pubs.acs.org/NanoLett
Effect of Annealing on P3HT:PCBM Charge Transfer and Nanoscale Morphology Probed by Ultrafast Spectroscopy R. Alex Marsh, Justin M. Hodgkiss,*,† Sebastian Albert-Seifried, and Richard H. Friend* Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom ABSTRACT We employ sub-picosecond TA spectroscopy on operating P3HT:PCBM devices to probe the effect of annealing on charge transfer dynamics and nanoscale morphology. Our measurement configuration allows us to remove the effect of high excitation densities that would otherwise dominate. Charge transfer in pristine P3HT:PCBM devices proceeds on a sub-picosecond time scale, implying molecular level intermixing and explaining the more localized character of excitons and charges. In annealed devices, annealing results in diffusion-limited charge generation with a half-life time of ∼3 ps, complete only after 30 ps. This is the result of exclusion of PCBM molecules and ordering of P3HT domains and is correlated with improved photovoltaic efficiency. We are able to use the spectra and dynamics of optical excitations themselves to interpret blend morphologies on the appropriate time- and length scales of photoinduced charge generation. KEYWORDS Photovoltaic diode, semiconducting polymer, optical spectroscopy
T
occupying π-conjugated systems become more spatially delocalized when polymer chains are planar, thus increasing the volume instantaneously sampled by an exciton while decreasing the optical coupling and driving force available for transfer to another site,19 and (ii) charge mobilities are also known to be correlated with the degree of planarity in polymers like P3HT.20
he promise of competitively priced solar energy drives the development of organic photovoltaic (OPV) cells.1-6 One of the most efficient OPV material combinations to date is P3HT:PCBM.7,8 This serves as a useful model system because of the role annealing plays in achieving superior efficiencies. Thermal annealing has been linked to P3HT reordering and improvement of nanoscale morphology,9-11 but the precise mechanism by which it improves charge photogeneration remains unclear. Only a small number of materials have been developed which can improve on P3HT:PCBM,12-14 despite a large and growing research effort in the field. This Letter seeks to understand what effect annealing has on the nanoscale morphology in P3HT:PCBM blends and relate this to improved photovoltaic efficiency. Bulk heterojunction OPV devices provide for efficient conversion of excitons to charge pairs when heterojunction interfaces are distributed throughout the device on a length scale commensurate with the exciton diffusion length. At ∼4-15 nm, reported exciton diffusion lengths15-18 define an optimal blend morphology that evades the resolution of microscopic techniques capable of interrogating ∼100 nm thick polymer films. Moreover, the chain conformation of conjugated polymers in the vicinity of interfaces becomes an increasingly important consideration in the conversion of excitons to charges. Reasons for this include (i) excitons
Here, we directly probe the nanoscale morphology experienced by optical excitations in P3HT:PCBM blend devices via transient absorption (TA) spectroscopy. Charge transfer time scales of ∼45 fs are often assumed in OPV devices based on previous ultrafast spectroscopy of MEHPPV:PCBM blends.21 However, charge generation must be slower in blends with a higher degree of phase separation since excitons must first diffuse to a heterojunction. The combination of time and spectral resolution at wavelengths corresponding to well-defined signatures of polymer chain conformations provides a detailed picture of how thermal annealing affects nanoscale blend morphologies. Ultrafast TA spectroscopy has been successfully employed to probe nanoscale morphology in less efficient polyfluorene blends.18,22 However, there has been very little use of ultrafast spectroscopy to probe charge generation in the P3HT:PCBM system. One example concludes that the principal effect of annealing may be to accelerate hole transfer away from the heterojunction.23 However, this conclusion is difficult to reconcile with the rapid ground state recovery kinetics presented in the same study. Moreover, it is often difficult to be confident that TA dynamics measured for thin films are representative of the steady state physics of corresponding devices owing to high pulse excitation densities often required to perform TA spectroscopy. Therefore we perform low intensity TA
* To whom correspondence should be addressed,
[email protected] and
[email protected]. †
Present address: MacDiarmid Institute of Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, New Zealand. Received for review: 11/15/2009 Published on Web: 02/02/2010 © 2010 American Chemical Society
