University of Groningen

Charge, energy and bond dynamics at molecular interfacesBakulin, Artem Alekseevich

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Chapter 7

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction6

In this Chapter, we investigate ultrafast dynamics of the hole-transfer process from a methanofullerene to polymer in a polymer:PCBM bulk heterojunction. In spectroscopic experiments, we observed that injection of holes into MDMO-PPV is markedly delayed with respect to the excitation. The fastest component of the delayed response was attributed to the PCBM-polymer hole transfer with a time constant of 30±10 fs, followed by the slower 150 fs component, assigned to the hole transfer after energy transfer inside PCBM nanoclusters. We show that the harvesting efficiency after PCBM excitation crucially depends on the concentration of the methanofullerene in the blend due to the changes in the morphology. Ultrafast charge generation is steadily effective while the characteristic scale of phase separation in the blend, derived from AFM measurements, did not exceed ~20 nm. At a larger scale of phase separation, the exciton harvesting dramatically declined. On the basis of combined ultrafast spectroscopy – AFM data, a general picture of charge dynamics after PCBM excitation is developed. The reported results underline the importance of the acceptor component in the light-harvesting process in bulk heterojunction photovoltaics.

6 The current chapter is based on the following publication:

Bakulin, A. A., Hummelen, J. C., Pshenichnikov, M. S. & Loosdrecht, P. H. M. v. Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction. Submitted for publication.

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7.1. Introduction

Plastic photovoltaics is one of the most promising candidates for renewable energy sources, sparking extensive research and development over the past decade 1-3. In the most widely-studied and effective plastic solar cells, light is harvested and converted to electricity in a “bulk-heterojunction” active layer, composed of a mixture of organic electron donor and acceptor materials 4. In such devices, the role of the donor can be played by a great variety of conjugated molecules, ranging from the extensively studied MDMO-PPV 5, 6 and regioregular P3HT 7, 8 to more advanced ‘narrow-bandgap’ polymers.9, 10 At the same time, the overwhelming majority of the photovoltaic blends utilizes the same molecule as electron acceptor: the soluble fullerene derivative (6,6)-phenyl-C61-butyric acid methyl ester ([60]PCBM, or simply PCBM).11 Taking into account that the mass content of PCBM in modern state-of-art solar cells varies from 50 to 80% 4, PCBM is currently the most widely used material in the field of organic photovoltaics.

The success of PCBM (and other similar fullerene derivatives) in solar cell applications originates from the excellent exciton-to-charge conversion properties of polymer/PCBM blends 12 and from the fairly high electron mobility in PCBM 13. In addition to this, the relatively small HOMO-LUMO energy difference of fullerenes and their derivatives makes them suitable materials for photovoltaics 14. Record energy conversion efficiencies have recently been achieved using solar cells which spectral region was extended into the near infrared (IR) by utilizing a combination of low-bandgap polymers and [70]PCBM absorbers.9, 15 The advantage of [70]PCBM over [60]PCBM being the stronger absorption in the visible region.14

The mostly studied process in polymer/PCBM blends is the charge generation after donor excitation. In this case, a photon, absorbed by the polymer, produces an excitonic state which subsequently diffuses to the donor/acceptor interface. At the interface a photoinduced electron transfer ET occurs from the LUMO-related HSOMO (highest semioccupied molecular orbital) of excited donor to the lower energy LUMO of the acceptor. The ET process is very efficient due to its extremely high speed: for instance, ~100% efficient ET in the MDMO-PPV:PCBM blend has been shown to occur at the ~50 fs time scale.16

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

165 165

However, the polymer is not the only absorber of photons in these blends: PCBM (or other fullerene derivatives) also contributes substantially to the blend absorption in the blue 17, red 18, and even visible 19 parts of the solar spectrum. Direct optical or energy-transfer mediated 18 excitation of PCBM in PCBM:polymer blends can be followed by the so-called hole-transfer (HT) process 14, 20 resulting in the generation of long-lived charges. The HT process corresponds to an electron transfer from the HOMO level of the polymer donor to the lower-lying HOMO-derived L-SOMO level of the fullerene-derivative acceptor that is formed upon optical excitation. The HT time and efficiency should be closely connected to the structure of donor and acceptor valence bands and, in particular, to the polymer-PCBM HOMO-HOMO energy offset which should be sufficient to overcome exciton binding energy.21 In this respect, HT is very different from the usually discussed ET process which is determined by LUMO-LUMO energetics.

