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Keywords:

  • bulk heterojunction;
  • recombination;
  • polymer:fullerene;
  • PBDTTT-C;
  • organic solar cell

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

We explore the interrelation between density of states, recombination kinetics, and device performance in efficient poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno[3,4-b]thiophene-2,6-diyl]:[6,6]-phenyl-C71-butyric acid methyl ester (PBDTTT-C:PC71BM) bulk-heterojunction organic solar cells. We modulate the active-layer density of states by varying the polymer:fullerene composition over a small range around the ratio that leads to the maximum solar cell efficiency (50–67 wt% PC71BM). Using transient and steady-state techniques, we find that nongeminate recombination limits the device efficiency and, moreover, that increasing the PC71BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a significant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents significant progress in understanding what limits these high-efficiency polymer:fullerene systems.


1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

Polymer:fullerene bulk heterojunction (BHJ) solar cells have recently witnessed tremendous advancements in active-layer performance due to materials and processing optimization.1–7 The development of novel interlayers and polymers with favorable absorptive, electrical, and morphological properties have enabled single-junction efficiencies to exceed 9%.8 In a number of efficient BHJ systems, investigators have shown that nongeminate recombination is the key loss mechanism that limits the power conversion efficiency.9, 17 The influence of nongeminate recombination on the shape of the current/voltage curve has even been demonstrated quantitatively in several polymer:fullerene systems.14–, 19 Nongeminate recombination is controlled by the recombination kinetics—usually experimentally represented through charge-carrier lifetimes—and the density of states (DoS) of the semiconductor. While the theoretical relation between the DoS, recombination kinetics, and device performance (e.g., open-circuit voltage (VOC)) have been discussed,20, 22 there have been few studies on polymer:fullerene systems that monitor the DoS explicitly, and study the impact on recombination kinetics, VOC, and overall device performance.23–, 25 In addition, there has b een comparatively little experimental work on understanding the precise physical origin of the recombination dynamics, and how the kinetics of recombination depends on the kinetics of transport.26–30

In this work, we explicitly examine the DoS, recombination dynamics, device performance, and their interdependence in a representative, high-efficiency, BHJ organic photovoltaic (OPV) active layer composed of poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT-C) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (Figure 1).31 We do so by combining electroluminescence spectroscopy (EL), transient photovoltage (TPV), and charge extraction (CE) measurements.11–13 To modulate the DoS, we examine different blend compositions around the composition at which the photovoltaic performance of PBDTTT-C:PC71BM solar cells is optimized. We find that the principal recombination mechanism in this system is of nongeminate character, and that the recombination kinetics scale with drift-mobility in the opposite way that would be predicted by a Langevin-type model. Namely, the highest fullerene loading leads to both the highest mobility and the longest charge-carrier lifetime. The improved carrier lifetimes and drift-mobility with increasing PC71BM content are accompanied by a significant change in the magnitude or width of the DoS. Our assignment of dominant carrier losses to nongeminate recombination is validated by reconstructions of the J-V curves and VOC predictions at different light intensities and blend compositions. In addition, we find a strong correlation between the blend CT-state energy, VOC, and PC71BM content, which has previously been observed in several other polymer systems over a wide composition range, despite vastly different processing conditions, and molecular structures.32–35, 55 In this regard, our results are consistent with the hypothesis that a major reason behind the success of PCBM-based acceptors is the tendency to form a unique morphology ideal for charge separation and extraction.32, 72, 74

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Figure 1. This work examines the PBDTTT-C:PC71BM system at three different compositions, 50 wt% PC71BM, 60 wt% PC71BM, and 67 wt% PC71BM wt. ratio PBDTTT-C:PC71BM (50–67 wt% PC71BM). The device architecture chosen for study was ITO/PEDOT:PSS/PBDTTT-C:PC71BM/Ca/Al.

