Primary Dynamics of Exciton and Charge ... - ACS Publications

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Primary Dynamics of Exciton and Charge Photogeneration in Solvent Vapor Annealed P3HT/PCBM Films Wei Zhang,†,‡ Rong Hu,‡ Dan Li,‡ Ming-Ming Huo,‡ Xi-Cheng Ai,‡ and Jian-Ping Zhang*,†,‡ †

Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin, 150001, People's Republic of China ‡ Department of Chemistry, Renmin University of China, Beijing, 100872, People's Republic of China S Supporting Information *

ABSTRACT: The primary dynamics of exciton and charge photogeneration in the neat P3HT (poly(3-hexylthiophene)) and the blend P3HT/PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) films were investigated by the use of nearinfrared femtosecond transient absorption spectroscopy with varying excitation photon energy and fluence. The effects of film morphology were examined by comparing the solvent vapor annealed (SVA) and the carbon disulfide (CS2) cast films. Spectroelectrochemistry was employed to characterize the cationic polaron P3HT•+ to facilitate the assignments of the transient spectra. Time-resolved spectroscopy revealed two different types of polarons, the delocalized (DP, absorbing over 630−830 nm) and the localized (LP, 750−1100 nm) ones inhabiting the crystalline and the disordered P3HT phases, respectively. In addition, the characteristic absorption of the longsought anionic polaron P3HT•− is proposed to be a broad-band spectrum extending up to ∼1300 nm with a maximum at ∼1080 nm. For SVA neat P3HT films under the low-fluence photoexcitation, ∼1012 photons·cm−2·pulse−1, the overall polaron yield (DP + LP) at 1.45 ns was determined to be 90%, Mw = 43 462, Mw/Mn = 2.9) and PCBM (∼99%) were used as received from Aldrich. For film preparation, neat P3HT or mixed P3HT/PCBM (1:1, wt) were dissolved in o-dichlorobenzene (o-DCB) or CS2 to obtain the solutions of 20 mg mL−1 (2%, wt). Quartz substrates were treated by ultrasonication in detergent and washed successively with deionized water, acetone, ethanol, and isopropyl alcohol, and on which the neat P3HT or the blend P3HT/PCBM films were spin-coated (1000 rpm; 30 s). The SVA treatment was performed by keeping the coated substrates in Petri dishes under the o-DCB atmosphere for 1 h. The films coated from the CS2 solutions were dried in an open atmosphere to obtain the unannealed photoactive layer referred to as the CS2-cast film. The preparations were conducted in a glovebox filled with argon (oxygen concentration below 0.1 ppm). The typical thickness of the photoactive layer was ∼190 nm as determined with an Alpha-Skep surface profiler (KLATencor). 2.2. Morphological and Steady-State Spectroscopic Characterization. AFM images of the films were taken with the tapping mode in air at room temperature (Veeco, D3100). UV−visible absorption spectra were recorded on a Cray-50 (Varian) absorption spectrometer. SEC measurements for the neat or the blend films were carried out with a trielectrode configuration consisting of a working electrode (P3HT or 4299

dx.doi.org/10.1021/jp211653x | J. Phys. Chem. C 2012, 116, 4298−4310

The Journal of Physical Chemistry C

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P3HT/PCBM films on ITO glass), a counter electrode (platinum wire), and a reference one (Ag+/Ag, AgNO3 (0.01 M) and hexafluorophosphate (0.1 M) in acetonitrile), which were assembled in a quartz cell with an effective optical path length of 2 mm. The electrolyte solution was hexafluorophosphate (0.1 M) in acetonitrile. The applied potential across the reference and the working electrodes was 0.5 V. The electrochemical cell was mounted on a spectrometer (Hitachi, U-4100) with a spectral response range of 200−2200 nm. 2.3. Time-Resolved Spectroscopic Measurements. The apparatus with a temporal resolution of ∼160 fs is briefly described below. An optical parametric amplifier (OPA-800 CF-1, Spectra Physics) pumped by a regenerative amplifier (SPTF-100F-1KHPR, Spectra Physics) provided the actinic laser pulses at desired wavelengths (∼120 fs, full width at halfmaximum). A white light continuum probe (430−1400 nm) was generated from a 3 mm thick sapphire plate and was detected after interrogating the excited sample either by a CCD detector (Spec-10:400B/LN) for the visible region or by an InGaAs detector (OMA-V, Princeton Instruments) for the NIR region attached to individual imaging spectrographs (SpectraPro 2300i, USA). To ensure that each laser shot excites the sample relaxed fully from the previous excitation, the laser system was run at a repetition rate of 100 Hz. To avoid photodegradation, the films sandwiched with quartz slices were kept in vacuum. A mechanical chopper (model 75158, Newport) was set in the pump beam to regulate pump “on” and “off” for a pair of sequential actinic pulses. To improve the signal-to-noise ratio, each transient spectrum was obtained by averaging 200 individual measurements, and the typical detection sensitivity of the difference absorption (ΔOD) was better than 10−4. The time-resolved absorption spectra were corrected against group velocity dispersion. All measurements were carried out at room temperature (296 K).

