Electronic States in Dilute Ternary Blend Organic Bulk Heterojunction

Article pubs.acs.org/JPCC

Electronic States in Dilute Ternary Blend Organic Bulk Heterojunction Solar Cells Robert A. Street,*,† Petr P. Khlyabich,‡ Andrey E. Rudenko,‡ and Barry C. Thompson*,‡ †

Palo Alto Research Center, Palo Alto, California 94304, United States Department of Chemistry and Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089-1661, United States

‡

S Supporting Information *

ABSTRACT: Electronic states and electronic excitations in a molecular solid such as an organic bulk heterojunction solar cell either may reflect the properties of individual molecules or may be delocalized over several molecules, exhibiting alloy properties of the average composition. Measurements of a variety of dilute ternary blend organic solar cells based on either two polymer donors and one fullerene acceptor or one polymer donor and two fullerene acceptors provide information about the degree of localization in different situations. In the two polymer case, where the polymers are well intermixed, excitons have molecular characteristics. Despite their localization, excitons from the dilute low band gap component readily diffuse to the heterojunction interface and generate mobile charge, and their diffusion is attributed to rod percolation. Mobile holes are delocalized, and the blend concentration dependence suggests delocalization over about 10 polymer molecules. In contrast, with poorly intermixed polymers, low band gap excitons are unable to diffuse and exhibit no charge generation. With fullerene mixtures, two different behaviors are also observed. Mixtures of PC61BM and ICBA exhibit delocalized alloy states, while dilute PC84BM in PC61BM mixtures exhibits localized trap states. The difference is attributed to the size mismatch of the larger PC84BM molecule.

1. INTRODUCTION Organic bulk heterojunctions (BHJ)1−8 comprise a phaseseparated blend of a polymer donor D and a fullerene acceptor A. Nanoscale phase separation9−14 is the key enabler of the organic solar cell because it allows excitons to dissociate at the internal heterojunction interface and generate mobile free carriers.15−22 The addition of a second polymer or fullerene to make ternary blends of the form D1XD2(1−X):A23−41 or D:A1XA2(1−X)24,26,42−48 allows additional control over the optoelectronic properties and optimization of the solar cell. Previous studies found that the electronic properties of the blend depend critically on the mixing properties of the blend, with two limiting cases.23−25,42 The blend may exhibit alloy properties, characterized by electronic states that reflect the average composition of the synergistic components (D1XD2(1−X) or A1XA2(1−X)), which requires well intermixed blends of these components and delocalized electronic states.23,24,26,42 The other limiting case is a blend with molecular properties, characterized by electronic states reflecting the individual molecules unmodified by the presence of the second material. 25 This situation occurs when intermixing is incomplete and/or when the electronic states are highly localized to the individual molecules.25 In blends that exhibit molecular electronic states, the component with the smaller energy band gap will introduce a trap state within the band gap of the higher gap component, with adverse effects on © XXXX American Chemical Society

the solar cell properties. Blends that have alloy properties will not have these trap states but may show disorder effects at low concentrations of the small band gap component, when the average separation of molecules of the dilute component exceeds the localization length. Studies of the low band gap component at dilute concentrations are therefore of interest. The present measurements study ternary blends with low concentrations (i.e., 1−10%) of one of the components. The specific properties of dilute mixtures provide further insight into the electronic states, including the mechanisms of exciton diffusion as well as electron and hole localization. One system is of the type D1XD2(1−X):A, with both well23 and poorly intermixed polymers.25 The well intermixed system is of interest because the polymer HOMO level showed alloy behavior, while the polymer exciton absorption exhibits molecular behavior, attributed to its stronger localization on a single molecule, compared to the more delocalized holes.23 A second system is of the type D:A1XA2(1−X), and we also compare two alternative fullerene mixtures: one shows alloy properties42 and the other shows localized molecular properties for the LUMO states.49 Received: September 2, 2014 Revised: October 24, 2014

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dx.doi.org/10.1021/jp5088724 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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The miscible D1XD2(1−X):A system23 consists of the low band gap polymer poly(3-hexylthiophene−thiophene−diketopyrrolopyrrole) (P3HTT-DPP-10%)50 as D1 and large band gap poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25)51 as D2. The similarity of the polymer chemical structures, which are shown in Figure 1a, supports

component adds discrete electronic states or is incorporated as an alloy.

