PCBM

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Femtosecond Time-Resolved Fluorescence Study of P3HT/PCBM Blend Films Yu Xie,† Yong Li,† Lixin Xiao,†,‡ Qiquan Qiao,*,† Rabin Dhakal,† Zhiling Zhang,† Qihuang Gong,‡ David Galipeau,† and Xingzhong Yan*,† Center for AdVanced PhotoVoltaics, Department of Electrical Engineering & Computer Science, South Dakota State UniVersity, Brookings, South Dakota 57006, and State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking UniVersity, Beijing 100871, China ReceiVed: December 31, 2009; ReVised Manuscript ReceiVed: July 19, 2010

In order to understand the dependence of photoinduced initial processes on thermal annealing, the femtosecond time-resolved fluorescence dynamics of regioregular poly(3-hexylthiophene) (P3HT) in (thermally) annealed P3HT/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend films has been studied by using the fluorescence up-conversion technique. For comparison, a P3HT solution, pristine P3HT, and unannealed P3HT/ PCBM blend films have been investigated as well. The fluorescence dynamics of the P3HT solution showed wavelength dependence. Excitation energy transfer between the segments and torsional relaxation possibly occurred in a time scale of several ps in the solution. Observed rise times at longer wavelength emission suggested the formation of these relatively lower emission states (at 650 and 700 nm). Charge transfer (or excitonic quenching) was the dominant process in the fs time scale with emission at 650 nm in the unannealed blend film. In the annealed blend film, the charge transfer (334 fs) and downhill relaxation (942 fs) of selftrapped (dynamic localized) excitons were competitive processes due to the well aligned nanodomains in the P3HT/PCBM blend films. There were different charge transfer rates at different excited states (650 and 700 nm) in the annealed film. The charge transfer process occurred faster at a lower excited state, and a stronger electronic and vibrational coupling in the annealed P3HT/PCBM films was revealed within these measurements as well. The ultrafast anisotropy decays suggested that a strong and ultrafast reorientation of the molecular dipole moments occurred at excited states. The anisotropy decay was mainly determined by the ultrafast process, whereas the energy could continuously migrate along or between P3HT chains in a time scale of ∼100 ps. The ultrafast process suggested that there was an excitation delocalization associated with vibrational modes, as was consistent with the observation from steady-state measurements. On the basis of the understanding of the mechanisms above, the optimized cell performance has been established. 1. Introduction Conjugated polymer-based solar cells are currently attracting extensive attention and interest due to their potential low-cost fabrication and mechanical flexibility.1-6 Among them, bulk heterojunction (BHJ) solar cells show potential as an inexpensive clean and alternative energy source.7,8 The BHJ solar cells comprising nanoscale interpenetrating networks have already achieved efficiencies of 6-7% in recent years.9,10 Solar cells fabricated from a regioregular poly(3-hexylthiophene) (P3HT) and a fullerene derivative, [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), have been exhibited as promising BHJ devices with efficiencies of ∼5%.11-14 The P3HT/PCBM blend becomes a typical electron donor/acceptor system for understanding the internal charge and energy migration processes via the study of photoinduced carrier generation at fast and ultrafast time scales.15,16 This investigation can help researchers find ways to enhance the performance of BHJ solar cells. Currently, the relatively low efficiencies of these cells are usually caused by the short exciton diffusion length (typically 4-10 nm), which is related to both the low carrier mobility and the short exciton lifetime.1,17,18 * To whom correspondence should be addressed. E-mail: [email protected] (X.Y.); [email protected] (Q.Q.). † South Dakota State University. ‡ Peking University.

Since the early 1990s, charge and energy migration processes in polymer/fullerene blends have been studied by several transient spectroscopy techniques with a time resolution from the sub-ps to ms range [for simplicity, ms (millisecond), ns (nanosecond), ps (picosecond), and fs (femtosecond) will be used in the following text of this paper].3,15,18-24 A slow decay (100 ns to 10 ms) was found to be independent of the excitation intensity in poly(phenylene vinylene) (PPV) derivatives/fullerene blends,25,26 while a decay with tens of ns scale ( annealed blend film > pristine film (Figure 6b). As discussed above, the vibrational cooling and dynamic localization occur in a time scale shorter than IRF, while the EET occurs in a few to tens of ps time scales. Thus, the observed fs processes should be dominated by the charge transfer and the relaxation of selftrapped (dynamic localized) exciton. In the pristine P3HT film, there is ignorable charge transfer and no exciton quenching in the sub ps time scale. The fluorescence decay with a time scale of 942 fs was attributed to the downhill relaxation of the selftrapped (dynamic localized) excitons, which was close to the previously reported results for the relaxation of self-trapped excitons with a time of 800 fs measured by transient absorption.68 The relaxation rate was calculated to be ∼1.06 × 1012 s-1. In the unannealed blend film, P3HT chains are more