Figure 2. UV−visible absorption spectra of SVA (red) and CS2-cast (black) neat P3HT films, and those of SVA (blue) and CS2-cast (green) blend P3HT/PCBM films (1:1, w/w). The spectra are normalized at 550 nm. Arrows point to the blue-edge (460 nm) and the red-edge (620 nm) excitation wavelengths used in the timeresolved measurements.

macromolecules with an interplane interchain distance of 0.38 nm and the resultant high planarity of the polythiophene backbone are responsible for the prominence of the 0−0 transition.29,49,50 In the time-resolved spectroscopic studies, we have made use of the SVA and the CS2-cast films to examine the morphological effects on the primary dynamics of excitation and charge photogeneration. 3.2. Spectral Characterization of Polaron P3HT•+ in the Neat and the Blend Films. To assist the spectral assignments of the cationic species P3HT•+ (hereafter referred to as polaron unless otherwise specified) to be probed with time-resolved spectroscopy, we performed SEC measurements on the neat and the blend films, and the results are depicted in Figure 3a. For comparison, Figure 3b shows the photoinduced transient absorption spectra of the SVA blends recorded ∼0.5 ns after the pulsed photoexcitation at 620 and 530 nm, respectively. The SEC spectra of both neat and blend films in Figure 3a exhibit the electrooxidation-induced bleaching of the groundstate absorption with distinct vibronic bands at 510, 550, and 605 nm. To the longer wavelength side of the ground-state bleaching, the broad-band absorptions denoted as P1 and P2, respectively, are attributed to the HOMO-to-SOMO and SOMO-to-LUMO electronic transitions of polarons.17,45 Note that the spectral region around 1200 nm between the P2 and the P1 bands is nearly transparent, allowing the singlet exciton to be detected with minimized interference from the polaron absorption (vide infra). It is also worthy of noting that the SEC spectra probe the polarons thermally equilibrated among the ordered and disordered polymer phases, despite somewhat preferential electrooxidization of the crystalline P3HT, as indicated by the slightly more prominent 0−0 bands with reference to the UV−visible spectra (Figure 2). The transient spectra in Figure 3b suggest that photoexcitation preferentially bleaches the crystalline P3HT phase, as indicated by the sharp and negative vibronic bands. Particularly, the photoexcitation at 620 nm, compared with that at 530 nm, is more selective to the crystalline phase, as reflected by the clearer and stronger 0−1 and 0−2 bands. Interestingly, under the 620 nm excitation, the polaron absorption in 630−830 nm is considerably stronger than that in 830−1100 nm, whereas under the 530 nm excitation, the intensities of these sub-bands are comparable. Since the crystalline phase is preferentially excited at 620 nm, whereas both ordered and disordered phases

3. RESULTS 3.1. Morphological Characterization of the Blend P3HT/PCBM Films. Figure 1 shows the representative AFM images of the SVA and the CS2-cast blend films. The surface of the SVA film is substantially coarser (1.49 nm, root-meansquare (rms), Figure 1a) than that of the CS2-cast one (0.43 nm, rms, Figure 1c), indicating the nanoscale phase separation of P3HT and PCBM upon SVA treatment, as also supported by the distinct difference in the phase contrast (panel b vs panel d in Figure 1). Notably, the CS2-cast film mainly exhibits round shape PCBM aggregates with rather weak phase contrast, which is suggestive of poorer phase separation. We also characterized the neat films, and the SVA film with fibrillar nanostructures was much coarser than the CS2-cast one (6.58 vs 1.02 nm rms, Figure S1, Supporting Information). Thus, for either the blend or the neat films, the SVA treatment promoted the formation of P3HT nanocrystallites via self-organization.41,42 The significant improvement of the P3HT crystallinity upon SVA treatment is further manifested by the results of X-ray diffractometry; that is, the diffraction peaks at 2θ = 5.3° of the SVA films are much stronger than those of the CS2-cast ones (Figure S2, Supporting Information). In addition, the in-plane interchain separation was derived to be 1.67 nm, consistent with the documented values of 1.3−2.7 nm.43−45 Figure 2 shows the UV−visible absorption spectra of the SVA and the CS2-cast films. As established both experimentally and theoretically, the 0−0 vibronic features peaking at ∼600 nm originate from the crystalline phase of P3HT.46−48 Specifically, the π-stacked 4300