2. RESULTS In order to probe the influence of the dilute component, measurements are made of the solar cell current−voltage characteristics, the external quantum efficiency spectrum (EQE), and photocurrent spectral response (PSR). These experimental techniques are described elsewhere.53−56 Both PSR and EQE measure the absorption spectrum of those transitions that generate mobile free carriers.56 These are similar measurements differing only in that EQE provides an accurate absolute magnitude in the high absorption range, and PSR provides high sensitivity to observe weak absorption at low energy. Hence PSR, EQE, and short-circuit current density (Jsc) provide information about the relative optical absorption and charge generation contributions of the synergistic components of the ternary blends. The PSR measurement of the chargetransfer (CT) absorption20−22 along with open-circuit voltage (Voc)57 provides information about the electronic states, and the solar cell fill factor (FF)58 reflects recombination in the mixed system.59,60 2.1. Miscible Polymer:Polymer:Fullerene Blends. Figure 1b,c shows the PSR spectra of the miscible D1XD2(1−X):A system23 for small admixtures of P3HTT-DPP-10% in the range 1−10% in P3HT75-co-EHT25. These two polymers have bulk exciton absorption at well separated energies, and previous studies show that the excitons are molecular, retaining their unique spectral response across the composition range, even though mobile holes display alloy properties with the HOMO energy shifting continuously with composition.24 The onset of exciton absorption for P3HT75-co-EHT25 is 1.9 eV51 and for P3HTT-DPP-10% is ∼1.5 eV.50 In the dilute composition regime, the P3HTT-DPP-10% absorption decreases in intensity with composition x, but is easily observed at 3% composition, and just detectable at 1%. Figure 2a,b shows the strength of the PSR signal arising from the P3HTT-DPP-10% as a function of composition. The values are obtained from the PSR data at two different photon energies within the low band gap exciton absorption band, with the signal from P3HT75-co-EHT25 subtracted. The log plot in Figure 2b shows that the PSR signal decreases linearly with composition down to about x = 5% and then decreases faster, approximately as the square of the composition (solid line), below 5%. The data therefore show that roughly 50% of the excitons reach the interface and generate a photocurrent at a dilution of 3% and about 20% of the excitons at 1% dilution. Table 1 shows the solar cell performance of the miscible polymer system, with the average values of Jsc, Voc, FF, and efficiency (η) obtained under simulated AM 1.5G illumination (100 mW/cm2). The Voc changes slightly upon incorporation of just 1 wt % of P3HTT-DPP-10% and decreases continuously with the increase of the P3HT-DPP-10%, as shown in Figure 3a. Jsc decreases slightly with composition and then recovers, while the FF remains constant, indicating that there is no significant introduction of traps that might cause recombination or reduced carrier mobility. EQE measurements, shown in Figure 3b, are similar to PSR but give more detail in the region of high absorption. The incorporation of a small amount (less than 5%) of the low band gap P3HTT-DPP-10% in the ternary blend leads to a small increase of the photoresponse in the near-IR, which is accompanied by the small decrease in the visible part of the

Figure 1. (a) Structures of P3HT75-co-EHT25, P3HTT-DPP-10%, and PC61BM. Photocurrent spectral response (PSR) spectra in the region of the polymer exciton absorption for various dilute concentrations of P3HTT-DPP-10%: (b) log vertical scale and (c) linear vertical scale.

their intermixing. The nonmiscible system (Figure 4a) comprises poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT)52 as the high band gap component of P3HTT-DPP10%X:PCDTBT(1−X):PC61BM. For the systems based on fullerene mixtures, one system of the type D:A1XA2(1−X) consists of poly(3-hexylthiophene) (P3HT) as the donor, with phenyl-C61-butyric acid methyl ester (PC61BM) as A1 and indene-C60 bisadduct (ICBA) as A2 (Figure 6a).42 This is compared with a previously published second system in which the fullerenes are PC61BM and phenyl-C84-butyric acid methyl ester (PC84BM).49 The studies focus on low concentrations of the low band gap component to address whether the dilute B



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Figure 2. (a) Concentration dependence of the additional PSR signal at 1.6 eV (crosses) and 1.7 eV (circles) arising from low concentrations of P3HTT-DPP-10%. The data are normalized to the PSR value at 20% P3HTT-DPP-10% concentration. (b) Same data plotted on a log−log scale. The solid line has a slope 2, and the dashed line has slope 1.

Figure 3. (a) Open-circuit voltage (Voc) of the ternary blend BHJ solar cells as a function of the amount of P3HT75-co-EHT25 in the blends. (b) External quantum efficiency of ternary blend BHJ solar cells (P3HTT-DPP-10%:P3HT75-co-EHT25:PC61BM), where black line is 0:1:8; red line is 0.01:0.99:0.81; blue line is 0.03:0.97:0.81; orange line is 0.05:0.95:0.84; and green line is 0.1:0.9:0.9.