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We chose to investigate PBDTTT-C because of its facile synthesis, excellent performance, and close similarity to some of the highest efficiency BHJ donor polymers known to date.3, 36–45 The PBDTTT:PC71BM BHJ system is interesting because it achieves high photovoltaic performance despite detailed morphological studies suggesting that it forms a complex and mostly amorphous morphology with relatively weak polymer/fullerene aggregation where it is presently unclear how such excellent photovoltaic performance manifests.46–50

2. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

2.1. J-V Characteristics

Herein we study devices made from three different PBDTTT-C:PC71BM compositions; namely, 50 wt% PC71BM, 60 wt% PC71BM, and 67 wt% PC71BM. In order to understand the differences in performance from a general standpoint, we optimized each blend individually for active-layer thickness (Figure 2). For plots with error bars we refer the reader to the supplementary information. In agreement with previous reports,31 we find that the optimal blend weight ratio has 60 wt% PC71BM. In Figure 2 the active-layer spin speed was used as a relative unit for thickness because a measurement of the absolute thickness of all active layers was not necessary for the following J-V comparisons. From the trends in Figure 2 we see that both the 60 wt% PC71BM and 67 wt% PC71BM devices are able to maintain high fill factors at thicknesses where the JSC is locally maximized. The 50 wt% PC71BM blend, on the other hand, begins to suffer losses in fill factor while the JSC is still increasing with increasing thickness. Based on typical absorbed photon flux vs. active-layer thickness plots for BHJ OPVs,51, 54 we hypothesize that, in this case, the charge-collection efficiency is greater in films with higher fullerene content, allowing the active layers to be optimized at the first absorption maximum.51–54 Thus, it appears that the favorable overall absorption of higher PBDTTT-C content active layers (50 wt% PC71BM) cannot be exploited due to excessive nongeminate losses with modest increases in active-layer thickness. This assessment of the J-V characteristics and device performance is supported by our transient optoelectronic measurements discussed below.

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Figure 2. (a) Fill Factor, (b) JSC, (c) VOC, and (d) PCE dependence on spin speed for the compositions examined in this study. All data points are averaged over 6 devices, and the solid lines are added as guides to the eye.

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We also observed a robust trend in VOC with fullerene loading for the compositions examined herein; namely, that the open-circuit voltage is always higher for lower fullerene content (Figure 2c). Though the differences in VOC between blends is only on the order of ∼10 mV, this trend was consistently reproduced from different polymer and PC71BM batches, as well as from many devices fabricated at different laboratories (ICL and UCLA). The observation that VOC decreases with higher PCBM content over a wide composition range has also been found in other polymer systems.32, 33, 35, 55–, 58 Sparing contact effects, the VOC is a function of energy levels and the steady-state carrier concentration determined by the balance between generation and recombination.9, 18, 22, 59, 60 Earlier investigations into the reason for the composition dependence of VOC found that the change in VOC with fullerene content can be explained with changes in the CT-state energy and that kinetic arguments were not necessary to explain the trends.32, 35 To verify whether this is the case in our devices as well, and to better understand how the PBDTTT-C:PC71BM composition affects energy levels, recombination, and the overall device performance, we performed transient optoelectronic measurements and electroluminescence studies.

2.2. Transient Optoelectronic Analysis

We performed TPV and CE experiments on the different blend-ratio devices; descriptions of these techniques and their use in characterizing organic solar cells are well-documented elsewhere.11–18, 22, 61 These techniques allow us to probe the typically non-linear relationship between carrier density and recombination, which plays a major role in determining the shape of the J-V curve and thus also the power conversion efficiency. Table 1 summarizes the typical J-V characteristics and device parameters relevant to the following analysis. We note that the carrier concentration data was obtained from CE measurements by correcting for both the charge on the electrodes (using the geometric capacitance) and any incurred carrier losses during extraction.12 We analyze our transient data using the empirical and approximate relations62