There are two main processes involved in the light-to-charge conversion in the blend after PCBM excitation.20, 22 First, because of the phase separation in the active layer, an exciton created on PCBM has to diffuse to the fullerene/polymer interface. Second, at the interface, charge generation through HT should occur. The efficiency of these processes is determined by the PCBM exciton diffusion length and by the charge separation efficiency. While the former is intricately connected to the blend morphology and diffusion constants, the latter is directly linked to PCBM-to-polymer HT time.

Despite the obvious fundamental and practical importance, HT dynamics have received only little attention so far. For instance, the HT times have only roughly been estimated for [70]PCBM:MDMO-PPV 14 and PCBM:P3HT 17 blends as being faster than the experimental time resolution (500 fs and 250 fs, respectively). Importantly, for a general understanding of the PCBM performance in the blends, it is not enough to study the transfer times; it is also required to obtain detailed knowledge on how the conversion of PCBM excitons to charges depend on the content and morphology of polymer:methanofullerene materials.

In this Chapter, we employ ultrafast visible-pump/IR-probe spectroscopy to study the dynamics of light-harvesting by PCBM in MDMO-PPV:PCBM blends. From the time delay between PCBM excitation and the response measured through the MDMO-PPV polaron band, we evaluate the HT time as 30±10 fs. An additional (~150 fs) component in the signal growth is assigned to energy transfer/relaxation

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inside PCBM nanoclusters. The results of ultrafast spectroscopy are correlated with information on the blend morphology, as obtained from atomic force microscopy (AFM) imaging. We observe that the efficiency of the ultrafast light-to-charge conversion is high when the scale of phase separation in the blend does not exceed 20 nm. For stronger phase separation, the efficiency declines dramatically because in this case not all PCBM excitons are able to reach the acceptor/donor interface. The obtained results on the efficiency of the ultrafast charge generation after PCBM excitation and its dependence on blend composition and morphology are instrumental for future design of fullerene-derivatives-based photovoltaic devices.

7.2. Experimental

Blends of MEH-PPV and the soluble C60 derivative (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) with 1-70% of methanofullerene content (by weight) were prepared by dissolving each component separately in chlorobenzene with a concentration of 5 g/L and subsequent mixing of appropriate volumes. Films were made by drop-casting these mixed solutions on 180-mm thick fused silica microscope cover slides (UQG-Optics). Optical densities of the blends at the excitation wavelength of 630 nm were kept below 0.3. All optical experiments were performed under a nitrogen atmosphere to avoid sample degradation during measurements. The very same films were used throughout linear absorption, ultrafast spectroscopy, and AFM experiments.

Time-resolved spectroscopy was performed with a setup described in the Chapter 5. The linearity of the PIA response with pump intensity was carefully checked to rule out the two-photon absorption processes. The density of absorbed photons in the sample did not exceed ~3x1017 cm-3. For the PIA isotropic component measurements, polarization of the IR probe beam was set under 55o (magic angle) with respect to the polarization of the visible pump. For the anisotropy measurements 26, the probe beam was rotated by 45o with respect to the polarization of the pump and, after the sample, the probe component parallel or perpendicular to the pump was analyzed by a wire-grid polarizer (1:100 extinction). The anisotropy values were then calculated on basis of conventional equations. 27

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

167 167

For accurate determination of the zero delay time between excitation and probe pulses we first measured the instantaneous 28 response in a thin (thickness less than 0.5 µm) Ge film. The film was prepared by laser deposition onto the substrate identical to those used for the blends. The PIA rise time in Ge film exactly corresponded to the 80 fs cross-correlation between excitation and probe pulses which, in turn, was consistent with independently measured durations of the pulses (70 and 40 fs, respectively). The high accuracy of the sample positioning (±20 mm ) and fast switching from the reference Ge film to a sample warranted the accuracy to be better than ±3 fs in delay time determination for all samples.

