Materials Design Considerations for Charge Generation in Organic

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Materials Design Considerations for Charge Generation in Organic Solar Cells Stoichko D. Dimitrov and James R. Durrant* Centre for Plastic Electronics and Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom ABSTRACT: This article reviews some of our recent progress on materials design guidelines for photoinduced charge generation in bulk-heterojunction organic solar cells. Over the last 7 years, our group has employed transient absorption measurement to determine the relative quantum yields of longlived polaron pairs for over 300 different organic Donor/ Acceptor blend films. We have shown that this optical assay of charge separation can be a strong indicator of photocurrent generation efficiency in complete devices. In this review, we consider the lessons that can be drawn from these studies concerning the parameters that determine efficiency of this photoinduced charge separation in such solar cells. We consistently find, from studies of several materials series, that the energy offset driving charge separation is a key determinant of the efficiency of this charge generation, and thereby photocurrent generation. Moreover, we find that the magnitude of the energy offset required to drive charge separation, and the strength of this energetic dependence, varies substantially between materials classes. In particular, copolymers such as diketopyrrolopyrrole- and thiazolothiazole-based polymers are found to be capable of driving charge separation in blends with PCBM at much lower energy offsets than polythiophenes, such as P3HT, while replacement of PCBM with more crystalline perylene diimide acceptors is also observed to reduce the energy offset requirement for charge separation. We go on to discuss the role of film microstructure in also determining the efficiency of charge separation, including the role of mixed and pure domains, PCBM exciton diffusion limitations and the role of material crystallinity in modulating material energetics, thereby providing additional energy offsets that can stabilize the spatial separation of charges. Other factors considered include the role of Coulombically bound polaron pair or charge transfer states, device electric fields, charge carrier mobilities, triplet excitons, and photon energy. We discuss briefly a model for charge separation consistent with these and other observations. We conclude by summarizing the materials design guidelines for efficient charge photogeneration that can be drawn from these studies. KEYWORDS: polymer, fullerene, transient absorption spectroscopy, charge transfer states, photocurrent, charge separation

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INTRODUCTION Organic solar cells (OSC) are showing rapid advances in efficiency and potential for technological application. The most efficient single-junction polymer/fullerene OSC are now approaching 10% power conversion efficiencies under laboratory conditions,1−8 with increasing confidence that efficiencies of 15% or higher will be achievable at least with multijunction devices. These advances in efficiency have largely been achieved by the synthesis and testing of new materials and device architectures, including, in particular, new photoactive layer materials whose energy levels have been modulated to achieve higher output voltages and/or improved photocurrent generation due to lower optical bandgaps.1,2,4,7,9 However, at present, our ability to predict device function from materials structure remains relatively limited. As a consequence, the vast majority of new materials tested have not performed better than the ongoing state of the art, typically achieving significantly lower device performance than that predicted by simple, energy level based, device models.10,11 A key challenge for this field is therefore the development of improved materials design guidelines to increase the effectiveness of materials © 2013 American Chemical Society

synthesis and screening programmes. In this review, we address one aspect of this materials design challenge, namely, the materials design requirements for efficient photocurrent generation and, in particular, the requirements for a high quantum yield photoinduced charge separation. In practice, for most new photoactive layer materials, the quantum efficiency of photocurrent generation in optimized solar cells is found to be significantly below unity. Identifying the cause(s) of this underperformance in photocurrent generation is therefore a key consideration for materials design. The efficiency of photocurrent generation in OSC can be broken down into four factors, the efficiencies of photon absorption (ηabs), exciton diffusion to the donor/acceptor (D/A) interface (ηdiff), charge separation at this interface (ηsep), and charge collection by the external circuit (ηcoll) (see Figure 1).12−14 The Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 17, 2013 Revised: September 9, 2013 Published: September 9, 2013 616

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0.3 eV.11,30,31 Such a simplistic view of photocurrent generation, however, is clearly unable to explain the large variations in photocurrent generation efficiency observed between different photoactive layer materials. One additional factor that has been identified as potentially limiting photocurrent generation is the formation of Coulombically bound charges at the D/A interface.12,14,32−38 The low dielectric constant of organic semiconductors (εr = 3−4) suggests that such bound states could have coulomb binding energies of several hundred meV (≫ kBT). There have been extensive studies of such bound states, which we will refer to herein as bound polaron pair states (BPP). We note that such states have also been referred to as charge transfer (CT) states (due to the observation of sub-bandgap optical absorption or emission) or exciplex states (due to the observation of exciplex-like emission). Several studies have provided evidence that these states can undergo relatively rapid charge recombination (on time scales from picoseconds to ∼100 ns),35,36,39−42 with this recombination pathway potentially being a key limitation on the efficiency of charge separation, and thereby photocurrent generation.12,37,38,41−59 The extent to which this charge recombination channel at the D/A interface indeed competes with the charge separation process in OSC, and the consequences of this for materials design, is a subject of extensive discussion in the current OSC literature.35,37,53,54,60−63 In this review, we discuss recent advances in our understanding of charge photogeneration in polymer/fullerene OSC, with the aim of developing empirical materials design rules for efficient photocurrent generation. We start by introducing the primary experimental approach we have employed, transient absorption spectroscopy (TAS), and considering the evidence that this approach is indeed an effective assay of charge separation efficiency in OSC. We then focus on a key consideration for the thermodynamic efficiency of such solar cells, namely the correlation between energetic offset driving charge separation and the quantum efficiency of this charge separation process ηsep. This is followed by a discussion of the role of film microstructure and material crystallinity, again focusing upon ηsep. We then discuss other factors we observe to influence ηsep. These considerations are then summarized and discussed in terms of empirical materials design guidelines for efficient charge photogeneration for OSC. Background to Photoinduced Charge Generation in OSC. Photocurrent generation in OSC is fundamentally different from that of the standard p−n junction solar cells. The origin of this difference lies in the low permittivity of organic materials and their inherent energetic and structural disorder. Unlike most inorganic semiconductors, light absorption by polymer and fullerene films generates localized excited states that are tightly bound electron and hole pairs. The binding energies of these so-called excitons can be more than an order of magnitude higher than the thermal energy, therefore making their separation a key issue for efficient photocurrent generation. This issue was successfully addressed by Tang, who in 1986 first used a heterojunction of two organic materials with different electron donor and acceptor properties to provide an energetic driving force for the dissociation of the photogenerated excitons.64 This breakthrough device was based on a bilayer heterojunction of copper phthalocyanine and a perylene tetracarboxylic derivative sandwiched between transparent ITO and Ag electrodes, and achieved ∼1% power conversion efficiency (PCE).