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spectroscopy in situ on operating devices where we can directly assess the importance of density-dependent effects. Devices comprise a bottom electrode of indium tin oxide (ITO) and a top electrode of aluminum. The ITO covered glass substrates were cleaned first using acetone and isopropyl alcohol and then oxygen plasma treatment. PEDOT: PSS (40 nm thick) was deposited by spin-coating onto the plasma-treated substrates and then annealed at 120 °C for 30 min before being transferred to a nitrogen glovebox for further fabrication steps. The active layer for P3HT:PCBM devices was spin-coated from a 50:50 weight ratio solution of regioregular P3HT (Merck) and PCBM (Aldrich) dissolved in chloroform at a concentration of 17 mg/mL to give an active layer of 95 ( 5 nm thickness. Where applicable, annealing was performed after encapsulation, at 140 °C for 5 min in an inert atmosphere. The P3HT-only diode was prepared in an identical way, using a solution of 17 mg/mL P3HT to give a film thickness of 90 ( 5 nm. Each device was completed by the evaporation of 80 nm of aluminum at a pressure of less than 10-6 mbar through a mask that defined an active area of 4.5 mm2. The 60 fs, 800 nm output of a Ti:sapphire oscillator and amplifier system (Tsunami and Spitfire Pro, Spectra Physics) was used to generate a tunable pulsed excitation via a TOPAS and a broad band probe (500-800 nm) via a homebuilt noncollinear optical parametric amplifier.24,25 The effect of excitation is examined by comparing the spectrally resolved intensity of alternate broad band probe pulses that are synchronized with excitation pulses chopped at half the frequency. Dynamics are compiled through variation of the time delay between excitation and probe pulses. The full probe spectrum is read out on a shot-to-shot basis, and the noise resulting from fluctuations in probe pulse intensities is mitigated by referencing each pulse to a second branch of the broad band beam that does not overlap with excited volume of the device. The system is described in more detail in ref 26. TA measurements were performed in reflection mode on devices rather than films: pump and probe beams are incident on the ITO face of the device, pass through the active layer, and are reflected at the aluminum cathode. They pass through the active layer a second time and are collected upon exiting at the ITO face. The measurement configuration combined with excellent detection sensitivity (∼10-4) permits photovoltaic and TA measurements to be made simultaneously under a low-intensity regime approaching that of solar PV operation. Figure 1 illustrates the absorption and EQE (external quantum efficiency) spectrum of P3HT:PCBM blends spun from chloroform and the effect of annealing. As expected, annealing produces a dramatic increase in efficiency. It should be noted that the annealed film exhibits a more structured absorption spectrum, in particular featuring a shoulder at 600 nm, corresponding to the 0-0 vibronic transition that is enhanced in planarized P3HT. This is a clear © 2010 American Chemical Society
FIGURE 1. Pristine (orange) and annealed (purple) P3HT:PCBM: solid lines indicate absorption spectra of films spun on spectrosil substrates, and dotted lines indicate EQE of complete OPV device.
indication of increased crystallinity within the P3HT domains, consistent with previous studies.9,27,28 P3HT Exciton Dynamics. In order to analyze and understand TA data in P3HT:PCBM blends, it is necessary to first examine ultrafast excitation dynamics in P3HT alone. Identification of charge-generation times in blends requires knowledge of the spectral signature of P3HT excitons. Once excitons’ relaxation dynamics are also known, this may also permit an estimate of the exciton-to-charge conversion efficiency in blends. Previous research shows that regioregular P3HT may order into H aggregates.27,28 This has been shown to dramatically affect charge transport properties,29 increasing hole mobilities from 10-11 m2/(V s) to 10-8 m2/(V s) in devices blended with PCBM. This ordering also affects excited-state spectra of regioregular P3HT, as seen by comparison with the excited-state spectra of regiorandom P3HT where the random placement of side chains disrupts the formation of ordered lamallae.30,31 It is important to consider the effect of exciton density when examining dynamics, as high densities can introduce additional decay mechanisms such as bimolecular exciton annihilation.32 In the case of P3HT:PCBM diodes, the effect of increasing fluence may be easily measured via the suppression of photovoltaic quantum efficiency. However, P3HTonly diodes have very low quantum efficiencies, less than 1%, such that the photocurrent is too low to employ this method. Therefore a low fluence was chosen corresponding to the value found to be reasonable in studies on P3HT:PCBM blend devices (vide infra). Under these conditions, the likelihood of second-order effects influencing excitation dynamics is minimized, while still obtaining a good signalto-noise ratio. The results of these measurements are given in Figure 2. At early times, there is a stimulated emission (SE) feature in a wavelength region corresponding to photoluminescence (λmax ∼750 nm), which is clear evidence of a singlet exciton. It is clearly possible to discern three ground state bleach (GSB) contributions, which produce a positive signal. These 924