  • equation image((1))
  • equation image((2))
  • equation image((3))
  • equation image((4))

where τΔn is the small perturbation lifetime obtained from fitting TPV decays to a monoexponential decay, τ the total carrier lifetime, and n is the average excess carrier concentration in the active layer relative to short circuit in the dark. The slope of τ(Voc) and n(VOC) are defined by m and ϑ, and δ is the reaction order.62, 63

Table 1. J-V Characteristics and device parameters of different blend compositions processed under identical conditions. The device characteristics were averaged over 4 devices (± standard deviation). Thickness measurements were averaged over ≥ 4 locations and taken with an AFM. The built-in voltage was derived from the intersection of light and dark curves.
CompositionVOC [mV]JSC [mA/cm2]FF [%]PCE [%]Active Layer Thickness [nm]Relative Dielectric Constant (ϵr)Built-in Voltage [V]
50 wt% PC71BM715 ± 311.4 ± 0.161.3 ± 0.54.98 ± 0.167 ± 43.70.79
60 wt% PC71BM708 ± 113.6 ± 0.264.5 ± 0.56.22 ± 0.182 ± 23.80.805
67 wt% PC71BM698 ± 314.6 ± 0.360.8 ± 1.26.18 ± 0.1105 ± 53.90.78

Figure 3a shows n(VOC) as measured by CE, and (c) gives the total carrier lifetime as a function of average carrier concentration. We first see from Figure 3a that for a given average carrier density, the VOC(n) is consistently different across blend ratios, where VOC(50 wt% PC71BM) > VOC(60 wt% PC71BM) > VOC(67 wt% PC71BM) for a given n = constant. Thus, for identical average charge densities n, the quasi-Fermi level splitting is larger for lower fullerene content, which indicates a significant composition-induced shift in the density of states. For lower fullerene loadings, the band tails of the polymer HOMO and PC71BM LUMO are either further separated from each other in energy, lower in magnitude, or a combination of both when compared to the higher PC71BM loading cases. We have schematically indicated this apparent shift in the density of states in Figure 3b. While the magnitude or energetic separation of the density of states around the band edges changes with composition, the slope of n vs. VOC, and therefore the shape of the DoS, is not significantly affected.64

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Figure 3. (a) The average carrier concentration as a function of VOC and (c) the total carrier lifetime as a function of average carrier concentration for all PBDTTT-C:PC71BM weight ratios studied herein. (b) is a schematic showing how the density of states shifts with composition. Note that an increase in the absolute density of states at given energy or a shift of the DoS in energy are indistinguishable from a charge extraction measurement. We cannot discern whether the change is primarily in the HOMO or LUMO or whether both change equally.

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In order to analyze the recombination kinetics, we examine the total carrier lifetime as a function of average carrier density (Figure 3c). Much like Figure 3a, Figure 3c shows that τ(n) is also shifted across compositions. However, despite the larger VOC values, τ(n = constant) goes as τ(50 wt% PC71BM) < τ(60 wt% PC71BM) < τ(67 wt% PC71BM). Thus, nongeminate recombination is actually fastest in the 50 wt% PC71BM blend, followed by the slower 60 wt% PC71BM and 67 wt% PC71BM blends, respectively. This indicates that the small differences in VOC are due to two effects that partly compensate each other, namely a change in DoS and a shift in recombination dynamics. The higher magnitude and/or energetic shift in DoS induced by higher fullerene content is partly compensated by a concomitant reduction of the recombination rate, which, taken together, leads to only a slightly lower open-circuit voltage when compared to a lower fullerene content case.

