Impact of Acceptor Crystallinity on the Photophysics of Nonfullerene

Jun 16, 2014 - To date, nonfullerene acceptors for organic solar cells have yet to reach the overall device performance achieved with fullerene deriva...
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Impact of Acceptor Crystallinity on the Photophysics of Nonfullerene Blends for Organic Solar Cells Paul E. Shaw,* Pascal Wolfer, Benjamin Langley, Paul L. Burn, and Paul Meredith Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, Queensland 4072, Australia ABSTRACT: To date, nonfullerene acceptors for organic solar cells have yet to reach the overall device performance achieved with fullerene derivatives. Power conversion efficiencies are low, even when combined with narrow optical gap polymers, with the processing conditions and blend microstructure difficult to optimize. To understand the potential origins of the decreased performance, we performed a photophysical study on a model system of blends of poly(3-n-hexylthiophene) (P3HT) with the planar small molecule electron acceptor 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12), focusing, in particular, on the effects of the crystallinity of the K12 phase on exciton dissociation and charge generation. Our results show that the microstructure of the blends can be manipulated by the processing conditions to give amorphous through to (semi)crystalline films. The amorphous blends show strong quenching of the photoluminescence, which indicates that there is a fine mixture of P3HT and K12. After annealing, the blends all showed increased photoluminescence and signs of phase separation with the formation of large-scale crystalline K12 domains. Photoinduced absorption spectroscopy confirmed the presence of positive polarons in the P3HT and revealed triplet excitons in blends containing crystalline K12 domains, indicating that exciton harvesting from the K12 phase was inefficient. However, charge generation in the 1:2 blend (the best for devices) was enhanced in the (semi)crystalline films, which suggests that aggregation of the P3HT and K12 aids charge separation. Hence, the complex phase behavior of K12 results in a trade-off between charge generation and collection against exciton harvesting from the K12 phase.

1. INTRODUCTION Organic solar cells are the focus of intense research effort with single junction device power conversion efficiencies approaching 10%.1,2 The majority of solution-processed devices are based on a bulk heterojunction (BHJ) architecture with the active layer comprising a mixture of a conjugated polymer or macromolecule (known as the donor) and a fullerene (the acceptor).3,4 This approach has been very successful with early devices based on P3HT:PC60BM blends being superseded by blends of narrow optical gap polymers with PC70BM.5,6 While a number of donor materials have been developed, there has been comparatively little evolution of the acceptor material with fullerenes still the predominant choice.7 Part of the reason for this is that it has proven difficult to achieve performance comparable to that of fullerene blends with alternative compounds, an issue that is compounded by the fact that many donor materials were designed to work with fullerenes. The performance advantage of fullerenes can be attributed to their high electron affinities and mobilities combined with their fortuitous tendency to form blends with a microstructure that is able to combine high exciton dissociation efficiency with favorable charge extraction. However, their light absorption is relatively weak in the solar window (particularly in the case of C60-based compounds) and their electron affinity is higher than it potentially needs to be, resulting in losses to the opencircuit voltage. Alternative acceptor materials include conjugated polymers and small molecules,8−12 both of which allow for a greater © XXXX American Chemical Society

versatility in the molecular structure than is normally possible with fullerenes. In particular, they enable greater tuning of the optical absorption of the acceptor so that it can better compliment that of the donor, maximizing the contribution of each component toward the device photocurrent. In addition, the energy levels of the nonfullerene acceptors can be tailored to maximize the open-circuit voltage.11 Finally, it is also possible to explore a greater range of microstructures and, in particular, develop highly crystalline acceptor materials. The potential benefits of highly crystalline acceptors are not yet known, but recent research has shown that, even in BHJ blends of polymers with PCBM, the presence of ordered domains appears to be a key driver for exciton dissociation and charge generation.13−15 In this paper, we demonstrate the impact of acceptor crystallinity on the photophysics of an acceptor−donor combination by considering a range of blend ratios of poly(3n-hexylthiophene) (P3HT) with 2-[{7-(9,9-di-n-propyl-9Hfluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (referred to henceforth as K12) in amorphous and crystalline phases. Devices incorporating a BHJ layer of P3HT:K12 with an optimum ratio of 1:2 have achieved maximum power conversion efficiencies of 0.73 ± 0.01%,9 although it is not entirely clear what limits the device Received: March 30, 2014 Revised: June 2, 2014