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is most pronounced for the 0-0 absorption transition, where a shift of 0.035 eV or 10 nm is observed over the first 100 ps. It is also evident from the longer time scale kinetics that the 0-0 bleach persists longer relative to the higher energy contributions. The loss of SE may reflect an increase in the delocalized nature of excitons, as this is associated with a reduction in the matrix element for the radiative transition to the ground state.35 The small red shift observed and the longer-lived 0-0 GSB contribution are consistent with movement of excitons toward more ordered sites and associated relaxation within the density of states. As noted above, we find also that a small population of directly photogenerated charges contributes to the long-lived 0-0 GSB feature in pure P3HT,33 in agreement with other recent reports.36,37 Since there is relatively little spectral shift in the excitons’ TA spectra over time, it is possible to read their lifetime directly from the integrated kinetics. This gives a half-life of approximately 50 ps, although the radiative lifetime, measured from the stimulated emission, is slightly shorter. This result will prove useful in the analysis of excitation dynamics in P3HT:PCBM blends, as it may be used to relate charge transfer times to the exciton-to-charge conversion efficiency. Charge Transfer in Pristine P3HT:PCBM. Having investigated the ultrafast TA dynamics of P3HT excitons, it is possible to use the same technique in P3HT:PCBM blends. The P3HT:PCBM system is designed to produce good photovoltaic performance and is an example of a type II heterojunction. Therefore, the introduction of PCBM molecules results in an additional decay channel for photoexcitations: charge separation at the interface and subsequent charge extraction or recombination. This results in the conversion of a P3HT exciton into a positive P3HT polaron and, hence, may be expected to produce an associated change in the TA signature. The dynamic evolution of the TA from predominantly excitonic to polaronic signatures therefore offers information on the rate of exciton diffusion to the heterojunction and resulting electron transfer. This in turn offers information regarding the blend morphology. It should be noted that PCBM radical anions are optically transparent in the spectral regions probed here,38 so only P3HT excitations are expected to contribute to the TA signal. In a blend, a variety of excitation-density dependent decay channels are possible. In addition to exciton-exciton annihilation,32 second-order effects are possible following electron transfer. These include exciton-charge annihilation,39,40 nongeminate charge recombination,41 space-charge effects42 and density-dependent mobilities. It is important in an investigation of blend devices to take this into account since instantaneous excitation densities are significantly higher as a result of pulsed excitation. Since even as-spun devices exhibit good photovoltaic efficiencies (see Figure 1), it is possible to measure the fluence dependence of such secondorder effects directly from the extracted photocurrent. This is illustrated in Figure 3.
FIGURE 2. Low fluence TA spectra of P3HT device at 0 V: (a) normalized time slices and (b) kinetics of each Gaussian component (0-0, 0-1, 0-2 GSB and electroabsorption/photoinduced absorption.) The device was excited at 500 nm, 6.25 µJ cm-2.
correspond to 0-0, 0-1, and 0-2 (in order of increasing energy) absorption transitions. The relative prominence of the 0-0 peak at early times is consistent with a good degree of P3HT ordering,27,28 as expected in P3HT-only films. The spectrum also exhibits a sharply peaked photoinduced absorption (PIA) feature at 660 nm, which we attribute to a small population of charge pairs that we find to be directly photogenerated.33 Its shape and close proximity to the GSB features, combined with the accentuated vibronic structure in the GSB region, is strong evidence for a contribution of electroabsorption (EA) from ground-state chromophores.34 In this case, EA is not induced by the field across the device, but by the local fields of charge pairs. The shape of the PIA peak is clearly also affected by the overlapping GSB and SE features associated with excitons. TA spectra and dynamics of the P3HT-only diode are unaffected by interfaces with the aluminum electrode and PEDOT:PSS, as confirmed by transmission mode TA measurements on films comprised of P3HT without electrodes and on P3HT films on PEDOT:PSS (see Figures S1 and S2 in Supporting Information). From the normalized time slices, it can be seen that there is very little evolution in the nature of the exciton with time. There is a reduction in the relative strength of the stimulated emission band over tens of picoseconds. The position of the GSB peaks is found to red-shift slightly over time. This effect © 2010 American Chemical Society
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FIGURE 3. Intensity dependence of EQE (solid lines) and photocurrent (dashed lines) for pristine (blue) and annealed (red) P3HT:PCBM devices. The slopes corresponding to linear photocurrent are shown as a guide for the eye (gray lines). Devices were excited with a 1 kHz train of 500 nm, 60 fs laser pulses.