The dominating recombination mechanism responsible for the trends in τ(n) of Figure 3c is of fundamental interest to understanding what limits the efficiency of a given active layer. A common approach for describing nongeminate recombination is via a Langevin-type mechanism, where the recombination coefficient krec is directly proportional to the carrier mobility (μ).65 To examine the validity of a Langevin-type model, we used the technique described in Reference 66 to evaluate carrier drift mobilities as a function of average carrier concentration (Figure 4a). From Figure 4a we see that the mobility is weakly carrier-density dependent, offsetting between compositions, and actually largest for the 67 wt% PC71BM blend, following μ(67 wt% PC71BM) > μ(60 wt% PC71BM) > μ(50 wt% PC71BM). For comparison, we plot the nongeminate recombination coefficient also in Figure 4a as calculated from Equation (4). The data in Figure 4a and Figure 3c have exactly the opposite trend one would expect if Langevin-type recombination were dominant. In this particular case, larger PC71BM content increases the mobility and simultaneously decreases the recombination rate. Moreover, it is clear that the nongeminate recombination coefficient and the mobility have significantly different dependencies on average carrier density, which has been previously noted by Rauh et al.30 In the following electroluminescence section, we provide further evidence that, in this case, phase-segregation effects are likely the dominating factor in determining the nongeminate recombination properties.

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Figure 4. (a) Average carrier mobilities of the different blend ratios under study measured by the method in Reference 66 and the effective nongeminate recombination coefficient as a function of average carrier concentration. (b) The resulting mobility-lifetime product (lifetime from Figure 3c) as a function of average carrier concentration. Note the different scales for the charge density n.

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To combine these dynamical constants into one effective assay of the electrical properties of the blend, we plot the mobility-lifetime product in Figure 4b using the data from Figure 3c and Figure 4a that overlap in carrier density. Clearly, higher PC71BM concentrations result in vastly superior electrical properties in this composition range. The differences in μτ(n) between the 50 wt.% PC71BM and 67 wt.% PC71BM blends approaches an order of magnitude over the measured carrier concentration range, which explains why the 50 wt% PC71BM blend has suboptimal photovoltaic properties.67 In this case, the relative improvements of μτ appear to afford acceptable fill factors at active-layer thicknesses corresponding roughly to the first absorption maximum for the 60 wt% PC71BM and 67 wt% PC71BM blends, but not the 50 wt% PC71BM blend (Figure 2).

Finally, in order to further understand the nature of these differences in recombination characteristics, we attempt to reconstruct the J-V curves and predict open-circuit voltages assuming that the generation rate is independent of voltage and that nongeminate recombination is the dominating loss process throughout the photovoltaic operating regime. Figure 5a–c shows the J-V reconstructions and Figure 5d the attendant VOC predictions of all blend ratios using the methods detailed in previous works (dark J-V reconstructions are provided on a logarithmic scale in Figure S1).14, 15, 18 The excellent agreement between measured and predicted values for all blend ratios in Figure 5 suggests that non-geminate recombination alone is sufficient to understand the shape of the J-V curves and the magnitudes of the open-circuit voltages for all compositions. In the final section below, we examine the electroluminescence spectra of the different blends to further specify the nature of this nongeminate, non-Langevin, type of recombination and, moreover, better understand why it is so sensitive to blend composition.

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Figure 5. (a–c) J-V reconstructions comparing experimentally measured (solid lines) and reconstructed values (open points) based on the method detailed in References 14 and 16. (d) The light intensity dependence of VOC for each blend and the corresponding predicted VOC using the methods detailed in Reference 18.

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2.3. Electroluminescence

Figure 6a shows the electroluminescence spectra for each of the PBDTTT-C:PC71BM weight ratios and the pure PBDTTT-C polymer. The normalized 50 wt% PC71BM blend EL resembles that of the pure polymer, but with the high-energy cutoff ∼0.1 eV redshifted and a stronger low-energy emission tail. When progressing through the composition range, the EL spectra continue to redshift and appear to be in a transitionary phase at the optimal 60 wt% PC71BM blend ratio. Further examination shows that the photon energy corresponding to the maximum emission intensity (Emax) for the 60 wt% PC71BM blend was strongly voltage-dependent, where under injection conditions of 40 mA/cm2 the spectrum resembles that of the 67 wt% PC71BM device, but with increased current it shifts rapidly to resemble the 50 wt% PC71BM (Figure 6b). This luminescence transition, consistent with our charge extraction data, is an indicator of the important DoS evolution that takes place over a relatively narrow composition range. While the optimal 60 wt% PC71BM device EL was sensitive to injection conditions, the relative spectral intensities of the 50 wt% PC71BM and 67 wt% PC71BM blends were insensitive to injection, which is similar to what has been found for other polymer:PCBM systems.68