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absorption spectroscopy to examine the effects of film morphology and excitation photon energy on the primary exciton and polaron dynamics. The NIR probe in 830−1350 nm allows us to differentiate the exciton and the polaron dynamics with minimized interference with each other, and the improved multicolor detection sensitivity (ΔOD < 10−4) enables us to apply a photon fluence as low as ∼1012 photons·cm−2·pulse−1. 3.3.1. Low Excitation Fluence. In Figure 4, the top and the bottom rows, respectively, show the time-resolved spectra after photoexcitation at the red- (620 nm) and the blue-edge (460 nm) of the ground-state absorption. As seen from Figure 4a,b for the SVA and the CS2-cast neat films, respectively, the transients spanning 830−1350 nm appeared promptly at the delay times of −0.08, 0.0,53 and 0.16 ps. These broad features peaking at ∼1200 nm are, therefore, safely assigned to the Sn ← S1 absorption of the P3HT singlet exciton, consistent with the recent NIR transient absorption studies on pristine P3HT films.18,31 For the SVA film, the corresponding kinetics at 1000 or 1200 nm in Figure 5a obey the monoexponential decay with a time constant of 290 ps, which agrees with the exciton lifetime of 200−600 ps determined by the use of time-resolved fluorescence spectroscopies.38,54,55 On the other hand, for the CS2-cast film, the exciton kinetics in Figure 5b exhibits a faster decay component with a time constant of 5 ps superimposed on a slower decay with 300 ps. This initial faster decay with a time scale comparable to that of exciton diffusion is attributed to the singlet−singlet (S−S) exciton annihilation. Note that, in Figure 4a,b, the transients at Δt = 1.45 ns with the absorption maxima at ∼1080 nm do not resemble the characteristic polaron spectra (Figure 3), and they are also not like the reminiscence of the Sn ← S1 spectra. We shall consider their spectral assignments in the Discussion section. Nevertheless, these results prove that, for either SVA or CS2-cast P3HT films, the low-fluence red-edge photoexcitation yields a negligible amount of polarons. In Figure 4c,d, the spectral dynamics under the blue-edge excitation are dominated by the Sn ← S1 absorption, similar to the case of red-edge excitation. However, the transients at Δt = 1.45 ns, which are relatively stronger to the shorter probing wavelengths, seem analogous to the characteristic spectra of polarons (see Figure S3b, Supporting Information, for a close comparison of the nanosecond transients to the SEC spectra), as further supported by the transient spectra (Δt = 1.45 ns) recorded under higher fluence photoexcitation (vide infra). The corresponding decay kinetics in Figure 5c,d exhibit a 5 ps initial decay component with substantial amplitudes, which is ascribable to the S−S exciton annihilation. Taking together the results of spectral dynamics and corresponding kinetics, we conclude that, for the SVA films under low-fluence photoexcitation, the polaron production is excitation-wavelength-dependent; that is, the polaron yield is negligible under the red-edge excitation and is ∼4% under the blue-edge excitation. Here, the value of ∼4% was estimated from the amplitude ratio of the transient at 1000 nm (Δt = 1.45 ns) over that at 1200 nm (Δt = 0.16 ps), for which comparable extinction coefficients were assumed for a polaron (LP) at 1000 nm and singlet exciton at 1200 nm.56 In view of the DP-to-LP population ratio of 0.8, the overall polaron yield (DP + LP) under the photoexcitation at 460 nm turns out to be ∼7%. Similar values were obtained for the CS2-cast films; that is, the overall polaron yields were negligible or ∼6%, respectively, under the photoexcitation at 620 or 460 nm. Thus, the direct

Figure 3. (a) Steady-state spectroelectrochemistry (SEC) of the SVA annealed neat P3HT film (black) and blend P3HT/PCBM (1:1, w/w) film (red). They were taken as the difference between the absorption spectra with and without the applied potential of 0.5 V and normalized at 500 nm. P1 and P2 denote the two characteristic absorption bands of the polaron P3HT•+. (b) Transient spectra at the delay time of Δt ∼ 0.5 ns for the SVA annealed P3HT/PCBM film recorded after photoexcitation at 530 nm (black) or 620 nm (red). DP and LP denote the sub-bands of the P2 ensemble. The spectra normalized at 650 nm were obtained by averaging the transients in the proximity of Δt ∼ 0.5 ns when the interference from exciton absorption vanished completely.

are excited at 530 nm, we attribute the shorter wavelength absorption (630−830 nm) to the delocalized polaron (DP) inhabiting the P3HT crystallites, whereas the longer wavelength absorption (830−1100 nm) to the localized polaron (LP) residing on the disordered polymer molecules. The polaron spectra in Figure 3b can be decomposed into the LP spectrum by subtracting the 620 nm spectrum from the 530 nm one, and the DP spectrum by operating reversely (Figure S3a, Supporting Information), and the resultant spectra agree well with the reported transient spectra of localized and delocalize polarons recorded in the microsecond regime for the blend films.19,20 Taking into account the extinction coefficients of ε700 nm = 35 000 M−1·cm−1 and ε1000 nm = 30 000 M−1·cm−1 for DP and LP, respectively,20 we estimated from the polaron spectra in Figure 3b the DP-to-LP population ratio to be 1.2 (0.8) under the 620 nm (530 nm) photoexcitation. Interestingly, the dominance of the DP absorption after 620 nm excitation retains up to Δt ∼ 0.5 ns, implying a higher HOMO energy of the crystalline P3HT than that of the disordered P3HT, in accordance with the relatively lower ionization potential of the crystalline phase.30,51,52 3.3. Primary Exciton and Charge Dynamics in the Neat P3HT Films. We carried out femtosecond time-resolved 4301



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Figure 4. Transient spectra at the indicated delay times recorded after photoexcitation at 620 and 460 nm, respectively, for (a, c) the SVA and (b, d) the CS2-cast neat P3HT films. The excitation photon fluence was 2.0 × 1012 photons·cm−2·pulse−1. No smoothing was applied to the transients.

Figure 5. Kinetics at the indicated probing wavelengths plotted from the corresponding spectral dynamics of Figure 4 for (a, c) the SVA annealed and (b, d) the CS2-cast neat P3HT films. Solid lines are fitting curves based on mono- or biexponential model functions. The kinetics traces are vertically shifted for clearness.

3.3.2. Higher Excitation Fluence. In Figure 6, the instantaneously appeared transient absorptions are originated from the P3HT singlet exciton. Their spectral dynamics differ distinctively from those under the low-fluence excitation: For both SVA and CS2-cast films and under either red- or blue-edge photoexcitation, a sizable transient signal appeared promptly in the shorter wavelength region of 830−1000 nm, as also seen clearly from the corresponding kinetics in Figure S4 (Supporting Information). Notably, all of the transients at Δt = 1.45 ns in Figure 6 resemble closely the characteristic P2 band of the SEC spectra in Figure 3a. The overall polaron yields for the SVA and the CS2-cast films under the red-edge

polaron photogeneration in the neat P3HT film is found to be dependent strongly on the photon energy, but weakly on the film morphology, in accordance with the neat-film polaron dynamics under moderate excitation photon fluence reported by Guo et al.18 Here, we note that the estimation of polaron yield under the blue-edge photoexcitation is robust: Although the effect of S−S exciton annihilation could not be avoided even under a fluence as low as 2.0 × 1012 photons·cm−2·pulse−1, the polaron yield of 6−7% remained unchanged on varying the fluence over 2.0 × 1012 to 5.5 × 1013 photons·cm−2·pulse−1, as also reported in ref 18. Therefore, the influence from the S−S annihilation was minimized. 4302



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Figure 6. Transient spectra at the indicated delay times recorded after photoexcitation at 620 and 460 nm for (a, c) the SVA and (b, d) the CS2-cast neat P3HT films. The excitation photon fluence was 5.5 × 1013 photons·cm−2·pulse−1. No smoothing was applied to the transients.

