Charge Photogeneration in Low Band Gap Polyselenophene

Apr 2, 2010 - Tracey M. Clarke,† Amy M. Ballantyne,‡ Steve Tierney,§ Martin Heeney,§ Warren Duffy,§. Iain McCulloch,§ Jenny Nelson,‡ and Jam...
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Charge Photogeneration in Low Band Gap Polyselenophene/Fullerene Blend Films Tracey M. Clarke,† Amy M. Ballantyne,‡ Steve Tierney,§ Martin Heeney,§ Warren Duffy,§ Iain McCulloch,§ Jenny Nelson,‡ and James R. Durrant*,† Departments of Chemistry and Physics, Imperial College London, Exhibition Road, London SW7 2AZ, England, and Merck Chemicals, Chilworth Science Park, Southhampton SO16 7QD, England ReceiVed: December 22, 2009; ReVised Manuscript ReceiVed: March 18, 2010

In this paper, we use transient absorption spectroscopy to examine the charge photogeneration yields of a series of low band gap polythiophenes and polyselenophenes in blend films with 6,6-phenyl C61-butyric acid methyl ester (PCBM). The polymers are selected to have approximately matched ionization potentials, allowing us to focus upon the importance of the polymer lowest unoccupied molecular orbital (LUMO) level in determining photogeneration efficiency. Data are collected as a function of PCBM composition. A correlation is observed between the yield of dissociated polarons, as measured by the amplitude of the transient absorption signal, and the polymer LUMO level. Lower band gap polyselenophenes produce lower polaron yields, in quantitative agreement with a previously proposed model in which the excess thermal energy of initially generated bound radical pairs determines their dissociation efficiency. Increasing the PCBM concentration from 5 to 50 wt % results in an increase in charge photogeneration. Photoluminescence data demonstrate that this dependence is not primarily associated with an increase in exciton quenching; instead, this increase is assigned to the additional influence of PCBM domain size and/or electron mobility on the dissociation efficiency of the bound radical pairs. These observations are then discussed in terms of their implications for the development of polymer semiconductor materials for organic photovoltaics, and in particular the development of guidelines for the design of polymers for efficiency charge photogeneration in such devices. 1. Introduction Conjugated polymer-based organic photovoltaic (OPV) devicesbased upon a bulk heterojunction active layer have received significant attention in recent years.1 Poly(3-hexylthiophene) (P3HT) has shown particular promise, with device power conversion efficiencies of approximately 5 wt % in blends with 6,6-phenyl C61-butyric acid methyl ester (PCBM).2–5 The high OPV efficiencies achieved by P3HT:PCBM-based devices have been attributed to the high crystallinity of P3HT,6–9 good hole mobility,10,11 and, when blended with PCBM, its high charge photogeneration yields,12 low bimolecular recombination losses,13 and formation of selective device contacts.14 However, the efficiency of P3HT:PCBM devices is limited by the relatively poor overlap of P3HT’s absorption spectrum with the solar irradiation spectrum and, in particular, by its lack of red/ near-infrared absorption, a result of its relatively high lying highest occupied molecular orbital (HOMO) energy level.15 This has led to extensive studies of lower optical band gap polymers toenhancephotonabsorption,16 includingtheuseofdonor-acceptor copolymers incorporating benzothiadiazole,17 thienothiophene,18 thienylthienopyrazine,19 and thienylene vinylene16 monomeric units. A key limitation of such studies to date is that devices employing lower optical band gap polymers typically produce lower device internal quantum efficiencies than those achieved with P3HT-based devices.16 In general, the use of lower optical band gap polymers reduces the energy of photogenerated singlet excitons and therefore also decreases the free energy available to drive charge separation and photovoltaic function. In this * To whom correspondence should be addressed. E-mail: j.durrant@ imperial.ac.uk. † Department of Chemistry, Imperial College London. ‡ Department of Physics, Imperial College London. § Merck Chemicals.

