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Phase-Dependent Photocurrent Generation in Polymer/Fullerene Bulk Heterojunction Solar Cells Thomas J. K. Brenner,† Zhe Li,† and Christopher R. McNeill*,‡ † ‡
Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom Department of Materials Engineering, Monash University, Wellington Rd, Clayton, VIC 3800, Australia
bS Supporting Information ABSTRACT: Voltage-dependent white-light bias external quantum efficiency (EQE) measurements are employed to compare the relative voltage-dependence of photocurrent generation of polymer and fullerene phases in polymer bulk heterojunction solar cells. By measuring EQE spectra as a function of applied bias, voltage dependent changes in the shape of EQE spectra can be detected and compared with the absorption profiles of polymer and fullerene. Several high-efficiency systems are studied including poly(3-hexylthiophene):PC60BM ([6,6]-phenylC61-butyric acid methyl ester) blends and blends of low band gap polymers with PC70BM. For a number of systems (but not all), voltage-dependent but light-intensity-independent changes in the shape of the EQE spectrum are observed that originate from differences in the voltage dependence of photocurrent for polymer and fullerene absorption. For one system, a stronger voltagedependence of photocurrent generation following fullerene absorption is found, whereas for another system a stronger voltagedependence of photocurrent generation following polymer absorption is observed. We attribute this effect to differences in the character of interfacial electronhole pairs formed immediately after charge transfer, sensitive to differences in the nature of the initial excitonic state and the conformation nature of the interface.
’ INTRODUCTION Polymer-based bulk heterojunction solar cells have now achieved power conversion efficiencies of >8%1 closing in on the 10% efficiency milestone. Whereas device efficiencies are nearing those required for commercialization, much remains to be understood regarding device operation. Bulk-heterojunction solar cells are complicated systems derived partially from the need to use donoracceptor combinations to facilitate charge generation. Neat films of conjugated polymers do not produce a large yield of free charges following photoexcitation with tightly bound excitons predominantly produced instead that require a donor/acceptor interface to be split.2 The exciton diffusion length is generally ∼10 nm requiring donor/acceptor interfaces to be spaced 1020 nm apart to minimize exciton recombination losses. The bulk heterojunction architecture is an ingenious structure that allows for the separate needs of exciton dissociation and light absorption (a film of 100 nm or more is required for sufficient light absorption) to be met.3,4 Bulk heterojunction films consist of a blend of donor and acceptor materials with (ideally) interconnected phase-separated domains. Such blends are typically produced via solution casting of blends with subsequent annealing5 or with the use of solution additives6 to tune phase separation and film microstructure. A domain size of ∼1020 nm optimizes exciton dissociation, whereas interconnected networks of donor and acceptor facilitate efficient charge collection. The actual blend morphology as produced via r 2011 American Chemical Society
solution processing is in general much more complicated than this ideal scenario. Real morphologies typically possess impure phases,7 a hierarchy or broad distribution of domain sizes8 and a vertical composition gradient.9,10 Varying degrees of crystallinity of one or both components further complicates this picture. For the well-studied poly(3-hexylthiophene) (P3HT):[6,6]-phenylC61-butyric acid methyl ester (PC60BM) system, it appears that a three-phase system consisting of pure PC60BM and (crystalline) P3HT phases along with a mixed amorphous P3HT:PC60BM phase exists.11,12 PC60BM has been observed to show a partial miscibility with several conjugated polymers,11,13 suggesting that mixed phases may be a common feature of polymer:fullerene systems. (The term “phase” is also used here loosely to refer to the blend constituents, for example, “light absorption by the fullerene phase”; however, it is acknowledged that the actual morphology is more complicated than a two-phase system.) Our understanding of interfacial processes is another area that requires further improvement. In addition to facilitating exciton dissociation via electron transfer from donor to acceptor (or hole transfer from acceptor to donor), the interfacial electronhole pair so produced must also be separated.2 The binding energy of relaxed interfacial electronhole pairs in organic semiconductor Received: September 2, 2011 Revised: October 4, 2011 Published: October 04, 2011 22075
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Figure 1. Chemical structures (a) and UVvis absorption spectra (b) of P3HT, PCDTBT, PCPDTBT, PTB7, PC60BM, and PC70BM.
