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ArticleApril 22, 2016

Influence of Blend Ratio and Processing Additive on Free Carrier Yield and Mobility in PTB7:PC₇₁BM Photovoltaic Solar Cells
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2016, 120, 18, 9588–9594

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https://doi.org/10.1021/acs.jpcc.6b01548

Published April 22, 2016

Abstract

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Charge separation and extraction dynamics were investigated in high-performance bulk heterojunction solar cells made from the polymer PTB7 and the soluble fullerene PC₇₁BM on a broad time scale from subpicosecond to microseconds using ultrafast optical probing of carrier drift and the integral-mode photocurrent measurements. We show that the short circuit current is determined by the separation of charge pairs into free carriers, which is strongly influenced by blend composition. This separation is found to be efficient in fullerene-rich blends where a high electron mobility of >0.1 cm² V^–1 s^–1 is observed in the first 10 ps after excitation. Morphology optimization using the solvent additive 1,8-diiodooctane (DIO) doubles the charge pair separation efficiency and the short-circuit current. Carrier extraction at low internal electric field is slightly faster from the cells prepared with DIO, which can reduce recombination losses and enhance a fill factor.

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Introduction

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The limited supply of fossil fuels and rising energy demands have encouraged research into renewable energy sources, with photovoltaics proving to be a suitable candidate. Interest in organic and hybrid solar cells has grown due to their ability to produce flexible lightweight devices with low manufacturing costs and an abundant supply of environmentally friendly materials. Small-scale single-junction organic solar cells now achieve power conversion efficiencies over 10%. (1-4) The best performance is achieved using a bulk heterojunction where an electron donor (usually a conjugated polymer) and an acceptor (usually a fullerene) are blended to obtain a large interfacial area for charge carrier generation. Progress has been spurred on by the synthesis of novel polymers and optimization of processing conditions. One of the most efficient blends is of the polymer PTB7 and the soluble fullerene PC₇₁BM, and it achieves high efficiency using a high-boiling-point solvent additive 1,8-di-iodooctane (DIO) for processing the active layer. (4-9) The influence of DIO on the morphology of PTB7:PC₇₁BM blends has previously been investigated in detail, which indicated that the blends prepared without additive show pure fullerene clusters of 20–60 nm in size which form large agglomerates embedded in a polymer-rich matrix containing about 30 wt % of fullerene. (10-13) The addition of DIO to the casting solvent improves the miscibility of PC₇₁BM with PTB7, dramatically shrinks the size of the clusters to several nanometers, and forms interpenetrating polymer-rich and fullerene-rich phases of tens of nanometers in size. (10, 12) Fluorescence quenching was found to be of similar efficiency in blends prepared with additive and without, suggesting that the increase of power conversion efficiency is not due to more efficient charge generation. (12) This is consistent with the power conversion efficiency enhancements occurring in both the polymer and fullerene absorption regions, suggesting that the improvement results from reduced carrier recombination. (10, 12) The origin of reduced recombination is not known; possible explanations include improved charge separation, higher carrier mobility, or reduced charge trapping. (12, 14-16) It is therefore clear that a detailed understanding of the free carrier generation, transport, and extraction in this important high-performance blend is lacking.

In this work, we combine transient photocurrent measurements with ultrafast optical probes of carrier mobility to investigate charge separation and motion dynamics in PTB7:PC₇₁BM solar cells in order to understand the reasons behind the increased efficiency of devices prepared with the additive DIO. We demonstrate that the increase of photocurrent in devices prepared with DIO occurs because of a higher dissociation efficiency of photogenerated charge pairs into free carriers. The enhancement of the carrier extraction rate is observed at a low built-in electric field and can contribute to a higher fill factor of DIO-prepared solar cells. We measured time-dependent carrier mobility in devices prepared using different blending ratios and show that carrier transport is limited by connectivity of fullerene/polymer domains in addition to trapping at low energy sites. These results give new insights into the influence of morphology on charge separation and transport, providing a full picture of the dynamics of these processes helping to understand the operation mechanisms and the ways to improve the performance.

Experimental Methods

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Solar Cell Preparation and Characterization

PTB7 with a molecular weight of 92000 and a polydispersity of 2.6 was obtained from 1-Material, Inc. PC₇₁BM of 99% purity was obtained from Solenne and DIO from Sigma-Aldrich. PTB7 and fullerene were dissolved in chlorobenzene (HPLC grade from Sigma-Aldrich) at ratios of 90:10, 40:60, 20:80, and 10:90 by weight and stirred at 50 °C for 4–5 h. We will call these blends [P90:F10], [P40:F60], [P20:F80], and [P10:F90], respectively, where the first number denotes the relative amount of the polymer. For the samples prepared with DIO, 3% of it by volume was added to the solution which was then stirred for a further 5 min. The blends were spin-coated on a ∼40 nm layer of PEDOT:PSS which was spin-coated on an indium–tin oxide coated glass substrate. The thickness of the PTB7:fullerene layer was ∼115 nm. The layers of calcium (∼20 nm) and aluminum (∼100 nm) were subsequently deposited by vacuum sublimation. The structure was encapsulated with a glass coverslip and epoxy. The current–voltage characteristics were measured under AM1.5 conditions using a solar simulator from Sciencetech and the intensity calibrated with an ORIEL reference cell with KG5 filter. The spectral mismatch factor was close to unity (0.995) for PTB7:PC₇₁BM. An aperture of the same size as the pixel was used to avoid contribution from stray light outside the device area.

Measurements of Carrier Drift Dynamics

To measure the carrier-drift dynamics, we have used the time-resolved electric-field-induced second-harmonic generation (TREFISH) technique, which was described in detail elsewhere. (17) Briefly, a pulsed p-polarized probe light is shone on a cell at ∼45° incidence angle. An applied electric field F breaks centrosymmetry of the blend for incident light so that the second harmonic of the probe light (SH) is generated and its intensity is proportional to F². An optical pump pulse which overlaps spatially with the probe light generates charge pairs which drift in response to electric field, and so they shield the internal electric field and reduce the SH intensity. In this way, the intensity of SH can be used to directly monitor carrier drift in the cell by changing the time delay between pump and probe pulses. The solar cell acts as a parallel plate capacitor; thus, F = (U_appl + ΔW/e)/L where U_appl is the applied reverse bias, ΔW = −0.8 eV is the difference of work functions of the electrodes, L is the distance between the electrodes, and e is the elementary charge. The SH dependence on the applied voltage without a pump pulse was measured first and used later to obtain the strength of the internal electric field from the TREFISH signal. Probe pulses at 800 nm and 1 kHz repetition rate were generated by a regenerative amplifier (Quantronix Integra-C), and SH was detected using a photomultiplier tube. The pump pulses were generated in an optical parametric amplifier TOPAS-C (Light Conversion) and excited the cell at 500 Hz, i.e., at every other probe pulse. The pump wavelength was 680 nm, which is the absorption maximum of the polymer. Pump and probe pulses were 150 fs long. The integral-mode photocurrent transients were measured simultaneously using the same pump pulses and recording the voltage on a 10 kΩ load resistor connected in series with the solar cell using a 500 MHz bandwidth Agilent Technologies oscilloscope DSO5054A. The pump pulse energy was adjusted to give photocurrent transients which were independent of pump energy, indicating negligible nongeminate recombination of charge pairs. The measurements were performed using pump pulse energy densities of 80, 120, 160, and 500 nJ cm^–2 in the solar cells prepared with the PTB7:PC₇₁BM blending ratios of 40:60, 20:80, 10:90, and 90:10 by weight, respectively.

Results

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Photovoltaic Response

Figure 1 shows the current–voltage characteristics of the solar cells at the optimum blend ratio [P40:F60], prepared with and without DIO measured under AM1.5 illumination conditions. The device prepared with DIO shows about two times higher short-circuit current (J_sc) and fill factor (FF). The open circuit voltage (V_oc) does not depend on the solvent additive. The power conversion efficiency (PCE) of the device prepared with DIO was 5.4% at AM1.5 conditions. It is important to note that our device stack has identical active layer properties (blend ratio, solution concentration, spin-coating deposition parameters, thickness, deposition onto PEDOT) as the best reported devices but suffers from lower overall performance owing to nonoptimized contacts, giving us efficiencies comparable to those which others report with similar nonoptimized contacts. (5, 10, 12) Nonetheless, we still see the same dramatic improvement in overall device performance with the addition of DIO and can use this as a basis for understanding in this article how the changes in morphology that are known to be occurring influence the electronic properties of the device.

Figure 1. Current–voltage characteristics of the PTB7:PC₇₁BM [40:60] solar cells under 1 Sun illumination.

Charge Separation and Extraction Dynamics in Optimized Cells

Figure 2 shows the photoinduced voltage drop ΔU in the devices with 60 wt % of PC₇₁BM which have the highest power conversion efficiency. It is measured by TREFISH in the time range from 0 to 3 ns using ΔU = ΔF(t)L where ΔF(t) is a decrease of the electric field strength due to carrier displacement and L is the sample thickness. The long-time data are obtained from the integral-mode photocurrent. The combination of the two techniques enables us to monitor carrier drift kinetics from subpicoseconds to tens of microseconds, starting from their photogeneration up to extraction from the devices. The photoinduced voltage drop is proportional to the amount of extracted charge in a time interval t

(1)where C is the sample capacitance and I(t) is photocurrent. The voltage drop at long times is proportional to the total amount of extracted charge from the device. At 1000 ns, it is two times bigger in devices prepared with DIO, which agrees well with the approximately two times higher photocurrent observed in current–voltage characteristics at the short-circuit condition and at reverse bias (Figure 1). In order to more clearly compare the extraction kinetics, Figure 2 presents the voltage kinetics for the sample without DIO normalized to that for the sample with DIO. They show that charge extraction at the 3 V bias is only marginally faster from the cell prepared with DIO. The difference is slightly stronger at 0 V bias. This indicates that the extraction efficiency of free carriers with a strong internal field is not particularly sensitive to morphology and suggests that the increased short circuit photocurrent in devices prepared with DIO is mainly due to more efficient charge separation rather than extraction. Combining this with previous observations that the charge generation efficiency is similar in blends prepared with additive and without, (12) we conclude that the increase of photocurrent in devices prepared with DIO occurs mainly because of a higher dissociation efficiency of photogenerated charge pairs. The charge extraction at 0 V bias is faster in a device prepared with DIO as the time taken to extract a half of the charge decreases from 60 ns without DIO to 40 ns with DIO. In this case, the internal electric field in the cell is lower than at the short circuit condition because the total voltage drop on the load resistor comes to nearly a half of the built-in voltage. A faster charge extraction from DIO-prepared devices at low built-in field reduces recombination losses and can explain the higher fill factor of solar cells prepared with DIO.

Figure 2. Photoinduced voltage drop in devices with 60 wt % of PC₇₁BM at 3 V reverse bias (a) and at the built-in field only (b) as a function of time after the pump pulse. The results are derived from TREFISH measurements up to 3 ns and from the integral-mode photocurrent at longer times (note log scale after the axis break). Black curves correspond to devices without DIO, blue to devices with DIO. Black dotted curves show the kinetics from devices without DIO which are scaled to match the voltage drop in devices with DIO at late times. Pump density was 4 × 10¹¹ absorbed photons/cm² at the peak of polymer absorption (680 nm).