FIGURE 4. Pristine P3HT:PCBM device TA spectra under flat-band conditions. Pump fluence 6.25 µJ cm-2, wavelength 500 nm. Time ranges over which spectra were integrated are indicated on the figure. Inset: Integrated TA kinetics in the 520-550 nm region.
The shape of the PIA measured in the as-spun blend and the lack of SE imply that it is charges, and not excitons, that are being detected. The assignment of the broad, featureless absorption to positive P3HT polarons is supported by previous charge-modulation spectroscopy measurements performed on P3HT diodes.43 The absence of intensity in the 0-0 region of the GSB indicates that the holes do not occupy planarized chain segments, consistent with the notion that PCBM disrupts the formation of ordered H aggregates. The lack of well-resolved vibronic structure in the pristine blend is also expected to obscure the detection of EA at the band edge.
The EQEs of both device types approach that measured under continuous wave (CW) conditions at the lowest fluences. This confirms that the inherent physics of solar photocurrent generation is retained under low-intensity pulsed excitation, highlighting the validity of our experimental approach. Retaining maximal EQEs is also a good indication that the excitation beam is well-aligned onto the active area of the pixel. At increased fluences, the EQEs begin to drop off and, thereafter, become heavily suppressed relative to the CW conditions. At the highest fluence measured, second-order effects reduce the EQEs by over an order of magnitude. The intensity-dependent suppression of photocurrent is clearly more pronounced in the case of the annealed device. This observation will be discussed later in view of the observed TA dynamics. Our detection sensitivity enables us to conduct TA measurements at a fluence of 6.25 µJ cm-2, where a short-circuit photocurrent collection is suppressed by less than a factor of 2 as a result of minor second-order effects. The TA spectra and kinetics of an as-spun P3HT:PCBM device are illustrated in Figure 4. These measurements were taken at an applied bias of 530 mV, which was chosen to produce approximately flat-band conditions, as it corresponds to the point of zero net photocurrent generation. The early time spectra consist of a positive signal in the region below 600 nm and a PIA at greater wavelengths. The positive signal corresponds approximately to the absorption spectrum of the film as illustrated in Figure 1 and is therefore interpreted as a GSB. There is no evidence of SE from P3HT in the blend, rather a broad and featureless PIA feature is observed between 600 and 800 nm from the subpicosecond time scale and beyond. There is very little spectral evolution over the 100 ps. The bleach does not noticeably red shift, and the shape of the PIA remains unchanged. The bleach drops by approximately 15% in the first 500 fs and is flat thereafter. The PIA is present from the first 100 fs and exhibits only a very weak rise over the following tens of picoseconds. © 2010 American Chemical Society
The kinetics strongly suggest that electron transfer to PCBM molecules happens extremely quickly. The fact that the GSB decay is stabilized within the first few hundred femtoseconds and no stimulated emission is detected implies that there are no remaining excitons after this time. From the initial magnitude of the photoinduced absorption, it appears that at least half the exciton population undergoes ultrafast electron transfer within the 100 fs instrument resolution limit. The rapid conversion of excitons to charges is consistent with the observation that only ∼15% of excitations appear to be vulnerable to bimolecular annihilation reactions; based on recovery of the GSB signal, the fraction of excitations that survives beyond the first picosecond is reduced from 85% to 70% when the fluence is increased ∼40-fold to create a sufficiently high exciton density (∼3 × 1019 cm-3; ∼3 nm spacing between excitations) to annihilate on the picosecond time scale (see figures S5 and S6 in Supporting Information).32 The observation that electron transfer proceeds on such fast time scales implies that the majority of excitons do not diffuse prior to undergoing charge transfer to a PCBM molecule. Rather, they must already be in close proximity to such an electron acceptor. These results therefore indicate that PCBM is molecularly dissolved throughout P3HT in asspun films. This configuration is strongly favored by entropic 926