systems has been measured to be 250 meV or more.14,15 For a number of systems, the currentvoltage characteristics have been correlated with suppression of charge-transfer (CT) or exciplex emission providing evidence of a voltage-dependent charge separation process.16,17 The high fill factors of efficient polymer/fullerene photovoltaic systems seem to preclude a strong voltage-dependence of charge separation with efficient charge separation via hot CT excitons a proposed mechanism.14,18 Recent measurements have found evidence of an interfacial dipole of ∼0.2 V nm1 in P3HT:PC60BM blends that may explain the high fill factor and charge separation efficiency in this system.19,20 Therefore the exact conformational21 and energetic nature of the donor/acceptor interface appears to be extremely important in determining device function yet is not well understood. In this contribution, we use voltage-dependent white-light bias EQE measurements to compare the photocurrent generation process for photons absorbed by polymer and fullerene phases. This study is largely motivated by the need for improved understanding of device operation and also by the need for accurate efficiency measurements because accurate EQE spectra of test cells are required to correctly calculate the spectral mismatch factor. Despite the fact that solar simulator measurements are typically performed at ∼100 mW cm2, EQE measurements are typically conducted under low light illumination of ∼1 mW cm2 or less. Because charge generation, recombination, and charge transport in bulk heterojunction solar cells, in general, can be dependent on charge density and applied voltage,22,23 it is important to verify the EQE spectrum of devices under high charge densities and for different applied bias. A number of previous publications have investigated the influence of white light bias on the EQE spectra of polymer:fullerene (including P3HT:PC60BM) and small molecule solar cells;24,25 however, only small changes in the magnitude of the EQE spectra were observed, and measurements were taken only at short circuit. Bias-dependent EQE measurements have been recently been reported on thick (2 μm) P3HT:PCBM cells; however, the intensity dependence was not investigated and this study was
motivated by a desire to understand the spatial extent of charge depletion regions.26 Our voltage-dependent EQE measurements, in particular, allow for the relative voltage dependence of photocurrent produced via fullerene absorption to be compared with that produced via polymer absorption. Whereas the process of electron transfer and charge separation following photoexcitation of the polymer has been well-studied, the corresponding processes following photoexcitation of the fullerene have been less wellstudied.27 Often overlooked, photocurrent generation resulting even from the weakly absorbing PC60BM can be significant in P3HT:PC60BM blends28 and is responsible for up to half the light absorption in poly(p-phenylenevinylene)-based solar cells that are optimized with a higher PC60BM loading.29 The stronger absorption cross-section of [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) has led to it being increasingly used as the electron acceptor to boost absorption in the green to blue region of the solar spectrum,30 particularly in combination with lowband gap polymers.31 The stronger absorption coefficient of PC70BM compared with PC60BM and the complementary absorption band of PC70BM with low band gap polymers present an opportunity for the contribution due to fullerene absorption to be more clearly discerned.
’ EXPERIMENTAL DETAILS Figure 1 presents the chemical structures and UVvis spectra of donor polymers and fullerene acceptors used in this study. P3HT was supplied by American Dye Source with a molecular weight (Mw) of 51 kg mol1 and a regioregularity of ∼94%. PCPDTBT, PCDTBT, and PTB7 were all supplied by 1-Material. PCPDTBT had a Mw of 178 kg mol1 and polydispersity (PDI) of 4.9, PCDTBT a Mw of 389 kg mol1, and PDI of 15, whereas PTB7 had a Mw of ∼200 kg mol1 and PDI of ∼4. Both PC60BM and PC70BM were supplied by Nano-C. Solar cells were fabricated on ITO-coated glass covered with a thin (∼60 nm) layer of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid). PEDOT:PSS films were dried at 150 °C 22076
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Figure 2. Schematic of the white-light bias external quantum efficiency experiment.
