Article pubs.acs.org/JPCC
Exciton and Charge Generation in PC60BM Thin Films Chaz Keiderling,† Stoichko Dimitrov,‡ and James R. Durrant*,†,‡ †
Centre for Plastic Electronics and Department of Chemistry, Imperial College London, London SW72AZ, United Kingdom SPECIFIC IKC, College of Engineering, Swansea University, Bay Campus, Swansea SA18EN, United Kingdom
‡
S Supporting Information *
ABSTRACT: Transient absorption spectroscopy is employed to contrast the photophysics of [6,6]-phenyl C61 butyric acid methyl ester (PC60BM) dispersed in a polystyrene matrix and as a neat film. For the dispersed PC60BM:polystyrene film, singlet excitons are observed that undergo intersystem crossing to triplet excitons. In contrast, in the neat PC60BM film, the transient absorption data indicate significant polaron generation, with photogenerated polarons exhibiting dispersive, bimolecular charge recombination on the nano- to microsecond time scales. These results are discussed in terms of their implications for charge generation from PC60BM light absorption in polymer/fullerene solar cells.
■
INTRODUCTION Since their discovery in 1985,1 fullerenes have attracted extensive interest, particularly due to their remarkable effectiveness as electron-acceptor and -transport materials in photochemical systems and (opto)electronic devices. The soluble fullerene derivative [6,6]-phenyl C60 butyric acid methyl ester (PC60BM), and its C70 equiv PC70BM, have been used almost ubiquitously as the electron-acceptor material in bulk heterojunction organic photovoltaic (OPV) cells.2 Indeed, in such devices, there has been increasing recognition that such fullerenes can not only play key roles as electron-acceptor and -transport materials but also contribute significantly to solar light absorption,3−5 leading to recent reports of efficient OPV devices comprising ∼95% PC60BM in the photoactive layer.6 Determining the properties that make fullerenes such good materials for OPV is important not only for optimization of fullerene-based OPV devices but also to guide the development of alternative electron-acceptor materials, an area of increasing interest.7,8 We focus on the photophysical properties of PC60BM thin films. The photophysical properties of fullerenes underlie many of their photochemical and optoelectronic applications, including, in particular, charge photogeneration from PC60BM singlet excitons in OPV devices, but have received relatively little attention to date.9 We focus, in particular, on the impact of fullerene aggregation upon its photophysics; such fullerene aggregation has become increasingly recognized as a key factor in the efficient function of fullerene-based OPV devices.10 Remarkably we find that while the photophysics of dispersed PC60BM molecules is dominated by singlet and triplet excited states, PC60BM aggregation results in a substantial increase in the charge transfer (CT) character of photogenerated excitons, leading to significant generation of dissociated charges in neat PC60BM films. © XXXX American Chemical Society
Previous photophysical studies of PC60BM have reported observation of both singlet and triplet excitons.9 Charge generation from fullerene absorption in bulk heterojunction OPV devices has been assigned to the formation of fullerene singlet excitons, followed by dissociation of the excitons into charges by hole transfer at polymer/fullerene interfaces. The efficiency of this process has been shown to depend on the efficiency of fullerene exciton diffusion to such interfaces and the avoidance of geminate recombination losses caused by the formation of interfacial “bound polaron pair” or CT states. The efficiency of exciton diffusion has been shown to be dependent on the degree of PC60BM aggregation in the blend film.5 PCxBM aggregation has also been suggested to be a key factor in minimizing recombination losses in OPV devices, providing an energy offset to stabilize the spatial separation of charges.10,11 Efficient intersystem crossing (ISC) from PC60BM singlet excitons to triplet exciton states has been reported,9 although such PC60BM triplets exciton do not appear to act as a significant contributors to photocurrent generation in OPV devices.12−16 The role of PC60BM as both a light absorber and an electron acceptor in OPV devices has gained increasing recognition in recent studies. For example, our own work reported BHJ devices based on blends of a lowband-gap polymer with PC70BM exhibiting power conversion efficiencies (PCEs) of 5.2, with 70% being deriving from PC70BM light absorption.5 Similarly, Burkhard et al. have shown, using excitation-dependent external quantum efficiency (EQE), that photocurrent can be generated directly from excitation within PCxBM domains.5 However, photophysical Received: April 27, 2017 Revised: June 2, 2017 Published: June 14, 2017 A