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considerations and is also found in MDMO-PPV:PCBM blends, where charge transfer proceeds on a similarly fast time scale.21,44 However, electron transport must remain relatively efficient in this morphology; otherwise the device would not exhibit good photovoltaic efficiencies, as illustrated in Figure 1. Single carrier electron mobility measurements elsewhere reinforce the conclusion that electron transport in these devices is efficient.29 The dissolved PCBM molecules must therefore be distributed sufficiently close to each other that charge hopping between them remains efficient. Since P3HT and PCBM have very similar densities (around 1.5 g cm-3),45,46 their volume fraction in the active layer is close to 50:50. This would suggest that the concentration of PCBM molecules is above the percolation threshold required to achieve continuous conducting pathways. Accommodating a continuous and finely spaced network of PCBM molecules within a P3HT matrix implies that the preferred conformation of P3HT chains must be disrupted. The lack of vibronic structure in the ground-state bleaching region is evidence for a pronounced reduction in P3HT crystallinity caused by the introduction of a PCBM network. Charge Transfer in Annealed P3HT:PCBM. The question of precisely how annealing affects the ordering of P3HT chains and PCBM molecules is crucial to understanding its overall impact on photovoltaic performance. Therefore, the same range of experiments used to investigate charge transfer dynamics and nanoscale morphology in as-spun devices was applied to annealed samples. The TA spectra and kinetics of an annealed P3HT:PCBM device are illustrated in Figure 5. These measurements were taken with the same excitation fluence as for the as-spun devices and at an applied bias of 640 mV. This bias was chosen to produce approximately flat-band conditions, as it corresponds to the point of zero net photocurrent generation. It may be noted that the built-in potential in the annealed device appears to be slightly larger than that measured in the as-spun case. The open-circuit voltage in such devices has been found elsewhere to vary upon annealing.29 It may be speculated that the origin of this effect is associated with annealing-induced vertical phase separation or surface energy effects at the electrodes. However, this study is concerned with charge transfer, and hence a more detailed study of the origin of the built-in potential falls outside its scope. It is sufficient for these purposes to establish that there is zero net photocurrent generation at 640 mV. The spectra are comprised of a positive signal due to bleaching of the ground state absorption at wavelengths below 630 nm and PIA overlapping with stimulated emission to the red of this region. The GSB signal is significantly redshifted and exhibits a more prominent 0-0 vibronic feature as a result of annealing, confirming that photogenerated excitations occupy a more crystalline form of P3HT than in the as-spun P3HT:PCBM blend.28 © 2010 American Chemical Society
FIGURE 5. TA of annealed P3HT:PCBM device under flat-band conditions: pump fluence, 6.25 µJ cm-2; excitation wavelength, 500 nm. Figures indicate (a) spectra and (b) integrated kinetics, with integration ranges as labeled.