for 30 min under flowing nitrogen prior to transfer to a nitrogen glovebox for further processig steps. P3HT:PC60BM was spin coated from a 36 mg mL1 chlorobenzene solution (1:0.8 by weight) to give an active layer ∼100 nm thick. PCPDTBT:PC70BM (1:2 by weight) was spin-coated from a 30 mg mL1 chlorobenzene solution and 2.5 vol % 1,8-octanedithiol as cosolvent, with a layer thickness of 8090 nm. PCDTBT:PC70BM (1:4 by weight) was spin-coated from a 25 mg mL1 dichlorobenzene solution to give a film ∼100 nm thick and additionally dried for 1 h at 70 °C prior to aluminum evaporation. PTB7:PC70BM (1:1.5 by weight) was spincoated from a 25 mg mL1 chlorobenzene solution with 3 vol % 1,8diiodooctane as cosolvent to give a film ∼100 nm thick. Solar cells were completed with the evaporation of a 100 nm thick aluminum top electrode. P3HT:PC60BM devices were additionally annealed at 140 °C for 10 min. All devices were fabricated in a nitrogen atmosphere and encapsulated prior to testing. Currentvoltage characteristics were measured with a Keithley 2635 source measure unit under ∼120 mW cm2 AM 1.5 G illumination (ABET Sun 2000 Class AAA). White-light bias EQE measurement was performed using the setup presented schematically in Figure 2. A 250 W tungsten halogen lamp (Newport, Simplicity QTH) dispersed through a monochromator (Oriel Cornerstone 130) was used to provide the monochromatic beam with a filter wheel used to block higher orders. Light from the monochromator (∼0.25 mW cm2 at 530 nm) was chopped at 635 Hz and a lock-in amplifier (Femto LIA-MV-200 with a Stanford Research SR 570 preamplifier) was used to distinguish the additional current contribution of the monochromatic light from a background photocurrent due to white-light bias. A ring of six white LEDs (Philips Lumiled) was used to provide the background light with the light intensity controlled by varying the LED current. The intensity of the white LED ring was calibrated as a function of LED current using a BPX 65 photodiode with the error associated with spectral changes in the emission profile of the white LEDs (examined using a spectrometer) over the range of LED current used found to be 100 nm. Whereas we do observe a voltage-dependent change in the relative fullerene contribution for this device, the sign of this change is opposite to that required by this mechanism. That is, we observe a relative decrease in the fullerene contribution as the applied bias is swept from open-circuit voltage into reverse bias 22080
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Figure 9. Schematic of the proposed mechanism. Left: Photoexcitation of donor to the state D*/A followed by charge transfer into the manifold of quasi-bound interfacial electronhole states and then charge separation with efficiency ηsep(F) with a given field dependence. Right: Photoexcitation of acceptor to the state D/A* leads to charge transfer into different states of the manifold with charge separation efficiency ηsep(F) that may possess a weaker or stronger electric field dependence.
that cannot be explained by the exciton-charge recombination mechanism. Indeed, the fact that we observe a relative increase and a relative decrease in fullerene contribution for different systems also precludes this effect being explained solely by exciton-charge recombination in the polymer phase alone. It is possible that for different systems exciton-charge recombination is occurring in opposite phases; however, this does not seem to match with the expectation of pronounced exciton-charge recombination in the large fullerene phases of the PTB7:PC70BM system processed without additive. This leaves us to consider asymmetries in the voltage dependence of interfacial charge separation, ηsep. We attribute the effects observed here to differences in the nature of interfacial charge pairs initially produced via fullerene exciton dissociation compared with polymer exciton dissociation. One may expect that the dissociation of polymer excitons and fullerene excitons will populate similar levels in the manifold of charge transfer states giving rise to identical charge separation rates. However, this neglects the fact that because of the different geometry of polymer and fullerene, the wave function of an exciton will be differently distributed over a fullerene molecule (or molecules) compared with on a polymer chain (or chains). Fullerene excitons and polymer excitons will also differ in other respects such as binding energy and polarizability. Therefore, the dissociation of polymer excitons, in principle, may lead to the creation of different interfacial CT states compared with the dissociation of fullerene excitons, even at the same donor/acceptor interface. For example, the dissociation of a polymer exciton may result in charge pairs that have a greater initial physical separation (and hence be easier to separate38) than the dissociation of a fullerene exciton or vice versa. Figure 9 schematically presents the proposed mechanism. Changes in the prominence of this phenomenon with processing can also be understood in terms of conformational changes that lead to different arrangement of excitonic wave functions at the donoracceptor interface. The nature of interfacial charge pairs is known to be sensitive to the precise conformational nature of donor and acceptor;39 therefore, the relative ease of charge-pair dissociation may also change with changing interface conformation. For the results above, the use of the additive octanedithiol to improve the efficiency of PCPDTBT:PC70BM cells may be seen as increasing the ease of polymer charge-pair dissociation (ηDdiss) relative to fullerene charge-pair dissociation (ηAdiss). This observation may be consistent with the associated changes brought about by the additive in the polymer phase, with the additive producing an