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Figure 7. Anisotropy depolarization of the P3HT pristine and P3HT/PCBM blend films with emission of 650 nm: (a) long-time decay; (b-d) deconvolution results for the short-time dynamics of pristine P3HT film, unannealed film of the P3HT/PCBM blend, and annealed film of the P3HT/PCBM blend, respectively. The red line indicates IRF. The deconvolution function consists of two exponential components: τ1 ∼30 ( 20 fs and τ2 ∼100 ps (estimation).

homogeneously mixed with PCBM, compared with those in the annealed blend. Therefore, the exciton quenching (or charge transfer induced quenching) was the dominant process in a femtosecond time scale in the unannealed blend film. A time scale of 334 fs was observed here, which was less than 400 fs. This was consistent with the report of others for the ultrafast charge transfer in P3HT/PCBM blends.29 The charge transfer rate for the 334 fs decay was then deduced to be 3.00 × 1012 s-1. Due to organized nanodomains (compared with unannealed blend) formed in the annealed blend film,79 charge transfer and the relaxation of self-trapped (dynamic localized) exciton can be competitive processes. Thus, if considering a homogeneous distribution of nanodomains in this case (P3HT/PCBM 1:1 wt/wt), the decay constant, k, was roughly estimated from both charge transfer (kCT) and self-trapped exciton relaxation (kSTE, downhill migration) rates: k ) 0.5kCT + 0.5kSTE (assuming the same probability for the two competitive cases). It was in the order of 2.03 × 1012 s-1, which was localized at a time scale of 493 fs. It was very close to the value 511 ( 20 fs found by the fitting process (Table 2), supporting the assumption. The charge transfer from P3HT to PCBM occurred during the downhill relaxation of selftrapped (dynamic localized) excitons. There were other possible reasons including reduction of traps and defects for elongating fluorescence decay time in the annealed blend film. However, the carrier recombination caused by defects with several hundred ps time scale in P3HT/PCBM systems was not considered because this was beyond the capability of the experiments.15,28 The time scale with several ps in the fluorescence decay at emission of 650 nm should be attributed to the EET between conjugated segments (Table 2). EET between the segments associated with torsional relaxation processes was observed to occur in a few to tens of ps in the emission transients in P3HT solutions (Figure 3).55 However, some torsional modes could be locked or restricted in assembled films.55 Nevertheless, EET between adjacent segments was favorable and EET via resonance energy transfer was still valid in the films. EET can

undergo redistribution of the populations in the adjacent segments due to intersegment and interchain (such as π-π stacking) interaction. On the other hand, diffusion-limited exciton-exciton annihilation was reported to have a time scale of >3 ps, which is similar to that of the EET process.18 However, from the experimental results and the discussion above, diffusion-limited exciton-exciton annihilation was neglected here. The observation of EET was supported by the following fluorescence anisotropy decay.58 For these EET processes observed at emission of 650 nm, the unannealed blend film showed the shortest time scale and lowest amplitude (Table 2). There was less intersegment and interchain interaction in the unannealed film.79 Annealing significantly strengthened this EET process with several ps time scale, which was likely due to the improvement of the packing order of nanostructures in the blend films.79 The results agreed well with the observation in the steady-state measurements and performances of real photovoltaic devices. The anisotropy decay of the pristine film (Figure 7a) showed a residual value of 0.07. However, the residual anisotropy was ∼0.11 in the blend films. This was likely caused by the local field change induced by PCBM. Interestingly, the depolarization was completed in the IRF time scale for all three films (Figure 7). In a short time scale, the anisotropy decay for these films was simulated by the same two-exponential function with time constants of τ1 ∼30 ( 20 fs and τ2 ∼100 ps (an estimation from simulation), as well as only a small change for each amplitude comparing the cases to each other (r1 compared with r1, r2 compared with r2, Table 3). The amplitude (r1) of the ultrafast process was much larger than that (r2) of the slow process. This indicated that the anisotropy decay was mainly determined by the ultrafast relaxation processes, whereas energy could continuously migrate along or between P3HT chains in a long time of ∼100 ps. The ultrafast depolarization behavior (τ1, ∼30 ( 20 fs) suggests that there was a strong and ultrafast reorientation of the dipole moments of excited states at many

