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Energy Express

  • Editor: Christian Seassal
  • Vol. 21, Iss. S2 — Mar. 11, 2013
  • pp: A241–A249
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Subnanosecond charge photogeneration and recombination in polyfluorene copolymer-fullerene solar cell: Effects of electric field

Wei Zhang, Ye Huang, Ya-Dong Xing, Yan Jing, Long Ye, Li-Min Fu, Xi-Cheng Ai, Jian-Hui Hou, and Jian-Ping Zhang  »View Author Affiliations


Optics Express, Vol. 21, Issue S2, pp. A241-A249 (2013)
http://dx.doi.org/10.1364/OE.21.00A241


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Abstract

Influence of electric field on the subnanosecond charge photogeneration dynamics in the polymer solar cell based on polyfluorene copolymer BisDMO-PFDTBT blended with PC61BM was examined with transient absorption spectroscopy. The charge dynamics showed no difference under short- or open-circuit conditions and under a forward bias of 0.79 V (1.6 × 105 V/cm), implying negligible field effects on the subnanosecond dynamics of charge photogeneration/recombination. However, under the reverse biases of −2 V (4.0 × 105 V/cm) and −5 V (1.0 × 106 V/cm), significant enhancement of charge photogeneration and apparent suppression of polaron pair recombination were observed, which agrees with the field-assisted enhancement of external quantum efficiency of the solar cell devices.

© 2013 OSA

1. Introduction

The primary charge photogeneration in the bulk heterojunction (BHJ) polymer solar cells involves the formation and diffusion of excitons in the polymer phases, and the subsequent dissociation of excitons at the BHJ interfaces yielding free charge carriers and Coulombically bound polaron pairs (PPs). PPs can further split into free charges or recombine back to the ground or relax to the triplet excited states [1

1. C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, “Plastic solar cells,” Adv. Funct. Mater. 11(1), 15–26 (2001). [CrossRef]

,2

2. T. M. Clarke and J. R. Durrant, “Charge photogeneration in organic solar cells,” Chem. Rev. 110(11), 6736–6767 (2010). [CrossRef] [PubMed]

]. In addition, free charges can be directly photogenerated with significant yields in the bulk polymer phases in ~100 fs [3

3. M. Tong, N. E. Coates, D. Moses, and A. J. Heeger, “Charge carrier photogeneration and decay dynamics in the poly(2,7-carbazole) copolymer PCDTBT and in bulk heterojunction composites with PC70BM,” Phys. Rev. B 81(12), 125210 (2010). [CrossRef]

6

6. J. Guo, H. Ohkita, H. Benten, and S. Ito, “Charge generation and recombination dynamics in poly(3-hexylthiophene)/fullerene blend films with different regioregularities and morphologies,” J. Am. Chem. Soc. 132(17), 6154–6164 (2010). [CrossRef] [PubMed]

]. Since the filling factor (FF) of an operating solar cell is determined by the comprehensive exciton and charge dynamics under the internal electric field, it is important to examine the influence of electric field on the elementary light conversion processes, especially the PP dissociation and the charge recombination.

To date, the literature works on the field dependence of the dynamics of charge photogeneration and recombination focus mostly to the ns-ms timeframes. However, the PPs generated directly from photoexcitation live as short as tens to hundreds of picoseconds as a result of dissociation and germinate recombination [3

3. M. Tong, N. E. Coates, D. Moses, and A. J. Heeger, “Charge carrier photogeneration and decay dynamics in the poly(2,7-carbazole) copolymer PCDTBT and in bulk heterojunction composites with PC70BM,” Phys. Rev. B 81(12), 125210 (2010). [CrossRef]

,10

10. D. Veldman, Ö. İpek, S. C. J. Meskers, J. Sweelssen, M. M. Koetse, S. C. Veenstra, J. M. Kroon, S. S. Bavel, J. Loos, and R. A. J. Janssen, “Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc. 130(24), 7721–7735 (2008). [CrossRef] [PubMed]

,15

15. A. A. Bakulin, A. Rao, V. G. Pavelyev, P. H. M. van Loosdrecht, M. S. Pshenichnikov, D. Niedzialek, J. Cornil, D. Beljonne, and R. H. Friend, “The role of driving energy and delocalized States for charge separation in organic semiconductors,” Science 335(6074), 1340–1344 (2012). [CrossRef] [PubMed]

19

19. S. K. Pal, T. Kesti, M. Maiti, F. Zhang, O. Inganäs, S. Hellström, M. R. Andersson, F. Oswald, F. Langa, T. Österman, T. Pascher, A. Yartsev, and V. Sundström, “Geminate charge recombination in polymer/fullerene bulk heterojunction films and implications for solar cell function,” J. Am. Chem. Soc. 132(35), 12440–12451 (2010). [CrossRef] [PubMed]

], and the directly photogenerated PPs may differ from those formed via free charge recombination in the behavior of electric field dependence. The present work is intended to examine the subnanosecond PP dynamics, that is, to focus on the field dependence of the initially formed PPs. We target the polyfluorene copolymer poly([2,7-(9,9-bis-(3,7-dimethyl-octyl)- fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)]) (BisDMO-PFDTBT), a promising light harvester and electron donor giving rise to a power conversion efficiency (PCE) of 4.5% when blended with the electron acceptor PC70BM [20

20. M.-H. Chen, J. Hou, Z. Hong, G. Yang, S. Sista, L.-M. Chen, and Y. Yang, “Efficient polymer solar cells with thin active layers based on alternating polyfluorene copolymer/fullerene bulk heterojunctions,” Adv. Mater. (Deerfield Beach Fla.) 21(42), 4238–4242 (2009). [CrossRef]

]. Note that the BisDMO-PFDTBT/PC70BM device exhibits moderate FF (~50%) and external quantum efficiency (EQE, ~50%) in the visible spectral region, implying significant dependence of the photocurrent on the internal electric field. Such device is thus suitable for investigating the possible field dependence of the dynamics of PP and polymer radical cation (P●+, also referred to as polaron in literature). Because in a nanosecond the free charges would not fully arrive at the electrodes to build static electric fields [21

21. A. Pivrikas, N. S. Sariciftci, G. Juška, and R. A. Österbacka, “A review of charge transport and recombination in polymer/fullerene organic solar cells,” Prog. Photovolt. Res. Appl. 15(8), 677–696 (2007). [CrossRef]

], we biased the solar cell with an external potential to mimic the macroscopic internal electric field of operating device. Furthermore, since the excitation photon fluence used in the TA measurements can be several orders of magnitudes higher than that of the terrestrial solar irradiation, which may introduce unwanted nonlinear effects such as singlet-singlet exciton annihilation and bimolecular charge recombination [12