DOI: 10.1021/acs.jpcc.7b03962 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C studies of PC60BM and PC70BM neat films to date have only reported exciton formation,9 with no reports of photoinduced charge generation in such neat films. Despite the widespread success and use of PCxBM in OPV devices, there are few studies that probe its photophysics in thin films3,13,14,17 and even fewer that probe the dependence of this photophysics upon the aggregation state of the fullerene. Neat fullerene films have been shown to exhibit substantially increased visible-light absorption relative to fullerene molecules either dissolved in solution or dispersed in a polystyrene thin film matrix. This increase in absorption has been assigned to the appearance of an intermolecular CT transition.17,18 The appearance of this additional visible absorption is important to the solar light-harvesting properties of PC60BM thin films. It has also been correlated with a reduction in the yield of longlived PC60BM triplet states.17 We employ transient absorption (TA) spectroscopy on femtosecond−millisecond time scales to address the issue of whether fullerene aggregation can result in the photogeneration of charge carriers, even in the absence of donor polymers, and the potential relevance of this to the function of BHJ organic solar cells.
Figure 1. XRD spectra of neat PC60BM as a powder and a film contrasted with a 4:1 PS:PC60BM blend film (upper). Corresponding UV−vis absorption spectra of neat and blend films (lower).
absorption spectra of neat and blended films (film thicknesses of ∼50 nm), normalized to the 240 nm absorption peak. It is apparent that the neat PC60BM film shows increased light absorbance relative to the PS:PC60BM blend, particularly in the visible spectral region, clearly indicative of increased electronic interactions between fullerene molecules in the neat film. Femto- to microsecond TA spectroscopy measurements were undertaken to track the dynamics of excited species in these films. Figure 2 shows TA spectra for both the neat and blend films on microsecond (upper plots) and ultrafast (lower plots) time scales. Figure 3 shows representative TA kinetics across all time scales at 550/530, 750/710, 1030, and 1300 nm (upper plots) and their oxygen dependency at 720 and 1300 nm (lower plots) for neat and blend films. For the dispersed PS:PC60BM film, the ultrafast TA spectra are dominated by a positive photoinduced absorption signal at 950 nm, assigned previously to the singlet exciton state of PC60BM9 and similar to that observed for C60.19,20 This feature decays with a single exponential decay with lifetime 1.3 ns, evolving into a weaker photoinduced absorption peaking at 720 nm that extends onto the microsecond time scale. The decay dynamics of this 720 nm absorption fits reasonably well to single exponential, with a lifetime of 33 μs in nitrogen, accelerating to 3 μs in oxygen (Figure 3d). This strong quenching in oxygen, which was reversed upon return to a nitrogen atmosphere, allows us to assign this 720 nm absorption to PC60BM triplet exciton absorption, in agreement with previous measurements by Cook et al.14 As with the singlet exciton, the dispersed PC60BM triplet energetics and kinetics are well matched with those reported for monomeric C60 molecules.19−23 Similar 1.3 ns time constants were obtained for the decay of singlet exciton absorption and the rise of triplet exciton absorption (see traces at 1030 and 710 nm, respectively, Figure 3b), in agreement with analogous data for PC60BM in solution9 and confirming triplet exciton formation by ISC from singlet excitons. A weak additional photoinduced absorption is observed at 500 nm, extending into the microsecond time frame; this feature is discussed further below. TA data on neat PC60BM films (Figure 2a,c) show strikingly dissimilar spectral features to the dispersed PS:PC60BM. The singlet exciton absorption peak at 950 nm no longer dominates the early time data. Instead, a strong positive absorbance feature is observed at 550 nm signal, accompanied by a broad
■
METHODS PS:PC60BM blend films and pristine PC60BM films were prepared by spin-coating at a spin rate of 1000 rpm for 60 s from chlorobenzene solution (25 mg/mL) onto fused silica substrates. All substrates were cleaned by sonication for 5 min in acetone, water, and 2-isopropanol successively. Film thickness were approximately 70−100 nm. Absorption spectra were measured under ambient conditions with a spectrophotometer (Perkin Elma Lambda 25 UV−vis spectrometer). Microsecond and femtosecond TA data were collected employing pulsed laser excitation at 355 and 350 nm; experimental details have been previously reported.4,14 Unless stated otherwise, excitation densities were 15 μJ/cm2.14 Neat PC60BM X-ray diffraction (XRD) measurements were done on thicker films produced by spin coating five times on the same substrate producing films ∼250 nm thick.