Figure 7. Transient spectra at the indicated delay times recorded after photoexcitation at 620 and 460 nm for (a, c) the SVA and (b, d) the CS2-cast P3HT/PCBM (1:1, w/w) films. The excitation photon fluence was 5.0 × 1012 photons·cm−2·pulse−1. No smoothing was applied to the transients.

excitation were estimated to be ∼8% and ∼11%, respectively, and the corresponding values under the blue-edge excitation were ∼7%. Upon the red-edge excitation with the higher fluence of 5.5 × 1013 photons·cm−2·pulse−1, the polaron yields are fairly large, which is in drastic contrast to the negligible polaron production upon the red-edge excitation with 2.0 × 1012 photons·cm−2·pulse−1. This is to be explained by the S−S exciton annihilation in terms of the relatively large delocalization size of the exciton created by the 620 nm photoexcitation. The above results show that the higher fluence photoexcitation yields a significant amount of polarons at Δt = 1.45

ns irrespective of the film morphology and photon energy. Particularly, the significant polaron production under the rededge excitation is obviously due to the S−S exciton annihilation and the successive autoionization:57−60 S1 + S1 → S0 + Sn and Sn → P3HT•+. Direct formation of a polaron from the higherlying singlet excited state (Sn) was previously reported for P3HT,18 and for other conjugated polymers, such as polyfluorene.61 We note that, within the initial 100 ps, the polaron dynamics over 830−1000 nm are overwhelmed by the exciton absorption in the same region, and consequently, the differentiation of polarons and excitons is rather hard. This is 4303



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Figure 8. Kinetics at the indicated probing wavelengths plotted from the corresponding spectral dynamics in Figure 7 for (a, c) the SVA and (b, d) the CS2-cast P3HT/PCBM (1:1, w/w) films. Solid lines are fitting curves based on mono- or biexponential model functions. Kinetics traces are vertically shifted for clearness.

Figure 9. Transient spectra at the indicated delay times recorded after photoexcitation at 620 and 460 nm for (a, c) the SVA and (b, d) the CS2-cast P3HT/PCBM (1:1, w/w) films. The excitation photon fluence was 5.0 × 1013 photons·cm−2·pulse−1. No smoothing was applied to the transients.

morphologies and excitation photon energy. Herein, the spectral dynamics and the kinetics under the low and the higher fluences are to be characterized. 3.4.1. Low Excitation Fluence. In Figure 7a, the transient spectra in the rise phase (−0.08, 0.00, and 1.60 ps) upon the red-edge excitation show sizable absorption in 830−1000 nm, which are not seen for the neat films under similar excitation conditions. As the delay time elapsed, the broad-band exciton absorption peaking at ∼1170 nm decayed rapidly and the absorption in 830−1000 nm became prominent after Δt ∼10 ps and eventually dominant. From the corresponding kinetics at 860 or 1000 nm in Figure 8a, the initial polaron dynamics

especially true when the process of polaron formation entangles with the diffusion-limited exciton annihilation. On the other hand, upon the blue-edge excitation, the direct photogeneration of polarons and the nearly unchanged yield over the fluence regime of 2.0 × 1012 to 5.5 × 1013 photons·cm−2·pulse−1 strongly supports the mechanism of “hot” exciton dissociation;17,18 that is, the lowest excited state dissociates into polarons with the assistance of the vibrational excess energy. 3.4. Primary Exciton and Charge Dynamics in the Blend P3HT/PCBM Films. Having examined the primary dynamics of the neat P3HT films, we now come to inspect those of the P3HT/PCBM blends with varying film 4304



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row, we see that the relative contribution of the prompt polaron formation, as measured by the absorption amplitude in 830− 1000 nm, is considerably higher in the lower row, suggesting that the high excitation photon energy favors the prompt polaron formation. (iii) Comparing the kinetics traces of the SVA films (Figure S5a,c, Supporting Information) to those of the CS2-cast films (Figure S5b,d, Supporting Information), we notice that, upon SVA treatment, the initial fast decay becomes more pronounced, which is indicative of more sever S−S exciton annihilation. This is understandable in view of the higher delocalization extent and/or the higher exciton diffusion constant of the crystalline P3HT phase (vide infra). 3.4.3. Estimation of the Exciton Delocalization Sizes. The kinetics of Sn ← S1 exciton absorption at 1200 nm was probed with minimized interference from the polaron absorption, which allows us to derive the exciton delocalization size from the photon-fluence-dependent maximal exciton absorption at Δt = 0.16 ps. At this delay time, the diffusion of excitons is negligible, and hence any annihilation must be caused by direct contact of excitons. Specifically, the exciton delocalization sizes of the SVA blends were obtained from the threshold photon fluence leading to the appearance of S−S exciton annihilation (cf. Figure S6, Supporting Information). They are found to be slightly dependent on the excitation wavelengths, that is, 7.6 and 6.6 nm upon the photoexcitation at 620 and 460 nm, respectively. These values for the blend P3HT/PCBM films are considerably smaller than those obtained for thermally annealed neat P3HT films,18 that is, 13.4 and 8.6 nm under the photoexcitation at 600 and 400 nm, respectively.