At early times, the PIA is peaked at ∼660 nm, as in the case of the P3HT-only device. Again, this spectral shape arises from a combination of several features; GSB at energies corresponding to the ground-state absorption, EA at the band edge, an SE feature from excitons further to the red (see Figure 2), and a broad PIA feature associated with charge absorption. However, while excitonic SE is distinctly visible as a positive ∆T/T signal in the P3HT-only diode in Figure 2a, the SE feature is overwhelmed by the ultrafast appearance of PIA from charges in the annealed blend. Taken together, this spectrum implies populations of both excitons and charges within the first picosecond of excitation in the annealed device. Over a period of the next few picoseconds, the spectral shape evolves with the loss of the SE contribution to leave an underlying broad absorptions consistent with the completion of electron transfer leaving only polarons in P3HT. The integrated kinetics shown in Figure 5b therefore give an indication of the time scale for electron transfer. The 700-750 nm integrated kinetics show that a population of charges is formed promptly within the instrument resolution while the delayed phase of charge generation occurs with a half-maximum rise time of approximately 3 ps. Charge transfer is complete within 30 ps, and the 927
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sion of excitons to charges. The ∼50% loss in signal in the first 10 ps is substantially faster than that observed in P3HT only under similar fluence. This could result either from the population of excitations decaying more quickly or from spectral effects reducing the GSB magnitude per excitation. Regarding the first possibility, we consider exciton-exciton annihilation: exciton densities must be comparable in the blend and the P3HT-only device for the same excitation fluences, even when the lower volume fraction of P3HT is taken into account for the blend. It therefore seems implausible that exciton-exciton annihilation could account for such a large reduction in population in the blend. It is possible that excitons could be quenched via Fo¨rster transfer to charges that had already been generated nearby, since the emission of P3HT excitons overlaps strongly with absorption from P3HT hole polarons in the 650-800 nm region. The distance dependence of Fo¨rster transfer (R-6) suggests that quenching by charges (exciton-charge annihilation) could become significant at the fluence used here and may contribute to the intensity-dependent suppression of photocurrent under pulsed excitation. This mechanism has been recently invoked in charge generating blends examined under similar excitation fluences.39,40 We note that exciton-charge annihilation would accelerate the observed exciton population decay, meaning that the 3 ps halflife attributed to charge transfer would in fact reflect the sum of rates of exciton-charge annihilation and charge transfer. If exciton-charge annihilation is operative and entirely responsible for the ∼50% reduction of photocurrent collection efficiency at this fluence, then both channels would contribute approximately equally to exciton decay, meaning that the inherent half-life for charge transfer time scale was ∼6 ps. Taking the potential for exciton-charge annihilation into account to revise the upper limit of the charge transfer time scale does not substantially affect the preceding discussion regarding exciton delocalization and diffusion. A further contribution to the GSB decay could result from the subsequent fate of the population of charge pairs formed directly in P3HT phases. If a charge pair has a greater bleaching cross section than a single-hole polaron in P3HT, then subsequent transfer of an electron into PCBM will be associated in a reduction in GSB signal. However, this cannot be a major effect because the yield of charge pairs directly generated in P3HT is only on the order of 10%.33,36,37 Two spectral effects could also account for the apparent accelerated decay in the GSB. Since the decay occurs on the same time scale as the generation of charges, both are associated with the bleach and absorption cross section of charges compared to excitons. It may be the case that polarons have a smaller bleach cross section than excitons in P3HT. This would imply that they are more localized, as bleach magnitude is proportional to spatial extent of excitation.53 A second explanation is that the PIA of charges extends substantially into the blue and overlaps with both resolved GSB peaks. It is not a simple matter to distinguish
associated absorption signal remains flat in magnitude thereafter. The ultimate decay of this signal was found to proceed on multiple time scales with a half-life of approximately 100 ns, as is resolved in a forthcoming publication. The time scale for charge transfer in annealed P3HT: PCBM is found to be at least an order of magnitude slower than that in the as-spun blend. The significant amplitude of a slower phase of electron transfer suggests that most excitons are not generated adjacent to PCBM molecules. Instead, charge transfer is limited by diffusion of excitons through P3HT domains toward the heterojunction.18 Taking an exciton diffusion coefficient of 2 × 10-7 m2 s-1 measured using luminescence quenching experiments,17 excitons might be expected to diffuse only 1 nm in the first 10 ps. This is a very small value compared to the length scales of phase separation reported from TEM measurements elsewhere,47 which are of the order 10 nm. It also makes it difficult to envisage how P3HT chains could effectively order into lamellae if PCBM molecules were so intimately dispersed. Further insight into the length scale of phase separation may be gained from a consideration of the charge conversion efficiency in these blends. Since charge transfer on average takes 3 ps, some fraction of P3HT excitons must decay monomolecularly to the ground state before they reach a PCBM molecule. Taking