paper, we study a series of polythiophenes with optical band gaps ranging from 1.9 eV (analogous to P3HT) down to 1.6 eV. These polymers have relatively similar ionization potentials, allowing us to focus specifically upon the role of singlet exciton energy in determining charge photogeneration yield and thus photovoltaic device performance. At present, there is no clear consensus on the parameters that determine charge photogeneration in polymer/fullerene solar cells. It has been widely reported that a lowest unoccupied molecular orbital (LUMO) level offset of 0.3 eV between the polymer and the PCBM is sufficient to overcome the Coulomb binding energy of the singlet exciton,20 allowing electron transfer from the polymer to the fullerene. We note, however, that experimental data to support the validity of this 0.3 eV LUMO level offset requirement is very limited, and it is primarily based upon a theoretical study of PPV polymer blends which stated 0.35 eV as the energy required for a transition between intrachain and interchain excitons.21,22 An important concern is that the initially generated electron-hole pair may continue to experience significant electrostatic attraction (of the order of 0.1 - 0.4 eV) after electron transfer, potentially resulting in the formation of a Coulombically bound species at the polymer/ fullerene interface (referred to herein as “bound radical pairs”, but variously referred to in the literature as “charge transfer states”, “exciplexes” (when radiatively coupled to the ground state), or “geminate ion pairs”). This is a particular concern for organic photovoltaic devices due to the relatively low dielectric constant of organic semiconducting materials relative to inorganic materials (εr ∼ 3-4 compared to 12 for silicon). In addition, the physical constraint of the domain sizes of the polymer and fullerene components in the blend may further limit the ability of the initially generated charges to move away from each other. As a result, geminate recombination of the initially

10.1021/jp9120782  2010 American Chemical Society Published on Web 04/02/2010

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Figure 2. Molecular structures of the polymers studied herein. Figure 1. Energy diagram for charge formation via a bound radical pair (P+ · · · C60-) proposed for polymer/PCBM blend films. The initially formed bound radical pair (P+ · · · C60-)hot can either undergo thermalization or dissociation into the free charge carriers P+ + C60-, depending on the magnitude of ∆GCSeff.

generated electrons and holes is thought to be a significant loss pathway in polymer/fullerene solar cells, competing with dissociation of the charges and subsequent collection by the device electrodes. Several studies have addressed the importance of geminate recombination in limiting OPV device performance. Such studies have particularly focused upon the effect of domain size; for example, Janssen et al. investigated poly[2,7-(9,9-didecylfluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)(PF10TBT), which forms a charge transfer emissive state in blends with PCBM. It was concluded that the presence of large PCBM domains at high PCBM concentrations reduces the probability of geminate recombination.23 This correlation between phase segregation and geminate recombination has been noted in a number of experimental24–26 and modeling studies.11,27 In addition, some studies have considered the role of macroscopic electric fields in driving the dissociation of the bound radical pairs23,27 and the importance of the molecular structure at the donor/acceptor interface in determining the strength of their Coulombic attraction.28,29 Of particular relevance to the development of low band gap polymers for OPV devices, we have recently addressed the correlation between the free energy available to drive charge separation, ∆GCSeff, and the yield of photogenerated charges, as measured by transient absorption spectroscopy. This change in free energy, ∆GCSeff, is defined most simply as the difference between the singlet exciton energy (S1) and the energy of the dissociated charges, where the latter is estimated from the difference between the ionization potential (IP) of the donor and electron affinity (EA) of the acceptor, ∆GCSeff ) S1 - (IP - EA); see Figure 1. We have reported a close correlation between this free energy difference and the yield of dissociated charges for a series of polythiophene/PCBM blend films (5 wt % PCBM), where the polythiophenes vary in ionization potential.30 It was suggested that this correlation may originate from the dissociation efficiency of the initially formed bound radical pairs being dependent upon their thermal energy. As such, excess thermal energy (a high ∆GCSeff) would be required to overcome the Coulombic binding energy to generate the fully separated charge carriers, as summarized in Figure 1. The efficiency of charge photogeneration is therefore likely to be a critical issue in the selection of new, lower optical band gap polymers for polymer/fullerene solar cells. One approach to reduce the optical band gap of conjugated polymers is to use charge transfer copolymers. Such charge transfer motifs provide an additional strategy to modulate the