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Figure 6. (a) Electroluminescence spectra of the different blend films and pure PBDTTT-C (all at J = 160 mA/cm2). (b) Shift in the EL emission maximum with injection conditions. (c) The linear relationship between CT emission band peak position (at J = 80 mA/cm2) and VOC at equal carrier concentrations.

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Looking closer at the blend EL spectra in Figure 6a, they appear to be composed of primarily two emission bands: one with peak intensity around ∼1.0–1.15 eV and the other at ∼1.4 eV. In order to quantitatively assess this observation, we reduced and deconvoluted each spectrum into two Gaussian emission profiles,34 attributing the higher-energy Gaussian at ∼1.4 eV to pure polymer emission and the low-energy Gaussian centered around ∼1.1 eV to CT-state emission (Figure S2). The presence of PBDTTT-C singlet activation in the electroluminescence spectra is consistent with the findings of Faist et al.69 for the case where the donor has an optical band gap of <1.7 eV (Eg,PBDTTT-C = 1.6 eV) and the donor/acceptor CT-energy is near the singlet energy of one of the pure materials. Based upon this finding, we hypothesize that, in this case, the CT energy is quite close to the polymer's singlet energy and the DoS modulation from fullerene loading significantly affects the singlet activation process. Faist et al.69 qualitatively found that singlet activation of one component correlated with reduced photovoltaic performance, which is in agreement with the case here, where the poorly performing 50 wt% PC71BM blend shows by far the most singlet activation, followed consecutively by the electrically superior 60 wt% PC71BM and 67 wt% PC71BM compositions (Figure S2). A number of photophysical processes can be activated when the blend energy levels are brought in too close proximity;70, 71 however, in this instance, it appears that injected electrons on the PC71BM can more easily transfer onto the PBDTTT-C for cases with less PC71BM loading. This effect would partially nullify the physical separation of charges by material phase segregation, leading to the observed higher nongeminate recombination rates for lower fullerene loadings. It may also seem likely that, in addition to the nongeminate losses detailed above, the 50 wt% PC71BM blend suffers more geminate recombination due to a smaller driving force for interfacial charge separation. However, we note that transient absorption measurements (Figures S3–S5) found strong generation yields for all the compositions studied herein and even a strong polaron yield for neat PBDTTT-C.

For further verification of the connection between our EL and optoelectronic measurements, we take the reduced CT emission band's center of gravity (ECT) and plot it against the VOC at equivalent active-layer charge densities (Figure 6c). We find for the case of VOC(n = 2 × 1016 cm−3) and for EL injection conditions of J = 80 mA/cm2 that the CT emission band's center of mass and VOC scale on precisely a 1-to-1 basis. We note that the CT emission band's center of mass varied weakly with injection conditions (Figure S6) and that the slopes also varied marginally with different VOC(n = constant). Despite these variations, we found that plots of VOC(n = constant) vs. ECT were always linear with slopes near unity. Thus, the offset between open-circuit voltages of different compositions at equivalent carrier densities are reproduced well by our EL analysis, indicating that EL reflects the same DoS that is measured with charge extraction.