Table 1. Photovoltaic Properties of P3HTT-DPP10%:P3HT75-co-EHT25:PC61BM Ternary Blend BHJ Solar Cells at Different Polymer Ratios P3HTT-DPP-10%:P3HT75-coEHT25:PC61BMa

Jscb (mA/cm2)

Voc (V)

FF

η (%)

0.1:0.9:0.9 0.05:0.95:0.84 0.03:0.97:0.81 0.01:0.99:0.81 0:1:0.8

9.43 9.15 9.28 9.37 9.52

0.642 0.646 0.648 0.658 0.663

0.59 0.57 0.57 0.58 0.57

3.56 3.36 3.44 3.55 3.58

a All devices were spin-coated from o-dichlorobenzene (o-DCB) and dried under N2 for 30 min before aluminum deposition. bMismatch corrected.

solar spectrum. The addition of 10% of P3HTT-DPP-10% in the ternary blend causes a noticeable enhancement of the photocurrent in the near-IR as well as almost full recovery of the EQE response in the visible. The solar cell, PSR, and EQE results show that excitons generated in the diluted phase (P3HTT-DPP-10%) reach the donor:acceptor interface with good efficiency and dissociate to produce free charge carriers, beginning from as low as 1% of the P3HTT-DPP-10% in the three component blend. 2.2. Nonmiscible Polymer:Polymer:Fullerene Blends. Figure 4 shows the polymers and the contrasting PSR spectra in the P3HTT-DPP-10%X:PCDTBT(1−X):PC61BM blend that is known not to intermix well.25 Figure 4b shows PSR data of the 5% and 10% compositions of the low band gap polymer and the

Figure 4. (a) Structures of PCDTBT, P3HTT-DPP-10%, and PC61BM. (b) PSR spectra for the P3HTT-DPP10%X:PCDTBT(1−X):PC61BM composition for various values of X.

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two end point materials. Instead of the low gap material increasing the measured PSR, as in Figure 1, there is a region from 1.6 to 2 eV where the PSR decreases significantly below that of the cell with the 100% high band gap material, showing that absorption in the low band gap polymer does not generate a photocurrent and even suppresses the photocurrent from the high band gap material. The ternary P3HTT-DPP-10%X:PCDTBT(1−X):PC61BM solar cells also show a striking difference in current−voltage characteristics and EQE compared to P3HTT-DPP10%:P3HT75-co-EHT25:PC61BM solar cells, as shown in Table 2 and Figure 5. The Voc decreases abruptly (see Figure 5a) and Table 2. Photovoltaic Properties of P3HTT-DPP10%:PCDTBT:PC61BM Ternary Blend BHJ Solar Cells at Different Polymer Ratios P3HTT-DPP10%:PCDTBT:PC61BMa

Jscb (mA/cm2)

Voc (V)

FF

η (%)

0.1:0.9:1.3 0.05:0.95:1.3 0.04:0.96:1.3 0.03:0.97:1.3 0.02:0.98:1.3 0.01:0.99:1.3 0:1:1.3

6.30 6.44 6.47 6.94 9.06 9.14 9.89

0.594 0.605 0.637 0.671 0.846 0.888 0.924

0.34 0.37 0.34 0.34 0.45 0.45 0.50

1.21 1.30 1.32 1.62 3.61 3.70 4.47

a

All devices were spin-coated from o-dichlorobenzene (o-DCB) and dried under N2 for 30 min before aluminum deposition. bMismatch corrected.

Figure 5. (a) Open-circuit voltage (Voc) of the ternary blend BHJ solar cells as a function of the amount of PCDTBT in the blends. (b) External quantum efficiency of ternary blend BHJ solar cells (P3HTTDPP-10%:PCDTBT:PC61BM) where (i) is 0:1:1.3 (black line), (ii) is 0.01:0.99:1.3 (red line), (iii) is 0.02:0.98:1.3 (green line), (iv) is 0.03:0.97:1.3 (blue line), (v) is 0.04:0.96:1.3 (cyan line), (vi) is 0.05:0.95:1.3 (orange line), and (vii) is 0.1:0.9:1.3 (olive line).

almost reaches the Voc of corresponding binary blend of 0.582 V for the P3HTT-DPP-10%:PC61BM upon adding of just 5% of the low band gap polymer in the three component blend. Jsc also decreases rapidly with the increase of P3HTT-DPP10% in the blend and drops 35% at x = 5%. The EQE data in Figure 5b show that the incorporation of the low band gap polymer does not lead to a photoresponse in the near-IR (700−850 nm), while the photocurrent in the visible is significantly decreased. The large decrease in FF shows that the current−voltage characteristics are greatly degraded in this nonmiscible polymer blend. In general, a low FF typically reflects an increase in recombination or an increase in series resistance,61 and in either case the likely cause is the introduction of trapping states. These changes therefore strongly indicate that the low band gap polymer forms a localized trap state rather than exhibiting alloy behavior of the HOMO levels. The sharp drop in Voc also indicates the introduction of trap states. 2.3. Polymer:Fullerene:Fullerene Blends. Figure 6 shows PSR data for two examples of BHJ structures with fullerene blends D:A1XA2(1−X) at low concentrations of A1. Figure 6b is the blend of PC61BM and ICBA with a P3HT donor. As the concentration of PC61BM increases up to 10%, there is a steady shift of the CT absorption band to lower energy, but no change in the shape of the CT absorption band. The shift is consistent with the trend previously observed over the full composition range which was explained by the alloy behavior of the fullerene LUMO level.24 The new data show that the alloy behavior continues to low concentrations of PC61BM. Previous data also show that the FF remains constant indicating no introduction of additional localized trap states.42 In contrast, Figure 6c shows comparable PSR data for a blend of PC84BM in PC61BM, with a PCDTBT polymer,