■
RESULTS AND DISCUSSION Films of PC60BM were spin-coated onto glass substrates, either as neat films or as blends with polystyrene (PS, Mw ≈ 1M). Polystyrene has previously been shown to be an electronically inactive dispersing agent for PC60BM, allowing comparison of aggregated PC60BM in neat films versus molecularly dispersed PC60BM in blend films. A 4:1 PS to PC60BM blend ratio was employed for all blend film data reported herein; qualitatively similar data were obtained for both higher (8:1) and lower (2:1) ratio blends. Figure 1 (upper) presents the results of XRD spectra of neat PC60BM and PS:PC60BM blend spin-coated films (film thicknesses of ∼250 nm). These results are compared with the XRD of as purchased powder PC60BM without further treatment. It is apparent that the neat PC60BM films show clear XRD peaks matching those of the powder PC60BM and indicative of significant PC60BM crystallinity. In contrast, the PS:PC60BM films exhibited no detectable XRD signal, indicative of an amorphous film structure. We note that such clear XRD from neat PC60BM films has not been reported previously; they were obtained herein by using multiple spincoated layers to form a thicker PC60BM film with detectable XRD signal. Figure 1 (lower) shows the corresponding UV−vis B
DOI: 10.1021/acs.jpcc.7b03962 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Transient absorption spectroscopy of neat PC60BM (a,c) and PS:PC60BM (b,d) films on ultrafast (c,d) and microsecond (a,b) time scales, measured in a nitrogen atmosphere. Ultrafast time scales (c,d) have a small area of missing data where signal was unable to be collected (800−825 nm).
Figure 3. Transient absorption kinetics for neat PC60BM film (a,c) and PS:PC60BM film (b,d). Complete time scales from picosecond to millisecond shown with indicative fits in the upper plots for data collected in nitrogen (a,b). The lower plots compare data at longer time scales collected in oxygen and nitrogen atmospheres (c,d).