The AFM images of the blend surfaces were recorded using a NanoScope MultiMode Atomic Force Microscope (Digital Instruments) in the tapping mode with transversal (XY) spatial resolution of 4 nm. For all samples, the analyzed surface was at least 103 times larger than the area of characteristic surface features. Image analysis was performed with the WSxM 4.0 software. First, flattering of images was made using second-order polynomial. After this, a two-dimensional (2D) autocorrelation function was computed and fitted by a 2D Gaussian. The phase separation scale was calculated as the averaged peak width along X and Y axis. We verified the estimate for the domain size with 2D Fourier transformation of AFM images recorded in the phase mode, and with the grain analysis function 29. All methods employed gave the similar outcome within 20% error margins.

7.3. Ultrafast spectroscopy results

Figure 7-1 (left) shows the absorption spectra of MDMO-PPV:BCBM blends with

different PCBM mass fractions. All samples were prepared using an identical

procedure which insures similar film thicknesses to facilitate direct comparison of

spectroscopic data. As expected, with increasing concentration of PCBM, the

polymer absorption diminishes while the PCBM-associated absorption increases.

All absorption spectra are dominated by the strong MDMO-PPV absorption band

below 600 nm with a shoulder due to PCBM absorption extending up to 750 nm,

clearly visible at high PCBM concentrations. The kinks at the wavelengths of 630

and 710 nm are similar to those reported previously for C60 30 and PCBM 18

solutions.

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For the selective excitation of PCBM in these blends, the wavelength of 630 nm provides the best trade-off between the polymer and PCBM absorption, while being sufficiently far from the MDMO-PPV/PCBM charge transfer complex (CTC) absorption band situated at ~800 nm.31, 32 The right panel in the figure 7-1 shows that the optical density near 630 nm scales linearly with the PCBM concentration, in a good agreement with earlier observations that absorption at 630 nm is highly dominated by PCBM absorption and MDMO-PPV absorption plays a minor role only31. However, it is important to note that even pure MDMO-PPV films display nonzero optical density around 630 nm which is probably associated with sub-bandgap absorption on defects and/or aggregated chains.33 From the linear fit to experimental data points (solid line in figure 7-1, right) it was deduced that the MDMO-PPV mass extinction coefficient at 630 nm is approximately times lower than that of PCBM.

To study charge dynamics after the PCBM excitation, we utilized a time-resolved photo-induced absorption (PIA) technique.14, 23, 34, 35 PCBM was selectively excited in the blend by tuning the wavelength of the excitation pulse into the PCBM absorption shoulder at 630 nm (figure 7-1). The appearance of positive charges in the polymer was registered by an IR probe which monitors the charge-associated low-energy (LE) absorption of MDMO-PPV at 3 µm.23, 36 In such an experiment, the response of the polymer should be retarded from the moment of PCBM excitation

Figure 7-1. Absorption spectra of MDMO-PPV:PCBM blends at different PCBM weight

concentrations (left).The shaded area is the spectrum of the 40-fs excitation pulse. The right panel

presents absorption values at the excitation wavelength of 630 nm as a function of PCBM

content. The solid line in the right panel shows a linear fit to the experimental data points.

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

169 169

due to the finite time of the fullerene-polymer HT and due to PCBM-exciton diffusion. This allows for extraction of both the HT times and the exciton diffusion from the delayed PIA transients. The spontaneous charge dissociation in the pure PCBM phase and the subsequent charge injection into the polymer should occur at a much longer (ns) timescale 37 and will not be considered here.

Figure 7-2 (a) presents a number of PIA transients recorded on MDMO-PPV:PCBM films with varying PCBM concentration. Clearly, the shape of the transients significantly varies with the PCBM content. At low (1-5%) PCBM concentrations, the PIA signal rapidly grows immediately after the excitation and then partially decays on a sub-ps time scale. Such behavior is typical for polymer-fullerene blends excited into the polymer band 34, 35 and is usually associated with the direct photogeneration of carriers and their partial recombination. 34 Such dynamics are consistent with the dominance of the polymer absorption at low PCBM concentrations (figure 1, right). The main part of the observed response can therefore be attributed to polymer intra- or inter-chain exciton formation 38 following by polymer-fullerene ET.