electronic structure of the polymer/fullerene interface and therefore to potentially enhance charge photogeneration.29 However, uncertainty over the detailed molecular structure of the interface complicates quantitative comparison of different copolymers. In this paper, we therefore focus only upon a novel series of conjugated polymers containing thiophene and/or selenophene that do not exhibit significant charge transfer character and have relatively invariant ionization potentials. In this polymer series, the optical band gap is primarily determined by varying the selenophene content; the selenium acts to stabilize the LUMO, thereby decreasing the band gap. This new polymer series is therefore different from the previous polymer series studied30 where it was the ionization potential that was the primary variant. We have already reported a study of one polymer of this series, regioregular poly(3-hexylselenophene) (P3HS), the selenium analogue of P3HT.31 This material exhibited a smaller optical gap (1.6 eV) than regioregular P3HT (1.9 eV), the result of a shift in the P3HS LUMO energy.32 The extended absorption range allows a greater spectral overlap with the solar emission spectrum, and thus, it was anticipated that P3HS might be a good candidate for polymer solar cells. Indeed, P3HS:PCBM devices exhibited a significant extension of photosensitivity into the red relative to P3HT:PCBM control devices. However, this enhancement was offset by a reduction in the photocurrent quantum efficiency, resulting in similar overall device performance under AM1.5 irradiation, analogous to that observed for many other lower optical band gap polymers. Mechanistic studies identifying the origin of these observed losses in photocurrent have been relatively limited to date. In this paper, we focus upon the origin of these quantum efficiency losses by studying the correlation between charge photogeneration yield and the driving force of charge separation for a series of P3HT analogues, with most involving selenophene comonomers. 2. Results The chemical structures of the polythio- and selenophenes studied in this paper are illustrated in Figure 2, and a summary of their key properties is shown in Table 1. P3HT was prepared using a proprietary method developed by Merck KGaA. P3HS was prepared using a Grignard metathesis method developed for polythiophenes33,34 that has been previously reported.32 The polymer materials P(T0TT16),35 P(Se0TT16), P(T10Se0T10), P(Se10T0Se10), and P(Se10Se0Se10) were synthesized using a previously reported microwave-assisted Stille coupling of the respective dibromide and distannane monomers.36 The absorption spectra of the pristine selenophene polymers show a progressive red-shift as the extent of selenium substitution increases. The absorption maximum of P(T10Se0T10), for example, is measured at 569 nm and shifts to 614 nm for P(Se10Se0Se10). This has the effect of improving the spectral

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TABLE 1: Characterization of Polymer Pristine Filmsa polymer

S1 (eV)b

IP (eV)c

∆GCSeff d

P(Se10Se0Se10) P3HS P(Se10T0Se10) P(T10Se0T10) P(Se0TT16) P3HT P(T0TT16)

1.8 1.8 1.9 1.9 2.0 2.0 2.1

4.92 4.80 4.91 4.87 5.0 4.77 5.0

0.58 0.70 0.69 0.73 0.70 0.93 0.80

a Note that molecular weights were in the range of 20 000-35 000 Da. b Estimated from crossover point between normalized absorption and emission spectra of pristine polymer. c Measured using ambient photoelectron spectroscopy. d Determined using ∆GCSeff ) S1 - (IP - EA), using as previously30 an electron affinity for PCBM of 3.7 eV.

Figure 4. Transient absorption decays of as-spun P(T0TT16) films as a function of PCBM concentration (0, 5, 50 wt %), measured using a pump wavelength of 530 nm with an excitation density of 63 µJ · cm-2 and a probe wavelength of 980 nm. The inset shows the same data on a log-log scale.

Figure 5. Transient absorption decays of an as-spun P(T0TT16):PCBM (1:1) film as a function of excitation density, measured using a pump wavelength of 530 nm and a probe wavelength of 980 nm.

Figure 3. Normalized absorption spectra of P(T0TT16) and P(Se0TT16) films in both the pristine state and in blends with 5 and 50 wt % PCBM (a) and the emission spectra of P(T10SeT10) and P(T10SeT10):PCBM (1:1) films using an excitation wavelength of 625 nm.

overlap with the solar spectrum; therefore, it is possible that those polymers containing more selenophenes may show enhanced device performance. The ionization potentials of these two polymers are very similar (approximately 4.90 eV), and thus, the decrease in band gap with increasing selenophene substitution is due to a lower LUMO level. The addition of 5 wt % PCBM did not significantly change the films’ absorption spectra. However, 50 wt % PCBM resulted in a blue-shift of the absorption for all polymers studied in this series (by a maximum of 50 nm), indicative of a loss of polymer crystallinity. This blue shift is exemplified for P(T0TT16) and P(Se0TT16) in Figure 3a. A fluorescence study of PT10Se0T10 reveals typical emission quenching behavior (Figure 3b). Upon addition of 50 wt % PCBM, the emission intensity of the pristine polymer was reduced by over 95%. This quenching is indicative of highly efficient exciton dissociation at the polymer/PCBM interface: the PCBM can efficiently remove an electron from the first excited singlet state of the polymer before the exciton can radiatively relax back to the ground state. Fluorescence studies

