.; Tong, G.; Yin, W.; Qi, Y. Atomic-Scale Insight into the Enhanced Surface Stability of Methylammonium Lead Iodide Perovskite by Controlled Deposition of Lead Chloride. Energy Environ. Sci. 2021, 14 (8), 4541– 4554,  DOI: 10.1039/D1EE01084K
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    Atomic-scale insight into the enhanced surface stability of methylammonium lead iodide perovskite by controlled deposition of lead chloride
    Jamshaid, Afshan; Guo, Zhendong; Hieulle, Jeremy; Stecker, Collin; Ohmann, Robin; Ono, Luis K.; Qiu, Longbin; Tong, Guoqing; Yin, Wanjian; Qi, Yabing
    Energy & Environmental Science (2021), 14 (8), 4541-4554CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    The incorporation of a certain amt. of Cl ions into methylammonium lead iodide (MAPbI3) perovskite films and how these incorporated Cl ions affect the structural and electronic properties of these films have been an intensively studied topic. In this study, we comprehensively investigated Cl incorporation in MAPbI3 at the at. scale by a combined study of scanning tunneling microscopy, XPS, UV and inverse photoemission spectroscopy, d. functional theory and mol. dynamics calcns. At a Cl concn. of 14.8 ± 0.6%, scanning tunneling microscopy images confirm the incorporation of Cl ions on the MAPbI3 surface, which also corresponds to the highest surface stability of MAPbI3 found from the viewpoint of both thermodn. and kinetics by d. functional theory and mol. dynamics calcns. Our results show that the Cl concn. is crucial to the surface bandgap and stability of MAPbI3.
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    Son, Y.; Li, M. Y.; Cheng, C. C.; Wei, K. H.; Liu, P.; Wang, Q. H.; Li, L. J.; Strano, M. S. Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging. Nano Lett. 2016, 16 (6), 3571– 3577,  DOI: 10.1021/acs.nanolett.6b00699
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    Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging
    Son, Youngwoo; Li, Ming-Yang; Cheng, Chia-Chin; Wei, Kung-Hwa; Liu, Pingwei; Wang, Qing Hua; Li, Lain-Jong; Strano, Michael S.
    Nano Letters (2016), 16 (6), 3571-3577CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chem. vapor deposition methods has provided an effective soln. to producing an in-plane p-n junction, which is a crit. component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral at. force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2-MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are obsd. to be selectively switched on and off, allowing for the realization of a hyper-resoln. array of the switchable photodiode pixels. This exptl. approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolns. Comparative 2D Fourier anal. of phys. height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.
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    Frammelsberger, W.; Benstetter, G.; Kiely, J.; Stamp, R. C-AFM-Based Thickness Determination of Thin and Ultra-Thin SiO 2 Films by Use of Different Conductive-Coated Probe Tips. Appl. Surf. Sci. 2007, 253 (7), 3615– 3626,  DOI: 10.1016/j.apsusc.2006.07.070
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    C-AFM-based thickness determination of thin and ultra-thin SiO2 films by use of different conductive-coated probe tips
    Frammelsberger, Werner; Benstetter, Guenther; Kiely, Janice; Stamp, Richard
    Applied Surface Science (2007), 253 (7), 3615-3626CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    The influence of the probe tip type on the elec. oxide thickness result was researched for four differently coated conductive tip types using SiO2 (oxide) films with optical thickness of 1.7-8.3 nm. For this purpose, conductive at. force microscopy (C-AFM) was used to measure more than 7200 current-voltage (IV) curves. The elec. oxide thickness was detd. on a statistical basis from the IV-curves using a recently published tunneling model for C-AFM application. The model includes parameters assocd. with the probe tip types used. The evolution of the tip parameters is described in detail. For the theor. tip parameters, measured and calcd. IV-curves showed excellent agreement and the elec. oxide thickness vs. the optical oxide thickness showed congruent behavior, independent of the tip type. However, differences in the elec. oxide thickness were obsd. for the different tip types. The theor. parameters were modified exptl. in order to reduce these differences. Theor. and exptl. tip parameters were compared and their effect on the differences in the elec. oxide thickness is discussed for the different tip types. Overall, it is shown that the proposed model provides a comprehensive framework for detg. the elec. oxide thickness using C-AFM, for a wide range of oxide thicknesses and for differently coated conductive tips.
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    Wang, H.; Kim, D. H. Perovskite-Based Photodetectors: Materials and Devices. Chem. Soc. Rev. 2017, 46 (17), 5204– 5236,  DOI: 10.1039/C6CS00896H
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    Perovskite-based photodetectors: materials and devices
    Wang, Huan; Kim, Dong Ha
    Chemical Society Reviews (2017), 46 (17), 5204-5236CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    While the field of perovskite-based optoelectronics has mostly been dominated by photovoltaics, light-emitting diodes, and transistors, semiconducting properties peculiar to perovskites make them interesting candidates for innovative and disruptive applications in light signal detection. Perovskites combine effective light absorption in the broadband range with good photo-generation yield and high charge carrier mobility, a combination that provides promising potential for exploiting sensitive and fast photodetectors that are targeted for image sensing, optical communication, environmental monitoring or chem./biol. detection. Currently, org.-inorg. hybrid and all-inorg. halide perovskites with controlled morphologies of polycryst. thin films, nano-particles/wires/sheets, and bulk single crystals have shown key figure-of-merit features in terms of their responsivity, detectivity, noise equiv. power, linear dynamic range, and response speed. The sensing region has been covered from UV-visible-near IR (UV-Vis-NIR) to gamma photons based on two- or three-terminal device architectures. Diverse photoactive materials and devices with superior optoelectronic performances have stimulated attention from researchers in multidisciplinary areas. In this review, we provide a comprehensive overview of the recent progress of perovskite-based photodetectors focusing on versatile compns., structures, and morphologies of constituent materials, and diverse device architectures toward the superior performance metrics. Combining the advantages of both org. semiconductors (facile soln. processability) and inorg. semiconductors (high charge carrier mobility), perovskites are expected to replace com. silicon for future photodetection applications.
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    Lin, Q.; Armin, A.; Lyons, D. M.; Burn, P. L.; Meredith, P. Low Noise, IR-Blind Organohalide Perovskite Photodiodes for Visible Light Detection and Imaging. Adv. Mater. 2015, 27 (12), 2060– 2064,  DOI: 10.1002/adma.201405171
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    Low Noise, IR-Blind Organohalide Perovskite Photodiodes for Visible Light Detection and Imaging
    Lin, Qianqian; Armin, Ardalan; Lyons, Dani M.; Burn, Paul L.; Meredith, Paul
    Advanced Materials (Weinheim, Germany) (2015), 27 (12), 2060-2064CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    An organohalide perovskite low noise, IR-blind visible photodiode with performance metrics equiv. to com. inorg. semiconductor devices was demonstrated. The MeNH3+PbI3- perovskite photodiode consists of a simple, soln.-processed homojunction capped with a PC60BM /C60 cathode interlayer. This interlayer: (i) behaves as a low shunt capacitor improving the diode temporal response; (ii) suppresses the dark current by conformally coating the perovskite homojunction and limiting the back injection of holes; and (iii) provides electrooptical control of the spectral response. These features are achieved without compromising the linear dynamic range (LDR) as the fullerene electron mobility is sufficient to prevent space charge forming in the junction up to light intensities of 0.4 W cm-2. A measurement-limited. LDR of -170 dB (at -0.5 V) was obsd., but noise calcns. would indicate it could be ≤250 dB.
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    Fang, Y.; Huang, J. Resolving Weak Light of Sub-Picowatt per Square Centimeter by Hybrid Perovskite Photodetectors Enabled by Noise Reduction. Adv. Mater. 2015, 27 (17), 2804– 2810,  DOI: 10.1002/adma.201500099
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    Resolving Weak Light of Sub-picowatt per Square Centimeter by Hybrid Perovskite Photodetectors Enabled by Noise Reduction
    Fang, Yanjun; Huang, Jinsong
    Advanced Materials (Weinheim, Germany) (2015), 27 (17), 2804-2810CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A highly sensitive perovskite photodetector is reported with low noise (16 fA Hz-1/2 at -0.1 V) close to the shot and thermal noise limits, high av. EQE approaching 90%, large LDR of 94 dB, and short response time of 120 ns. The interfacial electron transfer and hole transfer layers engineering of the device, esp. the trap passivation effect of the fullerene layer, enables its direct measurement of light irradiance down to sub-pW cm-2, which matches with the calcd. noise equiv. power. The excellent weak light sensing capability of the perovskite photodetector makes it potential in replacing com. Si photodiodes in the applications like defense, communication, and imaging.
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Cited By