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SP2300 spectrometer. A long-pass filter was used to block the scattered excitation. Phase-sensitive lock-in (SR530 amplifier) techniques were used with the phase set to give the maximum fluorescence signal from the sample. The PL signal was measured separately and subtracted from the PIAS data. PL spectra of the films were also measured using the PIAS setup at room temperature with the films in the cryostat under vacuum and were corrected for the response of the instrument. 2.3. Optical Light Microscopy. The optical micrographs and polarized optical micrographs were obtained with an Olympus BX61 microscope operated in transmission mode. No additional processing was applied to the images.

performance. K12 is a planar small molecule that can crystallize, a feature that enables tuning of the microstructure of the BHJ across a range of phases.16,17 This makes K12 an attractive model compound for building structure property relationships and understanding why, in principle, planar acceptors do not perform as well as fullerenes. One distinguishing characteristic of K12 compared to the majority of organic photovoltaic materials is that it is moderately luminescent, which enables the K12 singlet exciton population to be probed through measurements of the photoluminescence quantum yield (PLQY) and photoluminescence (PL) decay kinetics. In addition, we used photoinduced absorption spectroscopy (PIAS) to probe charge generation in P3HT:K12 blends in both the amorphous state when both materials are finely intermixed as well as the (semi)crystalline phase, in which there is a degree of phase separation. The results show that achieving a stable microstructure that balances exciton harvesting with charge generation is a challenge but that the presence of crystalline order aids charge separation.

3. RESULTS AND DISCUSSION The chemical structure of K12 and its absorbance and photoluminescence spectra for the amorphous and (semi)crystalline phases are shown in Figure 1a,b, respectively. The

2. EXPERIMENTAL SECTION 2.1. Film Preparation. The P3HT was purchased from Merck (Mw = 94 100, polydispersity = 1.9, regioregularity = 95.5%), and the K12 was synthesized using the previously reported method.18 Solutions were prepared in chloroform and stirred overnight at ∼40 °C. The solutions were then spincoated onto fused silica substrates immediately prior to measurements. Thermal annealing to accelerate crystallization was performed in a nitrogen-filled glovebox at 80 °C for 2 h. Films were also prepared with dichlorobenzene following the previously reported method for optimum device blends with a P3HT:K12 ratio of 1:2.9 These solutions were also stirred overnight and heated to ∼40 °C. The films cast from DCB were annealed at 65 °C for 20 min. 2.2. Spectroscopy. The absorbance spectra of the films were measured with a Varian Cary 5000. The time-resolved PL decays were measured with a Fluorolog 3 with integrated TCSPC capability. A pulsed LED with an emission wavelength of 441 nm and a pulse length of ∼1 ns was used, and the PL was measured at 575 nm. The film PLQY measurements were performed using the method described by Greenham et al.19 The 442 nm output of a HeCd laser was attenuated with neutral density filters to ∼0.2−0.3 mW and used to photoexcite the films. The interior of the integrating sphere was flushed with nitrogen to minimize photodegradation, and the photoluminescence signal was measured with a calibrated photodiode. The PLQY was measured at multiple points on each film and averaged. The films used to monitor the evolution of the PLQY over time were spin-coated onto 2.5 cm square silica substrates to ensure that there was a large area over which the measurements could be performed. The films were stored in air in the dark between measurements in an air-conditioned room with a steady temperature of 23 °C. PIAS was performed with the samples in a cryostat in a helium atmosphere at 77 K. The 442 nm output of the HeCd laser, modulated with a mechanical chopper at a frequency of ∼180 Hz, was used to pump the samples. Pump power was controlled with neutral density filters and was in the range of 0.1−34.0 mW. The probe beam was generated by passing the output from a halogen lamp through an Acton SP2300 spectrometer, which was then focused onto the sample. The transmitted probe beam intensity was measured with amplified Si (Thorlabs PDA100A) and InGaAs (Thorlabs PDA20CS) photodetectors coupled to a second