cannot be readily perceived owing to the superposition of exciton absorption. However, the kinetics at 790 nm does exhibit a distinct rise phase with a time constant of 10 ps superimposed on an instantaneous onset. The above results of spectral dynamics and kinetics indicate that, for the SVA blend, the red-edge excitation induced both instantaneous and delayed polaron formation. Because the kinetics at 790 and 860 nm preferentially probe the mixed DP/ LP and the pure LP, respectively, the delayed (instantaneous) polarons must originate from the delocalized (localized) polarons residing in the crystalline (disordered) P3HT phase. Importantly, the red-edge photoexcitation with low fluence resulted in the prompt polaron formation in the blend P3HT/ PCBM film, which is contradictory to a recent report on the thermally annealed P3HT/PCBM blend, where, under the 600 nm excitation, only delayed polarons were detected.19 This discrepancy is to be explained in the Discussion section. In Figure 8a, the exciton kinetics at 1200 nm decayed with a time constant of ∼10 ps, correlating intimately with the rise phase of the polaron kinetics at 790 nm. This decay-to-rise correlation is to be interpreted in terms of the exciton diffusion in the P3HT crystallite, followed by the interfacial charge separation. For the CS2-cast blend film, the prompt polaron formation with a relatively larger contribution is clearly seen from the spectral dynamics (Figure 7b) and the corresponding 790 nm kinetics (Figure 8b). The exciton decay probed at 1200 nm correlates with a time constant of ∼7 ps to the polaron rise detected at 790 nm. The apparently faster exciton relaxation with reference to the SVA annealed film may be due to the smaller P3HT crystallite that, in effect, shortens the time scale for exciton diffusion from the interior to the crystallite boundary. It is seen in Figure 7c,d that the blue-edge excitation induced prompt formation of polaron absorption in 830−1000 nm, as evidenced by the transients in the rise phase (−0.08, 0.00, and 1.60 ps). From the corresponding 790 nm kinetics for the SVA film, we saw both prompt and delayed polaron formation (Figure 8c), whereas only prompt polaron formation was detected (Figure 8d) for the CS2-cast film. In addition, the rise phase of the 790 nm kinetics in Figure 8c correlates with a time constant of ∼10 ps to the decay of the exciton absorption at 1200 nm. Assuming that the extinction coefficients of DP and LP are comparable at 790 nm, which is reasonable in view of the characteristic polaron spectra (Figure S3a, Supporting Information), the DP-to-LP population ratio of the SVA film was estimated to be 0.9 based on the amplitude ratio derived from curve fitting, in agreement with that obtained from Figure 3b (0.8). 3.4.2. Higher Excitation Fluence. In Figure 9a−d, the spectral dynamics under the higher fluence excitation illustrate clearly the instantaneous formation of polarons (LP), as evidenced by the sizable absorption in 830−1000 nm in the rise phase (Δt = −0.08 to 0.16 ps), as well as by the corresponding kinetics at 860 and 1000 nm in Figure S5 (Supporting Information). The kinetics difference with reference to those under the low-fluence excitation is summarized below. (i) Irrespective to the film morphology (SVA vs CS2-cast) or the excitation photon energy (460 vs 620 nm), all of the kinetics traces show an initial fast decay with a time constant of 0.5−1.5 ps, which were absent under the low-fluence excitation. This decay component is thus ascribed to the S−S exciton annihilation. (ii) In Figure 9, when we compare the transients at Δt = −0.08 or 1450 ps in the upper row to those in the lower

4. DISCUSSION The standard AM 1.5 terrestrial solar irradiation supplies a photon fluence of ∼1016 photons·cm−2·nm−1·s−1 over the visible spectral region,62 corresponding to a value of 105−106 photons·cm−2·pulse−1 for a laser pulse of ∼100 fs duration, which is hardly attainable in transient absorption spectroscopy. Fortunately, as demonstrated by Marsh et al.,21 the inherent photophysics of solar photocurrent generation is retained under sufficiently low excitation photon fluence, provided that the effects of bimolecular reactions, such as exciton annihilation, can be avoided.33,40 For dynamics studies, it is also crucial to spectrally identify the exciton and various charged species. In the present work, we have attempted to unravel the exciton/ polaron dynamics under the low-fluence excitation for the neat and the blend films subjected to SVA treatment. Two different types of polarons, DP and LP, were identified and the polaron photogeneration dynamics were examined with emphasis on the effects of film morphology and excitation photon energy. We now extend the discussion on these important issues and propose the characteristic absorption spectrum of the longsought radical anion, P3HT•−. 4.1. Spectral Characterization of Polarons P3HT•+and P3HT•−. The electronic absorption spectra of P3HT•+ are characterized by two broad bands referred to as P1 and P2 ranging in 1350−6000 and 600−1150 nm, respectively, as established by spectroscopic means, such as the photo- or doping-induced absorption spectroscopies17,45,56 and the ultrafast time-resolved absorption spectroscopy.18−20,30,37,63 Recently, the assignments of the spectrally superimposed transient species, polarons and excitons, have been reassessed by Guo et al. by means of time-resolved visible-to-NIR absorption spectroscopies.18−20 The present work has further consolidated 4305