This article is cited by 3 publications.

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  2. Rahim Abdur, Son Singh, Md Abdul Kuddus Sheikh, Md Aftab Ali Shaikh, Mohammad Shah Jamal, Jaegab Lee. Modified post-annealing process with N, N-dimethylformamide vapor to control the growth of hybrid perovskite microstructure. Results in Materials 2022, 16 , 100330. https://doi.org/10.1016/j.rinma.2022.100330
  3. Zewei Li, Yi-Teng Huang, Lokeshwari Mohan, Szymon J. Zelewski, Richard H. Friend, Joe Briscoe, Robert L. Z. Hoye. Elucidating the Factors Limiting the Photovoltaic Performance of Mixed Sb–Bi Halide Elpasolite Absorbers. Solar RRL 2022, 6 (11) , 2200749. https://doi.org/10.1002/solr.202200749
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  • Abstract

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    Figure 1

    Figure 1. Grain growth of MAPbI3 films induced by treatment with DMF–MACl aerosol. (a–c) Tilt-angle cross-section SEM images of untreated MAPbI3 film after antisolvent-assisted spin coating (a), treated MAPbI3 film for 5 min, (b) and overtreated MAPbI3 film for 10 min (c). Scale bars in all images are 500 nm. (d) Impact of MACl concentration in DMF on the statistical data of lateral grain size. (e) Impact of different halide compound additives in DMF (all with concentrations of 0.1 M) on the statistical data of lateral grain size. (f) Schematic drawing of MAPbI3 film morphological transition during MACl treatment and the process of aerosol ingression to the MAPbI3 films; growth of larger MAPbI3 grains at the expense of surrounding smaller grains.

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    Figure 2

    Figure 2. Characterization of MAPbI3 films. (a–d) Surface and cross-sectional SEM images of untreated MAPbI3 film (a, b) and MACl treated MAPbI3 film (c, d). Scale bars are 500 nm on all images. (e) X-ray diffraction patterns of untreated and MACl treated MAPbI3 films. (f) Zoom-in of the (110) diffraction peak. (g, h) Time-resolved PL spectra of untreated MAPbI3 (g) and MACl treated MAPbI3 (h) films, measured with a 435 nm pulsed laser under varied excitation density. (i) Steady-state PL spectra of MAPbI3 films measured with a 635 nm continuous-wavelength laser with an excitation density of 1.5 mW cm–2. The inset figure shows the spectra with normalized PL intensity, and the arrow indicates peak broadening of the MACl treated film.

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    Figure 3

    Figure 3. Photoluminescence mapping of MAPbI3 films. (a–f) Hyperspectral PL maps on an untreated MAPbI3 film (a–c) and a MACl treated MAPbI3 film (d–f) probed with broadband detector (a, d) and wavelength-selective detector at 775 nm (b, e) and at 800 nm (c, f). Maps were taken with 405 nm laser excitation with an intensity of 240 mW cm–2 in an ambient atmosphere. (g) Localized PL spectra from three different grains labeled “I”, “II”, and “III” in (d). On all images, a.u. refers to arbitrary units.

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    Figure 4

    Figure 4. pc-AFM measurement of perovskite films. (a–h) pc-AFM maps of untreated MAPbI3 film (a–d) and MACl treated MAPbI3 film (e–h): surface height (a, e), dark current Idark (b, f), and the photocurrent Iph (c, g) and photoresponse (d, h) under 0.03 mW cm–2. Both films are deposited on ITO substrate. All Idark and Iph data are measured with an applied bias of −1.5 V on ITO. Illumination is provided by a 633 nm laser. (i, j) Line-scan data of height, Idark, Iph, and photoresponse taken from untreated film (marked “I”) and from three different positions in MACl treated film (marked “II”, “III”, and “IV”, respectively) under varied light intensities from 0.03 to 160 mW cm–2. (k) Schematic drawing of the energy diagram of the ITO/perovskite/Au heterojunction under bias. (l) Area averaged photoresponse as a function of light intensity. The error bars indicate spatial variation of the photoresponse within the measured area.

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    Figure 5

    Figure 5. Characterization of the perovskite photodiodes (PPDs). (a) Schematic drawing of the architecture of the PPDs where poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) is used as the electron blocking layer, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is used as the hole blocking layer, poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide (PFN-Br) is used as the surface modifier of PTAA to reduce its hydrophobicity, and bathocuproine (BCP) is used as the interfacial dipole layer for the cathode. (b) Current density–voltage (J–V) scans of the PPDs under the dark and illumination of 100 mW cm–2 provided by an AM1.5G solar simulator. Scan rate is 25 mV s–1 in the forward direction from −2 to 1.5 V. (c) Responsivity spectra of the PPDs measured under an applied bias of −0.5 V. (d) Specific detectivity (D*) of the PPDs measured at −0.5 V, calculated from the responsivity and noise spectra. (e) Photocurrent measured at −0.5 V bias under varied light intensities provided by an AM1.5G solar simulator with neutral density filters. The solid line is a linear guideline, and the dashed lines indicate the Jdark of the respective PPDs.

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  • References

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    This article references 45 other publications.