of other thiophene-based polymers show, in general, a high degree of emission quenching of over 80%, even with small quantities of PCBM (5 wt %); therefore, this observation could be considered generic for this class of conjugated polymers.37 The ability of the polymer/PCBM blend films to achieve photogeneration of long-lived (dissociated) polarons was assayed by transient absorption spectroscopy (TAS). TAS was employed to directly monitor the optical absorption of photogenerated charges on the microsecond time scale. As we have shown previously, such measurements are a probe of charge generation yield and thus an indicator of device performance.30,38,39 Transient signals were measured for pristine polymer films and blend films with 5 and 50 wt % PCBM by weight. For all polymers, a broad photoinduced absorption band was observed in the near-infrared, primarily assigned to absorption of positive polythiophene polarons (as previously reported12,40). For comparison between polymers, a probe wavelength of 980 nm was used, corresponding to the absorption maximum of P3HT positive polarons.12 Control studies on pristine polymer film in the absence of PCBM indicated relatively small, or negligible, transient absorption signals, consistent with the expected lower (or negligible) charge photogeneration in such films.41 Typical transient decays of blend films are shown in Figure 4 for P(T0TT16):PCBM. These decays follow a power law (∆OD ∝ t-R) on time scales longer than ∼4 µs, as illustrated by the inset plotted on a double-logarithmic scale. Prior to this slow power law phase, an excitation density-dependent fast phase exists (Figure 5). This biphasic transient decay behavior is analogous to that we have reported previously for MDMO-PPV38,39 and P3HT2,12 blended with PCBM. As such, the slow phase

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Figure 6. Transient absorption decays of as-spun P(T0TT16), P(Se0TT16), P(T10Se0T10), and P(Se10Se0Se10) films with 5 wt % PCBM, corrected for ground state absorbance at the pump wavelength, measured using an excitation density of 63 µJ · cm-2 and a probe wavelength of 980 nm.

(the power law decay) is assigned to bimolecular recombination of dissociated polarons in the presence of an exponential distribution of localized (trapped) polaron states.42 The fast phase that appears at high laser intensities can be assigned, at least in part, to recombination of free charge carriers that are generated when the density of photogenerated polarons exceeds the density of localized states. It is evident from the TAS decays of the P(T0TT16) films that the value of R (the gradient of the power law, which provides an indication of the energetic distribution of the polaron trap states) changes as the quantity of PCBM in the films is altered from 0 to 50 wt % (Figure 4 inset). The pristine film has an R value of 0.65 which decreases to 0.40 and then to 0.27 for the 5 and 50 wt % PCBM films, respectively. This observation that the decay dynamics become more dispersive with increased PCBM content is indicative of an increased proportion of energetically deeper traps. This is consistent with the increased PCBM content reducing polymer crystallinity, in agreement with the film absorption spectra and previous reports on the effect of large PCBM concentrations disrupting the crystallinity of a pristine polymer.10 It also agrees with a recent study of unannealed P3HT:PCBM blend films that shows a continuous decrease in R as the PCBM content is increased from 0 to 70 wt %, consistent with the decrease in polymer crystallinity.26 Transient absorption data measured for polymer/PCBM blend films with the other polymers studied herein all revealed similar recombination dynamics to those shown in Figure 4. However, a large variation was observed in the transient signal amplitude between different polymers in blends, indicative of a large range in the yield of dissociated polarons. This is illustrated in Figure 6 for 5 wt % PCBM blend films. It is therefore of interest to investigate the possibility of a correlation between polaron yield (estimated from the amplitude of the transient absorption signal at 1 µs using 980 nm probe) and the free energy of charge separation, ∆GCSeff, for these new polymers, in accordance with the correlation between these two parameters previously observed for a different polymer series.30 Figure 7a shows an overlay of the data obtained for 5 wt % PCBM blends of the new selenophene polymer series with the data reported previously (also with 5 wt % PCBM). It is apparent that the new data are in agreement with the correlation we have reported previously, and provides confirmation of a dependence of the charge photogeneration yield and the free energy difference ∆GCSeff. We turn now to the effect of increasing the PCBM concentration in the blend film from 5 wt % to 50 wt %. The P(T0TT16) films show a clear increase in the transient signal amplitude,

Figure 7. Transient absorbance signal amplitude measured at 1 µs of polymer/PCBM (5 wt %) blend films of the five polymers studied here as a function of ∆GCSeff (O), with analogous data for a series of polythiophenes reported in ref 29 (9) shown for comparison (a). TAS data points for blend films with 50 wt % PCBM (4) are shown in (b), revealing a shallower dependence of the polaron yield on ∆GCSeff. All transient signals have been corrected for variation in the optical absorbance at the excitation wavelength.