previously reported by Street et al.49 In this material system, a distinct new feature in the CT absorption band is observed at lower energy, which increases in intensity proportional to the concentration of PC84BM. The feature is explained by the PC84BM forming a localized trap state about 0.2 eV below the PC61BM LUMO energy. In addition, the cell current−voltage properties degrade rapidly with the addition of PC84BM, indicating either enhanced recombination or enhanced series resistance. Hence, the alloy effect is not observed in this system in the 1−10% composition range, but instead PC84BM behaves as a molecular dopant. However, the PSR data show that optical transition to this localized trap state generate a photocurrent, since otherwise it would not be observed.

3. DISCUSSION 3.1. Miscible Polymer:Polymer:Fullerene Blends. The PSR and solar cell data show that the previously established properties of the miscible P3HTT-DPP-10%:P3HT75-coEHT25:PC61BM ternary blend23,24 extend to dilute alloys. The most interesting feature of the results is the contribution of the low band gap exciton absorption to the photocurrent. Although the polymers in Figure 1 are well intermixed, the exciton absorption of the individual polymers is observed because the exciton is highly localized on a single polymer molecule. Since PSR is a measurement of the mobile electrons and holes, it selectively measures the absorption of excitons that ultimately reach the interface and dissociate. At high concentrations of P3HTT-DPP-10%, excitons can readily D



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and rod-like. Calculations of rod percolation indicate that the conductivity increases as the square of the concentration.62,63 The low concentration data plotted in Figure 2b are consistent with this relation. The second diffusion mechanism is Forster energy transfer.64 This mechanism has a typical range of 1−2 nm, beyond which the transfer rate drops as the inverse sixth power of separation. The range is probably too small to allow the exciton to reach the heterojunction interface in a single transfer step, but the mechanism can greatly increase the percolation rate, since the exciton does not need to transfer to a nearest-neighbor molecule but can transfer significantly further. Figure 7 illustrates the proposed mechanism for a semicrystalline polymer morphology in which the polymers are well intermixed.

Figure 7. Schematic illustration of exciton percolation (blue dashed line) through dilute polymer molecules (red lines) and assuming a semicrystalline polymer structure.

Our previous study on this miscible polymer pair showed that the Voc changes continuously across the full composition range, characteristic of alloy properties.24 However, the previous data also indicated that there is a larger change in Voc between 1 and 10% composition than across the rest of the composition range,24 and this difference is confirmed by the present more detailed study of dilute mixtures (Table 1 and Figure 3a). The probable explanation is that the alloy model for the HOMO level starts to break down for compositions below about 10%. Breakdown of the model is expected when the concentration is such that the hole wave function delocalization65,66 is not sufficient to reflect the average composition. The observations therefore suggest that hole delocalization is on the order of 10 molecular units, which for the π-stacked polymer is about 3 nm. 3.2. Nonmiscible Polymer:Polymer:Fullerene Blends. The data in Figure 4 and 5 show a ternary blend in which the addition of a low band gap polymer suppresses the PSR and EQE rather than enhancing it. This situation arises because the low band gap polymer absorbs the incident light, but the exciton evidently has a very low probability to reach the interface and dissociate.25 The absorption coefficient of the low gap polymer is much larger than of the high gap polymer in the energy range below 2 eV. Hence, the presence of a relatively small low gap polymer concentration suppresses the absorption of the high gap polymer and therefore reduces the corresponding PSR signal (see the optical absorption model in the Supporting Information). The absorption of the low gap polymer does not result in a PSR signal because the blend is poorly intermixed. Instead, the low concentration polymer tends to aggregate and prevents percolation of the exciton to the interface. Excitons generated in the P3HTT-DPP-10% phase are unable to contribute to photocurrent generation

Figure 6. (a) Structures of ICBA, PC61BM, and PC84BM. (b) PSR spectra of P3HT:PCBMX:ICBA1−X, for X = 1−10%. The inset shows the shift of the CT band with composition. (c) Similar PSR data for PCDTBT:PC84BMX:PC61BM1−X. The inset shows the increase of the 1.2 eV band with composition (from ref 49).

diffuse from molecule to molecule to reach the interface. However, the low band gap P3HTT-DPP-10% excitons cannot diffuse through the high band gap P3HT75-co-EHT25 molecules because of the large absorption energy difference. The surprising implication of the dilute mixture data is that efficient exciton diffusion occurs through the network of P3HTT-DPP10% molecules, even when the P3HTT-DPP-10% concentration is very low. A more detailed analysis of the absorption in the blend, and a model of the observed absorption confirming these conclusions is given in the Supporting Information. Two mechanisms can account for the exciton diffusion between the low concentration molecules. One explanation is the low percolation threshold in rod-like systems. The threshold concentration for percolation in an assembly of approximately spherical particles is about 16%, and so diffusion at a concentration of 1−3% should not be possible. However, the percolation threshold concentration decreases dramatically for conduction between rods and can be much less than 1%.62,63 In the miscible system, the two polymers are randomly dispersed and the polymer molecules are approximately linear E