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      Organic-inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced 'brightening' of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance.
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      Halide perovskites are promising semiconductors for inexpensive, high-performance optoelectronics. Despite a remarkable defect tolerance compared to conventional semiconductors, perovskite thin films still show substantial microscale heterogeneity in key properties such as luminescence efficiency and device performance. However, the origin of the variations remains a topic of debate, and a precise understanding is crit. to the rational design of defect management strategies. Through a multi-scale investigation - combining correlative synchrotron scanning X-ray diffraction and time-resolved photoluminescence measurements on the same scan area - we reveal that lattice strain is directly assocd. with enhanced defect concns. and non-radiative recombination. The strain patterns have a complex heterogeneity across multiple length scales. We propose that strain arises during the film growth and crystn. and provides a driving force for defect formation. Our work sheds new light on the presence and influence of structural defects in halide perovskites, revealing new pathways to manage defects and eliminate losses.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtlWrsr4%253D&md5=a70f033b2bf5a38559defe36badaefc8
    16. 16
      de Quilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of Microstructure on Local Carrier Lifetime in Perovskite Solar Cells. Science 2015, 348 (6235), 683– 686,  DOI: 10.1126/science.aaa5333
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      16
      Impact of microstructure on local carrier lifetime in perovskite solar cells
      de Quilettes, Dane W.; Vorpahl, Sarah M.; Stranks, Samuel D.; Nagaoka, Hirokazu; Eperon, Giles E.; Ziffer, Mark E.; Snaith, Henry J.; Ginger, David S.
      Science (Washington, DC, United States) (2015), 348 (6235), 683-686CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The remarkable performance of hybrid perovskite photovoltaics is attributed to their long carrier lifetimes and high photoluminescence (PL) efficiencies. High-quality films are assocd. with slower PL decays, and it has been claimed that grain boundaries have a negligible impact on performance. We used confocal fluorescence microscopy correlated with SEM to spatially resolve the PL decay dynamics from films of nonstoichiometric org.-inorg. perovskites, CH3NH3PbI3(Cl). The PL intensities and lifetimes varied between different grains in the same film, even for films that exhibited long bulk lifetimes. The grain boundaries were dimmer and exhibited faster nonradiative decay. Energy-dispersive x-ray spectroscopy showed a pos. correlation between chlorine concn. and regions of brighter PL, whereas PL imaging revealed that chem. treatment with pyridine could activate previously dark grains.
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    17. 17
      Jiang, C.; Zhang, P. Crystalline Orientation Dependent Photoresponse and Heterogeneous Behaviors of Grain Boundaries in Perovskite Solar Cells. J. Appl. Phys. 2018, 123 (8), 083105  DOI: 10.1063/1.5007857
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      17
      Crystalline orientation dependent photoresponse and heterogeneous behaviors of grain boundaries in perovskite solar cells
      Jiang, Chuanpeng; Zhang, Pengpeng
      Journal of Applied Physics (Melville, NY, United States) (2018), 123 (8), 083105/1-083105/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Using photoconductive at. force microscopy and Kelvin probe force microscopy, we characterize the local elec. properties of grains and grain boundaries of org.-inorg. hybrid perovskite (CH3NH3PbI3) thin films on top of a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/ITO substrate. Three discrete photocond. levels are identified among perovskite grains, likely corresponding to the crystal orientation of each grain. Local J-V curves recorded on these grains further suggest an anti-correlation behavior between the short circuit current (JSC) and open circuit voltage (VOC). This phenomenon can be attributed to diffusion-limited surface recombination at the non-selective perovskite-tip contact, where a higher carrier mobility established in the perovskite grain results in an enhanced surface recombination and thus a lower VOC. In addn., the photoresponse of perovskite films displays a pronounced heterogeneity across the grain boundaries, with the boundaries formed between grains of the same photocond. level displaying even enhanced photocurrent and open circuit voltage compared to those of the adjacent grain interiors. These observations highlight the significance of controlling the microstructure of perovskite thin films, which will be a necessary route for further improving the efficiency of perovskite solar cells. (c) 2018 American Institute of Physics.
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    18. 18
      Leblebici, S. Y.; Leppert, L.; Li, Y.; Reyes-Lillo, S. E.; Wickenburg, S.; Wong, E.; Lee, J.; Melli, M.; Ziegler, D.; Angell, D. K. Facet-Dependent Photovoltaic Efficiency Variations in Single Grains of Hybrid Halide Perovskite. Nat. Energy 2016, 1 (8), 16093,  DOI: 10.1038/nenergy.2016.93
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      18
      Facet-dependent photovoltaic efficiency variations in single grains of hybrid halide perovskite
      Leblebici, Sibel Y.; Leppert, Linn; Li, Yanbo; Reyes-Lillo, Sebastian E.; Wickenburg, Sebastian; Wong, Ed; Lee, Jiye; Melli, Mauro; Ziegler, Dominik; Angell, Daniel K.; Ogletree, D. Frank; Ashby, Paul D.; Toma, Francesca M.; Neaton, Jeffrey B.; Sharp, Ian D.; Weber-Bargioni, Alexander
      Nature Energy (2016), 1 (8), 16093CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
      Photovoltaic devices based on hybrid perovskite materials have exceeded 22% efficiency due to high charge-carrier mobilities and lifetimes. Properties such as photocurrent generation and open-circuit voltage are influenced by the microscopic structure and orientation of the perovskite crystals, but are difficult to quantify on the intra-grain length scale and are often treated as homogeneous within the active layer. Here, we map the local short-circuit photocurrent, open-circuit photovoltage, and dark drift current in state-of-the-art methylammonium lead iodide solar cells using photoconductive at. force microscopy. We find, within individual grains, spatially correlated heterogeneity in short-circuit current and open-circuit voltage up to 0.6 V. These variations are related to different crystal facets and have a direct impact on the macroscopic power conversion efficiency. We attribute this heterogeneity to a facet-dependent d. of trap states. These results imply that controlling crystal grain and facet orientation will enable a systematic optimization of polycryst. and single-crystal devices for photovoltaic and lighting applications.
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    19. 19
      Du, T.; Burgess, C. H.; Lin, C.-T.; Eisner, F.; Kim, J.; Xu, S.; Kang, H.; Durrant, J. R.; McLachlan, M. A. Probing and Controlling Intragrain Crystallinity for Improved Low Temperature–Processed Perovskite Solar Cells. Adv. Funct. Mater. 2018, 28 (51), 1803943,  DOI: 10.1002/adfm.201803943
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    20. 20
      Li, J. J.; Ma, J. Y.; Ge, Q. Q.; Hu, J. S.; Wang, D.; Wan, L. J. Microscopic Investigation of Grain Boundaries in Organolead Halide Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7 (51), 28518– 28523,  DOI: 10.1021/acsami.5b09801
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      20
      Microscopic Investigation of Grain Boundaries in Organolead Halide Perovskite Solar Cells
      Li, Jiang-Jun; Ma, Jing-Yuan; Ge, Qian-Qing; Hu, Jin-Song; Wang, Dong; Wan, Li-Jun