∆OD, as the PCBM concentration is increased (Figure 4), indicating a progressive enhancement in the charge photogeneration yield. In particular, the initial ∆OD doubles from P(T0TT16) with 5 wt % PCBM to 50 wt % PCBM. This improvement increases to a factor of 5 when the laser excitation density is reduced to avoid potential saturation effects (as evident from Figure 5). This result indicates a progressively greater yield of charge carriers in each film as the PCBM concentration is increased; this is consistent with device studies that show considerably higher device efficiencies when 50 wt % PCBM is used in the active layer compared to 5 wt % (this increase in device efficiency may also be associated with improved electron transport). We note that photoluminescence quenching data indicate that this increase in polaron yield with PCBM concentration is too large to be assigned primarily to enhanced exciton quenching. In all cases studied, increasing the PCBM concentration from 5 to 50 wt % resulted in only a marginal increase in photoluminescence quenching (from ∼80% to >90%, see above). This observation strongly suggests that the enhanced yield of long-lived polarons observed in the TAS studies with increased PCBM concentration results not from improved exciton dissociation, but rather from a reduction in geminate recombination losses. Figure 7b overlays the resultant ∆GCSeff versus ∆OD plots for both the 5 wt % and 50 wt % PCBM blends. It is apparent that the 50 wt % PCBM blend films still show a clear correlation between polaron yield and ∆GCSeff. However, the magnitude of the enhancement in charge photogeneration yield observed when the PCBM concentration is increased from 5 to 50 wt % was found to be dependent upon the polymer studied. In general, the largest enhancements were observed for polymers with

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relatively low values of ∆GCSeff and therefore relatively low polaron yields. For the 5 wt % PCBM blend films, the polaron yield changes by ∼100 over the range of ∆GCSeff studied (0.3 eV), whereas for the 50 wt % PCBM blend film this dependence is reduced to approximately 10-fold. This observation is consistent with our ∆OD measurements being a yield rather than a kinetic measurement (vide infra). 3. Discussion The study we report herein builds upon our previous comparison of charge photogeneration for a series of polythiophenes with varying ionization potentials in blends with 5 wt % PCBM. This study revealed a strong correlation between charge photogeneration yield and ∆GCSeff,30 with an increase in ∆GCSeff by 0.3 eV resulting in an increase in the polaron yield by 2 orders of magnitude. A PCBM concentration of 5 wt % was utilized for this initial study in order to minimize the effect of PCBM on polymer crystallinity. The observed correlation was rationalized in terms of a model whereby the initially formed bound radical pairs dissociate with an efficiency dependent on their thermal energy: excess thermal energy (indicated by a high ∆GCSeff) is required to overcome their Coulombic binding energy and allow them to generate the fully separated charge carriers. This corresponds to the generation of thermally “hot” radical pairs (note that this model does not require the formation of “hot” excitons). If the hot radical pair does not have sufficient thermal energy to dissociate, then it will relax and subsequently undergo geminate recombination back to the ground state, thereby leading to a low yield of dissociated charges. The model is summarized in Figure 1 and is discussed in detail elsewhere.51 The concept of a bound radical pair state thermally relaxing and thus being unable to fully charge separate has been proposed previously43 to describe “exciplex” behavior in polymer/polymer blends. Our study therefore suggests that the efficiency of geminate recombination, a key loss mechanism in polymer photovoltaics devices,44 may be strongly related to ∆GCSeff. Given the substantial implications for strategies to optimize OPV efficiencies, it is important to consider the extent to which this correlation can be extended to other polymer/PCBM blends. The data we report herein using the new series of polyselenophenes provide clear evidence that this correlation is indeed predictive (at least for the class of polymers studied). The charge photogeneration yields determined for the new polymer series, again using 5 wt % PCBM, were overlaid upon the original data in Figure 7a, and show excellent agreement with the results of our previous study. This result is important, as it suggests that this correlation is generic for thiophene-based conjugated polymers and could potentially be used as a predictive tool for device performance. A particular limitation of the Ohkita et al.30 study is that it only addressed 5 wt % PCBM blend films, in an attempt to minimize the influence of PCBM on the polymer crystallinity. However, we note that efficient charge collection in OPV devices typically requires a minimum of PCBM content of ∼50 wt % in order to achieve continuous percolation pathways for efficient electron transport to the anode. For this reason, the transient absorption data of polymer blend films with 50 wt % PCBM (Figure 7b) are of particular interest in relation to device performance. The 50 wt % PCBM data clearly show the higher polaron yields for each polymer compared to the 5 wt % data. This effect does not appear to originate from differences in exciton quenching, which is already reasonably efficient (>80%), even for the 5 wt % blend films. Instead, the observed