Figure 1. Simplified illustration of the typical architecture of a BHJ polymer/fullerene organic solar cell. The diagram depicts the four processes leading to photocurrent generation in the photoactive layer: light absorption (by the polymer in this case), exciton diffusion to D/A interface, charge separation at this interface, and charge collection at the electrodes. The diagram illustrates both finely intermixed and relatively pure polymer and fullerene domains, as well as variations in local material crystallinity; the relative proportions of these structural features are strongly dependent upon the choice of photoactive layer materials and film processing conditions.

development of new, lower bandgap polymers has resulted in significant, and easily measured, improvements in ηabs, although at the expense of lower energies of the resultant photoexcited states (excitons). Similarly, the bulk-heterojunction (BHJ) strategy, based on an interpenetrating blend of donor and acceptor domains, has proved, for many photoactive layers, to be remarkably effective at enabling efficient exciton diffusion to the D/A interface. In addition, for many devices and, in particular, for blends of donor polymers with reasonable hole mobilities blended with the soluble fullerene acceptor PC60BM ([6,6]-phenyl C61-butyric acid methyl ester), the charge collection efficiency ηcoll is found to approach unity at short circuit (it should be noted that the efficiency of charge collection is, however, a key consideration for overall device efficiency, with the competing process of nongeminate recombination limiting the voltage output, and often the fill factor, of most devices).11,15−20 In some cases, a tension between ηcoll and ηabs is observed as a function of photoactive thickness, with efficient charge collection only being obtained for film thicknesses too thin for efficient light absorption.21−23 However, in general, it appears that, for many new photoactive layers and, in particular, blends of new donor materials with PCBM, the observation of suboptimal photocurrent densities often derives neither from inefficiencies in ηabs, ηdiff, nor ηcoll. Rather, as we discuss in this review, such suboptimal photocurrent densities often result from limitations in charge separation efficiency, ηsep. As such, this review focuses upon evidence that the efficiency of charge separation indeed limits photocurrent generation in many OSC and, more importantly, upon the material parameters that determine this efficiency. Understanding of the device physics of OSC has improved significantly in recent years, with semiempirical device models now able to reconstruct the open circuit voltage, and in some cases the fill factor, of a device relatively well.18,20,24−29 However, models to calculate the photocurrent density of devices from materials parameters are relatively limited. Most device efficiency predictions assume unity efficiency of exciton diffusion ηdiff to the D/A interface (i.e., that the domain sizes are much less than the exciton diffusion length) and that unity efficiency of charge separation ηsep is achieved provided that the energy offset between the polymer and fullerene lowest unoccupied molecular orbitals (LUMO) is at least 617

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nongeminate recombination occurs within the resolved time range; see, for example, ref 40). The simplicity of this approach has allowed us to undertake comparative studies of the efficiency of charge generation for a large number of photoactive layer materials, blend compositions, and processing conditions, now comprising over 300 such distinct film types (including 70 different donors and 20 different acceptors, as well as a range of blend compositions and film processing conditions). The results of many of these studies have been published previously.7,37,40,56,78−88 This large body of data has allowed us to identify statistically significant empirical materials correlations that can be difficult to observe from smaller data sets. In this review, we aim to summarize some of the key findings of these studies. We have reported strong evidence that our transient absorption assay of charge generation in blend films is indeed a good indicator of the efficiency of photocurrent generation in devices. In particular, we have observed a clear correlation between the amplitude of our transient absorption assay of polaron yield, ΔOD, and photocurrent density JSC/maximal IQE for a broad range of polymer/PCBM blend films over 3 orders of magnitude in signal size (Figure 2).89 This correlation

The photocurrent of most organic bilayer heterojunction devices, such as that reported by Tang et al.,64 is, however, limited by the short exciton diffusion length of organic materials. Polymers and fullerenes, for example, are typically reported to have exciton diffusion lengths shorter than 10 nm, thus substantially limiting the thickness of the active layer and its light harvesting properties.65−69 One approach to addressing this limitation is the use of BHJ films; this approach has proved particularly effective for polymer/fullerene-based devices.70 These are typically fabricated as random bicontinuous interpenetrating networks of a p-type polymer and n-type fullerene in which the D/A heterojunction area is very high; as a result, the likelihood of excitons reaching the D/A interface is substantially increased. Figure 1 shows a standard device architecture excluding hole and electron blocking layer, which are not discussed in this review. The active layer of the device is presented as a polymer/fullerene BHJ with pure and intermixed domains and variations in polymer crystallinity; the relative proportions of pure and intermixed domains, and material crystallinity, are thought to vary substantially between different donor and acceptor materials and film processing conditions. OSC based on such BHJ architectures are now achieving close to 10% PCE.1−8 Transient Absorption Assay of Charge Photogeneration Efficiency. The primary experimental technique we have employed to assay the efficiency of charge generation in D/A blend films and solar cells is nanosecond to millisecond TAS. This is a relatively straightforward pump−probe technique that monitors the transient absorption of photogenerated charge carriers, with the primary requirement being a high detection sensitivity, enabling signal magnitudes to be measured reliably with low light excitation densities, thereby avoiding nonlinear and saturation effects in the active layer.56,71−73 Nanosecond laser pulses are used as an excitation source, while the output of a tungsten lamp probes the changes in the optical density of the film or device induced by the laser excitation. A Si or InGaAs photodiode, connected to an amplifier and oscilloscope is used for detecting and recording the transient absorption signals on nanosecond to millisecond time scales. On this time scale, dissociated polarons can be identified by their characteristic power law decay dynamics and nonlinear behavior at high excitation densities, indicative of nongeminate recombination.72,74 In contrast, geminate recombination, for an example of Coulombically bound polaron pair states, exhibits distinct exponential (or stretched exponential) dynamics, linear behavior as a function of excitation density, and relatively fast decay times (picoseconds to ∼ a hundred nanoseconds).35−37,39,47,75,76 Triplet states, which can also contribute to the transient absorption signals on these time scales, can be often distinguished from these polaron states by their more exponential decay dynamics and quenching in the presence of molecular oxygen.77 As such, the magnitude of the transient polaron absorption (ΔOD = change in the optical density) can be employed as a direct, at least semiquantitative, assay of the quantum efficiency of generation of dissociated charges. There are, of course, limitations to this approach, including for example differences in polaron absorption coefficient (which means that comparison of polaron yields can only be done quantitatively when comparing blends of similar chemical composition) and the potential for some nongeminate recombination to occur on time scales faster than the instrument response (avoided or minimized by the use of sufficiently low excitation densities such that the temporal onset of

Figure 2. Correlation between a ΔOD assay of the yield of dissociated polarons measured by transient absorption spectroscopy of thin blend films and JSC (×) and IQErel (squares) measured in complete devices for a series of polymer/PCBM blends as a function of polymer, blend composition, and thermal annealing. Filled circles correspond to blends with two low hole mobility polymers, where the low hole mobility results in low photocurrent due to collection losses, the open circle corresponds to the indenofluorene polymer IF8TBTT,89 and, for this polymer, the deviation from the correlation line is assigned to field dependent charge generation. Reprinted with permission from ref 89. Copyright 2010, WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimQ.