      ACS Applied Materials & Interfaces (2015), 7 (51), 28518-28523CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Grain boundaries (GBs) play an important role in org.-inorg. halide perovskite solar cells, which have generally been recognized as a new class of materials for photovoltaic applications. To definitely understand the elec. structure and behavior of GBs, here the authors present Kelvin probe force microscopy and conductive at. force microscopy (c-AFM) measurements of both typical and inverted planar organolead halide perovskite solar cells. By comparing the contact p.d. (CPD) of these 2 devices in the dark and under illumination, a downward band bending exists in GBs that predominantly attract photoinduced electrons. The c-AFM measurements obsd. that higher photocurrents flow through GBs when a low bias overcomes the barrier created by the band bending, indicating that GBs act as effective charge dissocn. interfaces and photocurrent transduction pathways rather than recombination sites.
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    21. 21
      Doherty, T. A. S.; Winchester, A. J.; Macpherson, S.; Johnstone, D. N.; Pareek, V.; Tennyson, E. M.; Kosar, S.; Kosasih, F. U.; Anaya, M.; Abdi-jalebi, M. Performance-Limiting Nanoscale Trap Clusters at Grain Junctions in Halide Perovskites. Nature 2020, 580 (4), 360– 366,  DOI: 10.1038/s41586-020-2184-1
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      21
      Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites
      Doherty, Tiarnan A. S.; Winchester, Andrew J.; MacPherson, Stuart; Johnstone, Duncan N.; Pareek, Vivek; Tennyson, Elizabeth M.; Kosar, Sofiia; Kosasih, Felix U.; Anaya, Miguel; Abdi-Jalebi, Mojtaba; Andaji-Garmaroudi, Zahra; Wong, E. Laine; Madeo, Julien; Chiang, Yu-Hsien; Park, Ji-Sang; Jung, Young-Kwang; Petoukhoff, Christopher E.; Divitini, Giorgio; Man, Michael K. L.; Ducati, Caterina; Walsh, Aron; Midgley, Paul A.; Dani, Keshav M.; Stranks, Samuel D.
      Nature (London, United Kingdom) (2020), 580 (7803), 360-366CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Photoemission electron microscopy was used to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor luminescence efficiency, discrete, nanoscale trap clusters were obsd. By correlating microscopy measurements with scanning electron anal. techniques, these trap clusters appear at the interfaces between crystallog. and compositionally distinct entities. By generating time-resolved photoemission sequences of the photoexcited carrier trapping process, a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters was revealed. Managing structure and compn. on the nanoscale will be essential for optimal performance of halide perovskite devices.
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    22. 22
      Du, T.; Burgess, C. H.; Kim, J.; Zhang, J.; Durrant, J. R.; McLachlan, M. A. Formation, Location and Beneficial Role of PbI 2 in Lead Halide Perovskite Solar Cells. Sustain. Energy Fuels 2017, 1 (1), 119– 126,  DOI: 10.1039/C6SE00029K
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    23. 23
      Thompson, C. V. Secondary Grain Growth in Thin Films of Semiconductors: Theoretical Aspects. J. Appl. Phys. 1985, 58 (2), 763– 772,  DOI: 10.1063/1.336194
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      23
      Secondary grain growth in thin films of semiconductors: theoretical aspects
      Thompson, C. V.
      Journal of Applied Physics (1985), 58 (2), 763-72CODEN: JAPIAU; ISSN:0021-8979.
      Secondary grain growth in thin films can lead to grain sizes much greater than the film thickness. Surface energy anisotropy often provides an important fraction of the driving force for secondary grain growth, esp. in the early stages of growth. Surface-energy-driven secondary grain growth leads to the development of large grains with restricted crystallog. textures. A model is presented for growth of secondary grains into a uniform matrix of columnar normal grains. The model indicates that secondary grain growth rates should increase with grain boundary energy, surface energy anisotropy, grain boundary mobility, and temp. While final secondary grain sizes will decrease with film thickness, their growth rates will increase. The final secondary grain sizes and orientations will be strongly affected by grain sizes and orientations in the initial film. The models presented here provide anal. tools for exptl. study of secondary grain growth in thin films.
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    24. 24
      Kim, M.; Kim, G. H.; Oh, K. S.; Jo, Y.; Yoon, H.; Kim, K. H.; Lee, H.; Kim, J. Y.; Kim, D. S. High-Temperature-Short-Time Annealing Process for High-Performance Large-Area Perovskite Solar Cells. ACS Nano 2017, 11 (6), 6057– 6064,  DOI: 10.1021/acsnano.7b02015
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      24
      High-Temperature-Short-Time Annealing Process for High-Performance Large-Area Perovskite Solar Cells
      Kim, Minjin; Kim, Gi-Hwan; Oh, Kyoung Suk; Jo, Yimhyun; Yoon, Hyun; Kim, Ka-Hyun; Lee, Heon; Kim, Jin Young; Kim, Dong Suk
      ACS Nano (2017), 11 (6), 6057-6064CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Org.-inorg. hybrid metal halide perovskite solar cells (PSCs) are attracting tremendous research interest due to their high solar-to-elec. power conversion efficiency with a high possibility of cost-effective fabrication and certified power conversion efficiency now exceeding 22%. Although many effective methods for their application have been developed over the past decade, their practical transition to large-size devices has been restricted by difficulties in achieving high performance. Here we report on the development of a simple and cost-effective prodn. method with high-temp. and short-time annealing processing to obtain uniform, smooth, and large-size grain domains of perovskite films over large areas. With high-temp. short-time annealing at 400 °C for 4 s, the perovskite film with an av. domain size of 1 μm was obtained, which resulted in fast solvent evapn. Solar cells fabricated using this processing technique had a max. power conversion efficiency exceeding 20% over a 0.1 cm2 active area and 18% over a 1 cm2 active area. We believe our approach will enable the realization of highly efficient large-area PCSs for practical development with a very simple and short-time procedure. This simple method should lead the field toward the fabrication of uniform large-scale perovskite films, which are necessary for the prodn. of high-efficiency solar cells that may also be applicable to several other material systems for more widespread practical deployment.
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    25. 25
      You, P.; Li, G.; Tang, G.; Cao, J.; Yan, F. Ultrafast Laser-Annealing of Perovskite Films for Efficient Perovskite Solar Cells. Energy Environ. Sci. 2020, 13 (4), 1187– 1196,  DOI: 10.1039/C9EE02324K
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      25
      Ultrafast laser-annealing of perovskite films for efficient perovskite solar cells
      You, Peng; Li, Guijun; Tang, Guanqi; Cao, Jiupeng; Yan, Feng
      Energy & Environmental Science (2020), 13 (4), 1187-1196CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Perovskite solar cells have attracted much attention recently for their high efficiency, ease of prepn. and low cost. Here, we report a novel laser-annealing method for perovskite films at a low substrate temp. by scanning laser spots on the film surfaces. An ultrafast crystn. process within a few seconds is realized under a laser with a high intensity and a fast scanning speed. Because the cryst. perovskite phase has a stronger light absorption than the amorphous phase, the fast laser annealing can induce a higher temp. in the former and lead to the selective growth of large perovskite grains. Under optimum conditions, perovskite films with high crystallinity are successfully fabricated, resulting in perovskite solar cells with high power conversion efficiency and good stability. Moreover, a faster laser-annealing process of perovskite films is achieved by using a linear laser beam, which is expected to be a promising technique for the mass prodn. of large-scale perovskite solar cells.
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    26. 26
      Yang, M.; Zhang, T.; Schulz, P.; Li, Z.; Li, G.; Kim, D. H.; Guo, N.; Berry, J. J.; Zhu, K.; Zhao, Y. Facile Fabrication of Large-Grain CH3 NH3 PbI3-x Brx Films for High-Efficiency Solar Cells via CH3NH3 Br-Selective Ostwald Ripening. Nat. Commun. 2016, 7 (May), 12305,  DOI: 10.1038/ncomms12305
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      26
      Facile fabrication of large-grain CH3NH3PbI3-xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening
      Yang, Mengjin; Zhang, Taiyang; Schulz, Philip; Li, Zhen; Li, Ge; Kim, Dong Hoe; Guo, Nanjie; Berry, Joseph J.; Zhu, Kai; Zhao, Yixin