Influence of Internal Electric Field

In order to explore the influence of electric field, blend ratio, and the use of DIO on the charge separation efficiency, we studied the dependence of the amount of extracted charge in 1 μs on the applied external voltage (Figure 3). The amount of extracted charge from the cell with 10 wt % fullerene shows approximately linear dependence on the reverse bias with no saturation even at −4 V. At its maximum, ΔU from this cell is still about 20 times lower than from the cells with high fullerene content. This indicates that in the blend with low fullerene content the charge pairs are strongly bound and can only be separated by strong electric fields. It is interesting to note that the additive DIO decreases the amount of extracted charge from the polymer-rich blend, which is opposite to what is observed in the fullerene-rich blends, and the reduction factor is independent of the applied bias. In contrast, the amount of charge extracted from other three blends is much higher and shows a weak dependence on the bias between −1 and −4 V indicating that a weaker electric field is sufficient to drive charge separation in fullerene-rich blends. This result is consistent with previous observations of efficient free carrier generation in fullerene-rich blends (18, 19) and much higher initial electron mobility in the fullerene than the hole mobility in the electron donors observed by means of THz spectroscopy. (20) The amount of charge extracted from the blends with 80 and 90 wt % fullerene increases by about 30% with the increase of the negative bias between −1 and −4 V (Figure 3 c,d). We attribute this to carrier generation from excitons deep inside PC₇₁BM domains, a process which is assisted by an electric field. At high PC₇₁BM concentrations the absorption by fullerene at the excitation wavelength (680 nm) is comparable to that of the polymer and many excitons are generated deep inside the fullerene phase. The three-dimensional exciton diffusion length in PC₇₁BM can be estimated as

where D is exciton diffusivity and τ is the exciton lifetime. Using the reported D and τ values, (12) we estimate the exciton diffusion length of about 7 nm in the bulk PC₇₁BM. With polymer concentrations at only 10–20 wt % not all excitons created in fullerene domains reach the heterojunction. Charge carriers can be generated inside the fullerene domains with the assistance of an electric field as observed previously (21, 22) which can explain why the free carrier yield is bias-dependent. This suggests that carrier generation mechanism in the fullerene-rich blends could be different, depending if photon is absorbed by the polymer or by PC₇₁BM.

Figure 3. Photoinduced voltage drop at 1 μs from the integral-mode photocurrent measurements vs an applied external voltage in devices with different blend ratios for the excitation density of 4 × 10¹¹ absorbed photons/cm². Dotted black and blue curves correspond to the devices without and with DIO, respectively.

Effect of Blend Ratio and Solvent Additive on Carrier Mobility

In order to understand the role of electron and hole motion in charge separation and the effect of morphology we studied the time-dependence of carrier extraction from devices prepared with different blend ratios (Figure 4). The blends with high fullerene content show an enhancement of the amount of extracted charge when prepared with DIO. The enhancement factor is slightly lower than that observed in a blend with 60 wt % fullerene (Figure 2). All fullerene-rich blends show a fast extraction phase within 10 ns which is followed by a slow phase on the 10–100 ns time scale. The fast phase gets faster with increasing fullerene content. In the blend with 60 wt % fullerene a half of the mobile carriers are extracted in ∼10 ns at a 3 V reverse bias (Figure 2a), whereas in the blend with 80 wt % fullerene this happens in ∼2 ns (Figure 4b). Even faster carrier extraction is observed in the blend with 90 wt % fullerene, which is complete in ∼0.5 ns. This is similar to previous observations of very fast electron extraction from neat PC₆₁BM films. (21) In contrast, carrier extraction from the polymer-rich blend is much slower and occurs on a 100–1000 ns time scale. This trend allows us to assign the fast extraction phase in fullerene-rich blends to electrons and the slow phase to holes. The additive DIO enhances the electron extraction rate from the fullerene-rich blends, but slightly slows down carrier extraction from the polymer-rich blend.

Figure 4. Photoinduced voltage drop for the pump density of 4 × 10¹¹ absorbed photons/cm² in PTB7:PC₇₁BM devices with very low and high fullerene content at 3 V reverse bias. Black curves correspond to devices without DIO, blue to devices with DIO. Black dotted curves show the kinetics from devices without DIO which are scaled to match the voltage drop in devices with DIO at long times.

In order to evaluate the electron and hole mobilities and their dependence on the blend ratio, we have fitted the photoinduced voltage drop to a sum of integrated electron and hole photocurrents

(2)

Here, e is the electron and hole charge, F is the electric field strength, A is the area illuminated by the pump pulse, C is the cell capacitance, N_e,h(t) = N₀(1 – ⟨χ_e,h(t)⟩/L) are the average electron and hole concentrations in the cell with an account for carrier extraction, μ_e,h are their mobilities, N₀ is the density of generated charge pairs, and ⟨χ_e(t)⟩ and ⟨χ_h(t)⟩ are the average electron and hole drift distances ⟨χ_e,h(t)⟩ = F ∫₀^tμ_e,h(t) dt. The model assumes homogeneous carrier generation within the sample thickness. We approximated the electron and hole mobilities by functions μ_e,h(t) = μ_0e,ht^–α to account for their time dependences. A similar modeling procedure has been used in ref 18. Because of very low excitation densities, the losses to nongeminate carrier recombination are negligible. Figure 5 presents the fits to the voltage kinetics for blends without DIO as well as the electron and hole contributions to it. The simulations reproduce the experimental results well for the [P40:F60] and [P20:F80] samples, while the fits for the two extreme blend ratios are much worse. It is apparent that the power law functions cannot adequately describe the time dependences of the carrier mobilities at very low donor or acceptor concentrations. This is not surprising because these concentrations are at the percolation limit, and one type of carriers will inevitably encounter dead ends of charge transporting domains. (23) As Figures 2 and 4 show, normalized carrier motion dynamics in blends with and without DIO are very similar, giving very similar mobility values and kinetics; therefore, they are not presented and we will not specify it in discussion. The simulated electron and hole contributions to integrated current show that just about 50% of electrons are extracted in 1 μs from the blend with 10 wt % of fullerene (Figure 5a) and only about 2% of holes from the blend with 90 wt % of fullerene (Figure 5d). This indicates that the extraction of the rest of carriers takes much longer time than 1 μs in these blends with extreme ratios; thus, carrier extraction is strongly unbalanced. In solar cells with the optimal blend ratio of 60 wt % of fullerene electrons are extracted in ∼40 ns at a −3 V bias, whereas the hole extraction takes ∼200 ns. This indicates that carrier extraction is slightly unbalanced in optimized devices. The insets show the simulated electron and hole mobilities. In the blends with the fullerene content of 60, 80, and 90 wt %, the electron mobility in the first 10 ps after the pump pulse is >0.1 cm² V^–1 s^–1. High electron mobility drives fast dissociation of generated charge pairs into free carriers. The blend with 90 wt % of fullerene shows a weak μ_e ∝ t^–0.3 time dependence which can be explained by electron trapping at low energy sites. For comparison, the electron mobility in blends with 60 and 80 wt % fullerene shows a stronger time dependence μ_e ∝ t^–0.6, suggesting that there is another mechanism that slows down the electron mobility in addition to relaxation to low energy sites. It is natural to assume that the electron mobility decreases when electrons reach the boundaries of fullerene clusters and fullerene-rich domains. This assumption is supported by an about 10 times lower initial electron mobility in the blend with 10 wt % fullerene and its rapid decrease with time as μ_e ∝ t^–0.7, both showing that the electron transport is at a percolation limit in this blend. The highest initial hole mobility is observed in the optimized blend with 40 wt % of polymer and not in the blend with 90 wt % of polymer, indicating that the slow dissociation of bound electron–hole pairs limits the hole mobility in the polymer-rich blend. The time-dependence of hole mobility is very similar to that of electrons and follows μ_h ∝ t^–0.3 in the polymer-rich blend, which changes to μ_h ∝ t^–0.7 in the blend with 10 wt % of the polymer. Our results suggest comparable hole and electron mobilities of around 10^–4 cm² V^–1 s^–1 in the blends with 60 wt % fullerene at 100 ns. This is in good agreement with the previously reported electron and hole mobilities for space-charge-limited current in blends of PTB7 with the fullerene PC₆₁BM with the same blend ratio. (24) The same study has reported a sharp decrease of hole mobility when the fullerene concentration increased beyond 83 wt %, which also agrees with our results. This suggests that approximation of the time-dependent carrier mobility using the power law functions give a realistic separation of the integrated electron and hole photocurrents. Our results also suggest that the mobility of both types of carriers is controlled by low energy and spatial traps in the blends. The initial values of hole mobility, however, are not strongly dependent on the blend ratio. This can be explained by the faster hole transport along the conjugated polymer chains as compared to transport between chains. (25)

Figure 5. Modeling results of the carrier extraction kinetics from the cells prepared with different blend ratios without DIO at 3 V reverse bias. Black lines are the experimental data, green lines are the modeled kinetics, blue and red lines show electron and hole contributions, respectively. Insets show time-dependent electron (blue lines) and hole (red lines) mobility obtained from the fits. Shadows show the estimated error ranges.

Discussion

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Let us discuss our experimental findings by taking into consideration the results of previous studies of blend morphology and the influence of the solvent additive. In the most efficient blend of [P40:F60] with DIO, an optimum morphology of a finely interpenetrating network of PTB7 and PC₇₁BM is formed and the total amount of extracted charge is double that of a blend prepared without DIO. Here, we have been able to show that this enhancement is based on a higher dissociation efficiency of generated charge pairs into free carriers which is independent of the applied electric field. An internal quantum efficiency of >0.9 has been demonstrated in PTB7:PC₇₁BM solar cells prepared with DIO, (4-6, 12) implying that the free carrier generation efficiency is close to unity in optimized blends. Since the charge-generation efficiency was found to be similar in blends prepared with additive and without, (12) this indicates that about a half of generated charge pairs in the blends prepared without DIO never dissociate into free charge carriers. Previous studies of morphology and photophysics of PTB7:PC₇₁BM blends prepared without additive showed large pure fullerene clusters of 20–60 nm in size which were embedded in a polymer-rich phase with about 30 wt % of fullerene mixed in it. (10-13) Based on our observations of the pair dissociation efficiency being low in polymer-rich blends, we suggest that the geminate charge pairs generated between polymer and dispersed fullerene molecules in the polymer-rich phase never dissociate, as illustrated in Figure 6. This happens because the hole mobility in PTB7 is inherently low and fullerene molecules in the polymer-rich phase are too far from each other. PTB7:PC₇₁BM blends prepared using the additive DIO have been shown to consist of interpenetrating polymer-rich and fullerene-rich domains of tens of nanometers in size, and large pure domains were no longer observed. (10-12) These polymer- and fullerene-rich domains imply that there is a concentration gradient of donor and acceptor molecules between these domains. In such a morphology, charge separation can be thermodynamically driven as the entropy increases with carrier motion from the lower to higher concentration of acceptor molecules. For efficient charge separation all fullerene molecules have to be well connected to provide unperturbed electron motion. An insight into the development of a well-connected fullerene network in blends of PTB7 with a fullerene PC₆₁BM processed with DIO has been given by in situ measurements of the grazing incidence small-angle X-ray scattering during solvent evaporation. (26) These studies showed rapid formation of crystalline PTB7 aggregates driven by rapid evaporation of chlorobenzene but spontaneous phase separation was not observed presumably because of very slow evaporation of DIO which is a good solvent for fullerene molecules. Shrinkage of the fullerene domains in PTB7:fullerene blends with addition of DIO is different from DIO’s role in altering the morphology in other photovoltaic blends where it promotes phase separation of two intimately mixed materials. (27) Morphology optimization increases the free carrier yield, the short circuit current and the fill factor, whereas the open circuit voltage (V_OC) is not affected by it (Figure 1). These results are consistent with previous observations. (5, 10, 12) The contacts in these cells are nonselective (no charge blocking layers); therefore, V_OC is determined not only by the built-in potential and charge transport but is also reduced by surface recombination (extraction to wrong contacts by diffusion). We speculate that in our case the voltage loss to surface recombination is entirely determined by contacts which can explain why V_OC is not different.