12. F. C. Jamieson, T. Agostinelli, H. Azimi, J. Nelson, and J. R. Durrant, “Field-independent charge photogeneration in PCPDTBT/PC70BM solar cells,” J. Phys. Chem. Lett. 1(23), 3306–3310 (2010). [CrossRef]

,18

18. S. De, T. Pascher, M. Maiti, K. G. Jespersen, T. Kesti, F. Zhang, O. Inganäs, A. Yartsev, and V. Sundström, “Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc. 129(27), 8466–8472 (2007). [CrossRef] [PubMed]

,22

22. R. A. Marsh, J. M. Hodgkiss, S. Albert-Seifried, and R. H. Friend, “Effect of annealing on P3HT:PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy,” Nano Lett. 10(3), 923–930 (2010). [CrossRef] [PubMed]

], we applied low-fluence photoexcitation (~1.0 × 1013 photons·cm−2·pulse−1) in the TA measurements. Our results have revealed significantly enhanced charge photogeneration and substantially suppressed PP recombination under reverse but not forward potential biases, which agrees with the field dependence of FF and EQE of the BisDMO-PFDTBT/PC61BM device.

2. Materials and methods

2.1 Device fabrication

The polymer solar cells were constructed on patterned and ITO-coated glass substrates. The ITO surface was modified by spin-coating PEDOT:PSS (Baytron P VP A1 4083) with a thickness of ~40 nm, followed by baking in air at 150 °C for 15 min. BisDMO-PFDTBT blended with PC61BM (1:3 w/w) were dissolved in 1,2-dichlorobenzene. Photoactive layers were obtained by spin-coating (3000 rpm, 30 s), and were annealed at 110 °C for 10 min. The typical thickness of the BisDMO-PFDTBT:PC61BM layer was ~49 nm as determined by using a surface profilometer (Ambios Technology XP-2). The cathode consisted of 20 nm of calcium and 100 nm of aluminum, which were thermally evaporated to the top of polymer layer with a shadow mask to define an active area of 0.04 cm2. The current-voltage (I-V) curves were measured under 100 mW/cm2 standard AM 1.5 G spectrum using a solar simulator (XES-70S1, San-Ei Electric Co. Ltd.; AAA grade, 70 × 70 mm2 photo-beam size), for which a 2 × 2 cm2 monocrystalline silicon cell (SRC-1000-TC-Q; VLSI Standards Inc.) was used for reference. The EQE were measured, with dark currents deducted, by the use of the Solar Cell Spectral Response Measurement System (QE-R3011, Enli Technology Ltd., Taiwan), and the light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell.

2.2 Time-resolved near infrared absorption spectroscopy

3. Results and discussion

Figure 1(a)
Fig. 1 (a) UV-visible absorption spectra of the BisDMO-PFDTBT/PC61BM (1:3) photoactive layer in real solar cell device (thicker solid), the neat films of BisDMO-PFDTBT (thinner solid) and PC61BM (dashed). Insets are the molecular structures. Arrow points to the excitation wavelength for time-resolved measurements (610 nm). (b) Typical current-voltage (I-V) curve of the solar cells determined under AM 1.5 G illumination (▾) and in dark (◼).
shows the UV-visible absorption spectrum of the BisDMO-PFDTBT/PC61BM photoactive layer. The major absorption band at 554 nm (I) is attributed to the optical transition from the ground state to the intramolecular charge transfer state (S1←S0) shifting the electron density from the fluorene unit to the thienyl-benzothiadiazole (TBT) unit of BisDMO-PFDTBT, while the band at 389 nm (II) is ascribed to the absorptive transition to the excitonic state (S2←S0) with the π-electron delocalized over the fluorene-thiophene backbone [23

23. W. Zhang, Y.-W. Wang, R. Hu, L.-M. Fu, X.-C. Ai, J.-P. Zhang, and J.-H. Hou, “Mechanism of primary charge photogeneration in polyfluorene copolymer/fullerene blends and influence of the donor/acceptor lowest unoccupied molecular orbital level offset,” J. Phys. Chem. C 117(2), 735–749 (2013). [CrossRef]

,24

24. K. G. Jespersen, W. J. D. Beenken, Y. Zaushitsyn, A. Yartsev, M. Andersson, T. Pullerits, and V. Sundström, “The electronic states of polyfluorene copolymers with alternating donor-acceptor units,” J. Chem. Phys. 121(24), 12613–12617 (2004). [CrossRef] [PubMed]

]. In time-resolved measurements, we applied the bandgap photoexcitation at 610 nm to minimize the photoexcitation of PC61BM and the excess energy of the S1 exciton of BisDMO-PFDTBT, otherwise the former would introduce the diffusive dynamics of fullerene excitation (~100 ps) [25

25. F. Etzold, I. A. Howard, N. Forler, D. M. Cho, M. Meister, H. Mangold, J. Shu, M. R. Hansen, K. Müllen, and F. Laquai, “The effect of solvent additives on morphology and excited-state dynamics in PCPDTBT:PCBM photovoltaic blends,” J. Am. Chem. Soc. 134(25), 10569–10583 (2012). [CrossRef] [PubMed]

] and the latter would promote the PP dissociation despite a weak contribution [15

15. A. A. Bakulin, A. Rao, V. G. Pavelyev, P. H. M. van Loosdrecht, M. S. Pshenichnikov, D. Niedzialek, J. Cornil, D. Beljonne, and R. H. Friend, “The role of driving energy and delocalized States for charge separation in organic semiconductors,” Science 335(6074), 1340–1344 (2012). [CrossRef] [PubMed]

,26

26. J. Lee, K. Vandewal, S. R. Yost, M. E. Bahlke, L. Goris, M. A. Baldo, J. V. Manca, and T. V. Voorhis, “Charge transfer state versus hot exciton dissociation in polymer-fullerene blended solar cells,” J. Am. Chem. Soc. 132(34), 11878–11880 (2010). [CrossRef] [PubMed]

]. Figure 1(b) shows a typical I-V curve of the BisDMO-PFDTBT/PC61BM device, indicating an open-circuit voltage (VOC) of 0.79 V. Here, PC61BM instead of PC70BM was used, which resulted in a relatively low PCE of 1.47%.