Figure 8. EQE spectra for an annealed P3HT:PCBM (1:1) device and an as-spun P(Se0TT16):PCBM (1:1) device, showing both the increase in sensitivity in the red and the reduction in quantum efficiency for the latter device.

enhancement in polaron yield with PCBM content is assigned to an increase in PCBM domain size and/or crystallinity. At 5 wt % PCBM, the PCBM is likely to be more finely dispersed throughout the polymer phase and therefore expected to increase the likelihood of geminate recombination. In contrast, the higher PCBM content can be expected to result in the formation of large PCBM domains, and thus, the bound radical pair has a wider range of possible spatial separations and finding a configuration where separation into the fully dissociated charge carriers is more likely. Thus, the movement of dissociated electrons away from the polymer/PCBM interface is facilitated and geminate recombination reduced, thereby increasing the polaron yield. Thus, this observation is consistent with the dissociation of the bound radical pair (or “charge transfer”) states being dependent upon not only ∆GCSeff but also film nanomorphology and in particular material domain sizes. A more detailed discussion of this point is presented elsewhere.51 In support of the above rationale, a recent study of PBTTT: PCBM blend films (where PBTTT is one of the polythiophenes studied by Ohkita et al.) observed a strong dependence of the photocurrent density on PCBM content, with efficient photocurrent generation being correlated with the formation of large PCBM domains.46 This has been observed in other reports as well,23–25 and the same mechanism proposed by some.23,26 Furthermore, we have recently measured efficient charge photogeneration for polythiophene/perylene diimide (PDI) blend films even for relatively low values of ∆GCSeff,47 consistent with the relatively high electron mobility and/or large domain sizes of the perylene diimide facilitating electron movement away from the donor/acceptor interface. Preliminary device studies performed on this series of polyselenophenes revealed that only low power conversion efficiencies were obtained, with the highest being 0.88% for P(T0TT16):PCBM (2:3). The device data is presented in the Supporting Information (Table S1). As such, these efficiencies are substantially lower than that achievable for P3HT:PCBM. Figure 8 compares external quantum efficiency (EQE) spectra for P3HT:PCBM and P(Se0TT16):PCBM devices, clearly showing both the increase in red sensitivity and the decrease in efficiency for the latter compared to P3HT:PCBM. We therefore conclude that the decrease in the photocurrent quantum efficiencies observed for polyselenophenes compared to P3HT (resulting in low device efficiencies despite the observed enhancement in red sensitivity) has an appreciable contribution from a lower charge photogeneration yield, which in turn can be assigned to a reduction in the free energy of charge separation, ∆GCSeff. This conclusion has important implications