Figure 1. (a) Chemical structure of K12. (b) Absorbance and photoluminescence of P3HT (green triangles), amorphous K12 (red circles), and (semi)crystalline K12 (blue squares).

amorphous K12 shows an absorbance peak centered at 468 nm with emission peaking at 631 nm. After the K12 film was crystallized through thermal annealing, the long wavelength absorbance red-shifted slightly with the peak moving to 476 nm and a shoulder appearing at longer wavelengths (∼515 nm). These changes are reminiscent of those reported for molecules that exhibit crystalline ordering, such as P3HT,20 and have been assigned as resulting from J-aggregates.18 A red shift in absorption is usually reflected in a corresponding red shift in the PL, but for K12, there was a blue shift of the peak of 15 nm. The PL spectrum of P3HT is also included in Figure 1b and shows that there is only partial overlap with the emission of K12 and that, for wavelengths below ∼580 nm, there is a window in which only the K12 emission will be detected. This allows the K12 singlet exciton population kinetics to be measured and thus their quenching by P3HT to be probed. To probe the microstructure of the films, and, in particular, identify when it was amorphous or (semi)crystalline, optical microscopy was performed on neat films of K12. Micrographs of an as-cast film from a chloroform solution shown in Figure 2 displayed no visible signs of large-scale microstructural features, B

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Figure 2. Optical micrographs (unpolarized and polarized) of a K12 film as-cast and annealed. The polarizer/analyzer system is indicated with arrows.

including when viewed under cross-polarizers. The absence of a uniform bright background in the micrographs of the as-cast films also suggests that there is no significant small-scale microstructure. In contrast, the annealed film showed an extensive large-scale microstructure, confirming the effectiveness of thermal annealing for inducing crystallization. Films with P3HT:K12 weight ratios of 1:9, 1:2, 2:1, and 9:1 were prepared to investigate the effects of blend composition and crystallinity of the acceptor phase on the photophysics. The films were characterized promptly after spin-coating from chloroform and then imaged using optical microscopy before being annealed and recharacterized. The absorbance spectra are shown in Figure 3 for the as-cast and annealed films. The biggest changes in the absorbance spectra were observed for the blends with the greatest content of K12. As with the neat K12 films, there was a slight red shift of the absorbance peak and the emergence of a shoulder at longer wavelengths after annealing, which, in the case of the 1:2 blend (the optimum blend ratio for devices), significantly extends the absorption to longer wavelengths and thus improves absorption of the solar spectrum. The 9:1 blend ratio shows almost no change in the absorption spectrum upon annealing, which suggests that there is little crystallization of the K12. It also suggests that the P3HT is relatively unaffected by annealing at 80 °C. The polarized light micrographs of the as-cast and annealed films are shown in Figure 4 and confirm the amorphous character of the as-cast film and the (semi)crystalline microstructure of the annealed films. The micrographs also indicate that the large-scale microstructure of the films varies significantly with the blend ratio. The relatively low annealing temperature required to initiate crystallization, the changes observed in the absorbance of the films, and the general appearance of the crystal features in the micrographs all support the view that the crystalline domains are primarily composed of K12. Since the PLQY of K12 is moderately high and generally much higher than that of most electron acceptor materials for organic photovoltaics, the fluorescence from the blend can be used as a probe for monitoring changes to the microstructure of the film.21 This is because the exciton diffusion length of most solution-processed organic semiconductors is short, usually on