Figure 6. Schematic of the proposed charge separation mechanism in PTB7:PC₇₁BM solar cells. According to previous morphology studies, the blends prepared without additive form pure fullerene clusters of 20–60 nm in size which are embedded in a polymer-rich phase with about 30 wt % fullerene mixed in it. (10-13) The charge pairs generated at the dispersed fullerene molecules do not dissociate because of low hole mobility, and only charge pairs generated at the fullerene clusters give photovoltaic response. The addition of DIO to the casting solvent improves the miscibility of PC₇₁BM with PTB7 and forms interpenetrating polymer-rich and fullerene-rich phases of tens of nanometers in size. (10, 12) Charge separation in these blends is driven by fast electron motion in the fullerene-rich phase. Small-scale phase separation also improves charge transport at low built-in field.

The increase of free carrier yield with DIO is also observed in blends with 80 and 90 wt % fullerene, but the effect is less pronounced presumably because the fullerene molecules are better connected at higher concentration even without DIO. We also found that DIO increases the carrier extraction velocity at weak electric field which can be explained by improved connectivity of nanostructured polymer-rich and fullerene-rich domains providing faster percolation channels for extraction of both types of carriers. The DIO enhances the performance of all fullerene-rich blends, but it reduces the amount of extracted charge from the polymer-rich blend. Even though the charge separation in the polymer-rich blend shows strong electric field dependence, the reduction factor of extracted charge with DIO is field-independent, suggesting that DIO changes the morphology of this nonoptimum polymer-rich blend, too.

Conclusions

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Transient photocurrent and ultrafast optical measurements of carrier drift enabled us to measure charge-separation dynamics and electron and hole extraction in PTB7:PC₇₁BM solar cells with different blend ratios. We find that the short circuit current is determined by the separation of charge pairs into free carriers. This separation is found to be efficient in fullerene-rich blends and inefficient in the polymer-rich blends, suggesting that high mobility of one type of carriers is essential for efficient charge separation. The free carrier yield is higher by a factor of 2 in devices prepared with DIO than without at the optimal blend ratio. Half of the generated charge pairs in the blends prepared without DIO never dissociate into free charge carriers, which we attribute to charge pairs generated in the polymer-rich domains with molecularly dispersed fullerene molecules. DIO only weakly influences the carrier mobility. The carrier extraction rate slightly increases with morphology optimization with DIO, but only at a low built-in electric fields. These findings explain the higher photocurrent and fill factor of devices prepared with DIO. Our results also show that carrier extraction is unbalanced in optimized devices with electron extraction being about 10 times faster than the hole extraction.

Author Information

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Corresponding Authors
- Ifor D. W. Samuel - Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, United Kingdom; Email: [email protected]
- Vidmantas Gulbinas - Center for Physical Sciences and Technology, Savanoriu Ave 231, 02300 Vilnius, Lithuania; Email: [email protected]
Authors
- Vytenis Pranculis - Center for Physical Sciences and Technology, Savanoriu Ave 231, 02300 Vilnius, Lithuania
- Arvydas Ruseckas - Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, United Kingdom
- Dimali A. Vithanage - Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, United Kingdom
- Gordon J. Hedley - Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, United Kingdom
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.

Acknowledgment

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This work was supported by the Research Council of Lithuania (Project No. MIP-85/2015), the Engineering and Physical Sciences Research Council of the UK (Grant Nos. EP/J009016/1 and EP/L012294/1), and the European Research Council of the European Union (Grant No. 321305). I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award. D.A.V. is grateful to Supergen SuperSolar Hub for the travel grant. The research data supporting this publication can be accessed at http://dx.doi.org/10.17630/530a6c5a-fdb4-4944-af17-855d2b90e29b.