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the characteristic P3HT•+ spectra of neat and blend films by using steady-state SEC and time-resolved NIR spectroscopies. We show that the photoinduced P2 band of polaron absorption at Δt = 0.5 ns (Figure 3b) constitutes two subbands, DP and LP, correlating closely with the film morphology. Under the red-edge excitation at 620 nm, the DP sub-band absorption (630−830 nm) originating from the P3HT crystallites is dominant. The above-gap excitation at 460 nm excites both crystalline and disordered P3HT phases, and hence, the entire P2 ensemble (630−1100 nm) consisting of the DP and the LP sub-bands shows up. The spectral assignments of delocalized and localized polarons hold the theoretical basis that, owing to the interaction between polaronic and neutral P3HT molecules in the crystalline phase, the DP transition energy increases considerably with respect to that of LP.17,45 In addition, previous steady-state photomodulation spectroscopic studies on the thermally annealed P3HT/PCBM blends showed the prominence of the LP sub-band under the 496 nm excitation and the entire P2 broad band under the 800 nm excitation (1.55 eV, below gap).63 The salient spectral features of different types of polarons are useful in probing the morphology-dependent polaron dynamics. It is intriguing to see the difference spectrum (ΔΔOD) of the P2 absorption between the neat and the blend SVA films, as shown in Figure 10a (similar difference spectra were obtained for other cases with varying film morphologies and excitation photon energy; see Figure S7, Supporting Information). For

the following considerations, we ascribe the ΔΔOD spectrum to the radical anion P3HT•−. In the neat films lacking the exogenous electron acceptor PCBM, the P3HT excitons in “hot” S1 or higher-lying Sn states can readily dissociate into positive and negative P3HT polarons; however, both of them are confined in the bulk P3HT phase. In the blend films, on the other hand, the excrescent electron of P3HT•− can be scavenged by the adjacent PCBM aggregates or molecules owing to their high electron affinity. Such an electron-transfer reaction must be extremely fast, and hence, the negative polaron is too short-lived to be detected in the blends. The spectral assignment of the P3HT•− draws both theoretical and experimental support: In a theoretical work on oligothiophenes,64 P3HT•+ and P3HT•− were considered to have similar spectral features owing to the electron−hole symmetry, and for polythiophenes having 16 or 19 repeating units, the maximal oscillator strengths of P3HT•− absorption were placed at the NIR wavelengths of 1100−1150 nm, in rough agreement with the ΔΔOD maximum (1080 nm). In a recent transient absorption study, the initial yields of P3HT•− were proposed to be ∼15% and ∼10% for neat P3HT and blend P3HT/PCBM films, respectively.33 Our putative characterization of P3HT•− implies that, in neat films, both negative and positive P3HT polarons live beyond ∼1 ns without recombination, most likely due to the fully delocalized nature of the conduction-band electrons in P3HT crystallites. To the contrast, in the blend films, the electrons carried by P3HT•− are completely scavenged by PCBM (Figure 11, path 1). This finding is important for theoretically characterizing the electronic properties of this semiconducting polymer, and for deepening the understanding of the mechanisms of formation and transport of mobile charges in bulk P3HT phase. 4.2. Yield of Direct Polaron Photogeneration under Above-Gap Photoexcitation in Crystalline P3HT Phase. In the bulk P3HT crystallite, the excess-energy-assisted direct polaron formation (Figure 11, path 1) is well-documented,33,65 and the contribution to the solar photocurrent is considered to be significant.31 However, there has been no consensus with the quantum yield: In the regioregular polythiophene films, the yield of interchain charge pairs was reported to be 20% as measured by transient absorption,66 and the yield of direct polaron photogeneration is ∼1.7% (λex = 410−650 nm) as determined by microwave conductivity measurements,60 or ∼30% as derived by the use of transient photomodulation spectroscopy.56 Most recently, the yields have been estimated to be 6% (λex = 400 nm) or negligible (λex = 620 nm) based on femtosecond transient absorption spectroscopy.18 We have observed for neat P3HT films the wavelength-dependent direct photogeneration of polarons with significant (∼7%) or negligible quantum yields under the blue- or red-edge photoexcitation, respectively, in excellent agreement with ref 18. This moderate yield of polaron photogeneration in the bulk P3HT phase is of practical importance in device operation. 4.3. Photon-Energy-Dependent Mechanisms of Charge Photogeneration in Crystalline P3HT Phase of Blend Film. The photon energy of 2.7 eV (460 nm) is 3 vibrational quanta above the 0−0 transition energy, whereas that of 2.0 eV (620 nm) coincides with the optical band-gap energy of the semiconducting polymer (1.9 eV). It is then expected that the higher photon energy results in vibrationally “hot” excitons in bulk P3HT phases (Figure 11, path 1) or hot CT excitons at the donor−acceptor interfaces (Figure 11, path 3), which are prone to dissociate into mobile charges. These

Figure 10. (a) Difference spectrum between the transient at Δt = 1.45 ns of the neat P3HT film (Figure 6a) and that of the blend P3HT/ PCBM film (Figure 9a) recorded after photoexcitation at 620 nm. It is proposed to be the characteristic absorption of P3HT•−. (b) Blown up transient spectra at Δt = 1.45 ns in panels (a) (black) and (b) (red) in Figure 4 for neat P3HT films under the excitation wavelength of 620 nm with a fluence of 2.0 × 1012 photons·cm−2·pulse−1. They are proposed to be the characteristic absorption of the CT exciton [P3HTδ+···P3HTδ−]* and are presented for a close comparison to the spectrum in the upper panel. 4306