the kinetics of the exciton decay recorded at low fluences in the P3HT-only device (see Figure 2), it is possible to estimate that ∼6% of excitons decay by this kinetically competing channel in the annealed blend. This is consistent with the high EQE that these devices exhibit. However, if simple unbiased exciton diffusion as described in ref 17 is combined with a reported domain size of 10 nm,47 then the characteristic charge transfer time would be approximately 500 ps. Owing to the relatively short lifetime of P3HT excitons, this would produce a charge conversion efficiency below 30%. We must conclude that either excitons can sample 10 nm domain significantly faster than previously supposed or that pure P3HT domains are significantly smaller than the suggested 10 nm. The diffusion of excitons can only be treated within the point dipole approximation of Fo¨rster energy transfer when the spatial extent of the exciton is small compared to the Fo¨rster transfer distance.48 The failure of the Fo¨rster energy transfer model to explain exciton diffusion in dilute solutions of polythiophenes results from the delocalization of excitons over ∼7 monomer unitssa comparable or larger extent than the chromophore separation.49-51 Owing to the enhanced planarization of P3HT chains in solid films, particularly as a result of annealing, their spatial extent may be increased even further, in both intra- and interchain directions. The large spatial extent of excitons in well-ordered P3HT may explain how they apparently sample domains of order 10 nm in such short time scales.52 The rapid initial decay in the GSB requires further explanation since the ground state is not recovered upon conver© 2010 American Chemical Society
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between these effects because the PIA cannot be measured independently of the bleach. The observation of unusually rapid decay in the GSB is an intriguing observation that warrants further investigation under sufficiently low fluence to exclude the possibility of exciton-charge annihilation. Finally, we note that the relative intensity dependence of photocurrent extraction for the annealed device compared with the pristine device is consistent with our interpretation of the morphological evolution induced by thermal annealing. The low fluence limit of the EQE corresponds well with the level measured under steady-state conditions. The subsequent decline in EQE with increasing fluence is more pronounced for the annealed case than for the as-spun device. Slower conversion of excitons to charges as a result of the coarser morphology in the annealed device leaves excitons vulnerable to annihilation with other excitons or charges within the first few picoseconds at high intensity. The fraction of excitations that survive beyond the first 10 ps in the annealed device is halved upon increasing the fluence from 6.25 µJ/cm2 (50% survival based on GSB recovery) to 250 µJ/cm2 (25% survival, see Figures S7 and S8 in Supporting Information). As noted previously, only ∼15% of excitations are susceptible to annihilation on the time scale of charge generation for the as-spun device over the same intensity range, owing to the faster rate of charge generation in the finer blend morphology. Summary and Conclusions. In this Letter, ultrafast TA spectroscopy is performed on operating P3HT:PCBM devices. This is used to probe the dynamics of charge transfer and, hence, reveal the effect of annealing on the nanoscale morphology in these blends. This technique is applied to P3HT-only diodes to probe the dynamics of exciton motion and decay. Excitons are found to decay with a half-life of approximately 50 ps. In addition, the spectral shift in their TA spectra reveals that they move to more ordered P3HT sites over tens of picoseconds. This is accompanied by delocalization and an associated reduction in their radiative emission cross section. Electron transfer in pristine P3HT:PCBM devices is found to proceed on ultrafast time scales; 50% of excitons undergo charge transfer within 100 fs, and no excitons remain beyond 1 ps. This suggests that PCBM is molecularly dissolved throughout P3HT and accounts for the reduced crystallinity evident in P3HT and associated reduction in hole mobility. The effect of annealing is to increase the charge transfer time dramatically. A population of charges is formed promptly from excitons adjacent to interfaces, while the majority phase of charge generation proceeds to completion within 30 ps with a half-life time of ∼3 ps. This indicates that charge transfer rates are limited by the diffusion of excitons and suggests that the process of annealing causes P3HT to exclude PCBM molecules and form ordered domains. From a comparison with the dynamics of exciton decay in pristine P3HT, it is inferred that exciton motion cannot be considered as pointlike diffusion. Rather, excitons are delocalized and, hence, may sample © 2010 American Chemical Society
ordered domains efficiently in tens of picoseconds. Having determined that the conversion of excitons to charges is efficient in both as-spun and annealed devices, we conclude that improved photovoltaic efficiencies must result from suppressed charge recombination in the coarser and more crystalline morphologies found after annealing. Acknowledgment. This work was supported by the U.K. Engineering and Physical Sciences Research Council (EPSRC). R.A.M. would like to acknowledge the George and Lillian Schiff Foundation and S.A.-S. would like to thank the Ernest Oppenheimer Fund for financial support. Supporting Information Available. TA spectra for films of P3HT and P3HT deposited on a layer of PEDOT:PSS along with bias- and intensity-dependent TA spectra and kinetics for P3HT-only devices and pristine and annealed P3HT: PCBM blend devices. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
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DOI: 10.1021/nl9038289 | Nano Lett. 2010, 10, 923-–930