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blend. The obvious explanation is that PC61BM and ICBA are molecules of very similar size and can intermix easily, while PC84BM is substantially larger than PC61BM. The dilute PC84BM will therefore disrupt the local surroundings, disordering the neighboring PC61BM and hence creating a localized state, as illustrated in Figure 9. It is possible that the size difference also causes local aggregation of the PC84BM, further suppressing the alloy effect.

because the lack of miscibility suppresses exciton diffusion among the dilute phase. The PSR signal recovers below 1.6 eV because the photocurrent arises from the CT transition at the interface and therefore no longer depends on the exciton diffusion and dissociation. The solar cell characteristics support this interpretation. Jsc drops abruptly for small admixtures of the low band gap polymer, showing that this material does not contribute to the photocurrent and indeed suppresses the photocurrent from the high band gap material. Voc also drops abruptly and is 0.3 V lower at the 5% composition, suggesting that a deep localized trap state is formed in the HOMO level by the dilute polymer,. As noted earlier, the abrupt drop in FF also indicates the introduction of localized states. Figure 8 illustrates the expected effect of traps of different depth from the band edge transport energy on carrier transport,

Figure 9. Schematic diagram showing that the large PC84BM molecule disrupts the PC61BM local order (right), while ICBA has less effect (left) because it is nearly the same size as PC61BM. A simple cubic structure is assumed for convenience.

Excitation to the PC84BM localized state evidently generates a photocurrent, since the transition is observed in the PSR spectrum. Our proposed explanation for the difference between this behavior and the nonmiscible polymer case is that the PC84BM localized state is a shallow trap as illustrated in Figure 8. Electrons that are optically excited into the shallow traps by the CT absorption, are thermally excited into the electron transport states with high probability, and generate a photocurrent. Evidence for the shallower trap state is that a 10% concentration of the mixture shifts Voc by only 0.19 V in the case of the fullerenes49 but 0.33 V for the nonmiscible polymers.

Figure 8. Schematic diagram comparing the effects of shallow and deep traps on the transport and recombination. The relative probability of thermal excitation to the transport energy compared to recombination from the trap is much larger for carriers in shallow traps compared to deep traps.

photoconductivity, and recombination. The rate of thermal excitation out of a trap at temperature T is proportional to exp(−ET/kT), where ET is the trap depth. Hence, a carrier trapped in a shallow trap will be released quickly and the competing mechanism of recombination from this trap is less probable. The release rate from a deep trap is much smaller, and hence recombination is relatively more probable. When a CT optical transition excites a carrier directly into a shallow trap, then the trapped carrier is likely to be thermally excited to the transport energy, leading to the observation of photoconductivity. However, the trapped carrier is more likely to recombine before thermal excitation when the trap is deep, thus suppressing photoconductivity. The CT absorption in the PSR spectra of Figure 4 is hardly changed at 5% and only shifts to lower energy at higher compositions and hence does not track the much larger changes in Voc. Our proposed explanation is that the trap energy is sufficiently deep that photoconductivity is suppressed by the mechanism just discussed. 3.3. Polymer:Fullerene:Fullerene Blends. The uniform shift of the CT optical absorption in Figure 6b shows that the P3HT:PC61BMX:ICBA1−X blend evidently forms an alloy system, even at the small PC61BM compositions of 3−5%. The electron wave function must therefore encompass of order 20−30 fullerene molecules and hence extend out to between the second- and third-nearest neighbor of the central site. An electron delocalization of ∼1.5 nm is therefore indicated. In contrast, the data in Figure 6c show that PC84BM does not form an alloy with PC61BM, but instead creates localized trap states separated from the PC61BM LUMO level. Hence, these two fullerenes have different effects in a low concentration

4. CONCLUSIONS Dilute blends in ternary organic bulk heterojunction solar cells give detailed information about the electronic states of mobile carriers and excitons as well as the miscibility of the blend. Excitons originating from a dilute low band gap polymer in a polymer mixture diffuse easily in miscible systems23 but not in nonmiscible systems,25 which is a consequence of their morphological and/or aggregation properties. We attribute the exciton diffusion within the highly dilute low band gap polymer to a combination of rod percolation and Forster energy transfer. The dilute blends also probe the localization of the mobile carriers by indicating the transition from alloy properties when the electronic state is characteristic of the average composition to a more molecular state. This transition is strikingly different in miscible 23 and nonmiscible polymer blends.25 The distinction between alloy or molecular properties in fullerene mixtures25,49 is correlated to the difference in molecular size. The localization of electronic states and material miscibility have a very strong influence on the solar cells properties.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic, solar cell fabrication procedures, and optical modeling data. This material is available free of charge via the Internet at http://pubs.acs.org. F