Figure 3. Absorbance of the P3HT:K12 blends as-cast (red solid line) and after annealing (dashed blue line).

the order of 10 nm,22,23 so the degree of PL quenching in the blend will be very sensitive to small changes in the film microstructure occurring on a similar length scale.24 The PLQY of amorphous K12 was measured at 22 ± 2%, which is significantly higher than that of P3HT (2.5 ± 0.3%). The PLQY value for crystalline K12 was found to be lower than that for the amorphous films at 12 ± 1%, so, although the measured PLQY of P3HT:K12 blends will have a contribution from both components, it will be predominantly K12 emission. Figure 5a shows the change in the PLQY values of the P3HT:K12 blends as-cast (amorphous) and after thermal annealing [(semi)crystalline] as a function of the K12 weight fraction in the blend. For all the as-cast films, with the exception of the 1:9 blend, there is very strong quenching of both the P3HT and K12 emission. This is further confirmation that the as-cast blends consist of a fine mixture of the two compounds and excitons from both materials are undergoing dissociation. The annealed films all show higher PLQY values compared to their as-cast state. The 1:2 blend, in particular, shows the greatest PLQY change relative to the as-cast value, and both the K12-rich blends have PLQY values of ∼3% after annealing. The sensitivity of the PLQY to very small changes in the microstructure was then used to monitor the evolution of the blends over time and provide a time scale for the spontaneous short-range crystallization process. It is important to note that this technique cannot provide quantitative insights into the microstructure of the blend as multiple factors, such as the purity of the domains, the size of the domains, or the distribution of domain sizes, could easily result in the same degree of quenching for different microstructures. Figure 5b shows the values of the PLQY for 1:2 blend films cast from C

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Figure 5. PLQY values for (a) films with different blend ratios, as-cast and after annealing, and (b) the optimum P3HT:K12 device blend ratio of 1:2 over time when processed from DCB (with and without thermal annealing) and chloroform. The gray shaded area represents the PLQY value and the associated uncertainty for the 1:2 blend cast from chloroform after thermal annealing.

Figure 4. Polarized light optical micrographs of P3HT:K12 blends (ratios on the left side of each row) as-cast (left) and after annealing (right). The polarizer/analyzer system is indicated with arrows.

chloroform and DCB (as-cast and annealed) over about a month. DCB is the optimum solvent for device film deposition with the films annealed at 65 °C for 20 min. The film spincoated from chloroform shows a gradual increase in the PLQY over about 20 days until its value is comparable to that of the annealed film. This shows that the amorphous phase of P3HT: K12 blends is not thermodynamically stable and that the natural tendency of K12 to crystallize causes the blend microstructure to gradually evolve. The film cast from DCB with no annealing has a higher initial PLQY than the film cast from chloroform, which suggests that the use of the higher boiling point solvent results in partial phase separation. The PLQY of the film shows a much faster increase over time than the film cast from chloroform with a maximum PLQY of ∼3% reached after 9 days. The difference in the rate at which the maximum PLQY was reached could be caused by the presence of residual DCB (the higher boiling point solvent) in the films, which would assist the internal reorganization within the blend and the growth of P3HT and K12 domains. Alternatively, the use of the high boiling point DCB could introduce small (nanoscopic) crystalline domains during spin-coating, which can then act as nucleation sites for future crystal growth in the film. However, it was found that the K12 microstructure could be “locked in” by a short thermal anneal of the film, as is shown in Figure 5b for the film cast from DCB. This process increased