      Nature Communications (2016), 7 (), 12305CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technol. Structural and electro-optical properties of the perovskite absorber layer are most crit. to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH3NH3PbI3-xBrx (MAPbI3-xBrx) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concn. converts MAPbI3 thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI3-xBrx thin films following an Ostwald ripening process, which is strongly affected by MABr concn. and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concn. enhances I-Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.
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    27. 27
      Pham, N. D.; Tiong, V. T.; Yao, D.; Martens, W.; Guerrero, A.; Bisquert, J.; Wang, H. Guanidinium Thiocyanate Selective Ostwald Ripening Induced Large Grain for High Performance Perovskite Solar Cells. Nano Energy 2017, 41 (8), 476– 487,  DOI: 10.1016/j.nanoen.2017.10.006
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      27
      Guanidinium thiocyanate selective Ostwald ripening induced large grain for high performance perovskite solar cells
      Pham, Ngoc Duy; Tiong, Vincent Tiing; Yao, Disheng; Martens, Wayde; Guerrero, Antonio; Bisquert, Juan; Wang, Hongxia
      Nano Energy (2017), 41 (), 476-487CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      Org.-inorg. lead halide perovskite has become one of the most attractive materials for future low-cost high-efficiency solar technol. However, the polycryst. nature of perovskite thin-film often possesses an exceptional d. of defects, esp. at grain boundaries (GBs) and film surface, limiting further improvement in the power conversion efficiency (PCE) of the perovskite device. Here, we report a simple method to reduce GBs and to passivate the surface of a methylammonium lead tri-iodide (MAPbI3) film by guanidinium thiocyanate (GUTS)-assisted Ostwald ripening post treatment. High-optoelectronic quality MAPbI3 film consisting of micron-sized grains were synthesized by post-treating a MAPbI3 film with GUTS/isopropanol soln. (4 mg/mL, GUTS-4). Anal. of the electrochem. impedance spectra (EIS) of the solar cells showed that interfacial charge recombination resistance of the device based on a GUTS-4 post-treated MAPbI3 absorber film was increased by a factor of 1.15-2.6, depending on light illumination intensity, compared to the control MAPbI3 cell. This is consistent with results of the open-circuit voltage (Voc) decay and the light intensity dependent photovoltage evolution which shows device with GUTS treatment had one order longer charge carrier lifetime and was more ideal (ideality factor = 1.25). Further characterization by Kelvin probe force microscope indicated that GUTS-4 treatment shifted the energetics of the MAPbI3 film by ∼ 100 meV towards better energy level alignment with adjacent SnO2 electron transport layer, leading to a more favorable charge extn. process at the MAPbI3/SnO2 interface. As a result, the PCE of PSCs was enhanced from 14.59% to 16.37% and the hysteresis effect was mitigated.
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    28. 28
      Du, T.; Xu, W.; Daboczi, M.; Kim, J.; Xu, S.; Lin, C. T.; Kang, H.; Lee, K.; Heeney, M. J.; Kim, J. S. P-Doping of Organic Hole Transport Layers in p-i-n Perovskite Solar Cells: Correlating Open-Circuit Voltage and Photoluminescence Quenching. J. Mater. Chem. A 2019, 7 (32), 18971– 18979,  DOI: 10.1039/C9TA03896E
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      28
      p-Doping of organic hole transport layers in p-i-n perovskite solar cells: correlating open-circuit voltage and photoluminescence quenching
      Du, Tian; Xu, Weidong; Daboczi, Matyas; Kim, Jinhyun; Xu, Shengda; Lin, Chieh-Ting; Kang, Hongkyu; Lee, Kwanghee; Heeney, Martin J.; Kim, Ji-Seon; Durrant, James R.; McLachlan, Martyn A.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (32), 18971-18979CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Doping is a widely implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic (PV) devices. Here, in direct contrast to existing understanding, we find that a redn. in p-doping of the org. hole transport layer (HTL) leads to substantial improvements in PV performance in planar p-i-n perovskite solar cells (PSCs), driven by improvements in open circuit voltage (VOC). Employing a range of transient and steady state characterization tools, we find that the improvements of VOC correlate with reduced surface recombination losses in less p-doped HTLs. A simple device model including screening of bulk elec. fields in the perovskite layer is used to explain this observation. In particular, photoluminescence (PL) emission of complete solar cells shows that efficient performance is correlated to a high PL intensity at open circuit and a low PL intensity at short circuit. We conclude that desirable transport layers for p-i-n PSCs should be charge selective contacts with low doping densities.
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    29. 29
      Du, T.; Ratnasingham, S. R.; Kosasih, F. U.; Macdonald, T. J.; Mohan, L.; Augurio, A.; Ahli, H.; Lin, C.-T.; Xu, S.; Xu, W. Aerosol Assisted Solvent Treatment: A Universal Method for Performance and Stability Enhancements in Perovskite Solar Cells. Adv. Energy Mater. 2021, 11, 2101420,  DOI: 10.1002/aenm.202101420
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      29
      Aerosol Assisted Solvent Treatment: A Universal Method for Performance and Stability Enhancements in Perovskite Solar Cells
      Du, Tian; Ratnasingham, Sinclair R.; Kosasih, Felix U.; Macdonald, Thomas J.; Mohan, Lokeshwari; Augurio, Adriana; Ahli, Huda; Lin, Chieh-Ting; Xu, Shengda; Xu, Weidong; Binions, Russell; Ducati, Caterina; Durrant, James R.; Briscoe, Joe; McLachlan, Martyn A.
      Advanced Energy Materials (2021), 11 (33), 2101420CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      Metal-halide perovskite solar cells (PSCs) have had a transformative impact on the renewable energy landscape since they were first demonstrated just over a decade ago. Outstanding improvements in performance have been demonstrated through structural, compositional, and morphol. control of devices, with commercialization now being a reality. Here the authors present an aerosol assisted solvent treatment as a universal method to obtain performance and stability enhancements in PSCs, demonstrating their methodol. as a convenient, scalable, and reproducible post-deposition treatment for PSCs. Their results identify improvements in crystallinity and grain size, accompanied by a narrowing in grain size distribution as the underlying phys. changes that drive redns. of electronic and ionic defects. These changes lead to prolonged charge-carrier lifetimes and ultimately increased device efficiencies. The versatility of the process is demonstrated for PSCs with thick (>1 μm) active layers, large-areas (>1 cm2) and a variety of device architectures and active layer compns. This simple post-deposition process is widely transferable across the field of perovskites, thereby improving the future design principles of these materials to develop large-area, stable, and efficient PSCs.
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    30. 30
      Cao, X.; Zhi, L.; Li, Y.; Fang, F.; Cui, X.; Ci, L.; Ding, K.; Wei, J. Fabrication of Perovskite Films with Large Columnar Grains via Solvent-Mediated Ostwald Ripening for Efficient Inverted Perovskite Solar Cells. ACS Appl. Energy Mater. 2018, 1 (2), 868– 875,  DOI: 10.1021/acsaem.7b00300
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      30
      Fabrication of Perovskite Films with Large Columnar Grains via Solvent-Mediated Ostwald Ripening for Efficient Inverted Perovskite Solar Cells
      Cao, Xiaobing; Zhi, Lili; Li, Yahui; Fang, Fei; Cui, Xian; Ci, Lijie; Ding, Kongxian; Wei, Jinquan
      ACS Applied Energy Materials (2018), 1 (2), 868-875CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      Generally, residual solvent is embedded in perovskite precursor films fabricated from the Lewis adduct method. Most of the research focus on the ligand function of the solvent in forming a solvate complex for fabricating high quality perovskite films. However, little attention has been paid to the latent function of the solvent in the perovskite precursor films during the annealing process due to its fast extravasation at high temp. Here, we develop a sandwich configuration of substrate/perovskite precursor films/PC61BM to retard the extravasation of solvent during annealing. We find that the restrained solvent induces an obvious solvent-mediated dissoln.-recrystn. process, leading to high quality perovskite films with large columnar grains. There is mass transportation from small grains to large grains in the dissoln.-recrystn. process, which follows the Ostwald ripening model. Inverted planar solar cells are fabricated on the basis of this annealing method. The photovoltaic performance of the solar cells is improved significantly due to its high quality perovskite films with large columnar grains.