References

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He, Zhicai; Xiao, Biao; Liu, Feng; Wu, Hongbin; Yang, Yali; Xiao, Steven; Wang, Cheng; Russell, Thomas P.; Cao, Yong
Nature Photonics (2015), 9 (3), 174-179CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
Polymer solar cells are an exciting class of next-generation photovoltaics, because they hold promise for the realization of mech. flexible, lightwt., large-area devices that can be fabricated by room-temp. soln. processing. High power conversion efficiencies of ∼10% have already been reported in tandem polymer solar cells. Here, we report that similar efficiencies are achievable in single-junction devices by reducing the tail state d. below the conduction band of the electron acceptor in a high-performance photoactive layer made from a newly developed semiconducting polymer with a deepened valence energy level. Control over band tailing is realized through changes in the compn. of the active layer and the structure order of the blend, both of which are known to be important factors in cell operation. The approach yields cells with high power conversion efficiencies (∼9.94% certified) and enhanced photovoltage.
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Ouyang, X.; Peng, R.; Ai, L.; Zhang, X.; Ge, Z. Efficient Polymer Solar Cells Employing a Non-Conjugated Small-Molecule Electrolyte Nat. Photonics 2015, 9, 520– 524 DOI: 10.1038/nphoton.2015.126
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Efficient polymer solar cells employing a non-conjugated small-molecule electrolyte
Ouyang, Xinhua; Peng, Ruixiang; Ai, Ling; Zhang, Xingye; Ge, Ziyi
Nature Photonics (2015), 9 (8), 520-524CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
Polymer solar cells have drawn a great deal of attention due to the attractiveness of their use in renewable energy sources that are potentially lightwt. and low in cost. Recently, numerous significant research efforts have resulted in polymer solar cells with power conversion efficiencies in excess of 9% (ref. 1). Nevertheless, further improvements in performance are sought for com. applications. Here, we report polymer solar cells with a power conversion efficiency of 10.02% that employ a non-conjugated small-mol. electrolyte as an interlayer. The material offers good contact for photogenerated charge carrier collection and allows optimum photon harvesting in the device. Furthermore, the enhanced performance is attributed to improved electron mobility, enhanced active-layer absorption and properly active-layer microstructures with optimal horizontal phase sepn. and vertical phase gradation. Our discovery opens a new avenue for single-junction devices by fully exploiting the potential of various material systems with efficiency over 10%.
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Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4% Adv. Mater. 2010, 22, E135– E138 DOI: 10.1002/adma.200903528
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For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%
Liang, Yongye; Xu, Zheng; Xia, Jiangbin; Tsai, Szu-Ting; Wu, Yue; Li, Gang; Ray, Claire; Yu, Luping
Advanced Materials (Weinheim, Germany) (2010), 22 (20), E135-E138CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
A new semiconducting polymer, PTB7, for BHJ solar cells was developed and a cell based on the blend films of PTB7 with PC71BM exhibited an efficiency up to 7.4 %, the first polymer solar cell system showing a PCE >7 %. Both EQE and IQE obtained from such solar cells are high, implying that the harvest of solar energy is efficient. The results confirm that the thieno[3,4-b]thiophene-benzodithiophene polymers are a promising class of materials for high-performance polymer solar cells.
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He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure Nat. Photonics 2012, 6, 593– 597 DOI: 10.1038/nphoton.2012.190
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Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure
He, Zhicai; Zhong, Chengmei; Su, Shijian; Xu, Miao; Wu, Hongbin; Cao, Yong
Nature Photonics (2012), 6 (9), 593-597CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
Polymer-fullerene bulk heterojunction solar cells (PSCs) are currently attracting a great deal of attention and gaining increasing importance, having already shown great promise as renewable, lightwt. and low-cost energy sources. Recently, the power-conversion efficiency of state-of-the-art PSCs has exceeded 8% in the scientific literature. However, to find viable applications for this emerging photovoltaic technol., further enhancements in the efficiency towards 10% (the threshold for com. applications) are urgently required. Here, we demonstrate highly efficient PSCs with a certified efficiency of 9.2% using an inverted structure, which simultaneously offers ohmic contact for photogenerated charge-carrier collection and allows optimum photon harvest in the device. Because of the ease of use and drastic boost in efficiency provided by this device structure, this discovery could find use in fully exploiting the potential of various material systems, and also open up new opportunities to improve PSCs with a view to achieving an efficiency of 10%.
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Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y. Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells Adv. Funct. Mater. 2008, 18, 1783– 1789 DOI: 10.1002/adfm.200701459
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Effects of solvent mixtures on the nanoscale phase separation in polymer solar cells
Yao, Yan; Hou, Jianhui; Xu, Zheng; Li, Gang; Yang, Yang
Advanced Functional Materials (2008), 18 (12), 1783-1789CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
The mixed solvent approach has been demonstrated as a promising method to modify nanomorphol. in polymer solar cells. This work aims to understand the unique role of the additive in the mixt. solvent and how the optimized nanoscale phase sepn. develops laterally and vertically during the non-equil. spin-coating process. We found the donor/acceptor components in the active layer can phase sep. into an optimum morphol. with the additive. Supported by AFM, TEM and XPS results, we proposed a model and identified relevant parameters for the additive such as soly. and vapor pressures. Other additives are discovered to show the ability to improve polymer solar cell performance as well.
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Zhang, F.; Jespersen, K. G.; Björström, C.; Svensson, M.; Andersson, M. R.; Sundström, V.; Magnusson, K.; Moons, E.; Yartsev, a.; Inganäs, O. Influence of Solvent Mixing on the Morphology and Performance of Solar Cells Based on Polyfluorene Copolymer/Fullerene Blends Adv. Funct. Mater. 2006, 16, 667– 674 DOI: 10.1002/adfm.200500339
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Influence of solvent mixing on the morphology and performance of solar cells based on polyfluorene copolymer/fullerene blends
Zhang, Fengling; Jespersen, Kim G.; Bjoerstroem, Cecilia; Svensson, Mattias; Andersson, Mats R.; Sundstroem, Villy; Magnusson, Kjell; Moons, Ellen; Yartsev, Arkady; Inganaes, Olle
Advanced Functional Materials (2006), 16 (5), 667-674CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
The influence of the solvent on the morphol. and performance of polymer solar cells was studied in devices based on blends of the polyfluorene copolymer, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2', 1',3'-benzothiadiazole)), and [6,6]-phenyl-C61-butyric acid Me ester. The blends are spin-coated from CHCl3 or from CHCl3 mixed with small amts. of xylene, toluene, or chlorobenzene. The devices are characterized under monochromatic light and AM1.5 solar illumination. An enhancement of the photocurrent d. was obsd. in diodes made from CHCl3 mixed with chlorobenzene, and a decreased photocurrent d. is obsd. in diodes made from CHCl3 mixed with xylene or toluene, compared to diodes made from neat CHCl3. The open-circuit voltages are almost the same for all diodes. The surfaces of the active layers were imaged with AFM. Height images indicate that a finer and more uniform distribution of domains corresponds to the diodes with enhanced photocurrent that are made from CHCl3 mixed with chlorobenzene, while a structure with larger domains is assocd. with the lower photocurrents in the diodes made from CHCl3 mixed with xylene or toluene. The influence of the morphol. on the excited-state dynamics and charge generation was studied using time-resolved spectroscopy. Fast formation of bound charge pairs followed by their conversion into free charge carriers was resolved, and excitation-intensity-dependent non-geminate recombination of free charges was obsd. A significant enhancement in free-charge-carrier generation was obsd. on introducing chlorobenzene into CHCl3. Imaging photocurrent generation from the solar cells with a light-pulse technique shows an inhomogeneous photocurrent distribution, which is related to the undulations in the thickness of the active layer. Thicker parts of the diodes yield higher photocurrent values.
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Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells J. Am. Chem. Soc. 2008, 130, 3619– 3623 DOI: 10.1021/ja710079w
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Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells
Lee, Jae Kwan; Ma, Wan Li; Brabec, Christoph J.; Yuen, Jonathan; Moon, Ji Sun; Kim, Jin Young; Lee, Kwanghee; Bazan, Guillermo C.; Heeger, Alan J.
Journal of the American Chemical Society (2008), 130 (11), 3619-3623CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Two criteria for processing additives introduced to control the morphol. of bulk heterojunction (BHJ) materials for use in solar cells were identified: (i) selective (differential) soly. of the fullerene component and (ii) higher b.p. than the host solvent. Using these criteria, the authors have studied the class of 1,8-di(R)octanes with various functional groups (R) as processing additives for BHJ solar cells. Control of the BHJ morphol. by selective soly. of the fullerene component is demonstrated using these high b.p. processing additives. The best results are obtained with R = Iodine (I). Using 1,8-diiodooctane as the processing additive, the efficiency of the BHJ solar cells was improved from 3.4% (for the ref. device) to 5.1%.
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Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C. R.; Ade, H. Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC 71 BM Solar Cells Adv. Energy Mater. 2013, 3, 65– 74 DOI: 10.1002/aenm.201200377
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Absolute measurement of domain composition and nanoscale size distribution explains performance in PTB7:PC71BM solar cells
Collins, Brian A.; Li, Zhe; Tumbleston, John R.; Gann, Eliot; McNeill, Christopher R.; Ade, Harald
Advanced Energy Materials (2013), 3 (1), 65-74CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
The importance of morphol. to org. solar cell performance is well known, but to date, the lack of quant., nanoscale and statistical morphol. information has hindered obtaining direct links to device function. Here resonant X-ray scattering and microscopy are combined to quant. measure the nanoscale domain size, distribution and compn. in high efficiency solar cells based on PTB7 and PC71BM. The results show that the solvent additive diiodooctane dramatically shrinks the domain size of pure fullerene agglomerates that are embedded in a polymer-rich 70/30 wt.% molecularly mixed matrix, while preserving the domain compn. relative to additive-free devices. The fundamental miscibility between the species - measured to be equal to the device's matrix compn. - is likely the dominant factor behind the overall morphol. with the additive affecting the dispersion of excess fullerene. As even the mol. ordering measured by X-ray diffraction is unchanged between the two processing routes the change in the distribution of domain size and therefore increased domain interface is primarily responsible for the dramatic increase in device performance. While fullerene exciton harvesting is clearly one significant cause of the increase owing to smaller domains, a measured increase in harvesting from the polymer species indicates that the mol. mixing is not the reason for the high efficiency in this system. Rather, excitations in the polymer likely require proximity to a pure fullerene phase for efficient charge sepn. and transport. Furthermore, in contrast to previous measurements on a PTB7-based system, a hierarchical morphol. was not obsd., indicating that it is not necessary for high performance.
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Hammond, M. R.; Kline, R. J.; Herzing, A. a; Richter, L. J.; Germack, D. S.; Ro, H.; Soles, C. L.; Fischer, D. a; Xu, T.; Yu, L. Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells ACS Nano 2011, 5, 8248– 8257 DOI: 10.1021/nn202951e
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Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells
Hammond, Matthew R.; Kline, R. Joseph; Herzing, Andrew A.; Richter, Lee J.; Germack, David S.; Ro, Hyun-Wook; Soles, Christopher L.; Fischer, Daniel A.; Xu, Tao; Yu, Luping; Toney, Michael F.; DeLongchamp, Dean M.
ACS Nano (2011), 5 (10), 8248-8257CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
The authors report quant. measurements of ordering, mol. orientation, and nanoscale morphol. in the active layer of bulk heterojunction (BHJ) org. photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which yields a high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid Me ester (PC71BM). A low degree of order was found in the polymer - far lower in the bulk heterojunction than in pure PTB7. XRD yielded a nearly full orientation distribution for the polymer π-stacking direction within well-ordered regions, revealing a moderate preference for π-stacking in the vertical direction (face-on). By combining mol. orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, the authors propose a model describing the PTB7 mol orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (≈20%) of the polymer in the PTB7/PC71BM blend is ordered. Energy-filtered TEM shows that the morphol. of PTB7/PC71BM is composed of nanoscale fullerene-rich aggregates sepd. by polymer-rich regions. The addn. of diiodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphol., relative to blend films cast without DIO.