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Role of Structural and Electrostatic Disorder. Phys. Chem. Chem. Phys. 2014, 16, 20279−20290. (17) Deibel, C.; Dyakonov, V. Polymer−Fullerene Bulk Heterojunction Solar Cells. Rep. Prog. Phys. 2010, 73, 096401. (18) Kippelen, B.; Brédas, J.-L. Organic Photovoltaics. Energy Environ. Sci. 2009, 2, 251−261. (19) Dimitrov, S. D.; Durrant, J. R. Materials Design Considerations for Charge Generation in Organic Solar Cells. Chem. Mater. 2014, 26, 616−630. (20) Deibel, C.; Strobel, T.; Dyakonov, V. Role of the Charge Transfer State in Organic Donor-Acceptor Solar Cells. Adv. Mater. 2010, 22, 4097−4111. (21) Deibel, C. Photocurrent Generation in Organic Solar Cells. Semicond. Semimetals 2011, 85, 297−330. (22) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736−6767. (23) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Compositional Dependence of the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells Based on Two Donor Polymers. J. Am. Chem. Soc. 2012, 134, 9074−9077. (24) Street, R. A.; Davies, D.; Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Origin of the Tunable Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Organic Solar Cells. J. Am. Chem. Soc. 2013, 135, 986−989. (25) Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. Influence of Polymer Compatibility on the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 9913−9919. (26) Khlyabich, P. P.; Burkhart, B.; Rudenko, A. E.; Thompson, B. C. Optimization and Simplification of Polymer−Fullerene Solar Cells Through Polymer and Active Layer Design. Polymer 2013, 54, 5267− 5298. (27) Ameri, T.; Min, J.; Li, N.; Machui, F.; Baran, D.; Forster, M.; Schottler, K. J.; Dolfen, D.; Scherf, U.; Brabec, C. J. Performance Enhancement of the P3HT/PCBM Solar Cells through NIR Sensitization Using a Small-Bandgap Polymer. Adv. Energy Mater. 2012, 2, 1198−1202. (28) Yang, L.; Zhou, H.; Price, S. C.; You, W. Parallel-like Bulk Heterojunction Polymer Solar Cells. J. Am. Chem. Soc. 2012, 134, 5432−5435. (29) Koppe, M.; Egelhaaf, H.-J.; Dennler, G.; Scharber, M. C.; Brabec, C. J.; Schilinsky, P.; Hoth, C. N. Near IR Sensitization of Organic Bulk Heterojunction Solar Cells: Towards Optimization of the Spectral Response of Organic Solar Cells. Adv. Funct. Mater. 2010, 20, 338−346. (30) Honda, S.; Ohkita, H.; Benten, H.; Ito, S. Selective Dye Loading at the Heterojunction in Polymer/Fullerene Solar Cells. Adv. Energy Mater. 2011, 1, 588−598. (31) Huang, J.-H.; Velusamy, M.; Ho, K.-C.; Lin, J.-T.; Chu, C.-W. A Ternary Cascade Structure Enhances the Efficiency of Polymer Solar Cells. J. Mater. Chem. 2010, 20, 2820−2825. (32) Ananda Rao, B.; Sasi Kumar, M.; Sivakumar, G.; Singh, S. P.; Bhanuprakash, K.; Rao, V. J.; Sharma, G. D. Effect of Incorporation of Squaraine Dye on the Photovoltaic Response of Bulk Heterojunction Solar Cells Based on P3HT:PC70BM Blend. ACS Sustainable Chem. Eng. 2014, 2, 1743−1751. (33) Lim, B.; Bloking, J. T.; Ponec, A.; McGehee, M. D.; Sellinger, A. Ternary Bulk Heterojunction Solar Cells: Addition of Soluble NIR Dyes for Photocurrent Generation beyond 800 nm. ACS Appl. Mater. Interfaces 2014, 6, 6905−6913. (34) Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J. Organic Ternary Solar Cells: A Review. Adv. Mater. 2013, 25, 4245−4266. (35) Chen, Y.-C.; Hsu, C.-Y.; Lin, R. Y.-Y.; Ho, K.-C.; Lin, J. T. Materials for the Active Layer of Organic Photovoltaics: Ternary Solar Cell Approach. ChemSusChem 2013, 6, 20−35. (36) Goubard, F.; Wantz, G. Ternary Blends for Polymer Bulk Heterojunction Solar Cells: Polymer Bulk Heterojunction Solar Cells. Polym. Int. 2014, 63, 1362−1367.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel 650-812-4165 (R.A.S.). *E-mail [email protected]; Tel 213-821-2656 (B.C.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DESC0001013, specifically for partial support of P.P.K., A.E.R., and B.C.T.