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TABLE 3: Deconvolution Parameters for the Anisotropy Depolarization of the Pristine P3HT and P3HT/PCBM Films at Emission of 650 nm (Simulated by Two-Exponential Functions) anisotropy decay pristine P3HT film unannealed P3HT/PCBM film annealed P3HT/PCBM film

r1

r2

0.309 0.302 0.278

0.090 0.097 0.121

spectral units of P3HT.80 There might be a strong coupling between electronic and nuclear degree of freedom, which changed the delocalization properties of excitons. The excitons could relax in the femtosecond time scale along the high energy stretching modes of the polymer backbone. Such fast dissipation of the excess energy could ease exciton migration toward (low barrier or barrier-less) dissociation sites and cause charge separation. This observation (τ1) also suggested that there was excitation delocalization associated with vibrational modes.46,58 The long time depolarization due to exciton hopping in a time scale was beyond the capability of this measurement. Figure 8 illustrates the fluorescence dynamics of a lower emissive state at 700 nm. The transients were deconvoluted with the IRF by a two-exponential function as well. Table 4 shows the deconvoluted results. The decay tendency shown in Figure 8b was similar to that observed for the emission at 650 nm. As discussed above, the decay with several ps time scale was assigned to EET between the conjugated segments. The short time decay was attributed to the charge transfer and/or the relaxation of the self-trapped excitons. Considering no charge transfer in the pristine P3HT film, the relaxation of the selftrapped (dynamic localized) excitons should have a time constant of 480 fs (rate: 2.1 × 1012 s-1). Assuming that this downhill process in the unannealed film is ignorable, the strong charge transfer between P3HT and PCBM could then reach a rate of 4.2 × 1012 s-1 (with a time constant of 240 fs). Following the similar illustration of the case for the observation at 650 nm, a decay rate in the annealed film was estimated to be 3.2 × 1012 s-1. Its related time constant was ∼310 fs, which

Figure 9. Schematic illustration of several important processes, charge transfer, relaxation of self-trapped (dynamic localized) excitons, selftrapping (dynamic localization), and EET with corresponding time scales in the excited P3HT/PCBM blend films.

was close to the measured value of 290 ( 20 fs (Table 4) for the annealed film, supporting the illustration. This investigation suggested that there were different charge transfer rates at different excited states. Charge transfer occurred faster at the lower excited state (700 nm), indicating a stronger electronic and vibrational coupling. From the analysis above, two important events with fs time scales were observed by FFU (Figure 9). The charge transfer processes occurred in a time scale of 2.5 × 1012 s-1) with multiple charge injection states in P3HT and mostly dominated the ultrafast fluorescence quenching in the unannealed film. However, it was accompanied by the competitive ultrafast downhill relaxation of the self-trapped (dynamic localized) excitons in the annealed film because well aligned nanodomains were restored (annealing induced P3HT to push adjacent PCBM molecules away).79 3.5. Device Fabrication and Characterization. To investigate the dependence of photovoltaic effects on annealing based ultrafast charge transfer processes, we have constructed and compared solar cell performance using unannealed and annealed P3HT/PCBM films. The cell structure is illustrated in Figure

Figure 8. Fluorescence dynamics of pristine P3HT film and P3HT/PCBM (1:1 wt/wt) films with emission of 700 nm (∼1.8 eV): (a) actual fluorescence decay (empty circles, experimental data; solid lines, fitting data; red lines, IRF); (b) normalized fluorescence decay (thin lines, smoothed experimental data; thick lines, guiding eyes; red lines, IRF).