time scales, without the pronounced spectral shift associated with ISC from singlet to triplet excitons observed for the dispersed PS:PC60BM film. The 550 nm feature is analogous to
photoinduced absorption extending across the near-infrared, centered at 980−1050 nm (see also Supporting Figure 3). These absorption features extend from ultrafast to millisecond C
DOI: 10.1021/acs.jpcc.7b03962 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
near-unity ISC in C60 molecules in solution and attributed to the extensive conjugation of the C60 minimizing the singlet/ triplet energy splitting. In contrast, the neat PC60BM film shows little evidence of this spectral shift associated with ISC but rather the power law decay of spectral features more consistent with charge carrier absorption. While direct charge carrier generation has not been reported previously in photophysical studies of neat PC60BM films,9,17 it is in agreement with similar observations made on neat C60 films, assigned to self-trapped (polaronic) excitons and photogenerated charge carriers.24,25,27,28 The observation for the neat PC60BM film of a pronounced TA signal analogous to previously reported EA spectra for C60 is particularly striking. This feature is red-shifted and much stronger than an analogous feature observed at 500 nm in the PS:PC60BM film; it is also analogous to similar red shifts and strengthening for EA spectra of molecular and neat C60 film systems (Supporting Figure 2), which have previously been assigned to increased charge-transfer character of the neat film optical absorbance.18,24,28−30 We consider now the origin of the difference in the photophysics of dispersed PS:PC60BM and neat PC60BM films observed herein. The relatively high dielectric constant of PC60BM (3.9)31 when compared with polystyrene (2.6) and with typical OPV donor materials such as the donor polymer P3HT (3.0)32 will result in a higher screening of charges in the neat film compared with blends. This greater susceptibility to polarization and the resultant expected lower binding energy for singlet excitons formed in neat PC60BM films are consistent with the formation of more delocalized states with a higher degree of CT character. Bernado et al. have recently reported results supporting a model of high dielectric screening in fullerene nanocrystals, resulting in delocalized CT state dissociation.29 Blom et al. have also reported that lower loading of PC60BM in PPV:PC60BM blends results in a lower dielectric constant and related this to a reduced exciton separation efficiency.31 This lower dielectric constant and the increased spatial separation of PC60BM molecules reducing their electronic interactions can be expected to result in the relatively localized and more tightly bound excitons in blend films.18,29 The ability of neat PC60BM films to separate charges may also be associated with previous studies indicating a dependence of PC60BM LUMO level upon aggregation states, with local variations in the degree of aggregation/crystallization of PC60BM in neat films providing energetic offsets that may favor charge dissociation.10 In summary, for dispersed PS:PC60BM films, we observe the formation of localized singlet excitons, which undergo significant ISC to triplet exciton states. In contrast, in the neat PC60BM, we primarily observe the formation of CT/ polaron states. From the relative magnitude of residual triplet exciton absorption in the neat films, we estimate that the yield of charge-carrier generation in these neat films is ∼75%, although we note that most of these charges undergo relatively rapid recombination (with 75% recombining within 6 ns under the experimental conditions employed herein). As such, the yield of charge carriers with lifetimes long enough to be likely to enable photocurrent generation (i.e., microseconds) is relatively modest (∼20%). Our observation of dissociated charge generation in neat PC60BM contrasts with a previous photophysical study, which reported that photoexcitation of neat PC60BM films resulted in only singlet/triplet exciton formation9 but is consistent with the observation of photocurrent generation from neat PC60BM films by Burkhard et al.5