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Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C. T.; Martins, E. R.; Serrano, L. a; Cooke, G.; Ruseckas, A.; Samuel, I. D. W. Determining the Optimum Morphology in High-Performance Polymer-Fullerene Organic Photovoltaic Cells Nat. Commun. 2013, 4, 2867 DOI: 10.1038/ncomms3867
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Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells
Hedley Gordon J; Ward Alexander J; Howells Calvyn T; Martins Emiliano R; Ruseckas Arvydas; Samuel Ifor D W; Alekseev Alexander; Serrano Luis A; Cooke Graeme
Nature communications (2013), 4 (), 2867 ISSN:.
The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC71BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10-50 nm wide and 200-400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.
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Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen, L. X. Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells J. Am. Chem. Soc. 2011, 133, 20661– 20663 DOI: 10.1021/ja2085564
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Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells
Lou, Sylvia J.; Szarko, Jodi M.; Xu, Tao; Yu, Luping; Marks, Tobin J.; Chen, Lin X.
Journal of the American Chemical Society (2011), 133 (51), 20661-20663CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Processing additives are used in org. photovoltaic systems to optimize the active layer film morphol. However, the actual mechanism is not well understood. Using X-ray scattering techniques, the effects are analyzed of an additive diiodooctane (DIO) on the aggregation of a high-efficiency donor polymer PTB7 and an acceptor mol. PC71BM under solar cell processing conditions. It is concluded that DIO selectively dissolves PC71BM aggregates, allowing their intercalation into PTB7 domains, thereby optimizing both the domain size and the PTB7-PC71BM interface.
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Yao, E.-P.; Chen, C.-C.; Gao, J.; Liu, Y.; Chen, Q.; Cai, M.; Hsu, W.-C.; Hong, Z.; Li, G.; Yang, Y. The Study of Solvent Additive Effects in Efficient Polymer Photovoltaics via Impedance Spectroscopy Sol. Energy Mater. Sol. Cells 2014, 130, 20– 26 DOI: 10.1016/j.solmat.2014.05.049
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The study of solvent additive effects in efficient polymer photovoltaics via impedance spectroscopy
Yao, En-Ping; Chen, Chun-Chao; Gao, Jing; Liu, Yongsheng; Chen, Qi; Cai, Min; Hsu, Wei-Chou; Hong, Ziruo; Li, Gang; Yang, Yang
Solar Energy Materials & Solar Cells (2014), 130 (), 20-26CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier B.V.)
1, 8-Diiodooctane (DIO) has been known for its role of improving the polymer morphol. and enhancing performance of polymer bulk heterojunction (BHJ) solar cell. In this work, the impedance spectroscopy was used to investigate the interface of poly(4,8-bis-alkyloxybenzo(1,2-b:4,5-b')dithiophene-2,6-diyl-alt- (alkyl thieno(3,4-b) thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C):PC70BM in BHJ with DIO as additive. Based on our results, we were able to simulate the device into an equiv. circuit model, which allows us to conveniently analyze the org./org. interfacial contact in the org. photovoltaic (OPV) device. Thus, we demonstrate that the impedance spectroscopy can an effective approach in characterizing the donor/acceptor interfaces, such that a direct correlation can be established between the morphol. and the device performance of BHJ devices.
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Wang, Z.; Zhang, F.; Li, L.; An, Q.; Wang, J.; Zhang, J. The Underlying Reason of DIO Additive on the Improvement Polymer Solar Cells Performance Appl. Surf. Sci. 2014, 305, 221– 226 DOI: 10.1016/j.apsusc.2014.03.041
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The underlying reason of DIO additive on the improvement polymer solar cells performance
Wang, Zixuan; Zhang, Fujun; Li, Lingliang; An, Qiaoshi; Wang, Jian; Zhang, Jian
Applied Surface Science (2014), 305 (), 221-226CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
The effect of 1,8-diiodooctane (DIO) on the performance enhancement of polymer solar cells (PSCs) based on [6,6]phenyl-C71-butyric acid Me ester (PC71BM) as the acceptor and poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b'](dithiophene)-2,6-di-yl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-di-yl](PBDTTT-C) as the donor was studied from the device physics researches. The short circuit c.d. (Jsc) was increased from 8.84 to 12.11 mA/cm2, fill factor (FF) was increased from 44.5% to 63.1%, resulting in power conversion efficiency (PCE) with 80% improvement from 2.95% to 5.35% by adding DIO with 3% vol. ratio. The enhancement of performance of PSCs could be mainly attributed to the improved charge carrier transport and increased optical field strength in the active layer by adding DIO additive.
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Chang, S.-Y.; Liao, H.-C.; Shao, Y.-T.; Sung, Y.-M.; Hsu, S.-H.; Ho, C.-C.; Su, W.-F.; Chen, Y.-F. Enhancing the Efficiency of Low Bandgap Conducting Polymer Bulk Heterojunction Solar Cells Using P3HT as a Morphology Control Agent J. Mater. Chem. A 2013, 1, 2447– 2452 DOI: 10.1039/c2ta00990k
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Enhancing the efficiency of low bandgap conducting polymer bulk heterojunction solar cells using P3HT as a morphology control agent
Chang, Sheng-Yung; Liao, Hsueh-Chung; Shao, Yu-Tsun; Sung, Yu-Ming; Hsu, Sheng-Hao; Ho, Chun-Chih; Su, Wei-Fang; Chen, Yang-Fang
Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (7), 2447-2452CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
The development of low bandgap conducting polymers has made bulk heterojunction solar cells a viable low-cost renewable energy source. The high b.p. of 1,8-diiodooctane (DIO) is usually used to control the morphol. of the active layer consisting of a conducting polymer and PCBM, so that a high power conversion solar cell can be achieved. The authors report here an alternative approach using non-volatile, cryst. and conducting P3HT as an effective morphol. control agent. A model system of PCPDTBT/PC61BM was selected for this study. The change of optoelectronic properties with the introduction of P3HT was monitored by measuring the absorption spectra and charge carrier mobility, and the morphol. change with the introduction of P3HT in the active layer was monitored by AFM, TEM, and GIXRD. Favorable bi-continuous phase sepn. and appropriate domain size of each phase can be achieved to facilitate fast charge transport, and thus improve the power conversion efficiency of the solar cell. By adding 1% P3HT into the blend of PCPDTBT/PCBM, the power conversion efficiency can be improved by 20%. Also, with the incorporation of 1% P3HT to the blend of PCPDTBT/PC61BM with DIO, the power conversion efficiency can be further increased by 17%. The strategy of this study can be expanded to other low bandgap conducting polymers for high efficiency bulk heterojunction solar cells.
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Devižis, A.; Serbenta, A.; Meerholz, K.; Hertel, D.; Gulbinas, V. Ultrafast Dynamics of Carrier Mobility in a Conjugated Polymer Probed at Molecular and Microscopic Length Scales Phys. Rev. Lett. 2009, 103, 027404 DOI: 10.1103/PhysRevLett.103.027404
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Ultrafast Dynamics of Carrier Mobility in a Conjugated Polymer Probed at Molecular and Microscopic Length Scales
Devizis, A.; Serbenta, A.; Meerholz, K.; Hertel, D.; Gulbinas, V.
Physical Review Letters (2009), 103 (2), 027404/1-027404/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
The authors used time-resolved elec.-field-induced 2nd-harmonic generation to probe the charge-carrier-mobility dynamics in amorphous org. materials on an ultrafast time scale. The authors were able to show that the mobility in poly-spiro-bifluorene-co-benzothiadiazol decreases from 0.1 cm2/V s at 1 ps to 10-6 cm2/V s within 1 μs. The authors attribute this dramatic decrease to the relaxation of the charge carriers within the d. of states, clearly demonstrating the impact of disorder on the nanoscale charge transport in amorphous semiconductors.
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Pranculis, V.; Infahsaeng, Y.; Tang, Z.; Devižis, A.; Vithanage, D. A.; Ponseca, C. S.; Inganäs, O.; Yartsev, A. P.; Gulbinas, V.; Sundström, V. Charge Carrier Generation and Transport in Different Stoichiometry APFO3:PC61BM Solar Cells J. Am. Chem. Soc. 2014, 136, 11331– 11338 DOI: 10.1021/ja503301m
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Charge Carrier Generation and Transport in Different Stoichiometry APFO3:PC61BM Solar Cells
Pranculis, Vytenis; Infahsaeng, Yingyot; Tang, Zheng; Devizis, Andrius; Vithanage, Dimali A.; Ponseca, Carlito S.; Inganas, Olle; Yartsev, Arkady P.; Gulbinas, Vidmantas; Sundstrom, Villy
Journal of the American Chemical Society (2014), 136 (32), 11331-11338CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
In this paper we studied carrier drift dynamics in APFO3:PC61BM solar cells of varied stoichiometry (2:1, 1:1, and 1:4 APFO3:PC61BM) over a wide time range, from subpicoseconds to microseconds with a combination of ultrafast optical elec. field probing and conventional transient integrated photocurrent techniques. Carrier drift and extn. dynamics are strongly stoichiometry dependent: the speed of electron or hole drift increases with higher concn. of PC61BM or polymer, resp. The electron extn. from a sample with 80% PC61BM takes place during hundreds of picoseconds, but slows down to sub-microseconds in a sample with 33% PC61BM. The hole extn. is less stoichiometry dependent: it varies form sub-nanoseconds to tens of nanoseconds when the PC61BM concn. changes from 33% to 80%. The electron extn. rate correlates with the conversion efficiency of solar cells, leading to the conclusion that fast electron motion is essential for efficient charge carrier sepn. preventing their geminate recombination.
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Gelinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes Science (Washington, DC, U. S.) 2014, 343, 512– 516 DOI: 10.1126/science.1246249
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Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes
Gelinas, Simon; Rao, Akshay; Kumar, Abhishek; Smith, Samuel L.; Chin, Alex W.; Clark, Jenny; van der Poll, Tom S.; Bazan, Guillermo C.; Friend, Richard H.
Science (Washington, DC, United States) (2014), 343 (6170), 512-516CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
Understanding the charge-sepn. mechanism in org. photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here the authors report the time dependence of the sepn. of photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the elec. field generated between the charges, the authors measure ~200 millielectron volts of electrostatic energy arising from electron-hole sepn. within 40 fs of excitation, corresponding to a charge sepn. distance of at least 4 nm. At this sepn., the residual Coulomb attraction between charges is at or below thermal energies, so that electron and hole sep. freely. This early time behavior is consistent with charge sepn. through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.
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Lane, P. A.; Cunningham, P. D.; Melinger, J. S.; Esenturk, O.; Heilweil, E. J. Hot Photocarrier Dynamics in Organic Solar Cells Nat. Commun. 2015, 6, 7558 DOI: 10.1038/ncomms8558
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Hot photocarrier dynamics in organic solar cells
Lane, P. A.; Cunningham, P. D.; Melinger, J. S.; Esenturk, O.; Heilweil, E. J.
Nature Communications (2015), 6 (), 7558CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
Photocurrent in an org. solar cell is generated by a charge transfer reaction between electron donors and acceptors. Charge transfer is expected to proceed from thermalized states, but this picture has been challenged by recent studies that have investigated the role of hot excitons. Here we show a direct link between excess excitation energy and photocarrier mobility. Charge transfer from excited donor mols. generates hot photocarriers with excess energy coming from the offset between the LUMO of the donor and that of the acceptor. Hot photocarriers manifest themselves through a short-lived spike in terahertz photocond. that decays on a picosecond timescale as carriers thermalize. Different dynamics are obsd. when exciting the acceptor at its absorption edge to a thermalized state. Charge transfer in this case generates thermalized carriers described by terahertz photocond. dynamics consisting of an instrument-limited rise to a long-lived signal.
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Burkhard, G. F.; Hoke, E. T.; Beiley, Z. M.; Mcgehee, M. D. Free Carrier Generation in Fullerene Acceptors and Its Effect on Polymer Photovoltaics J. Phys. Chem. C 2012, 116, 26674– 26678 DOI: 10.1021/jp310821f
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Free carrier generation in fullerene acceptors and effect on polymer photovoltaics
Burkhard, George F.; Hoke, Eric T.; Beiley, Zach M.; McGehee, Michael D.
Journal of Physical Chemistry C (2012), 116 (50), 26674-26678CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Early research on C60 led to the discovery that the absorption of photons with energy greater than 2.35 eV by bulk C60 produces free charge carriers at room temp. We find that not only is this also true for many of the sol. fullerene derivs. commonly used in org. photovoltaics, but also that the presence of these free carriers has significant implications for the modeling, characterization, and performance of devices made with these materials. We demonstrate that the discrepancy between absorption and quantum efficiency spectra in P3HT:PCBM is due to recombination of such free carriers in large PCBM domains before they can be sepd. at a donor/acceptor interface. Since most theories assume that all free charges result from the sepn. of excitons at a donor/acceptor interface, the presence of free carrier generation in fullerenes can have a significant impact on the interpretation of data generated by numerous field-dependent techniques.
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Devižis, A.; Hertel, D.; Meerholz, K.; Gulbinas, V.; Moser, J.-E. Time-Independent, High Electron Mobility in Thin PC61BM Films: Relevance to Organic Photovoltaics Org. Electron. 2014, 15, 3729– 3734 DOI: 10.1016/j.orgel.2014.10.028