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    31. 31
      Zhou, Z.; Wang, Z.; Zhou, Y.; Pang, S.; Wang, D.; Xu, H.; Liu, Z.; Padture, N. P.; Cui, G. Methylamine-Gas Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells. Angew. Chemie Int. Ed. 2015, 54 (33), 9705– 9709,  DOI: 10.1002/anie.201504379
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      31
      Methylamine-Gas Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells
      Zhou, Zhongmin; Wang, Zaiwei; Zhou, Yuanyuan; Pang, Shuping; Wang, Dong; Xu, Hongxia; Liu, Zhihong; Padture, Nitin P.; Cui, Guanglei
      Angewandte Chemie, International Edition (2015), 54 (33), 9705-9709CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The discovery is reported of methylamine (CH3NH2) induced defect-healing (MIDH) of CH3NH3PbI3 perovskite thin films based on their ultrafast (seconds), reversible chem. reaction with CH3NH2 gas at room temp. The key to this healing behavior is the formation and spreading of an intermediate CH3NH3PbI3·xCH3NH2 liq. phase during this unusual perovskite-gas interaction. The versatility and scalability is demonstrated of the MIDH process, and dramatic enhancement is shown in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect-free films of hybrid perovskites.
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    32. 32
      Kutes, Y.; Zhou, Y.; Bosse, J. L.; Steffes, J.; Padture, N. P.; Huey, B. D. Mapping the Photoresponse of CH3NH3PbI3 Hybrid Perovskite Thin Films at the Nanoscale. Nano Lett. 2016, 16 (6), 3434– 3441,  DOI: 10.1021/acs.nanolett.5b04157
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      32
      Mapping the Photoresponse of CH3NH3PbI3 Hybrid Perovskite Thin Films at the Nanoscale
      Kutes, Yasemin; Zhou, Yuanyuan; Bosse, James L.; Steffes, James; Padture, Nitin P.; Huey, Bryan D.
      Nano Letters (2016), 16 (6), 3434-3441CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) hold unprecedented promise for low-cost, high-efficiency photovoltaics (PVs) of the future. While PV performance parameters of PSCs, such as short circuit current, open circuit voltage, and max. power, are always measured at the macroscopic scale, it is necessary to probe such photoresponses at the nanoscale to gain key insights into the fundamental PV mechanisms and their localized dependence on the OTP thin-film microstructure. Here we use photoconductive at. force microscopy spectroscopy to map for the first time variations of PV performance at the nanoscale for planar PSCs based on hole-transport-layer free methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) thin films. These results reveal substantial variations in the photoresponse that correlate with thin-film microstructural features such as intragrain planar defects, grains, grain boundaries, and notably also grain-aggregates. The insights gained into such microstructure-localized PV mechanisms are essential for guiding microstructural tailoring of OTP films for improved PV performance in future PSCs.
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    33. 33
      Chen, Y.; Lei, Y.; Li, Y.; Yu, Y.; Cai, J.; Chiu, M. H.; Rao, R.; Gu, Y.; Wang, C.; Choi, W. Strain Engineering and Epitaxial Stabilization of Halide Perovskites. Nature 2020, 577 (7789), 209– 215,  DOI: 10.1038/s41586-019-1868-x
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      33
      Strain engineering and epitaxial stabilization of halide perovskites
      Chen, Yimu; Lei, Yusheng; Li, Yuheng; Yu, Yugang; Cai, Jinze; Chiu, Ming-Hui; Rao, Rahul; Gu, Yue; Wang, Chunfeng; Choi, Woojin; Hu, Hongjie; Wang, Chonghe; Li, Yang; Song, Jiawei; Zhang, Jingxin; Qi, Baiyan; Lin, Muyang; Zhang, Zhuorui; Islam, Ahmad E.; Maruyama, Benji; Dayeh, Shadi; Li, Lain-Jong; Yang, Kesong; Lo, Yu-Hwa; Xu, Sheng
      Nature (London, United Kingdom) (2020), 577 (7789), 209-215CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Strain engineering is a powerful tool with which to enhance semiconductor device performance. Halide perovskites have shown great promise in device applications owing to their remarkable electronic and optoelectronic properties. Although applying strain to halide perovskites has been frequently attempted, including using hydrostatic pressurization, electrostriction, annealing, van der Waals force, thermal expansion mismatch, and heat-induced substrate phase transition, the controllable and device-compatible strain engineering of halide perovskites by chem. epitaxy remains a challenge, owing to the absence of suitable lattice-mismatched epitaxial substrates. Here, the authors report the strained epitaxial growth of halide perovskite single-crystal thin films on lattice-mismatched halide perovskite substrates. They investigated strain engineering of α-formamidinium lead iodide (α-FAPbI3) using both exptl. techniques and theor. calcns. By tailoring the substrate compn.-and therefore its lattice parameter-a compressive strain as high as 2.4 per cent is applied to the epitaxial α-FAPbI3 thin film. The authors demonstrate that this strain effectively changes the crystal structure, reduces the band gap and increases the hole mobility of α-FAPbI3. Strained epitaxy is also shown to have a substantial stabilization effect on the α-FAPbI3 phase owing to the synergistic effects of epitaxial stabilization and strain neutralization. As an example, strain engineering is applied to enhance the performance of an α-FAPbI3-based photodetector.
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    34. 34
      Wen, X.; Feng, Y.; Huang, S.; Huang, F.; Cheng, Y.; Green, M.; Ho-baillie, A. Defect Trapping States and Charge Carrier Recombination in Organic – Inorganic Halide. J. Mater. Chem. C 2016, 4, 793– 800,  DOI: 10.1039/C5TC03109E
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      34
      Defect trapping states and charge carrier recombination in organic-inorganic halide perovskites
      Wen, Xiaoming; Feng, Yu; Huang, Shujuan; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (4), 793-800CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Org.-inorg. perovskite solar cells have attracted huge research interest due to rapid improvement in device performance showing great potential to be the next generation flexible solar cells. Unique defect properties in perovskite have been considered as the possible mechanism for the superior performance, and closely relevant to the effects of hysteresis and light soaking. To date, the quant. correlation and in-depth understanding of defects in org.-inorg. perovskite are still lacking although extensive investigation have been undertaken. Here we study defect trapping states and carrier recombination dynamics in org.-inorg. halide perovskites. At low excitation the photoluminescence (PL) intensity exhibits a super-linear increase with increasing excitation, due to the slow depopulation rate of the defect states. The steady state and time-resolved photoluminescence (PL) carried out in this work reveal that the carrier recombination dynamics is ultimately correlated with both the defect d. and the relaxation rate of the carriers in defects. A model is established for the relationship between the properties of the defect trapping state and steady state PL intensity. Two key parameters, (i) the ratio of the trap-state d. to the depopulation rate of trapped states and (ii) ratio of the max. d. of covalence band electrons to the trapping rate, can be extd. from the model based on the excitation dependent steady state PL. This work demonstrates that the properties of defect trapping states are closely related to the fabrication technique, and suggests that the org.-inorg. halide perovskite is partly defect-tolerant.
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    35. 35