TABLE 4: Decay Parameters of the Fluorescence Dynamics of the Pristine P3HT and P3HT/PCBM Films at Emission of 700 nm (Fitted by Two-Exponential Functions) annealed P3HT/PCBM film unannealed P3HT/PCBM film pristine P3HT film

τ1 (fs)

A1

τ2 (ps)

A2

290 ( 20 240 ( 20 480 ( 25

0.33 ( 0.02 0.55 ( 0.01 0.46 ( 0.01

6.5 ( 0.10 5.1 ( 0.20 5.9 ( 0.20

0.67 ( 0.02 0.45 ( 0.01 0.54 ( 0.01

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Figure 10. Schematic structure of P3HT/PCBM heterojunction solar cells. Figure 12. Atomic force microscopy (AFM) images of the (a) unannealed and (b) annealed (150 °C for 5 min) P3HT/PCBM blend films (imaging dimensions: 1 µm × 1 µm).

atomic force microscopy (AFM) was used to characterize both unannealed and annealed P3HT/PCBM blend films. As shown in Figure 12, the nanoscale phase separation occurred for both unannealed and annealed films, whereas the latter has formed a well organized P3HT/PCBM nanocrystalline structure, leading to organized polymer nanodomains for more efficient carrier transport.81 This was consistent with the observation via the above optical spectroscopy techniques. A similar change in the morphology and crystalline nanodomain formation was also observed by others in the films.82-85 Savenije reported that, upon annealing, the transmission electron microscopy (TEM) images showed that the crystalline fibrils consisting of a more ordered packing of P3HT chains were formed, which was also reflected in a significant increase of photoconductivity from 0.0056 cm2/ (V · s) for the unannealed sample to 0.044 cm2/(V · s) after annealing due to enhancement of hole mobility in the crystalline domains.83 This agreed with the observation of a longer ps time scale for the EET process in the annealed blend film than those in the unannealed one. This was originated from a significant enhancement of intersegment and interchain interaction in the well organized nanodomains. In addition, the fill factor was increased from 0.44 to 0.56, in agreement with the previously reported results for P3HT/PCBM blends.86,87 Post annealing could also reduce the series resistance (Rs) caused by a significant increase of interfacial area between the active layer and the Al cathode.87 Figure 11. (a) Current density-voltage (J-V) plots and (b) incident photon to electron conversion efficiency (IPCE) of the P3HT/PCBM photovoltaic devices fabricated using unannealed and annealed blend films.

TABLE 5: Comparison of Photovoltaic Performances of Annealed and Unannealed P3HT/PCBM Solar Cells annealed unannealed

Voc (V)

Jsc (mA/cm2)

FF

η (%)

IPCE (%)

0.61 0.60

10.52 8.07

0.55 0.44

3.55 2.14

72.3 57.0

10. The cell with post annealing treatment exhibited a higher short circuit current density (JSC) with a value of 10.52 mA/ cm2 than a value of 8.07 mA/cm2 for the unannealed one, as shown in Figure 11a. This was consistent with improvement of incident photon to carrier conversion efficiency (IPCE) from 57 to 72.3% after thermal annealing (Figure 11b). Table 5 lists the comparison of these P3HT/PCBM solar cell parameters in terms of open circuit voltage (Voc), short circuit current density (JSC), fill factor (FF), energy conversion efficiency (η), and IPCE. To study reasons for the improved Jsc and IPCE, the P3HT/ PCBM composite morphology was investigated as well. The

4. Conclusions In conclusion, we have studied the dynamics of photoinduced initial processes in unannealed and annealed P3HT/PCBM blend films by the FFU technique. For comparison, some mechanisms, including EET processes, torsional relaxation, and downhill relaxation of self-trapped excitons, have been analyzed for the fluorescence dynamics of the P3HT solution, pristine, and the blend films as well. The fluorescence dynamics of the P3HT solution showed wavelength dependence and indicated that EET and/or torsional relaxation processes occurred. Exciton density within P3HT solid films still followed exponential decay instead of the power law decay in a finite system. There was an ultrafast exciton quenching rate at the P3HT/PCBM interface, providing evidence that exciton-exciton annihilation did not dominate the fluorescence quenching process in solid films. The excitonic quenching (or charge transfer) was the dominant process in the fs time scale in unannealed blend film. Due to more organized nanodomains formed in the annealed blend film, the charge transfer (334 fs) and downhill relaxation of dynamic localized (self-trapped) excitons (942 fs) were competitive processes with emission of 650 nm. Charge transfer occurred during the downhill energy relaxation of self-trapped (dynamic localized)