is clear evidence not only of the effectiveness of our empirical approach as an indicator of photocurrent generation but also that, for many such blend films, ηcoll is not a key factor determining JSC. It should be noted that this correlation between ΔOD and JSC is not observed for all photoactive layers; for example, it breaks down for donor/acceptor blends employing perylene diimide acceptors and for donor polymers with low hole mobilities (specifically FET mobilities of 0.3 eV is required to drive charge separation, although we note that there is very little experimental evidence for this specific energy offset requirement. This 0.3 eV energy offset requirement has been described as being necessary to overcome the binding energy of the excitons generated in organic semiconductor films, typically estimated at 0.3 eV.11,13 This exciton binding energy is also considered the reason for the optical bandgap (Eg) of most organic semiconductors being less than their electronic bandgap. It should be noted that, in this viewpoint, a ΔELUMO > 0.3 eV requirement corresponds to an approximately zero energy difference between the exciton energy (as approximated by Eg) and the blend electronic bandgap (IP−EA, where IP is the electron donor ionization potential and EA is the electron acceptor electron affinity). As such, this requirement ΔELUMO > 0.3 eV is equivalent to requiring that the energy offset driving charge separation, after accounting for

the exciton binding energy, to be greater than zero. An alternative viewpoint is to consider state energies, as illustrated in Figure 3b. As we discuss, we use the energy difference ΔECS between the polymer exciton, ES1, and the polaron energies IP and EA ΔECS = (IP−EA) − ES1, as a measure of the energy offset driving charge separation. ΔECS is related to the ΔELUMO by ΔECS = ΔELUMO − Eexcb. Eexcb represents the exciton binding energy. Also illustrated on both diagrams is the energy of BPP states EBPP, with their coulomb binding energy relative to IP−EA being given by EBPPb. The above discussion employs IP and EA as measures of the polaron energies. For semiconductors, these correspond to the conduction and valence band edge enthalpies. It is important to distinguish these energies from the free energies of electrons and holes in the blend, as defined by the electron and hole quasi-Fermi levels. The splitting of the quasi-Fermi levels by light irradiation determines the voltage output of the device (often referred to, at open circuit, as eVOC, in the absence of other voltage losses) and corresponds to the energy stored by the photogenerated electrons and holes following thermalisation with these Fermi levels (CSTR in Figure 3b). In general, the quasi-Fermi levels lie within the electronic bandgap of the film, such that the free energy of photogenerated polaron pairs is less than the electronic bandgap of the film. This difference in energy between IP−EA and eVOC derives most simply from the increase in entropy of the electrons and holes as they separate from the interface (readily calculated from simple degeneracy arguments to be several hundred meV),13 as well as thermal relaxation/trapping processes as the polarons relax down the intraband density of states discussed above. The free energy lost 619

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during this overall charge separation process to polarons thermalized with the film quasi-Fermi levels is illustrated in Figure 3b as ΔGCS.94 We note that in our initial discussion of the energetics of charge separation,13,56 we did not make this distinction between ΔECS and ΔGCS. In terms of experimental measurements of the various energy offsets illustrated in Figure 3 as driving charge separation, herein, we employ ΔECS = (IP−EA) − ES1 where ES1 is measured from optical spectroscopy data and IP and EA from photoelectron spectroscopy or cyclic voltammetry data. We note that these measurement techniques have the potential for significant systematic errors (for example due to the presence of an exponential density of tail states complicating the determination of the band ‘edge’); as such, we consider relative magnitudes of ΔECS rather than its absolute value. An alternative energy offset measurement used in the literature is based on electroluminescence of emissive interfacial charge transfer states, which determines the difference in energy between the emissive exciton and CT states.95 A third measure employs the difference between the optical bandgap and open circuit voltage of the device (Eg − eVOC), this assay corresponds (at open circuit) to ΔGCS, as defined above, and differs from ΔECS by the energy loss associated with entropy gain and trapping.96 We note that this latter, free energy assay of energetic offset depends upon the magnitude of the quasi-Fermi level splitting generated under solar irradiation and will therefore depend upon several additional factors, including the irradiation intensity and the nongeminate recombination rate constant. Exciton Quenching versus Polaron Generation: The Impact of Materials Energetics. Charge separation in donor/acceptor blend systems can be most simply described as electron transfer from donor excitons to generate positive and negative polarons (for acceptor excitons, this corresponds to a hole transfer process). In this simple picture, the efficiency of exciton quenching at the D/A interface should correlate directly with the yield of photogenerated charges. There is now extensive evidence that this correlation is not observed. In the extreme case of donor polymers with very low optical bandgaps, blending with PCBM does not result in strong polymer photoluminescence quenching, indicative of unfavorable energetics for exciton quenching at the D/A interface. However, for almost all the donor polymers we have studied, we have observed remarkably efficient polymer photoluminescence quenching (PLQ) following blending with PCBM, typically in excess of 90%, and often >98% (in this regard, P3HT is rather anomalous, exhibiting modest PLQ (70−90%) attributed to its more complete phase segregation and the resultant exciton decay losses during diffusion to the P3HT/PCBM interface).46,56,79,97 Despite this remarkably ubiquitous high efficiency of polymer PLQ, indicative that polymer excitons are indeed being quenched at polymer/fullerene interfaces, the yields of dissociated photogenerated charges, as assayed by our ΔOD measurement, are observed to vary very significantly between blend films. This distinction between consistently efficient PLQ and strongly varying yields of charge generation was first reported by Ohkita et al. in a study of a series of polythiophene based polymers blended with PCBM, as illustrated in Figure 4,56 and has since been observed for several other polymer/fullerene, polymer/perylene, and polymer/quantum dot systems.40,46,80,86,98 This observation is important for a number of reasons. First, it indicates that photoluminescence quenching is not a reliable assay of charge generation in organic blend films. Second, as the yield of dissociated charges does not

Figure 4. Spectroscopic analyses of exciton quenching and charge generation yields as a function of the energy offset driving charge separation ΔECS for a series of seven polythiophene/PCBM (19:1) blend films. (a) Polymer photoluminescence quenching data relative to neat polymer films, showing consistently high PLQ efficiency plotted as a function of ΔECS. (b) Transient absorption assay of the yield of dissociated charges determined from the magnitude of the ΔOD signals at 1 μs delay time. Molecular structures of four of the polymers are included in part b. All data have been corrected for variations in the absorption at the excitation wavelength. Adapted with permission from ref 13. Copyright 2010, American Chemical Society.

correlate with the efficiency of exciton quenching, it suggests the presence of a competing pathway following exciton quenching at the interface that competes with the generation of long-lived dissociated charges. As we discuss later on, there is now extensive evidence that this competing pathway involves the formation of interfacial BPP states.13,35,36,41,42,47 We have conducted several studies to investigate how the yield of dissociated charges varies with material energetics.40,79,80,86,88 We have not observed strong correlations with either the absolute exciton or polaron pair energies, nor with the magnitudes of these energies relative to estimates of material triplet state energies (further discussion of this point is found later on). However, we have observed a strong correlation between our polaron quantum yield assay, ΔOD, and the difference in energy between the exciton and polaron pairs, as defined as ΔECS = (IP−EA) − ES1. This is illustrated in Figure 4 for the data reported by Ohkita et al.,56 which shows that, for this series of donor polymers, the charge photogeneration yields increases with 2 orders of magnitude for a 300 meV increase in ΔECS. This observation strongly indicates that the energy offset ΔECS is a key determinant of charge separation efficiency ηsep and thereby photocurrent generation in OSC. We have since carried out studies with several series of polymer/acceptor films to investigate the generality of the 620