a similar spectral feature observed previously in optical studies of neat C60 films, assigned to an electroabsorbance signal deriving from the high polarizability of C60 in the presence of electric fields.24,25 The observation of this feature is therefore indicative of significant charge carrier generation in the neat PC60BM film (see also the Supporting Information (SI) and Supporting Figure 2 for further analysis of this assignment). The near-infrared absorption maximum observed in Figure 2c also coincides with previously reported 1030 nm maximum of fullerene anion absorption.26 These spectral features are therefore more indicative of charge rather than neutral exciton formation in the neat PC60BM film. Support for this conclusion also comes from consideration of the decay dynamics of these spectral features. The decay dynamics of these transient features are dominated by a single-phase power law decay process observable out to the millisecond time scale (see traces at 1030 and 1300 nm in Figure 3a). Similar power law decay kinetics were observed at all of these probe wavelengths for time delays greater than 100 ps, including the electroabsorption (EA) signal at 550 nm and the anion absorption at 1030 nm. At earlier times, a small, wavelength-dependent sub-10 ps rise/ decay phase is observed, as discussed further in the Supporting Information, as well as a modest (∼20%) decay of the EA signal on the 10−100 ps time scale, tentatively assigned to temporal evolution of the spatial separation of photogenerated charge carriers. The power law decay was observed to accelerate with higher excitation densities (Supporting Figure 1) but was not affected by the presence of oxygen (Figure 3c). These power law, excitation-density-dependent but oxygen-independent kinetics are typical of the nongeminate recombination kinetics of photogenerated charge carriers often observed in organic semiconductor films and as such are also strongly indicative that these spectral features result not from photoinduced singlet or triplet excitons but rather from polarons. We note that an additional weaker and oxygen-dependent exponential decay phase, with a decay time of ca. 1 μs was also observed at some probe wavelengths and particularly between 700 and 750 nm (Figures 2c, 3a, and 3c) and therefore assigned to residual triplet exciton formation in these neat films. From the amplitude of this triplet decay phase, we can conclude that triplet exciton generation in the PC60BM neat film is ∼25% of that observed in the dispersed PC60BM:PS films, suggesting a ∼75% yield of charge carriers in the neat films. Similar spectra were also reported by Chow et al.,9 although this study did not include oxygen quenching data and therefore was not able to discriminate between long-lived triplet and polaron absorption. The continuity in the kinetics we observe across ultrafast and microsecond time scales suggests the dominance of similar charged species on all time scales for neat PC60BM. Further data were collected at different PS:PC60BM blend ratios, showing qualitatively similar dependencies upon film to those shown in Figures 2 and 3. In particular, increasing the PS loading resulted in the increased prominence of the spectral shifts assigned to ISC and the appearance of oxygen-dependent microsecond decay kinetics, in agreement with those previously reported by Cook et al.17 The remarkable differences in the TA spectra of dispersed and aggregated PC60BM (i.e.: blend and neat films) suggests that PC60BM aggregation has a substantial impact upon its photophysics. Similar conclusions could also be drawn from a global kinetic analysis of the TA data, as detailed in the SI. The dispersed PS:PC60BM film yields large triplet signals, indicating an efficient ISC process in agreement with previous reports of D
DOI: 10.1021/acs.jpcc.7b03962 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