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REFERENCES

(1) Thompson, B. C.; Khlyabich, P. P.; Burkhart, B.; Aviles, A. E.; Rudenko, A.; Shultz, G. V.; Ng, C. F.; Mangubat, L. B. Polymer-Based Solar Cells: State-of-the-Art Principles for the Design of Active Layer Components. Green 2011, 1, 29−54. (2) Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia, S.; Williams, S. P. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2010, 22, 3839−3856. (3) Dou, L.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R. A.; Yang, Y. 25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 2013, 25, 6642−6671. (4) Schlenker, C. W.; Thompson, M. E. Current Challenges in Organic Photovoltaic Solar Energy Conversion. Top. Curr. Chem. 2012, 312, 175−212. (5) Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10−28. (6) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153−161. (7) Beaujuge, P. M.; Fréchet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009−20029. (8) Krebs, F. C.; Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. 25th Anniversary Article: Rise to Power - OPV-Based Solar Parks. Adv. Mater. 2014, 26, 29−39. (9) He, M.; Wang, M.; Lin, C.; Lin, Z. Optimization of Molecular Organization and Nanoscale Morphology for High Performance Low Bandgap Polymer Solar Cells. Nanoscale 2014, 6, 3984−3994. (10) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114, 7006−7043. (11) Van Bavel, S.; Veenstra, S.; Loos, J. On the Importance of Morphology Control in Polymer Solar Cells. Macromol. Rapid Commun. 2010, 31, 1835−1845. (12) Liu, F.; Gu, Y.; Jung, J. W.; Jo, W. H.; Russell, T. P. On the Morphology of Polymer-Based Photovoltaics. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 1018−1044. (13) Liu, F.; Gu, Y.; Shen, X.; Ferdous, S.; Wang, H.-W.; Russell, T. P. Characterization of the Morphology of Solution-Processed Bulk Heterojunction Organic Photovoltaics. Prog. Polym. Sci. 2013, 38, 1990−2052. (14) Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Controlling the Morphology and Performance of Bulk Heterojunctions in Solar Cells. Lessons Learned from the Benchmark Poly(3-hexylthiophene):[6,6]Phenyl-C61-Butyric Acid Methyl Ester System. Chem. Rev. 2013, 113, 3734−3765. (15) Gao, F.; Inganäs, O. Charge Generation in Polymer−Fullerene Bulk-Heterojunction Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 20291−20304. (16) Castet, F.; D’Avino, G.; Muccioli, L.; Cornil, J.; Beljonne, D. Charge Separation Energetics at Organic Heterojunctions: On the G



Article

(57) Qi, B.; Wang, J. Open-Circuit Voltage in Organic Solar Cells. J. Mater. Chem. 2012, 22, 24315−24325. (58) Qi, B.; Wang, J. Fill Factor in Organic Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 8972−8982. (59) Lakhwani, G.; Rao, A.; Friend, R. H. Bimolecular Recombination in Organic Photovoltaics. Annu. Rev. Phys. Chem. 2014, 65, 557− 581. (60) Proctor, C. M.; Kuik, M.; Nguyen, T.-Q. Charge Carrier Recombination in Organic Solar Cells. Prog. Polym. Sci. 2013, 38, 1941−1960. (61) Guerrero, A.; Ripolles-Sanchis, T.; Boix, P. P.; Garcia-Belmonte, G. Series Resistance in Organic Bulk-Heterrojunction Solar Devices: Modulating Carrier Transport with Fullerene Electron Traps. Org. Electron. 2012, 13, 2326−2332. (62) Obukhov, S. First Order Rigidity Transition in Random Rod Networks. Phys. Rev. Lett. 1995, 74, 4472−4475. (63) Bryning, M. B.; Islam, M. F.; Kikkawa, J. M.; Yodh, A. G. Very Low Conductivity Threshold in Bulk Isotropic Single-Walled Carbon Nanotube-Epoxy Composites. Adv. Mater. 2005, 17, 1186−1191. (64) Feron, K.; Belcher, W.; Fell, C.; Dastoor, P. Organic Solar Cells: Understanding the Role of Forster Resonance Energy Transfer. Int. J. Mol. Sci. 2012, 13, 17019−17047. (65) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335, 1340−1344. (66) Kaake, L. G.; Jasieniak, J. J.; Bakus, R. C.; Welch, G. C.; Moses, D.; Bazan, G. C.; Heeger, A. J. Photoinduced Charge Generation in a Molecular Bulk Heterojunction Material. J. Am. Chem. Soc. 2012, 134, 19828−19838.