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correlation between ΔECS and ηsep.37,40,86,89 While clear correlations were not observed in all cases, indicative of other factors influencing ηsep within the particular series studied in addition to ΔECS (such as differences in film microstructure, as discussed later), we have successfully reproduced the clear correlation between ΔECS and our ΔOD assay of ηsep for six distinct material series.40,56,78−80,86,88 Typical data from three such series are shown in Figure 5, a fourth, employing PDI

this striking effect derives from a reduction in coulomb binding energy of the BPP, an increase in local dielectric constant, a difference in polymer internal dipole,99 greater mixing of charge transfer and exciton states, or another parameter.50 Our observation that the energy offset required to drive efficient charge separation may be modulated by choice of class of donor polymers is of key significance for optimization of the solar cell device efficiency. It clearly indicates that the widely used assumption that ΔELUMO > 0.3 eV for efficient charge generation is material dependent. For the polythiophene-based polymers blended with PCBM, the energy offset ΔECS required to drive charge separation is ∼0.9 eV, almost half the photon energy (and corresponding to ΔELUMO = 1.2 eV, using ΔELUMO = ΔECS + Eexcb and assuming Eexcb = 0.3 eV). This energy loss is therefore the dominant energetic loss limiting device efficiency in such devices. In contrast, for several of the more recently developed low bandgap copolymers such as DPP-TT-T, the energy offset ΔECS required to drive charge separation approaches ∼0 eV (corresponding indeed to ΔELUMO = 0.3 eV, assuming Eexcb = 0.3 eV).1,37,88 This reduction in the energy offset with new donor polymers is likely to be one of the key factors behind the impressive advances in device efficiency in recent years. Dependence of Charge Photogeneration Yield upon Photon Energy. The previous section discusses data where the energetics of charge separation were modulated by changing the materials in the photoactive layer. We now consider data employing a complementary approach, modulating the photon energy used to excite a single material system. In this approach, we employed a low-band gap polymer/fullerene blend with a very low ΔECS, such that bandgap excitation resulted in relatively low yields of separated charges.37 The results of transient absorption experiments as a function of excitation wavelength (shown in Figure 6) revealed that, for this blend, the charge photogeneration yield increases with photon excitation energy above the optical bandgap. Complementing this observation, pump−push photocurrent spectroscopy demonstrated that this increase in the yield of dissociated charges with photon energy correlated with a reduction in the yield of interfacial BPP states. Bakulin et al. also compared directly the yield of dissociated charges vs BPP state formation in three different related polymer/fullerene blend films, thereby comparing the hole (PCBM excitation) and electron (polymer excitation) transfer contributions.36 A strong correlation between driving energy and bound state generation was observed independent of the material excited. Experimental evidence that the photon energy can also modulate the efficiency of charge separation, alongside the materials energetics discussed in the previous section, has also been reported in two other studies (although in at least one case rather controversially60−62).100,101 In our own studies, we have only observed a clear photon energy dependence for two blend systems with particularly small energy offsets. In general, such a photon energy dependency requires charge separation to proceed on a time scale faster than exciton thermal relaxation and therefore is unlikely to be observed in all systems, in agreement with our observation that, for most blend systems we have studied, we have not observed any photon energy dependence of charge separation yield. Notwithstanding this caveat, these data therefore provide further evidence that the yield of dissociated charges is strongly dependent upon the difference in energy between the exciton initiating charge separation and the dissociated polaron pairs.

Figure 5. Plots of the yield of dissociated charges, as determined from transient absorption signal amplitudes, against −ΔECS for three different series of donor polymers blended with PCBM. A strong correlation of the yields and ΔECS is observed for the three different series of D/A blend films, presented in the figure using different color codes. DPP-based copolymers blended with PC70BM at 1:2 w/w are shown with black triangles. Polythiophene-based polymers, such as P3HT, blended with PCBM at 1:1 w/w are shown with blue open squares. Thiazolothiazole-based polymers blended (1:1 w/w) with PCBM and ICBA are shown in red. For each series, the largest ΔOD signal is normalized to one to facilitate comparison between series, as differences in polaron extinction coefficients between polymer series are possible. The electron affinities of the PC70BM and PC60BM were assumed to be 3.7 eV. Data were taken from refs 40, 86 and 88. For the polythiophene series, the ΔOD data have been extrapolated to a time delay of 100 ns to facilitate comparison with the other data series.

acceptors is discussed later (Figure 8). Based on these results, we conclude that the energy offset ΔECS is indeed a key factor determining the charge separation yields in organic donor/ acceptor blends. Haque et al. have also recently reported a similar correlation for a series of polymer/quantum dot blends.98 In addition, Janssen et al. have reported a similar energetic correlation for a series of diketopyrrolopyrrole (DPP) polymers, employing an EQE assay for charge generation and Eg − eVOC as an assay of the energy offset driving charge separation.96 These observations provide clear evidence that ΔECS is, in general, a key determinant of charge separation efficiency in OSC. In addition to demonstrating the reproducibility of the correlation between ΔECS and ΔOD within a materials series, it is also apparent from Figure 5 that the magnitudes of ΔECS needed to achieve efficient charge photogeneration vary substantially between materials series. For example, it is apparent that the DPP-based polymer/PC70BM blend films (black-filled triangles in Figure 5) show efficient charge separation at energy offsets almost 1 eV lower than those required to achieve comparable yields of charge separation for the polythiophene/ PCBM blend films. We have reported a similar reduction in the energy offset required to drive charge separation for blend films employing the donor polymer PCPDTBT relative to polythiophene polymers.78,86,88 At present, it is unclear whether 621

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Figure 7. Energy level diagram depicting a model of charge photogeneration at organic D/A heterojunctions. The model is illustrated for two different initial exciton energies (SA and SB) relative to the polaron energies, corresponding for example to singlet excitons of two donor polymers with different optical bandgaps, or two different excitation wavelengths of the BTT-DPP polymer. Excitation of the higher energy exciton (SA) results in electron transfer (kET) to the acceptor with sufficient excess energy to avoid formation of a Coulombically bound interfacial polaron pair (BPPR) but rather leads to formation of charge separated states (CS), which are subsequently stabilized (kTR) by charge migration away from the D/A interface, resulting in an increase in state degeneracy (entropy) and electronic relaxation into lower energy polaron states in the film (CSTR). In contrast, for the lower energy exciton (SB), the injected electron does not have sufficient excess energy to escape the coulomb attraction of the polymer polaron and therefore results in formation of bound polaron pair states that subsequently recombine (kR) radiatively or nonradiatively to ground or to lower lying polymer/acceptor triplet states.