the initial PLQY value, which then remained stable over a period of at least a month. The PLQY measurements also show that, despite yielding the best performing devices, the annealed 1:2 blend cast from DCB is not optimized for exciton harvesting, as there is significant radiative singlet decay from the K12. To further investigate the harvesting of singlet excitons from the K12, time-resolved fluorescence measurements were performed on neat films of K12 and blends with P3HT in their amorphous and (semi)crystalline states with the PL decay data shown in Figure 6. In these measurements, we observed the PL at 575 nm, which is at a shorter wavelength than the PL of P3HT. This allows the effects of blend ratio and microstructure on the K12 singlet exciton population to be specifically probed. The PL decay of amorphous and (semi)crystalline K12 is nonexponential with the lifetime slightly longer in the crystalline phase. Combined with the decrease in the PLQY for the (semi)crystalline K12, the PL data indicate that there is a decreased radiative rate for the (semi)crystalline phase relative to the amorphous phase, although it is not possible to quantify the change as the PL decays are not exponential and the annealed films will also contain an amorphous component. All the amorphous blends with P3HT show fast decays of the K12 singlet population that D

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means that the crystal size is significantly larger than the K12 exciton diffusion length. In contrast, the annealed P3HT-rich blends clearly show quenching at later times, which would suggest that either the K12 domains are closer in size to the exciton diffusion length or they are not as pure as they are in the K12-rich blends. To investigate the dissociation of excitons into polarons (free carriers), we performed photoinduced absorption spectroscopy (PIAS) on the amorphous and (semi)crystalline blends as well as neat films of K12. PIAS is a quasi-steady-state pump-probe technique that detects small changes in the transmission of a film caused by the presence of long-lived excited states such as polarons and triplet excitons. It is, therefore, able to confirm the presence of polarons resulting from exciton dissociation as well as providing information on the local environment of the polaron, i.e., whether or not it is intrachain or interchain. The PIAS results are shown in Figure 7 for all four blend ratios before and after annealing as well as for amorphous neat K12. The two P3HT-rich blends (Figure 7c,d) exhibit a spectrum that closely resembles that normally observed in P3HT:fullerene blends, which consists of a peak at ∼1.25 eV and a broader peak at ∼1.8 eV. These features are generally associated with positive polarons on the P3HT with the low energy peak due to intrachain polarons and the broader higher energy peak due to interchain polarons. For both P3HT-rich blends, there is no significant change to the shape or relative intensities of both peaks upon annealing, just a decrease in the overall signal level, which is consistent with a loss of carrier generation due to the increase in phase separation that accompanies the crystallization of the K12. In the case of the annealed 2:1 blend, there is also a small peak at ∼1 eV, which does not appear in the ascast P3HT-rich blends or the annealed 9:1 blend. The peak at ∼1 eV was also observed in both the amorphous (see Figure 7a) and the (semi)crystalline films of neat K12. The intensity of this peak was found to vary with the fluorescence intensity and was, therefore, attributed to K12 triplet state absorption rather than a polaron state. Thus, the presence of this feature in the excited state spectrum is further evidence that excessive crystallization of K12 leads to reduced exciton harvesting and charge carrier generation. In contrast to the P3HT-rich blends, the excited state absorption spectra for the annealed K12-rich blends differ significantly from their initial as-cast state. Both as-cast films feature an excited state absorption spectrum with just a single peak at ∼1.25 eV, which suggests that the positive polaron population is predominantly intrachain in character. When the 1:2 blend, which is the optimum for devices, was annealed, the shape of the spectrum changed significantly, adopting the characteristic peak structure observed in the P3HT-rich blend with the K12 triplet peak at ∼1 eV (see Figure 7b). These changes are consistent with the microstructure of the blend changing from a finely intermixed state to one with a greater degree of phase separation and aggregation of the polymer chains. Overall, there is a net increase in the amount of polaron absorption in the 1:2 blend upon annealing that is not observed with any of the other blend ratios, which suggests that, in the optimum blend ratio, the phase separation aids charge carrier generation. In contrast, the annealed 1:9 blend shows a greatly reduced signal at ∼1.25 eV and strengthening of the peak at ∼1 eV, although no significant changes occurred at higher energies, which would suggest that aggregation of the P3HT is inhibited.