      Yamada, Y.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Spontaneous Defect Annihilation in CH3NH3PbI3 Thin Films at Room Temperature Revealed by Time-Resolved Photoluminescence Spectroscopy. J. Phys. Chem. Lett. 2015, 6 (3), 482– 486,  DOI: 10.1021/jz5026596
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      35
      Spontaneous Defect Annihilation in CH3NH3PbI3 Thin Films at Room Temperature Revealed by Time-Resolved Photoluminescence Spectroscopy
      Yamada, Yasuhiro; Endo, Masaru; Wakamiya, Atsushi; Kanemitsu, Yoshihiko
      Journal of Physical Chemistry Letters (2015), 6 (3), 482-486CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      We utilized time-resolved photoluminescence (PL) spectroscopy to investigate the photocarrier recombination dynamics in CH3NH3PbI3 thin films as a function of the time elapsed from the film's fabrication. We found that the PL lifetime gradually increased and began to level out once the age of the film reached ∼30 h. Even under weak excitation, the PL dynamics depended on the excitation intensity in the fresh sample, while the mature sample displayed no excitation-intensity dependence assocd. with the PL dynamics. We submit that this can be explained by the fact that a significant no. of defects are initially formed in CH3NH3PbI3 thin films fabricated by the sequential method and are spontaneously reduced by room-temp. annealing. Our results provide important insights for reducing the nonradiative recombination centers, which improves the power conversion efficiency of perovskite solar cells.
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    36. 36
      Andaji-Garmaroudi, Z.; Abdi-Jalebi, M.; Guo, D.; Macpherson, S.; Sadhanala, A.; Tennyson, E. M.; Ruggeri, E.; Anaya, M.; Galkowski, K.; Shivanna, R. A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites. Adv. Mater. 2019, 31 (42), 1902374,  DOI: 10.1002/adma.201902374
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      A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites
      Andaji-Garmaroudi, Zahra; Abdi-Jalebi, Mojtaba; Guo, Dengyang; Macpherson, Stuart; Sadhanala, Aditya; Tennyson, Elizabeth M.; Ruggeri, Edoardo; Anaya, Miguel; Galkowski, Krzysztof; Shivanna, Ravichandran; Lohmann, Kilian; Frohna, Kyle; Mackowski, Sebastian; Savenije, Tom J.; Friend, Richard H.; Stranks, Samuel D.
      Advanced Materials (Weinheim, Germany) (2019), 31 (42), 1902374CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Mixed-halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of soln.-processed triple-cation mixed-halide (Cs0.06MA0.15FA0.79)Pb(Br0.4I0.6)3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar-equiv. illumination. It is found that the illumination leads to localized surface sites of iodide-rich perovskite intermixed with passivating PbI2 material. Time- and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide-rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concn. of charges on the surface layer. This work reveals new insight into phase segregation of mixed-halide mixed-cation perovskites, as well as routes to highly luminescent films by controlling charge d. and transfer in novel device structures.
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    37. 37
      Kong, L.; Liua, G.; Gong, J.; Hu, Q.; Schaller, R. D.; Dera, P.; Zhang, D.; Liu, Z.; Yang, W.; Zhu, K. Simultaneous Band-Gap Narrowing and Carrier-Lifetime Prolongation of Organic-Inorganic Trihalide Perovskites. Proc. Natl. Acad. Sci. U. S. A. 2016, 113 (32), 8910– 8915,  DOI: 10.1073/pnas.1609030113
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      Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic-inorganic trihalide perovskites
      Kong, Lingping; Liu, Gang; Gong, Jue; Hu, Qingyang; Schaller, Richard D.; Dera, Przemyslaw; Zhang, Dongzhou; Liu, Zhenxian; Yang, Wenge; Zhu, Kai; Tang, Yuzhao; Wang, Chuanyi; Wei, Su-Huai; Xu, Tao; Mao, Ho-kwang
      Proceedings of the National Academy of Sciences of the United States of America (2016), 113 (32), 8910-8915CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The org.-inorg. hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a crit. factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chem. or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance.
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    38. 38
      Boopathi, K. M.; Martín-García, B.; Ray, A.; Pina, J. M.; Marras, S.; Saidaminov, M. I.; Bonaccorso, F.; Di Stasio, F.; Sargent, E. H.; Manna, L. Permanent Lattice Compression of Lead-Halide Perovskite for Persistently Enhanced Optoelectronic Properties. ACS Energy Lett. 2020, 5 (2), 642– 649,  DOI: 10.1021/acsenergylett.9b02810
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      38
      Permanent lattice compression of lead-halide perovskite for persistently enhanced optoelectronic properties
      Boopathi, Karunakara Moorthy; Martin-Garcia, Beatriz; Ray, Aniruddha; Pina, Joao M.; Marras, Sergio; Saidaminov, Makhsud I.; Bonaccorso, Francesco; Di Stasio, Francesco; Sargent, Edward H.; Manna, Liberato; Abdelhady, Ahmed L.
      ACS Energy Letters (2020), 5 (2), 642-649CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Under mild mech. pressure, halide perovskites show enhanced optoelectronic properties. However, these improvements are reversible upon decompression, and permanent enhancements have yet to be realized. Here, we report antisolvent-assisted solvent acidolysis crystn. that enables us to prep. methylammonium lead bromide single crystals showing intense emission at all four edges under UV light excitation. We study structural variations (edge-vs-center) in these crystals using micro-X-ray diffraction and find that the enhanced emission at the edges correlates with lattice compression compared to in the central areas. Time-resolved photoluminescence measurements show much longer-lived photogenerated carriers at the compressed edges, with radiative component lifetimes of ∼1.4μs, 10 times longer than at the central regions. The properties of the edges are exploited to fabricate planar photodetectors exhibiting detectivities of 3 × 1013 Jones, compared to 5 × 1012 Jones at the central regions. The enhanced lifetimes and detectivities correlate to the reduced trap state densities and the formation of shallower traps at the edges due to lattice compression.
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    39. 39
      Zhao, J.; Deng, Y.; Wei, H.; Zheng, X.; Yu, Z.; Shao, Y.; Shield, J. E.; Huang, J. Strained Hybrid Perovskite Thin Films and Their Impact on the Intrinsic Stability of Perovskite Solar Cells. Sci. Adv. 2017, 3 (11), eaao5616  DOI: 10.1126/sciadv.aao5616
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      Jamshaid, A.; Guo, Z.; Hieulle, J.; Stecker, C.; Ohmann, R.; Ono, L. K.; Qiu, L.; Tong, G.; Yin, W.; Qi, Y. Atomic-Scale Insight into the Enhanced Surface Stability of Methylammonium Lead Iodide Perovskite by Controlled Deposition of Lead Chloride. Energy Environ. Sci. 2021, 14 (8), 4541– 4554,  DOI: 10.1039/D1EE01084K
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      Atomic-scale insight into the enhanced surface stability of methylammonium lead iodide perovskite by controlled deposition of lead chloride
      Jamshaid, Afshan; Guo, Zhendong; Hieulle, Jeremy; Stecker, Collin; Ohmann, Robin; Ono, Luis K.; Qiu, Longbin; Tong, Guoqing; Yin, Wanjian; Qi, Yabing
      Energy & Environmental Science (2021), 14 (8), 4541-4554CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      The incorporation of a certain amt. of Cl ions into methylammonium lead iodide (MAPbI3) perovskite films and how these incorporated Cl ions affect the structural and electronic properties of these films have been an intensively studied topic. In this study, we comprehensively investigated Cl incorporation in MAPbI3 at the at. scale by a combined study of scanning tunneling microscopy, XPS, UV and inverse photoemission spectroscopy, d. functional theory and mol. dynamics calcns. At a Cl concn. of 14.8 ± 0.6%, scanning tunneling microscopy images confirm the incorporation of Cl ions on the MAPbI3 surface, which also corresponds to the highest surface stability of MAPbI3 found from the viewpoint of both thermodn. and kinetics by d. functional theory and mol. dynamics calcns. Our results show that the Cl concn. is crucial to the surface bandgap and stability of MAPbI3.
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      Son, Y.; Li, M. Y.; Cheng, C. C.; Wei, K. H.; Liu, P.; Wang, Q. H.; Li, L. J.; Strano, M. S. Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging. Nano Lett. 2016, 16 (6), 3571– 3577,  DOI: 10.1021/acs.nanolett.6b00699