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increase in crystallinity and change in crystallite polymorph and orientation.36 Di Nuzzo et al. have also found that octanedithiol helps to improve the free charge generation yield by suppressing the recombination of interfacial electronhole pairs into the lowest triplet excited state of the polymer.40 For the PTB7: PC70BM cells, the additive diiodooctane appears to facilitate the separation of fullerene exciton-produced charge pairs compared with polymer exciton-produced charge pairs, although the effect of the additive diiodooctane on the packing and morphology is not established at present. Another possibility that may give rise to differences in separation efficiency is a difference in the amount of excess thermal energy available to aid charge separation because in principle there will be different LUMO/LUMO and HOMO/HOMO energy level offsets. Whereas the actual energy levels at solid-state donor/acceptor interfaces are hard to determine directly, the HOMO and LUMO energy levels of PCPDTBT and PC70BM (based on measurements of the separate materials)41 suggest a LUMO/LUMO offset of 0.8 eV compared with a HOMO/ HOMO offset of 1.2 eV. Therefore, the higher HOMO/HOMO offset actually suggests that electronhole pairs produced after hole transfer should be easier to separate than those produced after electron transfer, which is not what is observed here. Furthermore, we have also investigated all-polymer blends42,43 of P3HT with the high electron mobility polymer P(NDI2ODT2)44 that have a predicted LUMO/LUMO offset of 0.8 eV compared with the HOMO/HOMO offset of 0.5 eV and see no change in spectral shape. (See Figure S9 of the Supporting Information.) Of course, the actual energy levels at the donor/ acceptor interface are likely to be different to those inferred from measurements like cyclic voltametry. The use of a processing additive may further affect the energy levels, similar to the observation of a change in the HOMO of P3HT in blends with PC60BM with thermal annealing.45 However, given the large inconsistency of the observed phenomena with that expected from HOMO/HOMO and LUMO/LUMO offsets derived from cyclic voltametry, we think that it is unlikely that this mechansism is the origin of the observed phenomenon. It should be noted that the changes in EQE spectra we observe are not insignificant. For PCPDTBT:PC70BM cells fabricated with octanedithiol, a 40% change in the relative contribution to photocurrent from the fullerene phase is observed in going from ∼V = +0.4 to 0.5 V. For the PTB7:PC70BM system, the magnitude is ∼10%. For the remaining systems where no apparent change is seen, there may still be differences that are on the order of a few percent that are harder to distinguish. Whereas there is still controversy surrounding the notion of voltagedependent photocurrent generation in organic solar cells,46,47 our observations are experimentally straightforward in contrast with ultrafast pumpprobe spectroscopy and are reproduced by EQE measurements with CW monochromatic illumination without white-light background. Although our measurements here can only detect differences in the voltage dependence of photocurrent generation for polymer and fullerene phases, they do show by way of there being a difference that a voltage-dependent photocurrent generation mechanism does exist in the polymer/ fullerene cells studied, if only for excitation of one phase. Finally, our observations demonstrate that in principle the mechanisms describing photocurrent generation are different depending on whether light is absorbed by the polymer or the fullerene component. Therefore, in general, it cannot be assumed that the species absorbing the light is irrelevant, as assumed in the 22081
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’ CONCLUSIONS We have observed changes in the shape of the EQE spectra of efficient polymer/fullerene solar cells due to differences in the voltage-dependence of photocurrent generation for fullerene and polymer phases. For PCPDTBT:PC70BM cells fabricated with the solvent additive octanedithiol, a stronger voltage dependence for fullerene photocurrent generation relative to polymer photocurrent generation is observed, whereas for PTB7:PC70BM devices fabricated without the use of a solvent additive a stronger voltage dependence for polymer photocurrent generation is observed. For other systems, including PCPDTBT:PC70BM cells fabricated without octanedithiol, this effect is not observed, demonstrating that it is not a universal phenomenon in polymer/ fullerene devices. The fact that we do not always observe photocurrent generation from one phase exhibiting a stronger voltage dependence than the other precludes this effect from originating from exciton-charge recombination from the fullerene phase alone. We attribute this phenomenon to differences in the nature of fullerene exciton-produced and polymer exciton-produced interfacial electronhole pairs dependent on the nature of the initial excitonic state. Our results demonstrate that the mechanisms of photocurrent generation should be separately considered for donor and acceptor and highlight the need for improved understanding of the nature of interfacial charge-pair formation and separation. ’ ASSOCIATED CONTENT
bS
Supporting Information. Full white-light bias intensity dependence of EQE; comparison of light intensity dependence of P3HT:PC60BM measured using the differential approach and conventional methods; comparison of the operation of PCPDTBT: PC70BM and PTB7:PC70BM devices with and without solvent additive; and white-light bias data of PCDTBT:PC70BM cells. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Engineering and Physical Sciences Research Council under grants EP/E051804/1 (T.J.K. B. and C.R.M.) and EP/G031088/1 (Z.L. and C.R.M.). Z.L. also thanks the Cambridge Overseas Trust and the Mott Physics of the Environment Award for financial support. ’ REFERENCES (1) He, Z.; Zhong, C.; Huang, X.; Wong., W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Adv. Mater. 2011, in press. DOI: 10.1002/adma. 201103006. (2) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Adv. Mater. 2007, 19, 1551.
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