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Figure 11. Schematic illustration of primary exciton and polaron dynamics initiated from the P3HT crystallite of the P3HT/PCBM blend. Processes 1, 3, and 4 constitute the prompt polaron formation, and process 2 represents the delayed polaron formation limited by exciton diffusion (7−10 ps). ED and CS stand for exciton diffusion and charge separation, respectively. (1): P3HT(S1*) + P3HT → P3HT•+ + P3HT•− (direct dissociation of hot exciton), followed by P3HT•− + PCBM → P3HT + PCBM− (interfacial electron scavenging). (2): P3HT* + P3HT → P3HT + P3HT* (ED), followed by P3HT* + PCBM → P3HT•+ + PCBM− (interfacial CS). (3): P3HT* + PCBM → P3HT•+ + PCBM− (interfacial CS). (4): [P3HTδ+···P3HTδ−]* + PCBM → P3HT•+ + PCBM− (interface-assisted CS of CT exciton).

existing crystallites is expected. In fact, thermal annealing had been shown to induce severe interpermeation of P3HT and PCBM materials.68 With the minimized noncrystalline phase between P3HT and PCBM crystallites, the excitons with an average size of ∼7.6 nm (19 repeating units) induced by the 620 nm excitation in P3HT crystallites and especially those in the proximity of crystallite boundaries can sense, without migration, the PCBM acceptors to dissociate instantaneously (Figure 11, path 3). The delayed polaron formation under the 600 nm photoexcitation in ref 19 was derived by subtracting the kinetics at 1200 nm (exciton) from that at 1000 nm (polaron + exciton). In the present work, this was directly probed from the kinetics at 790 nm under the low-fluence photoexcitation at 620 nm with minimized interference from the exciton absorption. Similar rise phase of polaron formation in the kinetics at ∼720 nm under above-gap photoexcitation was also detected directly for the P3HT/PCBM blends.21,30 Under the red-edge photoexcitation, the present work has demonstrated both prompt and delayed polarons in the SVA blend films, but none of them were detected in the SVA neat films. This immediately points to the essential role of the P3HT-PCBM interfaces in driving exciton dissociation. Here, the interfaces compromise those between the P3HT crystallite and the isolated or aggregated PCBM molecules, which are directly accessible by the del ocal i z ed e xci t on P3HT * o r t h e CT ex c i t o n [P3HT+δ···P3HT−δ]* in the P3HT crystallite in the vicinity of the boundary (Figure 11, path 3 or 4). The involvement of the CT exciton of P3HT, [P3HT+δ···P3HT−δ]*, is evidenced by the transient spectra at Δt = 1.45 ns for neat P3HT films after the 620 nm excitation with low fluence (Figure 10b) and is strongly supported by the complete disappearance of this unique spectral feature in the blend film in the presence of PCBM (Figure 7a,b). Here, we note that the signature spectra in Figure 10a,b were obtained in different ways and under different excitation conditions. In Figure 10a, the spectrum is the difference between the transient of the neat P3HT film and that of the P3HT/PCBM blend, and the above-gap excitation leads to direct photoionization in P3HT crystallites. In Figure 10b, however, the spectra are the original transients of neat P3HT films, and they were recorded under the low-fluence and red-edge excitation, which does not generate P3HT polarons. The above assignments of P3HT polarons and CT excitons,

processes were indeed observed for the neat or the blend films. Particularly, the electron carried by P3HT•− in the crystalline phase, which lives considerably longer (>1 ns) than the neutral exciton (∼300 ps), can be scavenged efficiently by PCBM in the blend films. In fact, it is shown that addition of 5% PCBM is sufficient to yield a large amount of long-lived polarons at 1 ns that are comparable to those of the 50% blend.33 On the other hand, the photoluminescence efficiency of neat P3HT film is inherently low (∼2%),31 which may be due to the optically forbidden character of the lowest-lying excited state of the Haggregated P3HT17,29,46 and the aforementioned prompt bulk exciton dissociation. In the blend films, these together with the interfacial exciton dissociation, taking place within the duration of optical pulse (∼100 fs),19,21,30,33,67 are responsible for the nearly unitary photoluminescence quenching efficiency. Besides the prompt polaron photogeneration that is free from the exciton diffusion, the mechanisms of exciton-diffusionlimited polaron formation (Figure 11, path 2) have been demonstrated by Guo et al. for the thermally annealed P3HT/ PCBM blends.19 These authors concluded that the optical excitation at 400 nm leads to both prompt and delayed polaron formation, whereas the excitation at 600 nm induces merely delayed polarons. In addition, they fit the primary polaron dynamics nicely to a morphological model featuring a noncrystalline polymer/PCBM phase (disordered and/or amorphous, ∼4 nm in dimension) between the P3HT and the PCBM crystallites. Within this framework, the excitons created inside the P3HT crystallite by the 600 nm photoexcitation have to migrate to meet the donor−acceptor interface before dissociation. The present work finds both instantaneous and delayed polaron formation for the SVA blend films even under the rededge excitation (section 3.4), implying that the optical excitation with the photon energy comparable to the optical band gap of P3HT can promote the prompt polaron formation. Herein, we explain the discrepancy from ref 19 in terms of the morphological difference between the thermally annealed and the SVA blends: In the SVA blends, the dimension of the noncrystalline intermediate phase is significantly thinner than that of the thermally annealed blends, which is understandable as the room-temperature SVA is thermodynamically much gentler than the annealing at elevated temperature (100−150 °C). Therefore, a lesser extent of phase mixing between the 4307