(10) Jamieson, F. C.; Domingo, E. B.; McCarthy-Ward, T.; Heeney, M.; Stingelin, N.; Durrant, J. R. Fullerene Crystallisation as a Key Driver of Charge Separation in Polymer/Fullerene Bulk Heterojunction Solar Cells. Chemical Science 2012, 3, 485−492. (11) Gehrig, D. W.; Howard, I. A.; Sweetnam, S.; Burke, T. M.; McGehee, M. D.; Laquai, F. The Impact of Donor−Acceptor Phase Separation on the Charge Carrier Dynamics in Pbttt:Pcbm Photovoltaic Blends. Macromol. Rapid Commun. 2015, 36, 1054−1060. (12) Distler, A.; Kutka, P.; Sauermann, T.; Egelhaaf, H.-J.; Guldi, D. M.; Di Nuzzo, D.; Meskers, S. C. J.; Janssen, R. A. J. Effect of Pcbm on the Photodegradation Kinetics of Polymers for Organic Photovoltaics. Chem. Mater. 2012, 24, 4397−4405. (13) Schlenker, C. W.; et al. Polymer Triplet Energy Levels Need Not Limit Photocurrent Collection in Organic Solar Cells. J. Am. Chem. Soc. 2012, 134, 19661−19668. (14) Cook, S.; Ohkita, H.; Durrant, J. R.; Kim, Y.; Benson-Smith, J. J.; Nelson, J.; Bradley, D. D. C. Singlet Exciton Transfer and Fullerene Triplet Formation in Polymer-Fullerene Blend Films. Appl. Phys. Lett. 2006, 89, 101128−3. (15) Benson-Smith, J. J.; Ohkita, H.; Cook, S.; Durrant, J. R.; Bradley, D. D. C.; Nelson, J. Charge Separation and Fullerene Triplet Formation in Blend Films of Polyfluorene Polymers with [6,6]-Phenyl C61 Butyric Acid Methyl Ester. Dalton Transactions 2009, 10000− 10005. (16) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Heeney, M.; Tierney, S.; McCulloch, I.; Bradley, D. D. C.; Durrant, J. R. Radical Ion Pair Mediated Triplet Formation in Polymer-Fullerene Blend Films. Chem. Commun. 2006, 3939−3941. (17) Cook, S.; Ohkita, H.; Kim, Y.; Benson-Smith, J. J.; Bradley, D. D. C.; Durrant, J. R. A Photophysical Study of Pcbm Thin Films. Chem. Phys. Lett. 2007, 445, 276−280. (18) Kazaoui, S.; Minami, N.; Tanabe, Y.; Byrne, H. J.; Eilmes, A.; Petelenz, P. Comprehensive Analysis of Intermolecular ChargeTransfer Excited States in C60 and C70 Films. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 7689−7700. (19) Ebbesen, T. W.; Tanigaki, K.; Kuroshima, S. Excited-State Properties of C60. Chem. Phys. Lett. 1991, 181, 501−504. (20) Guldi, D. M.; Prato, M. Excited-State Properties of C60 Fullerene Derivatives. Acc. Chem. Res. 2000, 33, 695−703. (21) Sension, R. J.; Phillips, C. M.; Szarka, A. Z.; Romanow, W. J.; McGhie, A. R.; McCauley, J. P.; Smith, A. B.; Hochstrasser, R. M. Transient Absorption Studies of Carbon (C60) in Solution. J. Phys. Chem. 1991, 95, 6075−6078. (22) Lee, M.; Song, O.-K.; Seo, J.-C.; Kim, D.; Suh, Y. D.; Jin, S. M.; Kim, S. K. Low-Lying Electronically Excited States of C60 and C70 and Measurement of Their Picosecond Transient Absorption in Solution. Chem. Phys. Lett. 1992, 196, 325−329. (23) Fujitsuka, M.; Kasai, H.; Masuhara, A.; Okada, S.; Oikawa, H.; Nakanishi, H.; Ito, O.; Yase, K. Laser Flash Photolysis Study on Photophysical and Photochemical Properties of C60 Fine Particles. J. Photochem. Photobiol., A 2000, 133, 45−50. (24) Dick, D.; Wei, X.; Jeglinski, S.; Benner, R. E.; Vardeny, Z. V.; Moses, D.; Srdanov, V. I.; Wudl, F. Transient Spectroscopy of Excitons and Polarons in ${\Mathrm{C}}_{60}$ Films from Femtoseconds to Milliseconds. Phys. Rev. Lett. 1994, 73, 2760−2763. (25) Lee, C. H.; Yu, G.; Moses, D.; Srdanov, V. I.; Wei, X.; Vardeny, Z. V. Transient and Steady-State Photoconductivity of a Solid C60 Film. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 8506−8509. (26) Yamamoto, S.; Guo, J.; Ohkita, H.; Ito, S. Formation of Methanofullerene Cation in Bulk Heterojunction Polymer Solar Cells Studied by Transient Absorption Spectroscopy. Adv. Funct. Mater. 2008, 18, 2555−2562. (27) Yonehara, H.; Pac, C. Dark and Photoconductivity Behavior of C60 Thin Films Sandwiched with Metal Electrodes. Appl. Phys. Lett. 1992, 61, 575−576. (28) Wei, X.; Dick, D.; Jeglinski, S. A.; Vardeny, Z. V. Quantum Efficiency Study of Photoexcited States in Fullerenes. Synth. Met. 1997, 86, 2317−2320.