Figures 2(a)
Fig. 2 Representative time-resolved spectra for the BisDMO-PFDTBT/PC61BM solar cell at indicated bias potentials and delay times. Excitation wavelength was 610 nm (~1.0 × 1013 photons·cm−2·pulse−1). The depleted sections around 1200 nm are due to the interference from the second order diffraction of the excitation pulses.
and 2(b) illustrate the TA spectra recorded for the solar cell under short circuit and the forward bias of Vf = 0.79 V, respectively, we see two major spectral components in common: The characteristic PP absorption peaking at ~910 nm in the transients at Δt = 0 ps, and the signature P●+ absorption at ~1050 nm in the transients at Δt = 2.0 ns [3

3. M. Tong, N. E. Coates, D. Moses, and A. J. Heeger, “Charge carrier photogeneration and decay dynamics in the poly(2,7-carbazole) copolymer PCDTBT and in bulk heterojunction composites with PC70BM,” Phys. Rev. B 81(12), 125210 (2010). [CrossRef]

,17

17. F. Etzold, I. A. Howard, R. Mauer, M. Meister, T.-D. Kim, K.-S. Lee, N. S. Baek, and F. Laquai, “Ultrafast exciton dissociation followed by nongeminate charge recombination in PCDTBT:PCBM photovoltaic blends,” J. Am. Chem. Soc. 133(24), 9469–9479 (2011). [CrossRef] [PubMed]

,18

18. S. De, T. Pascher, M. Maiti, K. G. Jespersen, T. Kesti, F. Zhang, O. Inganäs, A. Yartsev, and V. Sundström, “Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc. 129(27), 8466–8472 (2007). [CrossRef] [PubMed]

]. Note that the S1 exciton absorbing at ~1300 nm for the BisDMO-PFDTBT/PC70BM blend are rather weak even in the initial transients (0 ps, 0.13 ps), implying the extremely efficient dissociation of the S1 exciton. This is in accordance to the nanosecond-to-subpicosecond shortened fluorescence S1-exciton lifetime from the neat to the blend films [19

19. S. K. Pal, T. Kesti, M. Maiti, F. Zhang, O. Inganäs, S. Hellström, M. R. Andersson, F. Oswald, F. Langa, T. Österman, T. Pascher, A. Yartsev, and V. Sundström, “Geminate charge recombination in polymer/fullerene bulk heterojunction films and implications for solar cell function,” J. Am. Chem. Soc. 132(35), 12440–12451 (2010). [CrossRef] [PubMed]

], i.e. the nearly unitary exciton quenching efficiency. The temporal evolution of the TA spectra are similar from the short circuit (Fig. 2(a)) to the forward bias of 0.79 V (Fig. 2(b)). On the other hand, the charge carriers generated by pulsed photoexcitation cannot reach the electrodes within a few nanoseconds to build the macroscopic (internal) electric field, because the timescale of charge collection is 100~1000 ns depending on the hole mobility for a 100-nm thick active layer [18

18. S. De, T. Pascher, M. Maiti, K. G. Jespersen, T. Kesti, F. Zhang, O. Inganäs, A. Yartsev, and V. Sundström, “Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc. 129(27), 8466–8472 (2007). [CrossRef] [PubMed]

]. Therefore, the charge dynamics are governed by the diffusive charge translocation and the intrinsic electric fields localized to the polymer-fullerene interfaces. As the result, the spectral dynamics were similar under the conditions of short circuit (Fig. 1(a)) and open circuit (data not shown). However, in Fig. 2(b) the forward bias, comparable to the VOC, provides a macroscopic and static electric field across the photoactive layer, the spectral dynamics also show little difference from those under short circuit (Fig. 2(a)). These results strongly suggest that, under working condition, the macroscopic electric field does not influence the subnanosecond charge dynamics of the BisDMO-PFDTBT/PC61BM solar cell.

From Figs. 2(c) and 2(d), we see that the TA spectra under the reverse biases of Vr = −2 V and −5 V resemble closely each other. These transients, however, differ distinctly from those in Figs. 2(a) and 2(b): In Figs. 2(c) and 2(d), the relative intensity of the transients at 0.13 ps and 1.00 ps are much lower, and those later than 1 ps decay much slower. In addition, the TA amplitudes at Δt = 2.0 ns are considerably higher than those at Δt = 100 ps. The differences between the spectral dynamics in Figs. 2(c), 2(d) and those in Figs. 2(a), 2(b) are to be ascribed to the field-assisted PP-to-P●+ dissociation (vide infra).

Figure 3
Fig. 3 Time-evolution profiles probed at (a) 970 nm and (b) 1050 nm for the BisDMO-PFDTBT/PC61BM solar cell at indicated bias potentials (cf Fig. 2. Normalized to the amplitudes maxima at 0.13 ps). Solid lines were obtained by global fitting of the kinetics to the model function ΔOD=A1tα1+A2tα2,see text for details).
shows the kinetics plotted at the respective characteristic wavelengths of 970 nm and 1050 nm preferentially probing PP and P●+. It is seen that the kinetics under short circuit are close to those under Vf = 0.79 V, whereas those under Vr = −2 V and −5 V are similar to each other (Figs. 3(a) or 3(b)). The kinetics of PP and P●+ can be described by the power law that is applicable to the temporal evolution of the charge species in disordered polymeric solids [27

27. J. Nelson, “Diffusion-limited recombination in polymer-fullerene blends and its influence on photocurrent collection,” Phys. Rev. B 67(15), 155209 (2003). [CrossRef]

,28

28. J. Guo, H. Ohkita, S. Yokoya, H. Benten, and S. Ito, “Bimodal polarons and hole transport in poly(3-hexylthiophene):fullerene blend films,” J. Am. Chem. Soc. 132(28), 9631–9637 (2010). [CrossRef] [PubMed]

]. We therefore simultaneously fitted the kinetics at two different probing wavelengths for each case of electrical bias to the model function ΔOD=A1tα1+A2tα2, where the positive and the negative exponents, respectively, represent the decay and the formation of charge species [27

27. J. Nelson, “Diffusion-limited recombination in polymer-fullerene blends and its influence on photocurrent collection,” Phys. Rev. B 67(15), 155209 (2003). [CrossRef]

,28

28. J. Guo, H. Ohkita, S. Yokoya, H. Benten, and S. Ito, “Bimodal polarons and hole transport in poly(3-hexylthiophene):fullerene blend films,” J. Am. Chem. Soc. 132(28), 9631–9637 (2010). [CrossRef] [PubMed]

]. In curve fitting, the kinetics data before 0.4 ps were not taken into account, because in this temporal regime the kinetics are complicated by the dynamics of exciton dissociation and geminate charge recombination [23

23. W. Zhang, Y.-W. Wang, R. Hu, L.-M. Fu, X.-C. Ai, J.-P. Zhang, and J.-H. Hou, “Mechanism of primary charge photogeneration in polyfluorene copolymer/fullerene blends and influence of the donor/acceptor lowest unoccupied molecular orbital level offset,” J. Phys. Chem. C 117(2), 735–749 (2013). [CrossRef]

]. The exponents thus derived are listed in Table 1

Table 1. Power-law exponents (α) obtained by global fitting of the 970 nm and the 1050 nm kinetics under individual bias voltages (cfFig. 3).

table-icon
View This Table
.