Charge Photogeneration in Polymer Blend Films for the design of low band gap polymers for OPV applications. Current strategies to improve device efficiency generally involve attempts to tune the molecular orbitals of the polymer in order to obtain a smaller band gap.16 Lowering the polymer LUMO is the most common target for this because raising the HOMO has the additional detrimental effect of decreasing the open circuit voltage. This strategy assumes that the LUMO can be lowered without a negative effect on the efficiency: previous studies have indicated that a LUMO level offset of 0.3 eV is necessary for efficient charge separation,20 and therefore, as long as this minimum condition is maintained, the polymer LUMO can be lowered without sacrificing any efficiency. However, the ∆GCSeff plot suggests that this is not the case for homopolythiophenes; decreasing the polymer LUMO lowers ∆GCSeff which has a negative impact on charge photogeneration yield and often results in poor device performance. As such, strategies to improve OPV performance through lowering the polymer LUMO level to enhance light harvesting may need to consider additional strategies to avoid a concomitant loss of photogeneration yield and thus device photocurrent. Options to address this may include the use of alternative electron acceptors (as evidenced by the PDI results47) or “charge transfer” polymers (as exemplified by PCPDTBT29). A key motivation for transient absorption studies such as those presented herein is to develop design guidelines for polymer performance in organic photovoltaic devices. In general, polymer design for such devices is a significant challenge, since any change in polymer structure can impact upon overall device performance in a large number of ways. It is therefore typically very difficult to determine the specific origin of a change in device performance resulting from a change in polymer structure. The study reported herein focuses upon one particular aspect of device function: the efficiency of dissociation of charges photogenerated at the polymer/PCBM interface. Empirically we observe that this process is strongly dependent upon the energetics of this interface and, in particular, the excess energy of the polymer exciton relative to the energy of the dissociated charges (∆GCSeff). In addition to this energetic dependence, our observations indicate that charge dissociation can be enhanced by increasing the acceptor domain size (as reported by others23–26), by use of a charge transfer polymer,29 and by the use of a high electron mobility acceptor47 (a perylene diimide). Indeed, the presence of other factors influencing charge photogeneration apart from ∆GCSeff is demonstrated by the observation of efficient photocurrent generation in P3HT/ fullerence blend solar cells using a C60 derivative with a lower electron affinity than PCBM.48 Of equal interest are the factors which we observe to have a relatively minor, or no, effect on the charge dissociation efficiency: polymer hole mobility,30 alkyl side chain length (for a series of polythiophene-based polymers), and polymer crystallinity alone (apart from its effect upon energetics and/or nanomorphology). Clearly, this dependence analysis is not exhaustive, with different factors being more or less critical for different materials systems. For example, a recent modeling study by Huang et al.28 of polymer/polymer blends indicates that the Coulomb attraction of interfacial polaron pairs is strongly dependent upon the details of molecular packing at the interface. Furthermore, we have shown for polyfluorene/ PCBM blends that the energy of the PCBM triplet exciton can be a key determinant of charge photogeneration efficiency,49,50 most probably associated with the presence of relatively efficient energy transfer from the polyfluorenes to PCBM. Nevertheless, our analyses do start to provide some indication of the key factors to be considered in designing electron donor polymers

J. Phys. Chem. C, Vol. 114, No. 17, 2010 8073 for organic solar cells, which can be employed to aid rational materials and device optimization. Finally, we note some of the limitations of ∆GCSeff versus ∆OD plots such as those shown in Figure 7. First, it should be noted that the ∆OD assay is a yield-based rather than a kinetic assay. It is therefore only sensitive to the relative rates of charge dissociation, thermal relaxation, and geminate recombination shown in Figure 1. As such, it cannot readily be used to determine quantitatively the underlying dependence of the rate of any single process upon ∆GCSeff (or any other parameter). Direct measurements of the kinetics of these underlying processes in polymer/fullerene blend films have been relatively limited to date. Second, it should be emphasized that the observation of a correlation between ∆GCSeff and ∆OD does not imply that ∆GCSeff is the only factor determining the charge photogeneration yield. We have, for example, reported that the low band gap charge transfer polymer PCPDTBT exhibits efficient charge photogeneration with only a relatively small ∆GCSeff and suggested that this may be related to the intramolecular charge transfer nature of this polymer. Similarly, it has been widely reported that blend domain size may be closely linked with charge photogeneration efficiency23,26,44,45 and thus is a key issue affecting device performance.10 However, it should be noted that domain size and ∆GCSeff may themselves be correlated. In our previous study of the effect of thermal annealing on P3HT:PCBM blend films,12 we found that annealing increased both domain size and polymer crystallinity, with the increase in crystallinity resulting in a lower polymer ionization potential and consequently increasing ∆GCSeff (since S1 did not change in this system). The increase in ∆GCSeff was found to be of a sufficient magnitude to account for the observed increase in polaron yield and photocurrent generation. 4. Conclusions A series of low band gap polythiophenes and polyselenophenes have been investigated in this paper, primarily using transient absorption spectroscopy to examine their charge photogeneration yields in blend films as a function of PCBM concentration. A study of these new polymers in 5 wt % PCBM blend films revealed that the correlation between charge photogeneration yield and the free energy of charge separation is preserved compared with Ohkita’s original polythiophene study. This suggests that this correlation is generic for thiophenebased conjugated polymers and, as such, could possibly be used as a predictive tool for photovoltaic device performance. It was also observed that increasing the PCBM concentration from 5 to 50 wt % significantly enhanced the charge generation yield in polymer blend films while still maintaining a strong correlation between ∆GCSeff and polaron yield. The greater polaron yields are consistent with larger PCBM domain sizes at the higher concentration of PCBM, improving the spatial range available for the initially generated bound radical pair to escape the Coulombic attraction, thereby increasing their dissociation efficiency and decreasing geminate recombination losses. Photovoltaic devices fabricated using the lower band gap polyselenophenes in 1:1 blends with PCBM show an appreciable enhancement in red sensitivity compared to P3HT:PCBM devices but relatively low internal photocurrent generation efficiencies. Our results suggest that these low quantum efficiencies in polymer photovoltaic cells have an appreciable contribution from a smaller free energy of charge separation, producing a high probability of geminate recombination and a low charge photogeneration yield. This prevents the polysele-