The susceptibility of PC60BM excitons formed in aggregated domains, as present in neat films studied herein, to undergo charge dissociation may be a significant factor behind efficient photocurrent generation following PC60BM photoexcitation both in typical polymer:PC60BM BHJ solar cells4 and in recently reported PC60BM-rich organic solar cells.6
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03962. Excitation density dependence of transient absorption decays, transient spectra for neat PC60BM films plotted at progressive delay times, decay associated spectra and comparison against electroabsorption data. (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Stoichko Dimitrov: 0000-0002-1564-7080 James R. Durrant: 0000-0001-8353-7345 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We gratefully acknowledge the EPSRC for funding through grants EP/L016702/1 (Plastic Electronics CDT) and EP/ M023532/1 (APEX II) and the Welsh Assembly Government for funding through the Sêr Cymru Solar and Sêr Cymru II programmes, with partial funding by the European Regional Development Fund.
■
REFERENCES
(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (2) He, Y.; Li, Y. Fullerene Derivative Acceptors for High Performance Polymer Solar Cells. Phys. Chem. Chem. Phys. 2011, 13, 1970−1983. (3) Dimitrov, S. D.; Nielsen, C. B.; Shoaee, S.; Shakya Tuladhar, P.; Du, J.; McCulloch, I.; Durrant, J. R. Efficient Charge Photogeneration by the Dissociation of Pc70bm Excitons in Polymer/Fullerene Solar Cells. J. Phys. Chem. Lett. 2012, 3, 140−144. (4) Dimitrov, S. D.; Huang, Z.; Deledalle, F.; Nielsen, C. B.; Schroeder, B. C.; Ashraf, R. S.; Shoaee, S.; McCulloch, I.; Durrant, J. R. Towards Optimisation of Photocurrent from Fullerene Excitons in Organic Solar Cells. Energy Environ. Sci. 2014, 7, 1037−1043. (5) Burkhard, G. F.; Hoke, E. T.; Beiley, Z. M.; McGehee, M. D. Free Carrier Generation in Fullerene Acceptors and Its Effect on Polymer Photovoltaics. J. Phys. Chem. C 2012, 116, 26674−26678. (6) Yang, B.; Guo, F.; Yuan, Y.; Xiao, Z.; Lu, Y.; Dong, Q.; Huang, J. Solution-Processed Fullerene-Based Organic Schottky Junction Devices for Large-Open-Circuit-Voltage Organic Solar Cells. Adv. Mater. 2013, 25, 571−571. (7) Nielsen, C. B.; Voroshazi, E.; Holliday, S.; Cnops, K.; Rand, B. P.; McCulloch, I. Efficient Truxenone-Based Acceptors for Organic Photovoltaics. J. Mater. Chem. A 2013, 1, 73−76. (8) Lin, Y.; Zhan, X. Non-Fullerene Acceptors for Organic Photovoltaics: An Emerging Horizon. Mater. Horiz. 2014, 1, 470−488. (9) Chow, P. C. Y.; Albert-Seifried, S.; Gélinas, S.; Friend, R. H. Nanosecond Intersystem Crossing Times in Fullerene Acceptors: Implications for Organic Photovoltaic Diodes. Adv. Mater. 2014, 26, 4851−4854. E
DOI: 10.1021/acs.jpcc.7b03962 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (29) Bernardo, B.; Cheyns, D.; Verreet, B.; Schaller, R. D.; Rand, B. P.; Giebink, N. C. Delocalization and Dielectric Screening of Charge Transfer States in Organic Photovoltaic Cells. Nat. Commun. 2014, 5, 3245. (30) Pac, B.; Petelenz, P.; Slawik, M.; Munn, R. W. Theoretical Interpretation of the Electroabsorption Spectrum of Fullerene Films. J. Chem. Phys. 1998, 109, 7932−7939. (31) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551−1566. (32) Knipper, M. P.; Parisi, J.; Coakley, K.; Waldauf, C.; Brabec, C. J. J.; Dyakonov, v. Impedance Spectroscopy on Polymer-Fullerene Solar Cells. Z. Naturforsch., A: Phys. Sci. 2007, 62, 490−494.
F
DOI: 10.1021/acs.jpcc.7b03962 J. Phys. Chem. C XXXX, XXX, XXX−XXX