It is seen from Fig. 3 that the kinetics traces recorded under short circuit and under Vf = 0.79 V are similar, as also reflected by the similar α1 (0.20) or α2 (0.06) between the two different bias conditions (Table 1). However, on going to the reverse biases of −2 V or −5 V, α1 reduces from ~0.2 to ~0.1, suggesting significantly slowing down of PP depopulation, most probably owing to the field-assisted suppression of PP geminate recombination. In addition, under a reverse bias the absolute value of α1 is equal to α2, implying the decay-to-rise correlation between PP and P●+, i. e. the conversion of PP into P●+. Importantly, increase of the reverse bias voltage accelerates the PP-to-P●+ conversion, as indicated by the increased magnitudes of α1 and α2 from Vr = −2 V to Vr = −5 V (Table 1). The field effects are in accordance to those reported for solar cells based on other types of fluorene copolymers [10

10. D. Veldman, Ö. İpek, S. C. J. Meskers, J. Sweelssen, M. M. Koetse, S. C. Veenstra, J. M. Kroon, S. S. Bavel, J. Loos, and R. A. J. Janssen, “Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc. 130(24), 7721–7735 (2008). [CrossRef] [PubMed]

, 29

29. A. C. Morteani, P. Sreearunothai, L. M. Herz, R. H. Friend, and C. Silva, “Exciton regeneration at polymeric semiconductor heterojunctions,” Phys. Rev. Lett. 92(24), 247402 (2004). [CrossRef] [PubMed]

].

Because both polymer and fullerenes are agglomerated to different degrees [30

30. F. C. Jamieson, E. B. Domingo, T. McCarthy-Ward, M. Heeney, N. Stingelin, and J. R. Durrant, “Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells,” Chem. Sci. 3(2), 485–492 (2012). [CrossRef]

,31

31. B. A. Collins, E. Gann, L. Guignard, X. He, C. R. McNeill, and H. Ade, “Molecular miscibility of polymer−fullerene blends,” J. Phys. Chem. Lett. 1(21), 3160–3166 (2010). [CrossRef]

], upon photoexcitation a range of initial e-h+ separations are expected under a given polymer-fullerene LUMO level offset (−ΔEL). In addition, the delocalization lengths of electron and hole, respectively, rely on the molecular structures of polymers and the topology/bulkiness of the acceptor molecules [15

15. A. A. Bakulin, A. Rao, V. G. Pavelyev, P. H. M. van Loosdrecht, M. S. Pshenichnikov, D. Niedzialek, J. Cornil, D. Beljonne, and R. H. Friend, “The role of driving energy and delocalized States for charge separation in organic semiconductors,” Science 335(6074), 1340–1344 (2012). [CrossRef] [PubMed]

,32

32. R. D. Pensack, C. Guo, K. Vakhshouri, E. D. Gomez, and J. B. Asbury, “Influence of acceptor structure on barriers to charge separation in organic photovoltaic materials,” J. Phys. Chem. C 116(7), 4824–4831 (2012). [CrossRef]

]. According to Braun-Onsager’s model for e-h+ escape probability [33

33. L. Onsager, “Deviations from Ohm's Law in weak electrolytes,” J. Chem. Phys. 2(9), 599–615 (1934). [CrossRef]

], a larger initial e-h+ separation of the PP state leads to a higher dissociation probability. Considering a threshold e-h+ separation (rth) at which the −2 V bias (4.0 × 105 V/cm) can be effective in driving the e-h+ dissociation, we obtained a rth of 9.1 nm (by assuming that the biased field completely counteracts the field at the middle of an e-h+ pair), meaning that for the BisDMO-PFDTBT/PC61BM device biased with Vr = −2 V, a PP with initial separation larger than 9.1 nm can be converted into free charges by the macroscopic electric field. Under Vr = −5 V, the rth reduced to 5.8 nm, and hence the effect of field-assisted dissociation of PP would be more prominent.

As seen from Fig. 3 and Table 1, the PP depopulation is evidently speeded up (α1, 0.07→0.11) upon varying Vr from −2 V to −5 V, and the formation of P●+ is accelerated concomitantly (−α2, 0.06→0.10). These results once again prove that a reverse bias of −2 V is effective in promoting the PP-to-P●+ conversion. Here, we note that the internal field of a working device (1.6 × 105 V/cm), considerably lower than the field strength under the −2 V bias (4.0 × 105 V/cm), is consequently ineffective in the PP dynamics. Regarding the field effect on the dissociation of neutral excitons, either the applied or the intrinsic macroscopic fields may be too weak to be of any help. In this relation, for the ladder-type methylsubstituted poly(paraphenylene) (MeLPPP) film, the field strength for exciton dissociation was reported to be as high as 1.5 × 106 V/cm [34

34. V. Gulbinas, Y. Zaushitsyn, V. Sundström, D. Hertel, H. Bässler, and A. Yartsev, “Dynamics of the electric field-assisted charge carrier photogeneration in ladder-type poly(para-phenylene) at a low excitation intensity,” Phys. Rev. Lett. 89(10), 107401 (2002). [CrossRef] [PubMed]

]. However, recent studies suggest that the microscopic fields localized to the BHJ interfaces may play important roles in the interfacial charge photogeneration. E. g., the interfacial dipole can reduce the driving force of exciton dissociation and thereby improve VOC [35

35. J.-L. Brédas, J. E. Norton, J. Cornil, and V. Coropceanu, “Molecular understanding of organic solar cells: the challenges,” Acc. Chem. Res. 42(11), 1691–1699 (2009). [CrossRef] [PubMed]

,36

36. W. J. Potscavage Jr, S. Yoo, and B. Kippelen, “Origin of the open-circuit voltage in multilayer heterojunction organic solar cells,” Appl. Phys. Lett. 93(19), 193308 (2008). [CrossRef]

]. In addition, enhanced charge separation in the organic photovoltaic films doped with the ferroelectric dipole additive poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) was attributed to the field-assisted dissociation of excitons and PPs (field strength localized to the polymer-fullerene interface, 2.4 × 106 V/cm), although under working condition the macroscopic internal field showed little effect on the device performance [37

37. K. S. Nalwa, J. A. Carr, R. C. Mahadevapuram, H. K. Kodali, S. Bose, Y. Chen, J. W. Petrich, B. Ganapathysubramanian, and S. Chaudhary, “Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles,” Energy Environ. Sci. 5(5), 7042–7049 (2012). [CrossRef]

].