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electron scavenging of P3HT•− or P3HTδ− by the interfacial PCBM is emphasized, and the direct photoionization and the CT-exciton creation processes are shown to be excitationwavelength-dependent. On the other hand, the process of delayed polaron formation is governed by the relatively slow exciton diffusion (∼10 ps) in the interior P3HT crystallite, followed by the ultrafast interfacial charge separation (path 2), which is active upon either the red- or the blue-edge photoexcitation. It is important to point out that, in the disordered or amorphous phases of the blend film where P3HT and PCBM are contacted intimately, large amounts of instantaneously formed polarons are generated with a rather high quantum yield upon the above-gap photoexcitation, as discussed in detail in the literature.19,31,56 However, the interfaces in the random P3HT phases are also considered to be the polaron traps and carrier blocks resulting in the polaron recombination and the decrease of free charge mobilities. The afore-proposed mechanisms of charge photogeneration are important for a deeper understanding of the structure−function relationship of light conversion, as well as for modeling the device physics. Furthermore, the present work proves that SVA with respect to thermal annealing is advantageous in reducing the disordered or amorphous polymer/PCBM phases between the P3HT crystallite and the PCBM aggregate, which is in accordance with the adoption of combined thermal and SVA postdeposition treatments to realize high-performance P3HT/ PCBM devices.3,12,69 Most recently, Honda et al. have compared the thermally and the solvent annealed P3HT/ fullerene/dye ternary blends and have suggested the advantage of SVA in preparing the ideal P3HT−PCBM interfacial structures for efficient photovoltaic conversion.70

despite similar spectral appearance, may be rationalized by the influence of photon energy in the charge photogeneration in P3HT crystallites; that is, the above-gap excitation directly leads to full charge separation, whereas the red-edge excitation induces partial charge separation. In the literature, the signature absorptions of the P3HT−PCBM polaron pair (interfacial CT exciton)22 and the P3HT−P3HT germinate polaron pair (interchain exciton)17 were suggested to appear in 750−850 and 620−720 nm, respectively. In addition, the involvement of the oxygen-induced CT species [P3HT:O2]CT−, as recently identified by means of spin-sensitive detections,71 seems unlikely in the present cases since the transient absorption measurements were carried out under vacuum. On the basis of the transient spectra in Figure 4a,b recorded under the red-edge photoexcitation with low photon fluence, the [P3HT+δ···P3HT−δ]* yield was estimated to be 9% or 7% for the SVA or the CS2-cast neat P3HT films, respectively, by taking the amplitude ratio between the maximum of the 1.45 ns transient at 1080 nm (CT exciton) over that of the 0.16 ns transient at 1200 nm (exciton) with presumably the same extinction coefficient. Because of the efficient interfacial electron scavenging by PCBM, the relatively long-lived CT excitons (over 1 ns) are expected to fully dissociate, which, together with their significant yield, implies that the red-edge excitation produces polarons as efficiently as the blue-edge excitation does. This is corroborated by the fact that the internal quantum efficiency (IQE) of the P3HT/PCBM solar cell keeps 70−76% in the spectral region of 460−625 nm without significant variation.69

5. SUMMARY We have spectroscopically characterized the excitons and polarons in the neat P3HT and the blend P3HT/PCBM films and investigated the polaron photogeneration dynamics up to ∼1.5 ns. In the SVA and the CS2-cast blend films, the dimensions of exciton delocalization were found to be ∼7.6 nm (19 repeating units) and ∼6.6 nm (17 repeating units) under the excitation wavelengths of 620 and 460 nm, respectively. It is shown that the delocalized polarons characterized by the absorption in 620−830 nm inhabit the crystalline P3HT phase, whereas the localized polarons absorbing over 750−1100 nm reside on the disordered P3HT phase. Importantly, the characteristic absorption spectra of the negative polaron P3HT•− and the CT exciton [P3HTδ+···P3HTδ−]*, undetectable in the P3HT/PCBM blends owing to the extremely efficient electron scavenging by PCBM, are proposed to be a broad-band NIR absorption up to 1300 nm with a maximum at ∼1080 nm (Figure 10). As summarized in Figure 11, the polaron photogeneration processes of the P3HT crystallite in the P3HT/PCBM blend can be categorized, in terms of the photogeneration dynamics, into the prompt and the delayed ones. For the SVA blend film, these two mechanisms operating in different temporal regimes contribute comparably to the overall charge production under the visible photoexcitation, as manifested by the comparable amplitudes of the prompt and the delayed polaron formation (Figure 7a,c). The prompt polaron formation comprises the direct photoionization (path 1) in the P3HT crystallite taking place with a moderate quantum yield of 7% upon the above-gap photoexcitation, and the instantaneous interfacial charge separation occurring when the exciton (P3HT*, path 3) or the CT exciton ([P3HTδ+···P3HTδ−]*, path 4) can directly access the donor−acceptor interfaces. Here, the efficient

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ASSOCIATED CONTENT

S Supporting Information *

Data for AFM and X-ray diffractometry characterization of the neat and the blend films, P2 band decomposition and its comparison to the photoinduced transient, kinetics under the higher photon fluence excitation and estimation of exciton delocalization size, and comparison of transients between the neat and the blend films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-6251-6604. Fax: +86-10-6251-6444. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Grants in aid from the Natural Science Foundation of China (No. 20933010) and the National Basic Research Program of China (No. 2009CB20008) are acknowledged. We are grateful for the support from the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (No.10XNI007).

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REFERENCES

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