P3HT/PCBM Blend Films excitons. The decay transients at different wavelengths (650 and 700 nm) in the annealed film suggested that there were different charge transfer rates at different excited states. Charge transfer occurred faster at the lower excited state (700 nm) and indicated a stronger electronic and vibrational coupling in the annealed P3HT/PCBM film. The anisotropy decay was mainly determined by the ultrafast process, suggesting the strong and ultrafast reorientation of molecular dipole moments. There could be a strong coupling between electronic and nuclear degree of freedom, which changed the delocalization properties of the excitons. The improved photovoltaic performance was mainly attributed to improved carrier transport caused by the well organized and crystallized nanodomains, which agreed well with the fs time-resolved optical spectroscopy observations and previous reports using other methods, especially other ultrafast spectroscopy techniques.79 This effort demonstrates that the FFU technique can provide some imperative information to understand the thermal annealing effects on ultrafast processes in P3HT/PCBM blend films. Acknowledgment. X.Y. and Q.Q. acknowledge the financial support from the NSF (ECCS-0723114 and ECCS-0950731), NASA EPSCoR (NNX09AP67A), South Dakota Space Grant Consortium, and South Dakota NSF EPSCoR/PANS program. We also acknowledge Dr. M. F. Baroughi and Dr. V. Bommisetty, Department of Electrical Engineering & Computer Science, South Dakota State University, for their great help in the measurements of J-V, IPCE, and AFM images. References and Notes (1) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107 (4), 1324. (2) Sun, Q.; Park, K.; Dai, L. J. Phys. Chem. C 2009, 113 (18), 7892. (3) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (4) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (5) Qiao, Q.; Xie, Y.; McLeskey, J. T. J. Phys. Chem. C 2008, 112 (26), 9912. (6) Taranekar, P.; Qiao, Q.; Jiang, H.; Ghiviriga, I.; Schanze, K. S.; Reynolds, J. R. J. Am. Chem. Soc. 2007, 129 (29), 8958. (7) Mondal, R.; Ko, S.; Norton, J. E.; Miyaki, N.; Becerril, H. A.; Verploegen, E.; Toney, M. F.; Bre´das, J. -L.; McGehee, M. D.; Bao, Z. J. Mater. Chem. 2009, 19, 7195. (8) Mondal, R.; Miyaki, N.; Becerril, H. A.; Norton, J. E.; Parmer, J.; Mayer, A. C.; Tang, M. L.; Bre´das, J.- L.; McGehee, M. D.; Bao, Z. Chem. Mater. 2009, 21 (15), 3618. (9) Chen, H.-Y; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649. (10) Park, S. H; Roy, A.; Beaupre´, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297. (11) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (12) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 083506. (13) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (14) Darling, S. B. Energy EnViron. Sci. 2009, 2, 1266. (15) Hwang, I.-W.; Moses, D.; Heeger, A. J. J. Phys. Chem. C 2008, 112 (11), 4350. (16) Piris, J.; Dykstra, T. E.; Bakulin, A. A.; van Loosdrecht, P. H. M.; Knulst, W.; Trinh, M. T.; Schins, J. M.; Siebbeles, L. D. A. J. Phys. Chem. C 2009, 113 (32), 14500. (17) Shaw, P. E.; Ruseckas, A.; Samuel, I. D. W. AdV. Mater. 2008, 20, 3516. (18) Trotzsky, S.; Hoyer, T.; Tuszynski, W.; Lienau, C.; Parisi, J. J. Phys. D: Appl. Phys. 2009, 42, 055105. (19) Kraabel, B.; Lee, C. H.; McBranch, D.; Moses, D.; Sariciftci, N. S.; Heeger, A. J. Chem. Phys. Lett. 1993, 213, 389. (20) Nelson, J.; Choulis, S. A.; Durrant, J. R. Thin Solid Films. 2004, 451-452, 508. (21) Kraabel, B.; Hummelen, J. C.; Vacar, D.; Moses, D.; Sariciftci, N. S.; Heeger, A. J.; Wudl, F. J. Chem. Phys. 1996, 104, 4267.

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