Figure 6. (top graph) Charge photogeneration quantum yields of a BTT-DPP/PCBM blend film as estimated by TAS, recorded at the polymer polaron band (1150 nm) at a 0.2 μs time delay (red circles) as a function of excitation wavelength λexc. Data was normalized for differences in film absorption at λexc. IQE for photocurrent generation of the corresponding device is presented with a black line; it shows similar increases in the charge yield with excitation wavelength. Inset: PL quenching of the BTT-DPP/PCBM blend film plotted as a function of excitation wavelength. (bottom graph) Results of pump− push photocurrent (δJ/J) measurements on BTT-DPP/PCBM devices at different excitation wavelengths. The decays correspond to relaxed BPP state formation and recombination, thus showing that higher yields of relaxed BPP states are generated by longer wavelength excitation. Reprinted with permission from ref 37. Copyright 2012, American Chemical Society.

recombination loss pathway. The model assumes that the electron is transferred adiabatically from the polymer exciton to the fullerene acceptor without an initial loss of energy (neglecting any exciton diffusion processes). The electron is therefore injected with excess energy, corresponding to ΔECS above the bulk band edges, or ΔECS + EgBPP above the BPP state energy (we have previously referred to this initial state with excess energy as a ‘hot state‘). There is then a competition between motion of the electron away from the D/A interface, corresponding to spatial charge separation, and loss of the excess energy through thermalisation/electronic relaxation processes. This ‘hot’ electron motion versus thermalization picture is analogous to the Onsager model for autoionization in solution, as we have discussed previously.13,102 As for Onsager, stable charge separation is only achieved if the electron escapes the coulomb attraction of the positive polaron residing on the polymer before it loses its excess energy.102 A large energy offset ΔECS results in injection of electrons with a large excess energy, facilitating their escape from the coulomb attraction of the polymer polaron. This increased escape probability may derive both from the longer time taken to thermalize this large excess energy and from the potentially greater wave function delocalization present for states well above the band edge, as has been proposed by several groups.14,35,39,43,100,103−107 For example, theoretical calculations have provided evidence that higher energy BPP or CT states can be more delocalized and hence more prone to dissociation.8,100,103 In contrast, for a small energy ΔECS, the injected electron does not have sufficient excess energy to escape the coulomb attraction with the positive polaron, and therefore, it becomes trapped at the interface, forming a bound interfacial BPP state. This bound BPP subsequently either decays directly to ground or, for systems with material triplet excitons lying below the BPP state in energy, undergoes intersystem crossing

Model for Energetic Dependence. We turn now to discuss briefly a model of charge separation based on our experimental observation that the yield of dissociated charges increases with ΔECS. A key consideration for this model is the empirical observation that, even for blend films with small energy offsets such that the yield of dissociated polarons drops substantially, the efficiency of exciton quenching at the interface can still be very high (often approaching unity). This observation clearly implies a competing loss pathway, which results in charge carrier recombination after exciton dissociation and which appears to be particularly dominant for small energy offsets. For the purpose of this discussion, we will only consider charge separation across a D/A interface consisting of welldefined polymer and fullerene phases (see the following text for discussion of more realistic interface structures). The model, as illustrated in Figure 7, is based on that originally proposed in Ohkita et al.,56 but modified to take into account recent ultrafast spectroscopy and theoretical studies, including, in particular, those by the Friend group.33,35,100,101 A key component of the model is the presence of Coulombically bound polaron pair states at the D/A interface that function as a 622

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to the triplet BPP state, followed by charge recombination to triplet excitons. The model illustrated in Figure 7 suggests an inverse correlation between the yields of separated charges and interfacially bound states, as evidenced by the data in Figure 6. Analogous inverse correlations have also been observed between the intensity of CT state emission and EQE or polaron formation, again consistent with a competition between generation of bound interfacial states and free charges.33,35,37,41,57,76,108,109 We note, however, that there remains significant controversy in the literature over the nature and role of these BPP or CT states in organic solar cells, and significantly more experimental and theoretical work is necessary to establish fully the detailed mechanism of charge separation in these devices. For example, an optimum driving energy for charge separation, consistent with Marcus nonadiabatic electron transfer theory, has been observed for charge separation from fullerene excitons in dilute blends with donor polymers.111 In addition, there is increasing evidence that sub-bandgap excitation of low optical density charge transfer (CT) transitions can result in efficient photocurrent generation, suggesting that the states populated in such experiments do not correspond to the interfacially bound states responsible for the energetic dependencies of charge separation discussed above, as we discuss further towards the end of this article.53,112−115 We note that following the initial charge separation, the energy of the electron and hole is further reduced as the charge carriers thermalize to the film quasi-Fermi levels, as also illustrated in Figure 7. For efficient solar cell materials that achieve charge separation with ΔECS approaching 0 meV, this thermalization/trapping process is actually the dominant energy loss process associated with the overall charge photogeneration process13,32 and is a key factor in stabilizing the charge separation in these devices. The magnitude of this overall free energy loss, ΔGCS, is dependent upon device operating condition (being largest at short circuit conditions) and also depends (at open circuit conditions) upon the rate constant for nongeminate recombination. However, we do not discuss these energetic losses further in this review, as our focus is on the quantum efficiency of photocurrent generation, which is determined by efficiency of the initial charge separation, rather than by these energetic loss processes. In terms of materials design rules, the model illustrated in Figure 7 has several important implications. In addition to potentially explaining the origin of the energy offset dependence we observe empirically, it suggests several other parameters are likely to influence the efficiency of charge separation. These include the BPP state binding energy EgBPP, the extent of delocalization of the electron and hole wave functions after/ during charge transfer at the interface, the electron mobility, the thermalization time of the excess energy, the physical structure of the interface, including any local variations in energetics or composition, the domain structure and material composition in the blend film, and the presence of interface or molecular dipoles. In the following sections, we consider the extent to which our studies have provided experimental evidence for the importance of some of these parameters.