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nophenes from reaching photovoltaic device efficiencies equivalent to P3HT-based devices. This study has significant implications in terms of molecular design, as it suggests that strategies based around reducing the optical band gap to enhance light absorption need to consider carefully the potential impact of this loss in exciton energy upon the efficiency of charge dissociation at the polymer/acceptor interface. 5. Experimental Section P3HT,2 P3HS,31,32 and P(T0TT16)35 were prepared as previously reported. P(Se0TT16), P(T10Se0T10), and P(Se10T0Se10) were synthesized using a previously reported microwave-assisted Stille coupling of the respective dibromide and distannane monomers,36 and full details will be published elsewhere. Molecular weights of the polymers employed are detailed in the Supporting Information. Solutions and films were prepared in a clean room with a filtered air environment (Class 100). Polymer/PCBM (API Services Inc.) blend solutions were prepared using chlorobenzene (BDH Laboratory Supplies) and filtered (Gelman Acrodisc 0.45 µm PTFE membrane syringe filter) just before use. Devices were fabricated by first spincoating a layer of PEDOT:PSS (50 nm, Baytron P, H. C. Stark GmbH) onto patterned ITO-coated glass substrates (PsiOTc Ltd., UK) and annealing these films at 160 °C. Films were spin-coated at 1200-1500 rpm to give a thickness of 140-200 nm (measured using an Alpha Step 2000 surface profilometer, Tencor Instruments). Samples were then transferred to a glovebox where aluminum top electrodes were deposited in a vacuum system. Devices were also annealed in a dry nitrogenfilled glovebox. Ionization potentials were determined by an ambient photoelectron spectroscopy method with a Riken-Keiki AC-2 spectrometer. J-V curves and photocurrent were both measured using Keithley 238 Source Measure Units. Illumination for J-V curves was provided by a 300 W xenon arc lamp solar simulator (Oriel Instruments) with an output of 50 mW cm-2, and illumination for photocurrent measurements by the monochromated (Bentham monochromator) output from a Tungsten halogen source where the polymer photodiode response was calibrated using a Newport UV-818 photodiode. Photoluminescence quenching was determined using a steady state spectrofluorimeter (Horiba Jobin Yvon, Spex Fluoromax 1) relative to pristine polymer control films. Transient absorption decays were measured by exciting the sample film, under a nitrogen atmosphere, with a dye laser (Photon Technology International Inc., GL-301) pumped by a nitrogen laser (Photon Technology International Inc., GL-3300). The excitation wavelength used varied depending on the polymer (500-615 nm), with a pump intensities ranging between 0.7-75 µJ · cm-2 and a repetition frequency of 4 Hz. A 100 W quartz halogen lamp (Bentham, IL 1) with a stabilized power supply (Bentham, 605) was used as a probe light source, with a pump wavelength of 980 nm used. The probe light passing through the sample film was detected with a silicon photodiode (Hamamatsu Photonics, S1722-01). The signal from the photodiode was preamplified and sent to the main amplification system with an electronic band-pass filter (Costronics Electronics). The amplified signal was collected with a digital oscilloscope (Tektronics, TDS220), which was synchronized with a trigger signal of the pump laser pulse from a photodiode (Thorlabs Inc., DET210). To reduce stray light, scattered light, and sample emission, two monochromators and appropriate optical cutoff filters were placed before and after the sample.

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