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      Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging
      Son, Youngwoo; Li, Ming-Yang; Cheng, Chia-Chin; Wei, Kung-Hwa; Liu, Pingwei; Wang, Qing Hua; Li, Lain-Jong; Strano, Michael S.
      Nano Letters (2016), 16 (6), 3571-3577CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chem. vapor deposition methods has provided an effective soln. to producing an in-plane p-n junction, which is a crit. component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral at. force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2-MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are obsd. to be selectively switched on and off, allowing for the realization of a hyper-resoln. array of the switchable photodiode pixels. This exptl. approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolns. Comparative 2D Fourier anal. of phys. height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.
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    42. 42
      Frammelsberger, W.; Benstetter, G.; Kiely, J.; Stamp, R. C-AFM-Based Thickness Determination of Thin and Ultra-Thin SiO 2 Films by Use of Different Conductive-Coated Probe Tips. Appl. Surf. Sci. 2007, 253 (7), 3615– 3626,  DOI: 10.1016/j.apsusc.2006.07.070
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      C-AFM-based thickness determination of thin and ultra-thin SiO2 films by use of different conductive-coated probe tips
      Frammelsberger, Werner; Benstetter, Guenther; Kiely, Janice; Stamp, Richard
      Applied Surface Science (2007), 253 (7), 3615-3626CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      The influence of the probe tip type on the elec. oxide thickness result was researched for four differently coated conductive tip types using SiO2 (oxide) films with optical thickness of 1.7-8.3 nm. For this purpose, conductive at. force microscopy (C-AFM) was used to measure more than 7200 current-voltage (IV) curves. The elec. oxide thickness was detd. on a statistical basis from the IV-curves using a recently published tunneling model for C-AFM application. The model includes parameters assocd. with the probe tip types used. The evolution of the tip parameters is described in detail. For the theor. tip parameters, measured and calcd. IV-curves showed excellent agreement and the elec. oxide thickness vs. the optical oxide thickness showed congruent behavior, independent of the tip type. However, differences in the elec. oxide thickness were obsd. for the different tip types. The theor. parameters were modified exptl. in order to reduce these differences. Theor. and exptl. tip parameters were compared and their effect on the differences in the elec. oxide thickness is discussed for the different tip types. Overall, it is shown that the proposed model provides a comprehensive framework for detg. the elec. oxide thickness using C-AFM, for a wide range of oxide thicknesses and for differently coated conductive tips.
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      Wang, H.; Kim, D. H. Perovskite-Based Photodetectors: Materials and Devices. Chem. Soc. Rev. 2017, 46 (17), 5204– 5236,  DOI: 10.1039/C6CS00896H
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      Perovskite-based photodetectors: materials and devices
      Wang, Huan; Kim, Dong Ha
      Chemical Society Reviews (2017), 46 (17), 5204-5236CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      While the field of perovskite-based optoelectronics has mostly been dominated by photovoltaics, light-emitting diodes, and transistors, semiconducting properties peculiar to perovskites make them interesting candidates for innovative and disruptive applications in light signal detection. Perovskites combine effective light absorption in the broadband range with good photo-generation yield and high charge carrier mobility, a combination that provides promising potential for exploiting sensitive and fast photodetectors that are targeted for image sensing, optical communication, environmental monitoring or chem./biol. detection. Currently, org.-inorg. hybrid and all-inorg. halide perovskites with controlled morphologies of polycryst. thin films, nano-particles/wires/sheets, and bulk single crystals have shown key figure-of-merit features in terms of their responsivity, detectivity, noise equiv. power, linear dynamic range, and response speed. The sensing region has been covered from UV-visible-near IR (UV-Vis-NIR) to gamma photons based on two- or three-terminal device architectures. Diverse photoactive materials and devices with superior optoelectronic performances have stimulated attention from researchers in multidisciplinary areas. In this review, we provide a comprehensive overview of the recent progress of perovskite-based photodetectors focusing on versatile compns., structures, and morphologies of constituent materials, and diverse device architectures toward the superior performance metrics. Combining the advantages of both org. semiconductors (facile soln. processability) and inorg. semiconductors (high charge carrier mobility), perovskites are expected to replace com. silicon for future photodetection applications.
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      Lin, Q.; Armin, A.; Lyons, D. M.; Burn, P. L.; Meredith, P. Low Noise, IR-Blind Organohalide Perovskite Photodiodes for Visible Light Detection and Imaging. Adv. Mater. 2015, 27 (12), 2060– 2064,  DOI: 10.1002/adma.201405171
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      44
      Low Noise, IR-Blind Organohalide Perovskite Photodiodes for Visible Light Detection and Imaging
      Lin, Qianqian; Armin, Ardalan; Lyons, Dani M.; Burn, Paul L.; Meredith, Paul
      Advanced Materials (Weinheim, Germany) (2015), 27 (12), 2060-2064CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      An organohalide perovskite low noise, IR-blind visible photodiode with performance metrics equiv. to com. inorg. semiconductor devices was demonstrated. The MeNH3+PbI3- perovskite photodiode consists of a simple, soln.-processed homojunction capped with a PC60BM /C60 cathode interlayer. This interlayer: (i) behaves as a low shunt capacitor improving the diode temporal response; (ii) suppresses the dark current by conformally coating the perovskite homojunction and limiting the back injection of holes; and (iii) provides electrooptical control of the spectral response. These features are achieved without compromising the linear dynamic range (LDR) as the fullerene electron mobility is sufficient to prevent space charge forming in the junction up to light intensities of 0.4 W cm-2. A measurement-limited. LDR of -170 dB (at -0.5 V) was obsd., but noise calcns. would indicate it could be ≤250 dB.
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      Fang, Y.; Huang, J. Resolving Weak Light of Sub-Picowatt per Square Centimeter by Hybrid Perovskite Photodetectors Enabled by Noise Reduction. Adv. Mater. 2015, 27 (17), 2804– 2810,  DOI: 10.1002/adma.201500099
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      45
      Resolving Weak Light of Sub-picowatt per Square Centimeter by Hybrid Perovskite Photodetectors Enabled by Noise Reduction
      Fang, Yanjun; Huang, Jinsong
      Advanced Materials (Weinheim, Germany) (2015), 27 (17), 2804-2810CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A highly sensitive perovskite photodetector is reported with low noise (16 fA Hz-1/2 at -0.1 V) close to the shot and thermal noise limits, high av. EQE approaching 90%, large LDR of 94 dB, and short response time of 120 ns. The interfacial electron transfer and hole transfer layers engineering of the device, esp. the trap passivation effect of the fullerene layer, enables its direct measurement of light irradiance down to sub-pW cm-2, which matches with the calcd. noise equiv. power. The excellent weak light sensing capability of the perovskite photodetector makes it potential in replacing com. Si photodiodes in the applications like defense, communication, and imaging.
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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03839.

    • Experimental methods, SEM images, PL spectra, photocurrent and photoresponse maps, optimization of the layer thickness in the PPDs, noise power spectra of the PPDs, cyclic low-light Jph measurements of the PPDs (PDF)


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