In most text-book descriptions of polymer/fullerene bulk heterojunction solar cells, the film morphology is typically described as a two-component, bicontinuous blend comprising a polymer phase and a fullerene phase. The function of such films is then described as light absorption in one or both phases, exciton diffusion to the polymer/fullerene interface, exciton dissociation at this D/A interface to yield electrons and holes, and transport of these charges through the two phases to the device contacts. However, more detailed analyses are increasingly questioning whether this pure polymer phase/pure fullerene phase structural model is appropriate for the blend films typically employed in BHJ OSC, with increasing evidence for the importance to device function of, for example, D/A mixing on a molecular scale, and the role of variations of material crystallinity within and between blends.40,85,117,121−128 The former consideration, the presence of molecularly mixed regions of the films, is important with regard to charge separation, as it suggests that charge separation may be, at least in part, a molecular rather than interfacial process. Molecular mixed regions can also function as recombination centers in the film due to the lack of spatial separation of donor and acceptor molecules. Variations in material crystallinity can impact charge separation by, for example, causing changes in wave function delocalization or material energetics (increased crystallinity typically results in smaller electronic bandgaps). In this section, we consider some insights into these dependencies derived particularly from correlations with our transient optical studies. Exciton Dynamics. Film morphology can have a major impact on the efficiency of exciton diffusion to D/A junctions. In this regard, a particularly important parameter for excitons is their diffusion length; this is the average distance that an exciton can migrate during its lifetime. Typically this parameter is assessed by quantifying PLQ of bilayers as a function of a layer thickness using time-resolved or steady-state photoluminescence spectroscopy.65,129−133 For example, measurements of this type have yielded exciton diffusion lengths of 3−9 nm for P3HT and 5 nm for PC60BM.66,68,134−136 In general, achieving a high efficiency for diffusion of photogenerated excitons to the charge separation interface, ηdiff, requires domain sizes smaller than these diffusion lengths. We note that the exciton diffusion length will depend not only on the exciton diffusion constant but also on its lifetime, with shorter exciton lifetimes likely to result in more severe exciton diffusion requirements. For many of the polymer/PCBM blends discussed in this review, polymer photoluminescence quenching is remarkably high (often >95%), indicating that polymer excitons do not need to diffuse significant distances to reach a PCBM acceptor. Indeed, this high PLQ is indicative of the presence of significant molecular mixing of PCBM into the donor polymer on the length scale of the exciton wave function size (we note there is currently some discussion over the extent of exciton wave function delocalization at early times).85,137,138 Such molecular scale mixing is consistent with several recent reports of PCBM miscibility with, and diffusion into, donor polymers domains, including, in particular. relatively amorphous polymers.122,124,128,139−143 More modest polymer PLQ, indicative of significant polymer exciton diffusion on length scales approaching the exciton diffusion lengths, is only observed for highly crystalline polymers such as P3HT and DPP-TT-T, consistent with the formation of relatively pure crystalline polymer domains for these materials. This photoluminescence quenching has been most extensively studied for P3HT, which has PLQ, and therefore exciton

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IMPACT OF FILM MORPHOLOGY AND MATERIAL CRYSTALLINITY There is now extensive evidence that, in addition to energetics, film morphology and material crystallinity play key roles in determining the efficiency of charge separation.34,40,85,110,116−120 623

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dissociation yields, sensitive to changes in film morphology, for example, thermal annealing, which increases phase segregation in the P3HT/PCBM blends.79,97,144 In general, polymer photoluminescence quenching following blending with PCBM can derive from either electron or energy transfer to PCBM.67,145,146 In most cases, electron transfer appears to be the dominant pathway. We have observed efficient energy transfer from donor polymer excitons to PCBM, but only for highly fluorescent, high bandgap fluorene-based polymers, where the strong fluorescence efficiency and good spectral overlap between polymer emission and fullerene absorption results in a sufficiently high energy transfer rate constant to enable efficient energy transfer.81,87 In this case, this energy transfer can result in a loss of charge photogeneration, as the resultant PCBM excitons can be too low in energy to drive efficient charge generation. In contrast to the typically very efficient quenching of polymer photoluminescence in blend films, the quenching of fullerene photoluminescence is often less efficient.83,118,147−149 This cannot, in general, be attributed to unfavorable energetics, as often the PCBM exciton has a higher energy than the polymer exciton. Rather, this efficiency loss appears to derive from the tendency of both PC60BM and PC70BM to form aggregates on the length scale or larger than their exciton diffusion lengths, as we have recently reported in a study of photocurrent generation from PC70BM excitons in blends with a low-bandgap DPPbased polymer.83 In this case, clear correlations were observed between photocurrent density, PC70BM photoluminescence quenching, and PC70BM domain size, indicative of exciton diffusion limitations. This result suggests that fullerene aggregation and its miscibility with the polymer are a key consideration for optimization of photocurrent generation from fullerene excitons. Exciton Separation and Charge Carrier Mobility/ Delocalization. The molecular scale structure of the D/A junction can be expected to have a profound impact on the efficiency and dynamics of exciton separation. One consideration attracting particular interest is the potential for increased wave function delocalization and/or high carrier mobility to facilitate escape of charge carriers from the D/A junction (see proposed model in Figure 7). Several studies have reported both experimental and theoretical analyses of this issue.8,22,26−28,51,110,150 Our own studies comparing the charge separation efficiency of polymer/PCBM and polymer/PDI (perylene diimide) blends are particularly relevant to this.40,86 We have observed that replacement of PCBM in blend films with crystalline PDIs results in a large reduction in the energy offset dependence of charge separation (Figure 8). An analogous observation has also been made for polymer/CdS nanocrystals blends, where an increase in CdS crystallinity was also observed to reduce the energy offset dependence of charge separation.98 In both cases, the improved charge separation efficiency was assigned to increases in electron delocalization/mobility in the acceptor material facilitating the escape of the charge carriers from their mutual coulomb attraction. We have also observed that decreasing the fullerene content in the blend film increases the energy offset dependence of charge separation, attributed to more finely dispersed PCBM molecules within the blend making it more difficult for photogenerated electrons and holes to move away from each other.80 Interestingly, we have not observed a strong correlation between donor polymer hole mobility/crystallinity and charge separation efficiency, at least for charge separation from polymer excitons,56,86,89 suggesting

Figure 8. Comparison of the ΔOD signal amplitudes of polymer/ PCBM and polymer/PDI blend films. The substitution of PCBM with highly crystalline PDI acceptors reduces the dependence of charge photogeneration yields on energy offset driving charge separation. Adapted with permission from ref 86. Copyright 2010, American Chemical Society.

that it is the mobility/delocalization of charge carrier that is moving during exciton separation (e.g., the electron for LUMO to LUMO transfer), which is most important. Domain Energetics, Material Crystallinity, and the Spatial Separation of Charges. As discussed above, there is increasing evidence that, for many polymer/fullerene blends, there is extensive mixing of polymer and fullerene on a molecular scale, depending upon the details of film processing. This is particularly the case where the polymer is highly miscible with PCBM, either through formation of a mixed cocrystal or the formation of amorphous, mixed regions of film.85,122,128,151−153 However, there is also strong evidence that molecularly mixed films, in the absence of any pure domains, exhibit very rapid charge recombination, as charge carriers generated by exciton separation are not spatially well separated.39,54,90,153−157 We have, for example, observed that replacing PCBM with a more miscible fullerene in blends with P3HT results in appearance of rapid (≤100 ns) recombination and, consequently, negligible photocurrent generation.40,154 Similarly, fast recombination has been observed in films comprising covalently attached donor and acceptor.158 As such, it appears likely that molecularly mixed polymer/fullerene domains, while being effective at enabling efficient exciton quenching, can also function as recombination centers for photogenerated charge carriers. In this regard, it is striking that some of the highest device efficiencies reported to date have been achieved with relatively amorphous polymers (for example PTB7) that show evidence for rather high miscibility with PCBM and, consequently, significant polymer/PCBM mixing on a molecular scale.2,4,5,122,151 We recently undertook a number of studies to address this apparent contradictionnamely, that, for some blend films, the presence of significant fractions of molecular mixed material does not result in rapid electron/hole recombination and consequently poor device performance.40,85 The key to answering this conundrum appears to come from consideration of relative energetics of pure and mixed domains. In particular, we have observed that neat PCBM films exhibit an electron affinity approximately 100 meV greater than PCBM dispersed in a polymer matrix.85 In films comprising both molecularly mixed polymer/PCBM and pure PCBM domains, this energy difference provides an energy offset to localize electrons in the pure PCBM domains, thereby spatially separating the charges and enabling efficient charge collection, as illustrated in Figure 9c. A similar effect is likely for blend films exhibiting 624