Finally, to see whether the device performance can be enhanced with the assistance of electric fields, we measured the EQEs of the BisDMO-PFDTBT/PC61BM solar cell under various bias potentials, and the results are presented in Fig. 4
Fig. 4 The EQE curves of the BisDMO-PFDTBT/PC61BM device (1:3, w/w) determined under short circuit (0 V) and at the forwards (Vf) and the reverse (Vr) biases.
. Evidently, the bias at Vf = 0.8 V drastically suppressed the EQE with reference to the EQE of short circuit, which is due to the unfavorable charge transport and collection in the μs-ms regime, because in the subnanosecond timeframe the charge kinetics at 0 V resemble with those at Vf = 0.8 V (Fig. 3, Table 1). In contrast, the EQE under Vr = −2 V is enhanced dramatically, while only slight further improvement is seen for Vr = −5 V. (We have determined the I-V characteristics up to −5 V, under which no field-induced damage and irreversible modification of the device were observed.) Most importantly, the substantial enhancement of the EQEs under reverse biases tightly correlates to the field-assisted dissociation of PPs found in the subnanosecond timescale (vide supra). Recent study on the energetics factors driving the PP separation at organic-inorganic semiconductor interfaces has revealed that the band-bending, i. e. the higher shifted binding energy of organic molecules at the heterojunction with respect to that in the organic bulk, facilitates the separation of the photogenerated interfacial PPs, and hence the regeneration of interfacial holes by transferring to the consecutive bulk organic molecules plays an important role in free charge production [38

38. E. L. Unger, T. Edvinsson, J. D. Roy-Mayhew, H. Rensmo, A. Hagfeldt, E. M. J. Johansson, and G. Boschloo, “Excitation energy dependent charge separation at hole-transporting dye/TiO2 hetero interface,” J. Phys. Chem. C 116(40), 21148–21156 (2012). [CrossRef]

]. Similar mechanism may also work in organic solar cells, which is especially important when the effects of macroscopic internal electric field is taken into account.

4. Conclusions

The present work unambiguously shows that, under working condition, the macroscopic internal electric field of the BisDMO-PFDTBT/PC61BM solar cell is ineffective in the subnanosecond charge photogeneration/recombination dynamics. However, a reverse bias strength exceeding −2 V can significantly accelerate the processes of PP-to-P•+ conversion, and concomitantly suppress the PP geminate recombination. Polymer solar cells generally work with low operating voltage (<1 V), meaning relatively low internal field strength. To improve the device performance, it is important to optimize the phase morphologies of the BHJ layers so as to increase the initial e-h+ separation, and to promote the field-assisted PP separation.

Acknowledgments

Grants-in-aid from the Natural Science Foundation of China (Nos. 20933010; 21133001; 51173189) and the National Basic Research Program of China (No. 2009CB20008) are acknowledged.

References and links

1.

C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, “Plastic solar cells,” Adv. Funct. Mater. 11(1), 15–26 (2001). [CrossRef]

2.

T. M. Clarke and J. R. Durrant, “Charge photogeneration in organic solar cells,” Chem. Rev. 110(11), 6736–6767 (2010). [CrossRef] [PubMed]

3.

M. Tong, N. E. Coates, D. Moses, and A. J. Heeger, “Charge carrier photogeneration and decay dynamics in the poly(2,7-carbazole) copolymer PCDTBT and in bulk heterojunction composites with PC70BM,” Phys. Rev. B 81(12), 125210 (2010). [CrossRef]

4.

N. Banerji, S. Cowan, M. Leclerc, E. Vauthey, and A. J. Heeger, “Exciton formation, relaxation, and decay in PCDTBT,” J. Am. Chem. Soc. 132(49), 17459–17470 (2010). [CrossRef] [PubMed]

5.

J. Guo, H. Ohkita, H. Benten, and S. Ito, “Near-IR femtosecond transient absorption spectroscopy of ultrafast polaron and triplet exciton formation in polythiophene films with different regioregularities,” J. Am. Chem. Soc. 131(46), 16869–16880 (2009). [CrossRef] [PubMed]

6.

J. Guo, H. Ohkita, H. Benten, and S. Ito, “Charge generation and recombination dynamics in poly(3-hexylthiophene)/fullerene blend films with different regioregularities and morphologies,” J. Am. Chem. Soc. 132(17), 6154–6164 (2010). [CrossRef] [PubMed]

7.

V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom, “Photocurrent generation in polymer-fullerene bulk heterojunctions,” Phys. Rev. Lett. 93(21), 216601 (2004). [CrossRef] [PubMed]

8.

V. D. Mihailetchi, H. X. Xie, B. De Boer, L. J. A. Koster, and P. W. M. Blom, “Charge transport and photocurrent generation in poly(3-hexylthiophene): methanofullerene bulk-heterojunction solar cells,” Adv. Funct. Mater. 16(5), 699–708 (2006). [CrossRef]

9.

T. Offermans, P. A. van Hal, S. C. J. Meskers, M. M. Koetse, and R. A. J. Janssen, “Exciplex dynamics in a blend of π-conjugated polymers with electron donating and accepting properties: MDMO-PPV and PCNEPV,” Phys. Rev. B 72(4), 045213 (2005). [CrossRef]

10.

D. Veldman, Ö. İpek, S. C. J. Meskers, J. Sweelssen, M. M. Koetse, S. C. Veenstra, J. M. Kroon, S. S. Bavel, J. Loos, and R. A. J. Janssen, “Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc. 130(24), 7721–7735 (2008). [CrossRef] [PubMed]

11.

R. A. Marsh, J. M. Hodgkiss, and R. H. Friend, “Direct measurement of electric field-assisted charge separation in polymer:fullerene photovoltaic diodes,” Adv. Mater. (Deerfield Beach Fla.) 22(33), 3672–3676 (2010). [CrossRef] [PubMed]

12.

F. C. Jamieson, T. Agostinelli, H. Azimi, J. Nelson, and J. R. Durrant, “Field-independent charge photogeneration in PCPDTBT/PC70BM solar cells,” J. Phys. Chem. Lett. 1(23), 3306–3310 (2010). [CrossRef]

13.

S. Albrecht, W. Schindler, J. Kurpiers, J. Kniepert, J. C. Blakesley, I. Dumsch, S. Allard, K. Fostiropoulos, U. Scherf, and D. Neher, “On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM: influence of solvent additives,” J. Phys. Chem. Lett. 3(5), 640–645 (2012). [CrossRef]

14.