(37) Yang, L.; Yan, L.; You, W. Organic Solar Cells Beyond One Pair of Donor−Acceptor: Ternary Blends and More. J. Phys. Chem. Lett. 2013, 4, 1802−1810. (38) Ameri, T.; Heumüller, T.; Min, J.; Li, N.; Matt, G.; Scherf, U.; Brabec, C. J. IR Sensitization of an Indene-C60 Bisadduct (ICBA) in Ternary Organic Solar Cells. Energy Environ. Sci. 2013, 6, 1796−1801. (39) Koppe, M.; Egelhaaf, H.-J.; Clodic, E.; Morana, M.; Lüer, L.; Troeger, A.; Sgobba, V.; Guldi, D. M.; Ameri, T.; Brabec, C. J. Charge Carrier Dynamics in a Ternary Bulk Heterojunction System Consisting of P3HT, Fullerene, and a Low Bandgap Polymer. Adv. Energy Mater. 2013, 3, 949−958. (40) Cha, H.; Chung, D. S.; Bae, S. Y.; Lee, M.-J.; An, T. K.; Hwang, J.; Kim, K. H.; Kim, Y.-H.; Choi, D. H.; Park, C. E. Complementary Absorbing Star-Shaped Small Molecules for the Preparation of Ternary Cascade Energy Structures in Organic Photovoltaic Cells. Adv. Funct. Mater. 2013, 23, 1556−1565. (41) Min, J.; Ameri, T.; Gresser, R.; Lorenz-Rothe, M.; Baran, D.; Troeger, A.; Sgobba, V.; Leo, K.; Riede, M.; Guldi, D. M.; et al. Two Similar Near-Infrared (IR) Absorbing Benzannulated Aza-BODIPY Dyes as Near-IR Sensitizers for Ternary Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 5609−5616. (42) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Efficient Ternary Blend Bulk Heterojunction Solar Cells with Tunable OpenCircuit Voltage. J. Am. Chem. Soc. 2011, 133, 14534−14537. (43) Cheng, P.; Li, Y.; Zhan, X. Efficient Ternary Blend Polymer Solar Cells with Indene-C60 Bisadduct as an Electron-Cascade Acceptor. Energy Environ. Sci. 2014, 7, 2005−2011. (44) Xu, H.; Ohkita, H.; Benten, H.; Ito, S. Open-Circuit Voltage of Ternary Blend Polymer Solar Cells. Jpn. J. Appl. Phys. 2014, 53, 01AB10. (45) Jeltsch, K. F.; Schädel, M.; Bonekamp, J.-B.; Niyamakom, P.; Rauscher, F.; Lademann, H. W. A.; Dumsch, I.; Allard, S.; Scherf, U.; Meerholz, K. Efficiency Enhanced Hybrid Solar Cells Using a Blend of Quantum Dots and Nanorods. Adv. Funct. Mater. 2012, 22, 397−404. (46) Li, H.; Zhang, Z.-G.; Li, Y.; Wang, J. Tunable Open-Circuit Voltage in Ternary Organic Solar Cells. Appl. Phys. Lett. 2012, 101, 163302. (47) Chen, L.; Yao, K.; Chen, Y. Can Morphology Tailoring Based on Functionalized Fullerene Nanostructures Improve the Performance of Organic Solar Cells? J. Mater. Chem. 2012, 22, 18768−18771. (48) Kang, H.; Kim, K.-H.; Kang, T. E.; Cho, C.-H.; Park, S.; Yoon, S. C.; Kim, B. J. Effect of Fullerene Tris-Adducts on the Photovoltaic Performance of P3HT:Fullerene Ternary Blends. ACS Appl. Mater. Interfaces 2013, 5, 4401−4408. (49) Street, R. A.; Krakaris, A.; Cowan, S. R. Recombination Through Different Types of Localized States in Organic Solar Cells. Adv. Funct. Mater. 2012, 22, 4608−4619. (50) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Efficient Solar Cells from Semi-Random P3HT Analogues Incorporating Diketopyrrolopyrrole. Macromolecules 2011, 44, 5079−5084. (51) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Influence of the Ethylhexyl Side-Chain Content on the Open-Circuit Voltage in rrPoly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) Copolymers. Macromolecules 2012, 45, 3740−3748. (52) Beaupré, S.; Leclerc, M. PCDTBT: En Route for Low Cost Plastic Solar Cells. J. Mater. Chem. A 2013, 1, 11097−11105. (53) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Accurate Measurement and Characterization of Organic Solar Cells. Adv. Funct. Mater. 2006, 16, 2016−2023. (54) Snaith, H. J. How Should You Measure Your Excitonic Solar Cells? Energy Environ. Sci. 2012, 5, 6513−6520. (55) Gevorgyan, S. A.; Eggert Carlé, J.; Søndergaard, R.; Trofod Larsen-Olsen, T.; Jørgensen, M.; Krebs, F. C. Accurate Characterization of OPVs: Device Masking and Different Solar Simulators. Sol. Energy Mater. Sol. Cells 2013, 110, 24−35. (56) Street, R. A.; Song, K. W.; Northrup, J. E.; Cowan, S. Photoconductivity Measurements of the Electronic Structure of Organic Solar Cells. Phys. Rev. B 2011, 83, 165207. H


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