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Figure 9. Illustration of charge separation in blend films comprising relatively an amorphous, intimately mixed polymer/fullerene phase as well as relatively chemical pure, more crystalline phases. The crystalline/aggregated domains exhibit a smaller IP (for donor materials) and larger EA (for acceptor materials). The interface between the mixed and more crystalline domains thereby provides an energy offset that can stabilize the spatial separation of charge carriers. The model is illustrated for blend films comprising a relatively crystalline donor polymer (a), relatively crystalline acceptor fullerene (c), or where neither polymer nor fullerene exhibit significant crystallinity (b). In part b, the absence of a well-defined interface to separate spatially the polarons results in relatively rapid (≤100 ns) charge carrier recombination, preventing efficient photocurrent generation. Reprinted with permission from ref 40. Copyright 2010, Wiley-VCH Verlag GmbH & Co. KGaA.

However, the lifetime of charge carriers in such molecular mixed domains is relatively short (≤100 ns). As such, stabilization of this charge separation requires the presence of relatively pure domains, with a favorable energy offset between the mixed and pure domains being essential to drive this stabilization. The presence of such mixed domains is likely to be less important for polymers that show lower miscibility with PCBM, in particular, for more crystalline polymers. However, even for these polymers, it has been suggested that the interface between polymer and PCBM domains may be relatively amorphous and molecularly mixed, with the higher bandgap in this interface region helping to drive and stabilize charge separation.85,119,121,159−161 Relating to this, recent studies have suggested that relaxation into lower energy states resulting from local inhomogeneities may also be a key factor helping to separate polarons away from the donor/acceptor interface.162,163 The presence of at least two charge separation interfaces, and realization that crystalline and amorphous regions of the film may exhibit different energetics, clearly complicates both energetic and kinetic analyses of charge separation in blend films. We have, for example, suggested that photocurrent generation may be limited both by geminate recombination of tightly BPP (or CT) states and, for some systems that lack secondary charge separation interfaces to stabilize the spatial separation of charges, by recombination of more loosely bound polaron pair states, as illustrated in Figure 9.86 However, in general, experimental data on these issues are relatively limited to date.

significant fractions of pure, crystalline donor polymer; in general, increased polymer crystallinity reduces its ionization potential, again providing an energy offset to stabilize charge separation.79 Appreciation that differences in carrier energetics within a blend film deriving from differences in material crystallinity/ aggregation state can impact directly photovoltaic device performance has important implications for materials design. For example, the requirement of relatively pure, aggregated PCBM domains to stabilize charge separation is likely to be one origin of high PCBM weight fractions often required to achieve efficient device performance, and the reason this requirement is less pronounced for crystalline polymers (due to their typically lower immiscibility with PCBM). We have also recently shown that this additional energy offset requirement can explain empirical observations that the low electron affinity acceptor ICBA works well with some crystalline donor polymers but not with amorphous donor polymers.40 ICBA appears not to exhibit a difference in electron affinity between neat and molecularly dispersed films, attributed to its low tendency to aggregation/crystallization. As such, the energy offset between mixed and pure fullerene domains present in blends with PCBM is not present in blends with ICBA, such that the presence of molecularly mixed domains are more likely to result in rapid electron hole recombination (Figure 9b), as evidenced by observation of a rapid (100 ns) recombination decay phase in blends of amorphous donor polymers with ICBA.40,155 This is, however, avoided in blends with more crystalline donor polymers, which reduce the presence of molecular mixing, and results in the formation of pure polymer domains capable of stabilizing charge separation (Figure 9a); such blends do not exhibit this 100 ns recombination phase. A key consideration in the above discussion is that, for many blend films, we need to consider two distinct interfaces that drive charge separation. In many cases, and particularly for more amorphous donor polymers, the initial exciton quenching may occur primarily in rather molecularly mixed domains, without the presence of a well-defined physical D/A interface.

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OTHER FACTORS INFLUENCING CHARGE GENERATION In the previous sections, we focused on the roles of energetics and film microstructure/crystallinity in determining the efficiency of charge photogeneration. In this section, we consider the extent to which we observe empirical correlations between our ΔOD assay of charge generation and other potential 625

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In addition, from spin statistics well established in the organic light emitting diode field, nongeminate recombination of dissociated charges is likely to lead to high yields of triplets states (Figure 10). However, our studies to date have indicated

parameters, including polymer hole mobility, macroscopic electric fields, and population of triplet states. Hole mobility is one property of polymers that is often cited in the literature in relation to good device performance. It has indeed been shown that excessively low hole mobilities can limit device performance due to poor charge extraction and resultant space charge accumulation. However, relating to the efficiency of charge photogeneration, our investigations have not observed any significant correlation between hole mobility and charge separation from polymer excitons, at least in blends with PCBM and with FET hole mobilities >10−4 cm2 V−1 s−1,89 in agreement with other studies.19 Similarly, we have not observed any correlation between charge photogeneration and polymer crystallinity, beyond the impact of polymer crystallinity upon ionization potential and miscibility with PCBM, as we have discussed above and in more detail elsewhere.40 In this regard, it should be noted that charge photogeneration from polymer excitons corresponds to an electron rather than hole transfer process. As such, it is plausible that electron mobilities rather than hole mobilities might be the more relevant parameter related to charge separation. Indeed, our comparison of fullerene and PDI acceptors, and of blends as a function of fullerene composition support this argument, as we discuss above. Hole mobilities, on the other hand, may be important for charge separation mediated via fullerene exciton dissociation (hole transfer); however, to the best of our knowledge, evidence for the importance of polymer hole mobility to this process is not present to date. Another factor widely discussed in terms of its impact upon charge separation is the macroscopic electric fields generated by the difference in work function of the device electrodes.41,76,112,164−166 There is extensive evidence that these electric fields play a key role in sweeping out photogenerated charge carriers, and thereby ensuring a high collection efficiency ηcoll. However, our own transient absorption data on devices as function of applied bias, as well as transient optoelectronic device analyses,16,74,84 have indicated that, for most photoactive layers, the impact of these macroscopic electric fields on the efficiency of charge separation ηsep is relatively insignificant, at least under normal device operating conditions within the fourth quadrant of the device JV curve.166 In some cases, we have observed significant increases in the efficiency of charge separation between open circuit and short circuit and assigned these to the effect of macroscopic electric fields; so far, we have only observed this field dependence in very amorphous blend films where the morphology of the active layer is most probably dominated by well-mixed polymer/fullerene phases.90,167 This correlation between field-dependent generation and blend crystallinity/morphology has also been observed in elegant studies employing time-delayed collection field measurements by Neher et al.165 The origin of this correlation between field dependence and crystallinity/morphology is unclear, but it is plausible that electric fields may be particularly helpful in spatially separating charges generated in molecular mixed domains, which would otherwise undergo relatively rapid recombination. There have been several studies in the literature, which consider the role of low lying triplet states in accelerating charge recombination in OSC.168−171 Both we and others have shown that BPP state formation may result in high yields of polymer or fullerene triplet states, through intersystem crossing between singlet and triplet BPP states, as illustrated in Figure 10.29,41,56,75,77,169,172 This polaron pair mediated triplet mechanism is analogous to that observed in for example photosynthetic reaction centers.