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Devizis, A.; Hertel, D.; Meerholz, K.; Gulbinas, V.; Moser, J.-E.
Organic Electronics (2014), 15 (12), 3729-3734CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
Ultrafast optical probing of elec. field by means of electroabsorption combined with conventional photocurrent measurements was employed to investigate the drift and mobility dynamics of photo-generated charge carriers in the pristine PC61BM film and in the blend with a merocyanine dye. Electrons passed a 40 nm thick PC61BM film within a few picoseconds with time-independent and weakly dispersive mobility. The electron mobility is 1 cm2/(V s) at 1 MV/cm and an est. of the zero-field mobility yields 5 · 10-2 cm2/(V s). The initial electron mobility in the blend is of the order of 10-2 cm2/(V s) and decreases rapidly. We conclude that electron motion in PC61BM based org. bulk hetero-junction solar cells is limited by barriers between PC61BM domains rather than by intrinsic PC61BM properties.
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Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins, B. a.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H. The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells Adv. Energy Mater. 2013, 3, 364– 374 DOI: 10.1002/aenm.201200637
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Advanced Energy Materials (2013), 3 (3), 364-374CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
Most optimized donor-acceptor (D-A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ∼80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-co-octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) and [6,6]-phenyl-C61-butyric acid Me ester (PCBM) with 7.3% PCE and find that bimol. recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimol. recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphol. of these BHJs before and after thermal annealing and det. that thermal annealing drastically reduces the concn. of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concn. of PCBM may reduce the no. of percolating electron transport pathways within these mixed regions and create morphol. electron traps that enhance charge-carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concn. of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimol. recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.
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Chemical Physics Letters (2010), 498 (4-6), 302-306CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)
Motion of photogenerated charge carriers in π-conjugated polymer films with different disorder and chain orientation and also in a blend of conducting and insulating polymers has been investigated by means of time-resolved elec. field-induced second harmonic generation technique. Exptl. results and Monte Carlo simulation enabled us to distinguish three time domains of charge transport. The charge carriers become sepd. by about 10 nm in a strong elec. field within 1 ps. Subsequently, carriers drift another 10-15 nm on a picosecond time scale with a high mobility. The third and the slowest carrier motion phase is well described by the stochastic drift and dets. the macroscopic equil. mobility. We attribute the two ultrafast drift phases to carrier motion inside a conjugated segment and along a single polymer chain, resp., while the slow motion phase involves interchain jumps.
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Liu, F.; Zhao, W.; Tumbleston, J. R.; Wang, C.; Gu, Y.; Wang, D.; Briseno, A. L.; Ade, H.; Russell, T. P. Understanding the Morphology of PTB7:PCBM Blends in Organic Photovoltaics Adv. Energy Mater. 2014, 4, 1– 9 DOI: 10.1002/aenm.201301377
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Albrecht, S.; Schindler, W.; Kurpiers, J.; Kniepert, J.; Blakesley, J. C.; Dumsch, I.; Allard, S.; Fostiropoulos, K.; Scherf, U.; Neher, D. On the Field Dependence of Free Charge Carrier Generation and Recombination in Blends of PCPDTBT/PC70BM: Influence of Solvent Additives J. Phys. Chem. Lett. 2012, 3, 640– 645 DOI: 10.1021/jz3000849
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The authors have applied time-delayed collection field (TDCF) and charge extn. by linearly increasing voltage (CELIV) to study the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. Probably the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimol. recombination coeffs., which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a neg. field dependence of mobility, which the authors suggest to cause bias-dependent recombination coeffs.
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Abstract
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Figure 1
Figure 1. Current–voltage characteristics of the PTB7:PC₇₁BM [40:60] solar cells under 1 Sun illumination.
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Figure 2
Figure 2. Photoinduced voltage drop in devices with 60 wt % of PC₇₁BM at 3 V reverse bias (a) and at the built-in field only (b) as a function of time after the pump pulse. The results are derived from TREFISH measurements up to 3 ns and from the integral-mode photocurrent at longer times (note log scale after the axis break). Black curves correspond to devices without DIO, blue to devices with DIO. Black dotted curves show the kinetics from devices without DIO which are scaled to match the voltage drop in devices with DIO at late times. Pump density was 4 × 10¹¹ absorbed photons/cm² at the peak of polymer absorption (680 nm).
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Figure 3
Figure 3. Photoinduced voltage drop at 1 μs from the integral-mode photocurrent measurements vs an applied external voltage in devices with different blend ratios for the excitation density of 4 × 10¹¹ absorbed photons/cm². Dotted black and blue curves correspond to the devices without and with DIO, respectively.
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Figure 4
Figure 4. Photoinduced voltage drop for the pump density of 4 × 10¹¹ absorbed photons/cm² in PTB7:PC₇₁BM devices with very low and high fullerene content at 3 V reverse bias. Black curves correspond to devices without DIO, blue to devices with DIO. Black dotted curves show the kinetics from devices without DIO which are scaled to match the voltage drop in devices with DIO at long times.
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Figure 5
Figure 5. Modeling results of the carrier extraction kinetics from the cells prepared with different blend ratios without DIO at 3 V reverse bias. Black lines are the experimental data, green lines are the modeled kinetics, blue and red lines show electron and hole contributions, respectively. Insets show time-dependent electron (blue lines) and hole (red lines) mobility obtained from the fits. Shadows show the estimated error ranges.
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Figure 6
Figure 6. Schematic of the proposed charge separation mechanism in PTB7:PC₇₁BM solar cells. According to previous morphology studies, the blends prepared without additive form pure fullerene clusters of 20–60 nm in size which are embedded in a polymer-rich phase with about 30 wt % fullerene mixed in it. (10-13) The charge pairs generated at the dispersed fullerene molecules do not dissociate because of low hole mobility, and only charge pairs generated at the fullerene clusters give photovoltaic response. The addition of DIO to the casting solvent improves the miscibility of PC₇₁BM with PTB7 and forms interpenetrating polymer-rich and fullerene-rich phases of tens of nanometers in size. (10, 12) Charge separation in these blends is driven by fast electron motion in the fullerene-rich phase. Small-scale phase separation also improves charge transport at low built-in field.
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References
This article references 27 other publications.
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  Single-junction polymer solar cells with high efficiency and photovoltage
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  Polymer solar cells are an exciting class of next-generation photovoltaics, because they hold promise for the realization of mech. flexible, lightwt., large-area devices that can be fabricated by room-temp. soln. processing. High power conversion efficiencies of ∼10% have already been reported in tandem polymer solar cells. Here, we report that similar efficiencies are achievable in single-junction devices by reducing the tail state d. below the conduction band of the electron acceptor in a high-performance photoactive layer made from a newly developed semiconducting polymer with a deepened valence energy level. Control over band tailing is realized through changes in the compn. of the active layer and the structure order of the blend, both of which are known to be important factors in cell operation. The approach yields cells with high power conversion efficiencies (∼9.94% certified) and enhanced photovoltage.
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  4
  Efficient polymer solar cells employing a non-conjugated small-molecule electrolyte
  Ouyang, Xinhua; Peng, Ruixiang; Ai, Ling; Zhang, Xingye; Ge, Ziyi
  Nature Photonics (2015), 9 (8), 520-524CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
  Polymer solar cells have drawn a great deal of attention due to the attractiveness of their use in renewable energy sources that are potentially lightwt. and low in cost. Recently, numerous significant research efforts have resulted in polymer solar cells with power conversion efficiencies in excess of 9% (ref. 1). Nevertheless, further improvements in performance are sought for com. applications. Here, we report polymer solar cells with a power conversion efficiency of 10.02% that employ a non-conjugated small-mol. electrolyte as an interlayer. The material offers good contact for photogenerated charge carrier collection and allows optimum photon harvesting in the device. Furthermore, the enhanced performance is attributed to improved electron mobility, enhanced active-layer absorption and properly active-layer microstructures with optimal horizontal phase sepn. and vertical phase gradation. Our discovery opens a new avenue for single-junction devices by fully exploiting the potential of various material systems with efficiency over 10%.
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5. 5
  Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4% Adv. Mater. 2010, 22, E135– E138 DOI: 10.1002/adma.200903528
  5
  For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%
  Liang, Yongye; Xu, Zheng; Xia, Jiangbin; Tsai, Szu-Ting; Wu, Yue; Li, Gang; Ray, Claire; Yu, Luping
  Advanced Materials (Weinheim, Germany) (2010), 22 (20), E135-E138CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A new semiconducting polymer, PTB7, for BHJ solar cells was developed and a cell based on the blend films of PTB7 with PC71BM exhibited an efficiency up to 7.4 %, the first polymer solar cell system showing a PCE >7 %. Both EQE and IQE obtained from such solar cells are high, implying that the harvest of solar energy is efficient. The results confirm that the thieno[3,4-b]thiophene-benzodithiophene polymers are a promising class of materials for high-performance polymer solar cells.
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6. 6
  He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure Nat. Photonics 2012, 6, 593– 597 DOI: 10.1038/nphoton.2012.190
  6
  Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure
  He, Zhicai; Zhong, Chengmei; Su, Shijian; Xu, Miao; Wu, Hongbin; Cao, Yong
  Nature Photonics (2012), 6 (9), 593-597CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
  Polymer-fullerene bulk heterojunction solar cells (PSCs) are currently attracting a great deal of attention and gaining increasing importance, having already shown great promise as renewable, lightwt. and low-cost energy sources. Recently, the power-conversion efficiency of state-of-the-art PSCs has exceeded 8% in the scientific literature. However, to find viable applications for this emerging photovoltaic technol., further enhancements in the efficiency towards 10% (the threshold for com. applications) are urgently required. Here, we demonstrate highly efficient PSCs with a certified efficiency of 9.2% using an inverted structure, which simultaneously offers ohmic contact for photogenerated charge-carrier collection and allows optimum photon harvest in the device. Because of the ease of use and drastic boost in efficiency provided by this device structure, this discovery could find use in fully exploiting the potential of various material systems, and also open up new opportunities to improve PSCs with a view to achieving an efficiency of 10%.
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7. 7
  Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y. Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells Adv. Funct. Mater. 2008, 18, 1783– 1789 DOI: 10.1002/adfm.200701459
  7
  Effects of solvent mixtures on the nanoscale phase separation in polymer solar cells
  Yao, Yan; Hou, Jianhui; Xu, Zheng; Li, Gang; Yang, Yang
  Advanced Functional Materials (2008), 18 (12), 1783-1789CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The mixed solvent approach has been demonstrated as a promising method to modify nanomorphol. in polymer solar cells. This work aims to understand the unique role of the additive in the mixt. solvent and how the optimized nanoscale phase sepn. develops laterally and vertically during the non-equil. spin-coating process. We found the donor/acceptor components in the active layer can phase sep. into an optimum morphol. with the additive. Supported by AFM, TEM and XPS results, we proposed a model and identified relevant parameters for the additive such as soly. and vapor pressures. Other additives are discovered to show the ability to improve polymer solar cell performance as well.
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8. 8
  Zhang, F.; Jespersen, K. G.; Björström, C.; Svensson, M.; Andersson, M. R.; Sundström, V.; Magnusson, K.; Moons, E.; Yartsev, a.; Inganäs, O. Influence of Solvent Mixing on the Morphology and Performance of Solar Cells Based on Polyfluorene Copolymer/Fullerene Blends Adv. Funct. Mater. 2006, 16, 667– 674 DOI: 10.1002/adfm.200500339
  8
  Influence of solvent mixing on the morphology and performance of solar cells based on polyfluorene copolymer/fullerene blends