Despite significantly different chemical structures, polymer:PCBM stoichiometries, and optimization conditions, similar relationships between PCBM composition, VOC, and ECT have been reported for other conjugated polymer:PCBM blends.34, 35, 72 These prior studies have also found that VOC scales with the CT-state energy in an expected 1-to-1 fashion.35, 73 Primarily two explanations have emerged regarding the observed interrelation between PCBM content, VOC, and ECT,: (i) the VOC and CT-state energy decrease with increasing PCBM content due to an increase in the relative dielectric constant of the film,72 and/or (ii) nanoscale crystallization due to added PCBM stabilizes the LUMO energy76 and thus decreases the VOC and redshifts the CT-state emission.32, 34 Considering the work of Piersimoni et al.32 and Jamieson et al.,74 and because this effect is witnessed over several different polymer/fullerene systems, we believe that morphological differences are the primary cause for such trends. Additionally, Agostinelli et al.75 found that the refractive index of P3HT:PCBM BHJs was only weakly dependent on composition, which further suggests that a change in relative dielectric constant could not fully account for these trends. Composition-induced PC71BM aggregation would be consistent with our n vs. VOC and EL data for the reasons stated above; however, it is difficult to hypothesize about microstructural changes without direct evidence. It is presently unclear whether a change in the density of states with composition as observed for PBDTTT-C:PC71BM blends could be a general feature of polymer:PCBM systems. Given that the energy of the charge transfer state electroluminescence typically shifts with composition in various polymer:PCBM blends but not necessarily with other fullerene-based acceptors,32, 34, 35, 74 our data could be revealing as to why PCBM works so well with an array of different polymers.

3. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

In summary, we find that the dominant recombination mechanism in optimized PBDTTT-C:PC71BM solar cells is of non-Langevin and nongeminate character. Our assumptions regarding recombination are supported by accurate J-V reconstructions at multiple light intensities for all the blend compositions examined. Moreover, both the carrier lifetime and the effective drift mobility increase with PC71BM loading, directly opposite to what one would expect if Langevin-type kinetics dominated. Using electroluminescence spectroscopy and charge extraction measurements, we show that increased fullerene loading in this range energetically narrows and/or increases the magnitude of the DoS active in solar cell operation. This significant change in the DoS was not strongly reflected in VOC measurements because of a concomitant decrease in carrier lifetime. From a cell efficiency standpoint, we conclude that the poor electronic properties of the 50 wt% PC71BM blend do not permit higher active-layer thicknesses that would lead to increased light absorption, whereas the higher wt% PC71BM devices are able to reach the first active-layer absorption maximum without significant nongeminate losses.

4. Experimental Section

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

Device Fabrication and Characterization: PBDTTT-C:PC71BM is optimized with ∼6× higher molar ratios of fullerene to polymer-repeat unit than P3HT-like systems.76 The most effective optimization procedure for these high-performance PBDTTT:PC71BM BHJs often includes the addition of ∼3% v/v 1,8-Diiodooctane (DIO) as a solvent additive, thin active layers of ∼100 nm or less, compositions with higher weight fractions of PC71BM than polymer, and no need for solvent or thermal annealing.3, 31, 36, 77