D. Jarzab, F. Cordella, J. Gao, M. Scharber, H. J. Egelhaaf, and M. A. Loi, “Low-temperature behaviour of charge transfer excitons in narrow-bandgap polymer-based bulk heterojunctions,” Adv. Energy Mater. 1(4), 604–609 (2011). [CrossRef]

15.

A. A. Bakulin, A. Rao, V. G. Pavelyev, P. H. M. van Loosdrecht, M. S. Pshenichnikov, D. Niedzialek, J. Cornil, D. Beljonne, and R. H. Friend, “The role of driving energy and delocalized States for charge separation in organic semiconductors,” Science 335(6074), 1340–1344 (2012). [CrossRef] [PubMed]

16.

M. A. Loi, S. Toffanin, M. Muccini, M. Forster, U. Scherf, and M. Scharber, “Charge transfer excitons in bulk heterojunctions of a polyfluorene copolymer and a fullerene derivative,” Adv. Funct. Mater. 17(13), 2111–2116 (2007). [CrossRef]

17.

F. Etzold, I. A. Howard, R. Mauer, M. Meister, T.-D. Kim, K.-S. Lee, N. S. Baek, and F. Laquai, “Ultrafast exciton dissociation followed by nongeminate charge recombination in PCDTBT:PCBM photovoltaic blends,” J. Am. Chem. Soc. 133(24), 9469–9479 (2011). [CrossRef] [PubMed]

18.

S. De, T. Pascher, M. Maiti, K. G. Jespersen, T. Kesti, F. Zhang, O. Inganäs, A. Yartsev, and V. Sundström, “Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc. 129(27), 8466–8472 (2007). [CrossRef] [PubMed]

19.

S. K. Pal, T. Kesti, M. Maiti, F. Zhang, O. Inganäs, S. Hellström, M. R. Andersson, F. Oswald, F. Langa, T. Österman, T. Pascher, A. Yartsev, and V. Sundström, “Geminate charge recombination in polymer/fullerene bulk heterojunction films and implications for solar cell function,” J. Am. Chem. Soc. 132(35), 12440–12451 (2010). [CrossRef] [PubMed]

20.

M.-H. Chen, J. Hou, Z. Hong, G. Yang, S. Sista, L.-M. Chen, and Y. Yang, “Efficient polymer solar cells with thin active layers based on alternating polyfluorene copolymer/fullerene bulk heterojunctions,” Adv. Mater. (Deerfield Beach Fla.) 21(42), 4238–4242 (2009). [CrossRef]

21.

A. Pivrikas, N. S. Sariciftci, G. Juška, and R. A. Österbacka, “A review of charge transport and recombination in polymer/fullerene organic solar cells,” Prog. Photovolt. Res. Appl. 15(8), 677–696 (2007). [CrossRef]

22.

R. A. Marsh, J. M. Hodgkiss, S. Albert-Seifried, and R. H. Friend, “Effect of annealing on P3HT:PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy,” Nano Lett. 10(3), 923–930 (2010). [CrossRef] [PubMed]

23.

W. Zhang, Y.-W. Wang, R. Hu, L.-M. Fu, X.-C. Ai, J.-P. Zhang, and J.-H. Hou, “Mechanism of primary charge photogeneration in polyfluorene copolymer/fullerene blends and influence of the donor/acceptor lowest unoccupied molecular orbital level offset,” J. Phys. Chem. C 117(2), 735–749 (2013). [CrossRef]

24.

K. G. Jespersen, W. J. D. Beenken, Y. Zaushitsyn, A. Yartsev, M. Andersson, T. Pullerits, and V. Sundström, “The electronic states of polyfluorene copolymers with alternating donor-acceptor units,” J. Chem. Phys. 121(24), 12613–12617 (2004). [CrossRef] [PubMed]

25.

F. Etzold, I. A. Howard, N. Forler, D. M. Cho, M. Meister, H. Mangold, J. Shu, M. R. Hansen, K. Müllen, and F. Laquai, “The effect of solvent additives on morphology and excited-state dynamics in PCPDTBT:PCBM photovoltaic blends,” J. Am. Chem. Soc. 134(25), 10569–10583 (2012). [CrossRef] [PubMed]

26.

J. Lee, K. Vandewal, S. R. Yost, M. E. Bahlke, L. Goris, M. A. Baldo, J. V. Manca, and T. V. Voorhis, “Charge transfer state versus hot exciton dissociation in polymer-fullerene blended solar cells,” J. Am. Chem. Soc. 132(34), 11878–11880 (2010). [CrossRef] [PubMed]

27.

J. Nelson, “Diffusion-limited recombination in polymer-fullerene blends and its influence on photocurrent collection,” Phys. Rev. B 67(15), 155209 (2003). [CrossRef]

28.

J. Guo, H. Ohkita, S. Yokoya, H. Benten, and S. Ito, “Bimodal polarons and hole transport in poly(3-hexylthiophene):fullerene blend films,” J. Am. Chem. Soc. 132(28), 9631–9637 (2010). [CrossRef] [PubMed]

29.

A. C. Morteani, P. Sreearunothai, L. M. Herz, R. H. Friend, and C. Silva, “Exciton regeneration at polymeric semiconductor heterojunctions,” Phys. Rev. Lett. 92(24), 247402 (2004). [CrossRef] [PubMed]

30.

F. C. Jamieson, E. B. Domingo, T. McCarthy-Ward, M. Heeney, N. Stingelin, and J. R. Durrant, “Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells,” Chem. Sci. 3(2), 485–492 (2012). [CrossRef]

31.

B. A. Collins, E. Gann, L. Guignard, X. He, C. R. McNeill, and H. Ade, “Molecular miscibility of polymer−fullerene blends,” J. Phys. Chem. Lett. 1(21), 3160–3166 (2010). [CrossRef]

32.

R. D. Pensack, C. Guo, K. Vakhshouri, E. D. Gomez, and J. B. Asbury, “Influence of acceptor structure on barriers to charge separation in organic photovoltaic materials,” J. Phys. Chem. C 116(7), 4824–4831 (2012). [CrossRef]

33.

L. Onsager, “Deviations from Ohm's Law in weak electrolytes,” J. Chem. Phys. 2(9), 599–615 (1934). [CrossRef]

34.

V. Gulbinas, Y. Zaushitsyn, V. Sundström, D. Hertel, H. Bässler, and A. Yartsev, “Dynamics of the electric field-assisted charge carrier photogeneration in ladder-type poly(para-phenylene) at a low excitation intensity,” Phys. Rev. Lett. 89(10), 107401 (2002). [CrossRef] [PubMed]

35.