  Zhang, Fengling; Jespersen, Kim G.; Bjoerstroem, Cecilia; Svensson, Mattias; Andersson, Mats R.; Sundstroem, Villy; Magnusson, Kjell; Moons, Ellen; Yartsev, Arkady; Inganaes, Olle
  Advanced Functional Materials (2006), 16 (5), 667-674CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The influence of the solvent on the morphol. and performance of polymer solar cells was studied in devices based on blends of the polyfluorene copolymer, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2', 1',3'-benzothiadiazole)), and [6,6]-phenyl-C61-butyric acid Me ester. The blends are spin-coated from CHCl3 or from CHCl3 mixed with small amts. of xylene, toluene, or chlorobenzene. The devices are characterized under monochromatic light and AM1.5 solar illumination. An enhancement of the photocurrent d. was obsd. in diodes made from CHCl3 mixed with chlorobenzene, and a decreased photocurrent d. is obsd. in diodes made from CHCl3 mixed with xylene or toluene, compared to diodes made from neat CHCl3. The open-circuit voltages are almost the same for all diodes. The surfaces of the active layers were imaged with AFM. Height images indicate that a finer and more uniform distribution of domains corresponds to the diodes with enhanced photocurrent that are made from CHCl3 mixed with chlorobenzene, while a structure with larger domains is assocd. with the lower photocurrents in the diodes made from CHCl3 mixed with xylene or toluene. The influence of the morphol. on the excited-state dynamics and charge generation was studied using time-resolved spectroscopy. Fast formation of bound charge pairs followed by their conversion into free charge carriers was resolved, and excitation-intensity-dependent non-geminate recombination of free charges was obsd. A significant enhancement in free-charge-carrier generation was obsd. on introducing chlorobenzene into CHCl3. Imaging photocurrent generation from the solar cells with a light-pulse technique shows an inhomogeneous photocurrent distribution, which is related to the undulations in the thickness of the active layer. Thicker parts of the diodes yield higher photocurrent values.
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9. 9
  Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells J. Am. Chem. Soc. 2008, 130, 3619– 3623 DOI: 10.1021/ja710079w
  9
  Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells
  Lee, Jae Kwan; Ma, Wan Li; Brabec, Christoph J.; Yuen, Jonathan; Moon, Ji Sun; Kim, Jin Young; Lee, Kwanghee; Bazan, Guillermo C.; Heeger, Alan J.
  Journal of the American Chemical Society (2008), 130 (11), 3619-3623CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Two criteria for processing additives introduced to control the morphol. of bulk heterojunction (BHJ) materials for use in solar cells were identified: (i) selective (differential) soly. of the fullerene component and (ii) higher b.p. than the host solvent. Using these criteria, the authors have studied the class of 1,8-di(R)octanes with various functional groups (R) as processing additives for BHJ solar cells. Control of the BHJ morphol. by selective soly. of the fullerene component is demonstrated using these high b.p. processing additives. The best results are obtained with R = Iodine (I). Using 1,8-diiodooctane as the processing additive, the efficiency of the BHJ solar cells was improved from 3.4% (for the ref. device) to 5.1%.
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10. 10
  Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C. R.; Ade, H. Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC 71 BM Solar Cells Adv. Energy Mater. 2013, 3, 65– 74 DOI: 10.1002/aenm.201200377
  10
  Absolute measurement of domain composition and nanoscale size distribution explains performance in PTB7:PC71BM solar cells
  Collins, Brian A.; Li, Zhe; Tumbleston, John R.; Gann, Eliot; McNeill, Christopher R.; Ade, Harald
  Advanced Energy Materials (2013), 3 (1), 65-74CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
  The importance of morphol. to org. solar cell performance is well known, but to date, the lack of quant., nanoscale and statistical morphol. information has hindered obtaining direct links to device function. Here resonant X-ray scattering and microscopy are combined to quant. measure the nanoscale domain size, distribution and compn. in high efficiency solar cells based on PTB7 and PC71BM. The results show that the solvent additive diiodooctane dramatically shrinks the domain size of pure fullerene agglomerates that are embedded in a polymer-rich 70/30 wt.% molecularly mixed matrix, while preserving the domain compn. relative to additive-free devices. The fundamental miscibility between the species - measured to be equal to the device's matrix compn. - is likely the dominant factor behind the overall morphol. with the additive affecting the dispersion of excess fullerene. As even the mol. ordering measured by X-ray diffraction is unchanged between the two processing routes the change in the distribution of domain size and therefore increased domain interface is primarily responsible for the dramatic increase in device performance. While fullerene exciton harvesting is clearly one significant cause of the increase owing to smaller domains, a measured increase in harvesting from the polymer species indicates that the mol. mixing is not the reason for the high efficiency in this system. Rather, excitations in the polymer likely require proximity to a pure fullerene phase for efficient charge sepn. and transport. Furthermore, in contrast to previous measurements on a PTB7-based system, a hierarchical morphol. was not obsd., indicating that it is not necessary for high performance.
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11. 11
  Hammond, M. R.; Kline, R. J.; Herzing, A. a; Richter, L. J.; Germack, D. S.; Ro, H.; Soles, C. L.; Fischer, D. a; Xu, T.; Yu, L. Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells ACS Nano 2011, 5, 8248– 8257 DOI: 10.1021/nn202951e
  11
  Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells
  Hammond, Matthew R.; Kline, R. Joseph; Herzing, Andrew A.; Richter, Lee J.; Germack, David S.; Ro, Hyun-Wook; Soles, Christopher L.; Fischer, Daniel A.; Xu, Tao; Yu, Luping; Toney, Michael F.; DeLongchamp, Dean M.
  ACS Nano (2011), 5 (10), 8248-8257CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  The authors report quant. measurements of ordering, mol. orientation, and nanoscale morphol. in the active layer of bulk heterojunction (BHJ) org. photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which yields a high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid Me ester (PC71BM). A low degree of order was found in the polymer - far lower in the bulk heterojunction than in pure PTB7. XRD yielded a nearly full orientation distribution for the polymer π-stacking direction within well-ordered regions, revealing a moderate preference for π-stacking in the vertical direction (face-on). By combining mol. orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, the authors propose a model describing the PTB7 mol orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (≈20%) of the polymer in the PTB7/PC71BM blend is ordered. Energy-filtered TEM shows that the morphol. of PTB7/PC71BM is composed of nanoscale fullerene-rich aggregates sepd. by polymer-rich regions. The addn. of diiodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphol., relative to blend films cast without DIO.
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12. 12
  Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C. T.; Martins, E. R.; Serrano, L. a; Cooke, G.; Ruseckas, A.; Samuel, I. D. W. Determining the Optimum Morphology in High-Performance Polymer-Fullerene Organic Photovoltaic Cells Nat. Commun. 2013, 4, 2867 DOI: 10.1038/ncomms3867
  12
  Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells
  Hedley Gordon J; Ward Alexander J; Howells Calvyn T; Martins Emiliano R; Ruseckas Arvydas; Samuel Ifor D W; Alekseev Alexander; Serrano Luis A; Cooke Graeme
  Nature communications (2013), 4 (), 2867 ISSN:.
  The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC71BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10-50 nm wide and 200-400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.
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13. 13
  Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen, L. X. Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells J. Am. Chem. Soc. 2011, 133, 20661– 20663 DOI: 10.1021/ja2085564
  13
  Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells
  Lou, Sylvia J.; Szarko, Jodi M.; Xu, Tao; Yu, Luping; Marks, Tobin J.; Chen, Lin X.
  Journal of the American Chemical Society (2011), 133 (51), 20661-20663CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Processing additives are used in org. photovoltaic systems to optimize the active layer film morphol. However, the actual mechanism is not well understood. Using X-ray scattering techniques, the effects are analyzed of an additive diiodooctane (DIO) on the aggregation of a high-efficiency donor polymer PTB7 and an acceptor mol. PC71BM under solar cell processing conditions. It is concluded that DIO selectively dissolves PC71BM aggregates, allowing their intercalation into PTB7 domains, thereby optimizing both the domain size and the PTB7-PC71BM interface.
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14. 14
  Yao, E.-P.; Chen, C.-C.; Gao, J.; Liu, Y.; Chen, Q.; Cai, M.; Hsu, W.-C.; Hong, Z.; Li, G.; Yang, Y. The Study of Solvent Additive Effects in Efficient Polymer Photovoltaics via Impedance Spectroscopy Sol. Energy Mater. Sol. Cells 2014, 130, 20– 26 DOI: 10.1016/j.solmat.2014.05.049
  14
  The study of solvent additive effects in efficient polymer photovoltaics via impedance spectroscopy
  Yao, En-Ping; Chen, Chun-Chao; Gao, Jing; Liu, Yongsheng; Chen, Qi; Cai, Min; Hsu, Wei-Chou; Hong, Ziruo; Li, Gang; Yang, Yang
  Solar Energy Materials & Solar Cells (2014), 130 (), 20-26CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier B.V.)
  1, 8-Diiodooctane (DIO) has been known for its role of improving the polymer morphol. and enhancing performance of polymer bulk heterojunction (BHJ) solar cell. In this work, the impedance spectroscopy was used to investigate the interface of poly(4,8-bis-alkyloxybenzo(1,2-b:4,5-b')dithiophene-2,6-diyl-alt- (alkyl thieno(3,4-b) thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C):PC70BM in BHJ with DIO as additive. Based on our results, we were able to simulate the device into an equiv. circuit model, which allows us to conveniently analyze the org./org. interfacial contact in the org. photovoltaic (OPV) device. Thus, we demonstrate that the impedance spectroscopy can an effective approach in characterizing the donor/acceptor interfaces, such that a direct correlation can be established between the morphol. and the device performance of BHJ devices.
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15. 15
  Wang, Z.; Zhang, F.; Li, L.; An, Q.; Wang, J.; Zhang, J. The Underlying Reason of DIO Additive on the Improvement Polymer Solar Cells Performance Appl. Surf. Sci. 2014, 305, 221– 226 DOI: 10.1016/j.apsusc.2014.03.041
  15
  The underlying reason of DIO additive on the improvement polymer solar cells performance
  Wang, Zixuan; Zhang, Fujun; Li, Lingliang; An, Qiaoshi; Wang, Jian; Zhang, Jian
  Applied Surface Science (2014), 305 (), 221-226CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
  The effect of 1,8-diiodooctane (DIO) on the performance enhancement of polymer solar cells (PSCs) based on [6,6]phenyl-C71-butyric acid Me ester (PC71BM) as the acceptor and poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b'](dithiophene)-2,6-di-yl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-di-yl](PBDTTT-C) as the donor was studied from the device physics researches. The short circuit c.d. (Jsc) was increased from 8.84 to 12.11 mA/cm2, fill factor (FF) was increased from 44.5% to 63.1%, resulting in power conversion efficiency (PCE) with 80% improvement from 2.95% to 5.35% by adding DIO with 3% vol. ratio. The enhancement of performance of PSCs could be mainly attributed to the improved charge carrier transport and increased optical field strength in the active layer by adding DIO additive.
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16. 16
  Chang, S.-Y.; Liao, H.-C.; Shao, Y.-T.; Sung, Y.-M.; Hsu, S.-H.; Ho, C.-C.; Su, W.-F.; Chen, Y.-F. Enhancing the Efficiency of Low Bandgap Conducting Polymer Bulk Heterojunction Solar Cells Using P3HT as a Morphology Control Agent J. Mater. Chem. A 2013, 1, 2447– 2452 DOI: 10.1039/c2ta00990k
  16
  Enhancing the efficiency of low bandgap conducting polymer bulk heterojunction solar cells using P3HT as a morphology control agent
  Chang, Sheng-Yung; Liao, Hsueh-Chung; Shao, Yu-Tsun; Sung, Yu-Ming; Hsu, Sheng-Hao; Ho, Chun-Chih; Su, Wei-Fang; Chen, Yang-Fang
  Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (7), 2447-2452CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
  The development of low bandgap conducting polymers has made bulk heterojunction solar cells a viable low-cost renewable energy source. The high b.p. of 1,8-diiodooctane (DIO) is usually used to control the morphol. of the active layer consisting of a conducting polymer and PCBM, so that a high power conversion solar cell can be achieved. The authors report here an alternative approach using non-volatile, cryst. and conducting P3HT as an effective morphol. control agent. A model system of PCPDTBT/PC61BM was selected for this study. The change of optoelectronic properties with the introduction of P3HT was monitored by measuring the absorption spectra and charge carrier mobility, and the morphol. change with the introduction of P3HT in the active layer was monitored by AFM, TEM, and GIXRD. Favorable bi-continuous phase sepn. and appropriate domain size of each phase can be achieved to facilitate fast charge transport, and thus improve the power conversion efficiency of the solar cell. By adding 1% P3HT into the blend of PCPDTBT/PCBM, the power conversion efficiency can be improved by 20%. Also, with the incorporation of 1% P3HT to the blend of PCPDTBT/PC61BM with DIO, the power conversion efficiency can be further increased by 17%. The strategy of this study can be expanded to other low bandgap conducting polymers for high efficiency bulk heterojunction solar cells.
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17. 17