To make devices, indium-tin oxide (ITO) coated glass substrates were sequentially sonicated in acetone and isopropyl alcohol for 15 minutes, followed by drying with compressed N2. The substrates were then loaded into a UV ozone reactor for 7 minutes of further cleaning and surface treatment in a 0.2 mbar oxygen/ozone atmosphere. Immediately following ozone treatment, the PEDOT (CLEVIOS P VP Al 4083) solution was spin coated directly from a 0.2 μm GHP membrane filter to give a ∼30 nm thick film, which was then annealed at 140 °C for 15 min. While still at temperature, the PEDOT coated substrates were loaded into a N2 atmosphere glove box (<1 ppm H2O) and allowed to cool. At least one day prior to spin coating, the active-layer solutions were prepared from various weight ratios of PC71BM (Solenne or Nano-C) and PBDTTT-C (Solarmer). All solutions employed 1,2-dichlorobenzene (DCB) as the solvent and contained 1,8-diiodooctane (3% v/v) purchased from Sigma-Aldrich®. We held the concentration of PBDTTT-C in DCB constant at 10 mg/ml for all solutions, while the PC71BM concentration was varied from 10 mg/ml (50 wt% PC71BM), to 15 mg/ml (60 wt% PC71BM), to 20 mg/ml (67 wt% PC71BM). The dry materials for each solution were weighed in ambient conditions, and the solvents were added in a N2 atmosphere glove box. Devices made at ICL were spun coat from each of these blend solutions onto the PEDOT covered substrates at various spin speeds (Figure 2) for 40 s. Devices made at UCLA (Table 1) were made from identical procedures as described above, save for slightly different parameters due to inherent equipment variability. The presented transient optolectronic analysis is from the devices summarized in Table 1; however, the same trends in the transient data were found on devices made at ICL (Figure 2). After spin coating the active layer, we deposited Ca/Al electrodes to create devices with active areas of 4.5 mm2 or 10 mm2. To add the electrodes, we first deposited 10–20 nm of Ca at a pressure of 1–2.3 × 10−6 mbar and a rate of 0.5 Å/s, followed by a deposition of 100 nm at a rate of 1 Å/s. Before the shutter was opened in each case, ∼20 nm of metal was evaporated off to ensure purity. The final device structure was ITO/PEDOT/PBDTTT-C:PC71BM/Ca/Al.

The J-V characteristics in Figure 2 were obtained under simulated AM 1.5G illumination using a xenon lamp with a water IR filter. The J-V measuremtents in Table 1 were taken under simulated AM 1.5G illumination using an Oriel 9600 solar simulator. All J-V measurements were obtained with a Keithley 2400 sourcemeter, and the light intensity was calibrated with an IR cutoff filter equipped silicon photodiode.

TAS: Transient absorption spectroscopy was carried out in transmission mode on samples consisting of ITO/PEDOT/PBDTTT-C:PC71BM, which were processed in identical fashion as those made into devices. The pump beam was selected to be at 650 nm, exciting PBDTTT-C, while the probe beam was at 1190 nm, around the absorption maximum of polarons in the 60 wt% PC71BM system (Figure S4). The pump excitation was performed with a OPO Nd:YAG laser (Opotek LD 355) at 5 Hz and was varied from 0.25–6 μJ/cm2 pulse fluence (Figure S5). The probe beam was monitored by a InGaAs Hamamatsu G8370-82 photodetector amplified by a Costronics 2004 Optical Transient Amplifier connected to a Tektronix TDS220 digital oscilloscope.

TPC/TPV/CE: We performed charge extraction, transient photocurrent, and transient photovoltage measurements and analysis in exact accordance with that detailed previously.11–15, 17

Electroluminescence: Electroluminescence was measured using a Princeton Instruments Acton SP 2500 spectrograph combined with a liquid nitrogen-cooled InGaAs photodiode array (Acton OMAV:1024). The spectral intensity was corrected with the spectrum from a calibrated halogen lamp.68, 69

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

S. A. H acknowledges support by the NSF IGERT: Materials Creation Training Program (MCTP)-DGE-0654431 and NSF CHE-1112569 and would like to thank Dr. Chad Risko for useful discussions and Letian Dou for aid with Figure 1. G. Li thanks to the UCLA Henry Samuli School of Engineering and Applied Science for financial support. The author would like to thank Dr. Yue Wu (Solarmer Energy Inc.) and Prof. Jianhui Hou (Institute of Chemistry Chinese Academy of Science) for providing PBDTTT-C. T. K. acknowledges support via an Imperial College Junior Research Fellowship. F. D. acknowledges financial support by the UK Engineering and Physical Sciences Research Council (grant APEXEP/H040218/2).

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Results and Discussion
  5. 3. Conclusions
  6. 4. Experimental Section
  7. Supporting Information
  8. Acknowledgements
  9. Supporting Information

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