Figure 6. Photoluminescence decays of K12 measured at 575 nm in blends with P3HT at different weight ratios before and after annealing with the instrument response function (IRF). The PL decay of neat K12 is represented by a green band, which represents the range of lifetimes observed between the amorphous and the (semi)crystalline phases.

are beyond the temporal resolution of the TCSPC system used, indicating that the singlet excitons are rapidly dissociated and consistent with a finely intermixed system. There are two possible mechanisms for exciton dissociation in P3HT:K12 blends. The first is that the exciton on the K12 is dissociated by hole transfer to the polymer. The second is that the K12 exciton undergoes resonant energy transfer to the P3HT, followed by electron transfer from the P3HT to the K12. The rate of energy transfer is largely governed by the spectral overlap between the K12 PL and the molar extinction coefficient of the P3HT (see Figure 1) as well as the PLQY of the K12. It is, therefore, expected that the Förster radius between K12 and P3HT will be significant, so energy transfer is likely to dominate over Dexter transfer. After annealing the two K12-rich blends, there is a significant increase in the PL decay lifetime, which is, in fact, close to that measured for a neat K12 film (Figure 6). However, in spite of this, the PLQY of the films is only around 3%, which is about a quarter of that of the neat film. While some of the PL will be lost due to the inner filter effect, the magnitude of the decrease in the PLQY is greater than would be expected from the effect. Therefore, the low PLQY primarily arises from fast exciton dissociation (beyond the time resolution of our equipment) occurring either in amorphous regions of the film and/or at the interface of the crystallites and the P3HT. For the annealed blend films to have a component of the lifetime equivalent to the neat K12 film E

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Figure 7. Photoinduced absorption of P3HT:K12 blends as-cast (red circles) and after annealing (blue squares). (a) The photoinduced absorption spectrum of amorphous K12 is included as a solid green line with the region between 1.6 and 2.0 eV omitted due to residual artifacts from the PL subtraction.

4. CONCLUSIONS We have investigated the photophysical properties of blends of P3HT with the small molecule electron acceptor K12 for organic photovoltaic devices. K12 is an attractive model system for comparison with fullerenes as a range of (semi)crystalline BHJ microstructures can be obtained through control of the processing conditions. In particular, we have focused on the impact of the K12 microstructure by comparing films with amorphous and (semi)crystalline phases. The results indicate that, in the amorphous films, there is a fine mixture of P3HT with K12 that results in efficient dissociation of the photogenerated singlet excitons from both materials. After the blends were annealed, all showed signs of phase separation with the formation of large-scale crystalline K12 domains. Measurements of the PLQY of the blends and the PL decay kinetics of the K12 showed an increase in the PL relative to the amorphous films, particularly for the K12-rich blends. The results indicate that the K12-rich blends contain domains that are much greater in size than the K12 exciton diffusion length, which could limit exciton harvesting and thus power conversion efficiency in devices. This is particularly evident in the P3HT: K12 blend ratio of 1:2 (the best for device performance), where there is significant PL from the K12 as well as the signature of K12 triplet exciton in the photoinduced absorption. However, the results also show that charge generation in the 1:2 blend is enhanced in the (semi)crystalline films, which indicates that aggregation of the P3HT and K12 aids charge separation. This would suggest that, as with fullerene blends, the presence of

mixed and pure phases is also a driver for charge separation in nonfullerene blends.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61 7 334 67989. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.E.S. and this research are supported by the Australian Research Council through a Discovery Early Career Research Award (DE120101721). P.W. would like to thank the Swiss National Science Foundation (SNSF) for an Advanced Researcher Fellowship (PA00P2 145395). P.L.B. and P.M. are both supported by University of Queensland Vice Chancellor’s Strategic Research Fellowships, and P.M. an Australian Research Council Discovery Outstanding Researcher Award.

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