  Devižis, A.; Serbenta, A.; Meerholz, K.; Hertel, D.; Gulbinas, V. Ultrafast Dynamics of Carrier Mobility in a Conjugated Polymer Probed at Molecular and Microscopic Length Scales Phys. Rev. Lett. 2009, 103, 027404 DOI: 10.1103/PhysRevLett.103.027404
  17
  Ultrafast Dynamics of Carrier Mobility in a Conjugated Polymer Probed at Molecular and Microscopic Length Scales
  Devizis, A.; Serbenta, A.; Meerholz, K.; Hertel, D.; Gulbinas, V.
  Physical Review Letters (2009), 103 (2), 027404/1-027404/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  The authors used time-resolved elec.-field-induced 2nd-harmonic generation to probe the charge-carrier-mobility dynamics in amorphous org. materials on an ultrafast time scale. The authors were able to show that the mobility in poly-spiro-bifluorene-co-benzothiadiazol decreases from 0.1 cm2/V s at 1 ps to 10-6 cm2/V s within 1 μs. The authors attribute this dramatic decrease to the relaxation of the charge carriers within the d. of states, clearly demonstrating the impact of disorder on the nanoscale charge transport in amorphous semiconductors.
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18. 18
  Pranculis, V.; Infahsaeng, Y.; Tang, Z.; Devižis, A.; Vithanage, D. A.; Ponseca, C. S.; Inganäs, O.; Yartsev, A. P.; Gulbinas, V.; Sundström, V. Charge Carrier Generation and Transport in Different Stoichiometry APFO3:PC61BM Solar Cells J. Am. Chem. Soc. 2014, 136, 11331– 11338 DOI: 10.1021/ja503301m
  18
  Charge Carrier Generation and Transport in Different Stoichiometry APFO3:PC61BM Solar Cells
  Pranculis, Vytenis; Infahsaeng, Yingyot; Tang, Zheng; Devizis, Andrius; Vithanage, Dimali A.; Ponseca, Carlito S.; Inganas, Olle; Yartsev, Arkady P.; Gulbinas, Vidmantas; Sundstrom, Villy
  Journal of the American Chemical Society (2014), 136 (32), 11331-11338CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  In this paper we studied carrier drift dynamics in APFO3:PC61BM solar cells of varied stoichiometry (2:1, 1:1, and 1:4 APFO3:PC61BM) over a wide time range, from subpicoseconds to microseconds with a combination of ultrafast optical elec. field probing and conventional transient integrated photocurrent techniques. Carrier drift and extn. dynamics are strongly stoichiometry dependent: the speed of electron or hole drift increases with higher concn. of PC61BM or polymer, resp. The electron extn. from a sample with 80% PC61BM takes place during hundreds of picoseconds, but slows down to sub-microseconds in a sample with 33% PC61BM. The hole extn. is less stoichiometry dependent: it varies form sub-nanoseconds to tens of nanoseconds when the PC61BM concn. changes from 33% to 80%. The electron extn. rate correlates with the conversion efficiency of solar cells, leading to the conclusion that fast electron motion is essential for efficient charge carrier sepn. preventing their geminate recombination.
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19. 19
  Gelinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes Science (Washington, DC, U. S.) 2014, 343, 512– 516 DOI: 10.1126/science.1246249
  19
  Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes
  Gelinas, Simon; Rao, Akshay; Kumar, Abhishek; Smith, Samuel L.; Chin, Alex W.; Clark, Jenny; van der Poll, Tom S.; Bazan, Guillermo C.; Friend, Richard H.
  Science (Washington, DC, United States) (2014), 343 (6170), 512-516CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  Understanding the charge-sepn. mechanism in org. photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here the authors report the time dependence of the sepn. of photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the elec. field generated between the charges, the authors measure ~200 millielectron volts of electrostatic energy arising from electron-hole sepn. within 40 fs of excitation, corresponding to a charge sepn. distance of at least 4 nm. At this sepn., the residual Coulomb attraction between charges is at or below thermal energies, so that electron and hole sep. freely. This early time behavior is consistent with charge sepn. through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.
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20. 20
  Lane, P. A.; Cunningham, P. D.; Melinger, J. S.; Esenturk, O.; Heilweil, E. J. Hot Photocarrier Dynamics in Organic Solar Cells Nat. Commun. 2015, 6, 7558 DOI: 10.1038/ncomms8558
  20
  Hot photocarrier dynamics in organic solar cells
  Lane, P. A.; Cunningham, P. D.; Melinger, J. S.; Esenturk, O.; Heilweil, E. J.
  Nature Communications (2015), 6 (), 7558CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
  Photocurrent in an org. solar cell is generated by a charge transfer reaction between electron donors and acceptors. Charge transfer is expected to proceed from thermalized states, but this picture has been challenged by recent studies that have investigated the role of hot excitons. Here we show a direct link between excess excitation energy and photocarrier mobility. Charge transfer from excited donor mols. generates hot photocarriers with excess energy coming from the offset between the LUMO of the donor and that of the acceptor. Hot photocarriers manifest themselves through a short-lived spike in terahertz photocond. that decays on a picosecond timescale as carriers thermalize. Different dynamics are obsd. when exciting the acceptor at its absorption edge to a thermalized state. Charge transfer in this case generates thermalized carriers described by terahertz photocond. dynamics consisting of an instrument-limited rise to a long-lived signal.
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21. 21
  Burkhard, G. F.; Hoke, E. T.; Beiley, Z. M.; Mcgehee, M. D. Free Carrier Generation in Fullerene Acceptors and Its Effect on Polymer Photovoltaics J. Phys. Chem. C 2012, 116, 26674– 26678 DOI: 10.1021/jp310821f
  21
  Free carrier generation in fullerene acceptors and effect on polymer photovoltaics
  Burkhard, George F.; Hoke, Eric T.; Beiley, Zach M.; McGehee, Michael D.
  Journal of Physical Chemistry C (2012), 116 (50), 26674-26678CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Early research on C60 led to the discovery that the absorption of photons with energy greater than 2.35 eV by bulk C60 produces free charge carriers at room temp. We find that not only is this also true for many of the sol. fullerene derivs. commonly used in org. photovoltaics, but also that the presence of these free carriers has significant implications for the modeling, characterization, and performance of devices made with these materials. We demonstrate that the discrepancy between absorption and quantum efficiency spectra in P3HT:PCBM is due to recombination of such free carriers in large PCBM domains before they can be sepd. at a donor/acceptor interface. Since most theories assume that all free charges result from the sepn. of excitons at a donor/acceptor interface, the presence of free carrier generation in fullerenes can have a significant impact on the interpretation of data generated by numerous field-dependent techniques.
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22. 22
  Devižis, A.; Hertel, D.; Meerholz, K.; Gulbinas, V.; Moser, J.-E. Time-Independent, High Electron Mobility in Thin PC61BM Films: Relevance to Organic Photovoltaics Org. Electron. 2014, 15, 3729– 3734 DOI: 10.1016/j.orgel.2014.10.028
  22
  Time-independent, high electron mobility in thin PC61BM films: Relevance to organic photovoltaics
  Devizis, A.; Hertel, D.; Meerholz, K.; Gulbinas, V.; Moser, J.-E.
  Organic Electronics (2014), 15 (12), 3729-3734CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
  Ultrafast optical probing of elec. field by means of electroabsorption combined with conventional photocurrent measurements was employed to investigate the drift and mobility dynamics of photo-generated charge carriers in the pristine PC61BM film and in the blend with a merocyanine dye. Electrons passed a 40 nm thick PC61BM film within a few picoseconds with time-independent and weakly dispersive mobility. The electron mobility is 1 cm2/(V s) at 1 MV/cm and an est. of the zero-field mobility yields 5 · 10-2 cm2/(V s). The initial electron mobility in the blend is of the order of 10-2 cm2/(V s) and decreases rapidly. We conclude that electron motion in PC61BM based org. bulk hetero-junction solar cells is limited by barriers between PC61BM domains rather than by intrinsic PC61BM properties.
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23. 23
  Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins, B. a.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H. The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells Adv. Energy Mater. 2013, 3, 364– 374 DOI: 10.1002/aenm.201200637
  23
  The importance of fullerene percolation in the mixed regions of polymer-fullerene bulk heterojunction solar cells
  Bartelt, Jonathan A.; Beiley, Zach M.; Hoke, Eric T.; Mateker, William R.; Douglas, Jessica D.; Collins, Brian A.; Tumbleston, John R.; Graham, Kenneth R.; Amassian, Aram; Ade, Harald; Frechet, Jean M. J.; Toney, Michael F.; McGehee, Michael D.
  Advanced Energy Materials (2013), 3 (3), 364-374CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
  Most optimized donor-acceptor (D-A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ∼80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-co-octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) and [6,6]-phenyl-C61-butyric acid Me ester (PCBM) with 7.3% PCE and find that bimol. recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimol. recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphol. of these BHJs before and after thermal annealing and det. that thermal annealing drastically reduces the concn. of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concn. of PCBM may reduce the no. of percolating electron transport pathways within these mixed regions and create morphol. electron traps that enhance charge-carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concn. of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimol. recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.
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24. 24
  Foster, S.; Deledalle, F.; Mitani, A.; Kimura, T.; Kim, K.-B.; Okachi, T.; Kirchartz, T.; Oguma, J.; Miyake, K.; Durrant, J. R. Electron Collection as a Limit to Polymer:PCBM Solar Cell Efficiency: Effect of Blend Microstructure on Carrier Mobility and Device Performance in PTB7:PCBM Adv. Energy Mater. 2014, 4, 1400311 DOI: 10.1002/aenm.201400311
  There is no corresponding record for this reference.
25. 25
  Devižis, A.; Meerholz, K.; Hertel, D.; Gulbinas, V. Hierarchical Charge Carrier Motion in Conjugated Polymers Chem. Phys. Lett. 2010, 498, 302– 306 DOI: 10.1016/j.cplett.2010.08.071
  25
  Hierarchical charge carrier motion in conjugated polymers
  Devizis, A.; Meerholz, K.; Hertel, D.; Gulbinas, V.
  Chemical Physics Letters (2010), 498 (4-6), 302-306CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)
  Motion of photogenerated charge carriers in π-conjugated polymer films with different disorder and chain orientation and also in a blend of conducting and insulating polymers has been investigated by means of time-resolved elec. field-induced second harmonic generation technique. Exptl. results and Monte Carlo simulation enabled us to distinguish three time domains of charge transport. The charge carriers become sepd. by about 10 nm in a strong elec. field within 1 ps. Subsequently, carriers drift another 10-15 nm on a picosecond time scale with a high mobility. The third and the slowest carrier motion phase is well described by the stochastic drift and dets. the macroscopic equil. mobility. We attribute the two ultrafast drift phases to carrier motion inside a conjugated segment and along a single polymer chain, resp., while the slow motion phase involves interchain jumps.
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26. 26
  Liu, F.; Zhao, W.; Tumbleston, J. R.; Wang, C.; Gu, Y.; Wang, D.; Briseno, A. L.; Ade, H.; Russell, T. P. Understanding the Morphology of PTB7:PCBM Blends in Organic Photovoltaics Adv. Energy Mater. 2014, 4, 1– 9 DOI: 10.1002/aenm.201301377
  There is no corresponding record for this reference.
27. 27
  Albrecht, S.; Schindler, W.; Kurpiers, J.; Kniepert, J.; Blakesley, J. C.; Dumsch, I.; Allard, S.; Fostiropoulos, K.; Scherf, U.; Neher, D. On the Field Dependence of Free Charge Carrier Generation and Recombination in Blends of PCPDTBT/PC70BM: Influence of Solvent Additives J. Phys. Chem. Lett. 2012, 3, 640– 645 DOI: 10.1021/jz3000849
  27
  On the Field Dependence of Free Charge Carrier Generation and Recombination in Blends of PCPDTBT/PC70BM: Influence of Solvent Additives
  Albrecht, Steve; Schindler, Wolfram; Kurpiers, Jona; Kniepert, Juliane; Blakesley, James C.; Dumsch, Ines; Allard, Sybille; Fostiropoulos, Konstantinos; Scherf, Ullrich; Neher, Dieter
  Journal of Physical Chemistry Letters (2012), 3 (5), 640-645CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  The authors have applied time-delayed collection field (TDCF) and charge extn. by linearly increasing voltage (CELIV) to study the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. Probably the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimol. recombination coeffs., which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a neg. field dependence of mobility, which the authors suggest to cause bias-dependent recombination coeffs.
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Influence of Blend Ratio and Processing Additive on Free Carrier Yield and Mobility in PTB7:PC71BM Photovoltaic Solar CellsClick to copy article linkArticle link copied!

The Journal of Physical Chemistry C

Abstract

Introduction

Experimental Methods

Solar Cell Preparation and Characterization

Measurements of Carrier Drift Dynamics

Results

Photovoltaic Response

Figure 1

Charge Separation and Extraction Dynamics in Optimized Cells

Figure 2

Influence of Internal Electric Field

Figure 3

Effect of Blend Ratio and Solvent Additive on Carrier Mobility

Figure 4

Figure 5

Discussion

Figure 6

Conclusions

Author Information

Acknowledgment

References

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The Journal of Physical Chemistry C

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Figure 3

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Figure 5

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Influence of Blend Ratio and Processing Additive on Free Carrier Yield and Mobility in PTB7:PC₇₁BM Photovoltaic Solar Cells
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