Figure 10. Illustration of the charge separation and recombination processes in organic solar cells, analogous to the model shown in Figure 7, but including the presence of triplet BPP and exciton states. Adapted with permission from ref 56. Copyright 2008, American Chemical Society.

that while triplet states may indeed be formed by such recombination processes when these triplet states lie energetically below the relevant BPP or CS states, the presence of such low lying triplet states does not necessarily prevent charge generation. For example, charge recombination in PTB7/PCBM blend films appears to result in efficient PTB7 triplet generation, indicative of the presence of a lower energy triplet state; yet under short-circuit conditions, photocurrent generation can be remarkably efficient.75 Work on this topic is ongoing and will be presented in more detail elsewhere. Finally, we note that there is currently extensive discussion in the literature over whether interfacial BPP or CT states may undergo thermally activated dissociation.76,150,173−175 Our studies indicate that low energy offsets result in low charge generation yields due to the presence of a competing loss pathway after exciton separation, which prevents efficient charge dissociation. The formation and recombination of Coulombically bound polaron pairs are the most likely candidate for this loss pathway. Whether these Coulombically bound states are the same states as those involved in studies of ‘CT state’ emission and sub-bandgap absorption is not fully resolved. In this context, it is important to note the presence of both molecular mixed and phase segregated domains in many blend films, as well as modulation to the energetic landscape from variations in material crystallinity, such that it is possible that such ‘CT state’ absorption and electroluminescence studies may be probing states formed at different interfaces to those associated with photoinduced charge separation, as we have discussed elsewhere.40 It has, for example, been noted that studies of CT state photoluminescence and electroluminescence often reveal rather distinct emission peaks,46 indicating the presence of different CT states in the film populated differently by optical or electrical generation and suggested to be associated with CT states located within molecular mixed domains and at domain interfaces, respectively.153 Also relevant to this discussion, Zhou et al. have considered a similar model for donor/acceptor pairs in solution and concluded that charge separation following sub-bandgap excitation in such systems can derive primarily from direct excitation of loosely bound polaron ion pairs.176

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CONCLUSIONS The focus of this review is on identifying materials design guidelines that may facilitate the development of new 626

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photoactive layer materials or processing procedures to maximize the quantum efficiency of photoinduced charge separation ηsep and thus photocurrent generation in OSC. These guidelines can be summarized as follows: (1) A key determinant for high ηsep is a sufficiently large energy offset between the D and A material optical bandgaps, and the blend electronic bandgap IP−EA, ΔECS. However, a large value of ΔECS corresponds to a large loss of energy per photon; therefore, a key challenge for OSC is to achieve a high quantum efficiency for charge separation with as small a value for ΔECS as possible. Empirically, we have observed that materials design strategies to achieve this include a. the use of donor copolymers with a high degree of ‘D−A’ character. This has proved to be remarkably effective at reducing the energy offset requirement for charge separation and is likely to be a key factor behind recent advances in OSC device efficiencies. b. the use (for charge separation from polymer excitons) of acceptors with increased electron mobility/ delocalization (e.g.: by increasing acceptor crystallinity). We note, however, that excessive acceptor crystallinity may result in lower collection efficiency. (2) The generation of sufficiently long-lived charge carriers to enable efficient charge collection requires the spatial separation of charges into different domains. This typically requires the presence in the blend of reasonably pure domains of either donor or acceptor (or both). For such pure domains to stabilize charge separation, there must be an energy offset (of the order of 100 meV or greater) driving one charge carrier from mixed domains into these pure domains. This energy offset can derive from differences in crystallinity/aggregation between pure and mixed domains. (3) For most polymer/fullerene blends, the miscibility of PCBM with many donor polymers results in efficient polymer exciton quenching, with negligible exciton diffusion requirements. In contrast, PCBM exciton diffusion limitations can result in significant photocurrent losses. We note that, for many OSC blends, and particularly those with more amorphous donor polymers with PCBM, efficient device performance requires the formation of relatively pure PCBM domains, for reason 2 above. This can be achieved by appropriate film processing (e.g., by use of cosolvent additives) or the addition of excess PCBM to the blend. However, this formation of aggregated PCBM domains can also reduce the efficiency of PCBM exciton utilization. (4) Device macroscopic electric fields do not significantly impact upon photocurrent generation in most OSC (except at strong reverse biases), apart from in blend films employing highly amorphous donors. We also do not observe a significant correlation of charge separation efficiency with polymer hole mobility (for charge separation from polymer excitons). Clearly these materials design guidelines are limited both in scope and detail. However, we hope that they provide some insights that may aid synthetic chemists, materials scientists, and device physicists in their drive toward more efficient OSC materials and devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Stoichko Dimitrov has an MSc degree from Sofia University, Bulgaria (2003) and one year experience as a chemist at the Institute of Physical Chemistry of the Bulgarian Academy of Science. He obtained his PhD degree from Boston College, USA (2010), studying excited state dynamics of semiconductor nanocrystals and DNA, using femtosecond laser spectroscopy. In 2011, he joined Imperial College London, UK, as a postdoctoral researcher, where he works with James Durrant. His research focuses on characterizing polymers and fullerenes for solar cell applications, using various laser spectroscopy techniques. James Durrant is Professor of Photochemistry in the Department of Chemistry, Imperial College London, and Deputy Director of Imperial College’s Energy Futures Lab. His research employs photochemical studies to elucidate design principles which enable the development of solar conversion technologies. His group is currently addressing the development and functional characterization of organic solar cells and photoelectrodes for solar fuel generation. He has published over 280 research papers and was recently awarded the 2012 Tilden Prize by the Royal Society of Chemistry.

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ACKNOWLEDGMENTS We gratefully acknowledge the EPSRC projects EP/J500021/1 and EP/G037515/1 for funding. We thank our many colleagues and co-workers who have contributed to these studies over the last 8 years, and in particular Safa Shoaee, Eliza Collado Fregoso, and Yvonne Soon, who helped with the preparation of some figures in this manuscript, and Christian Nielsen, Hugo Bronstein, Jenny Nelson, and Dieter Neher for helpful discussions and comments. We also thank colleagues, and particularly Peter Würfel for discussions regarding ΔECS vs ΔGCS terminology.

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REFERENCES

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dx.doi.org/10.1021/cm402403z | Chem. Mater. 2014, 26, 616−630