J.-L. Brédas, J. E. Norton, J. Cornil, and V. Coropceanu, “Molecular understanding of organic solar cells: the challenges,” Acc. Chem. Res. 42(11), 1691–1699 (2009). [CrossRef] [PubMed]

36.

W. J. Potscavage Jr, S. Yoo, and B. Kippelen, “Origin of the open-circuit voltage in multilayer heterojunction organic solar cells,” Appl. Phys. Lett. 93(19), 193308 (2008). [CrossRef]

37.

K. S. Nalwa, J. A. Carr, R. C. Mahadevapuram, H. K. Kodali, S. Bose, Y. Chen, J. W. Petrich, B. Ganapathysubramanian, and S. Chaudhary, “Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles,” Energy Environ. Sci. 5(5), 7042–7049 (2012). [CrossRef]

38.

E. L. Unger, T. Edvinsson, J. D. Roy-Mayhew, H. Rensmo, A. Hagfeldt, E. M. J. Johansson, and G. Boschloo, “Excitation energy dependent charge separation at hole-transporting dye/TiO2 hetero interface,” J. Phys. Chem. C 116(40), 21148–21156 (2012). [CrossRef]

OCIS Codes
(160.5470) Materials : Polymers
(250.2080) Optoelectronics : Polymer active devices
(300.6500) Spectroscopy : Spectroscopy, time-resolved
(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors

ToC Category:
Photovoltaics

History
Original Manuscript: November 12, 2012
Revised Manuscript: January 13, 2013
Manuscript Accepted: January 17, 2013
Published: February 4, 2013

Citation
Wei Zhang, Ye Huang, Ya-Dong Xing, Yan Jing, Long Ye, Li-Min Fu, Xi-Cheng Ai, Jian-Hui Hou, and Jian-Ping Zhang, "Subnanosecond charge photogeneration and recombination in polyfluorene copolymer-fullerene solar cell: Effects of electric field," Opt. Express 21, A241-A249 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-S2-A241


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References

  1. C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, “Plastic solar cells,” Adv. Funct. Mater.11(1), 15–26 (2001). [CrossRef]
  2. T. M. Clarke and J. R. Durrant, “Charge photogeneration in organic solar cells,” Chem. Rev.110(11), 6736–6767 (2010). [CrossRef] [PubMed]
  3. M. Tong, N. E. Coates, D. Moses, and A. J. Heeger, “Charge carrier photogeneration and decay dynamics in the poly(2,7-carbazole) copolymer PCDTBT and in bulk heterojunction composites with PC70BM,” Phys. Rev. B81(12), 125210 (2010). [CrossRef]
  4. N. Banerji, S. Cowan, M. Leclerc, E. Vauthey, and A. J. Heeger, “Exciton formation, relaxation, and decay in PCDTBT,” J. Am. Chem. Soc.132(49), 17459–17470 (2010). [CrossRef] [PubMed]
  5. J. Guo, H. Ohkita, H. Benten, and S. Ito, “Near-IR femtosecond transient absorption spectroscopy of ultrafast polaron and triplet exciton formation in polythiophene films with different regioregularities,” J. Am. Chem. Soc.131(46), 16869–16880 (2009). [CrossRef] [PubMed]
  6. J. Guo, H. Ohkita, H. Benten, and S. Ito, “Charge generation and recombination dynamics in poly(3-hexylthiophene)/fullerene blend films with different regioregularities and morphologies,” J. Am. Chem. Soc.132(17), 6154–6164 (2010). [CrossRef] [PubMed]
  7. V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom, “Photocurrent generation in polymer-fullerene bulk heterojunctions,” Phys. Rev. Lett.93(21), 216601 (2004). [CrossRef] [PubMed]
  8. V. D. Mihailetchi, H. X. Xie, B. De Boer, L. J. A. Koster, and P. W. M. Blom, “Charge transport and photocurrent generation in poly(3-hexylthiophene): methanofullerene bulk-heterojunction solar cells,” Adv. Funct. Mater.16(5), 699–708 (2006). [CrossRef]
  9. T. Offermans, P. A. van Hal, S. C. J. Meskers, M. M. Koetse, and R. A. J. Janssen, “Exciplex dynamics in a blend of π-conjugated polymers with electron donating and accepting properties: MDMO-PPV and PCNEPV,” Phys. Rev. B72(4), 045213 (2005). [CrossRef]
  10. D. Veldman, Ö. İpek, S. C. J. Meskers, J. Sweelssen, M. M. Koetse, S. C. Veenstra, J. M. Kroon, S. S. Bavel, J. Loos, and R. A. J. Janssen, “Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc.130(24), 7721–7735 (2008). [CrossRef] [PubMed]
  11. R. A. Marsh, J. M. Hodgkiss, and R. H. Friend, “Direct measurement of electric field-assisted charge separation in polymer:fullerene photovoltaic diodes,” Adv. Mater. (Deerfield Beach Fla.)22(33), 3672–3676 (2010). [CrossRef] [PubMed]
  12. F. C. Jamieson, T. Agostinelli, H. Azimi, J. Nelson, and J. R. Durrant, “Field-independent charge photogeneration in PCPDTBT/PC70BM solar cells,” J. Phys. Chem. Lett.1(23), 3306–3310 (2010). [CrossRef]
  13. S. Albrecht, W. Schindler, J. Kurpiers, J. Kniepert, J. C. Blakesley, I. Dumsch, S. Allard, K. Fostiropoulos, U. Scherf, and D. Neher, “On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM: influence of solvent additives,” J. Phys. Chem. Lett.3(5), 640–645 (2012). [CrossRef]
  14. D. Jarzab, F. Cordella, J. Gao, M. Scharber, H. J. Egelhaaf, and M. A. Loi, “Low-temperature behaviour of charge transfer excitons in narrow-bandgap polymer-based bulk heterojunctions,” Adv. Energy Mater.1(4), 604–609 (2011). [CrossRef]
  15. A. A. Bakulin, A. Rao, V. G. Pavelyev, P. H. M. van Loosdrecht, M. S. Pshenichnikov, D. Niedzialek, J. Cornil, D. Beljonne, and R. H. Friend, “The role of driving energy and delocalized States for charge separation in organic semiconductors,” Science335(6074), 1340–1344 (2012). [CrossRef] [PubMed]
  16. M. A. Loi, S. Toffanin, M. Muccini, M. Forster, U. Scherf, and M. Scharber, “Charge transfer excitons in bulk heterojunctions of a polyfluorene copolymer and a fullerene derivative,” Adv. Funct. Mater.17(13), 2111–2116 (2007). [CrossRef]
  17. F. Etzold, I. A. Howard, R. Mauer, M. Meister, T.-D. Kim, K.-S. Lee, N. S. Baek, and F. Laquai, “Ultrafast exciton dissociation followed by nongeminate charge recombination in PCDTBT:PCBM photovoltaic blends,” J. Am. Chem. Soc.133(24), 9469–9479 (2011). [CrossRef] [PubMed]
  18. S. De, T. Pascher, M. Maiti, K. G. Jespersen, T. Kesti, F. Zhang, O. Inganäs, A. Yartsev, and V. Sundström, “Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends,” J. Am. Chem. Soc.129(27), 8466–8472 (2007). [CrossRef] [PubMed]
  19. S. K. Pal, T. Kesti, M. Maiti, F. Zhang, O. Inganäs, S. Hellström, M. R. Andersson, F. Oswald, F. Langa, T. Österman, T. Pascher, A. Yartsev, and V. Sundström, “Geminate charge recombination in polymer/fullerene bulk heterojunction films and implications for solar cell function,” J. Am. Chem. Soc.132(35), 12440–12451 (2010). [CrossRef] [PubMed]
  20. M.-H. Chen, J. Hou, Z. Hong, G. Yang, S. Sista, L.-M. Chen, and Y. Yang, “Efficient polymer solar cells with thin active layers based on alternating polyfluorene copolymer/fullerene bulk heterojunctions,” Adv. Mater. (Deerfield Beach Fla.)21(42), 4238–4242 (2009). [CrossRef]
  21. A. Pivrikas, N. S. Sariciftci, G. Juška, and R. A. Österbacka, “A review of charge transport and recombination in polymer/fullerene organic solar cells,” Prog. Photovolt. Res. Appl.15(8), 677–696 (2007). [CrossRef]
  22. R. A. Marsh, J. M. Hodgkiss, S. Albert-Seifried, and R. H. Friend, “Effect of annealing on P3HT:PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy,” Nano Lett.10(3), 923–930 (2010). [CrossRef] [PubMed]
  23. W. Zhang, Y.-W. Wang, R. Hu, L.-M. Fu, X.-C. Ai, J.-P. Zhang, and J.-H. Hou, “Mechanism of primary charge photogeneration in polyfluorene copolymer/fullerene blends and influence of the donor/acceptor lowest unoccupied molecular orbital level offset,” J. Phys. Chem. C117(2), 735–749 (2013). [CrossRef]
  24. K. G. Jespersen, W. J. D. Beenken, Y. Zaushitsyn, A. Yartsev, M. Andersson, T. Pullerits, and V. Sundström, “The electronic states of polyfluorene copolymers with alternating donor-acceptor units,” J. Chem. Phys.121(24), 12613–12617 (2004). [CrossRef] [PubMed]
  25. F. Etzold, I. A. Howard, N. Forler, D. M. Cho, M. Meister, H. Mangold, J. Shu, M. R. Hansen, K. Müllen, and F. Laquai, “The effect of solvent additives on morphology and excited-state dynamics in PCPDTBT:PCBM photovoltaic blends,” J. Am. Chem. Soc.134(25), 10569–10583 (2012). [CrossRef] [PubMed]
  26. J. Lee, K. Vandewal, S. R. Yost, M. E. Bahlke, L. Goris, M. A. Baldo, J. V. Manca, and T. V. Voorhis, “Charge transfer state versus hot exciton dissociation in polymer-fullerene blended solar cells,” J. Am. Chem. Soc.132(34), 11878–11880 (2010). [CrossRef] [PubMed]
  27. J. Nelson, “Diffusion-limited recombination in polymer-fullerene blends and its influence on photocurrent collection,” Phys. Rev. B67(15), 155209 (2003). [CrossRef]
  28. J. Guo, H. Ohkita, S. Yokoya, H. Benten, and S. Ito, “Bimodal polarons and hole transport in poly(3-hexylthiophene):fullerene blend films,” J. Am. Chem. Soc.132(28), 9631–9637 (2010). [CrossRef] [PubMed]
  29. A. C. Morteani, P. Sreearunothai, L. M. Herz, R. H. Friend, and C. Silva, “Exciton regeneration at polymeric semiconductor heterojunctions,” Phys. Rev. Lett.92(24), 247402 (2004). [CrossRef] [PubMed]
  30. F. C. Jamieson, E. B. Domingo, T. McCarthy-Ward, M. Heeney, N. Stingelin, and J. R. Durrant, “Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells,” Chem. Sci.3(2), 485–492 (2012). [CrossRef]
  31. B. A. Collins, E. Gann, L. Guignard, X. He, C. R. McNeill, and H. Ade, “Molecular miscibility of polymer−fullerene blends,” J. Phys. Chem. Lett.1(21), 3160–3166 (2010). [CrossRef]
  32. R. D. Pensack, C. Guo, K. Vakhshouri, E. D. Gomez, and J. B. Asbury, “Influence of acceptor structure on barriers to charge separation in organic photovoltaic materials,” J. Phys. Chem. C116(7), 4824–4831 (2012). [CrossRef]
  33. L. Onsager, “Deviations from Ohm's Law in weak electrolytes,” J. Chem. Phys.2(9), 599–615 (1934). [CrossRef]
  34. V. Gulbinas, Y. Zaushitsyn, V. Sundström, D. Hertel, H. Bässler, and A. Yartsev, “Dynamics of the electric field-assisted charge carrier photogeneration in ladder-type poly(para-phenylene) at a low excitation intensity,” Phys. Rev. Lett.89(10), 107401 (2002). [CrossRef] [PubMed]
  35. J.-L. Brédas, J. E. Norton, J. Cornil, and V. Coropceanu, “Molecular understanding of organic solar cells: the challenges,” Acc. Chem. Res.42(11), 1691–1699 (2009). [CrossRef] [PubMed]
  36. W. J. Potscavage, S. Yoo, and B. Kippelen, “Origin of the open-circuit voltage in multilayer heterojunction organic solar cells,” Appl. Phys. Lett.93(19), 193308 (2008). [CrossRef]
  37. K. S. Nalwa, J. A. Carr, R. C. Mahadevapuram, H. K. Kodali, S. Bose, Y. Chen, J. W. Petrich, B. Ganapathysubramanian, and S. Chaudhary, “Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles,” Energy Environ. Sci.5(5), 7042–7049 (2012). [CrossRef]
  38. E. L. Unger, T. Edvinsson, J. D. Roy-Mayhew, H. Rensmo, A. Hagfeldt, E. M. J. Johansson, and G. Boschloo, “Excitation energy dependent charge separation at hole-transporting dye/TiO2 hetero interface,” J. Phys. Chem. C116(40), 21148–21156 (2012). [CrossRef]

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