The shape of the transients becomes substantially different for blends with PCBM concentrations ¥15%. First, the sub-ps decay of PIA vanishes and even changes into the growing component. The former originates from the less pronounced recombination of charges while the latter corresponds to additional (delayed) charge generation. Second, in sharp contrast to blends with low PCBM concentration, the signal rise is noticeably retarded with respect to the zero-delay position.(figure 7-2b) To verify that our calibration of the zero delay was correct we also measured photogenerated-charge dynamics in a thin (<500 nm) Ge film (figure 7-2b, cross symbols) which are known to be almost instantaneous 28. The observed rise time excellently corresponds to the independently measured 80 fs cross-correlation between the excitation and probe pulses. The response of MDMO-PPV:PCBM blends with low (1%) PCBM weight content reproduces well the response in the Ge film in the first 100 fs, which confirms the immediate origin of the response at the LE peak after polymer excitation. For blends with a high PCBM concentration (30-70%), the PIA delay shows a saturation-like behavior.

The retardation times derived from the PIA responses at the half-maximum positions are presented in fig. 7-2c as a function of PCBM concentration. When the polymer absorption dominates, the delay is close to zero while when the PCBM

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absorption takes over, the delay approaches its saturation value of 30 fs. This clearly demonstrates that the generation of charges (holes) on the polymer after acceptor excitation is partially delayed, and we conclude that this delay is due to the HT process from the photoexcited interfacial PCBM molecules to the polymer. In other words, the observed delay of ~30 fs is the time required for electron to be transferred from the HOMO of the polymer to the HOMO-related L-SOMO of excited PCBM (i.e. hole transfer).

Figure 7-2. Real scale PIA (a) and normalized (b) transients for MDMO-PPV/PCBM films at

different PCBM weight fraction. Excitation was tuned into the PCBM absorption band (at

630nm) and the probe wavelength corresponds to the LE charge-associated band of the MDMO-

PPV at 3 µm. Pannel (c) shows the delay of response at half maxima as function of PCBM

concentration. Solid line in the panel (c) presents the relative contribution of PCBM in to the

absorption of the samples calculated for the ratio of 10 between the extinction coefficients of

PCBM and MEH-PPV.

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

171 171

The validity of such interpretation can be readily verified on the basis of the following two-component model. Let us assume that the delay at a certain concentration can be linearly interpolated in between the two limiting values of 0 fs (0% of PCBM) and (100%). However, because the optical (i.e. absorption) contributions of the both components should be considered, the mass content has to be corrected by the ratio of the extinction coefficients of PCBM and polymer at 630nm, that has already been determined from the linear absorption measurements as s=10 (figure 7-1). Therefore, we arrive at the following expression for the delay:

,)1( MAX

NN

Nδτ

σ

σδτ ⋅

−+= (7-1)

where N stands for PCBM concentration and dtMAX is the only scaling parameter. The calculated delay curve (figure 2c, the solid line) reproduces fairly well the experimental data points, which proves the validity of attribution of the 30 fs delay to the HT process.

Surprisingly, the observed HT time is quite similar or even faster than the reported ET time of 50 fs in a similar blend.16 This suggests that the efficiency of HT should be about that of ET, i.e. close to unity. Moreover, the extremely fast HT rate implies an unexpectedly large exchange integral between the corresponding PCBM and MDMO-PPV ‘valence-band’ wavefunctions. In this respect, it is important to note that, according to a number of recent studies, PCBM and other fullerene derivatives can form a ground-state CTC with certain conjugated polymers.31, 39, 40 Such CTCs are not as pronounced as in the blends with low-molecular acceptors 41-43, but the formation of an additional charge-transfer complex can noticeably effect the photophysics in the material 32, 44-48. Although direct excitation of the MDMO-PPV/PCBM CTC band was carefully avoided in the current experiments, efficient HT could still be mediated by CTC states in the same manner as was recently demonstrated for ET. 46

Careful inspection of the observed PIA transients reveals an additional, growing component in the signal. This ~150-fs component is most pronounced in the blends with high PCBM concentration and accounts for up to 25% of the amplitude. There can be several different origins of such a delayed response. It could be that the HT times have a broad distribution originating from inhomogeneity in the local environments of the donor-acceptor pairs. This would lead to a variation of the

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overlap between the HOMO orbitals of the polymer and PCBM, and result in a distribution of HT times. Such effect is, however, unlikely to result in a very broad distribution of HT times. It also does not seem to be in line with the observed clear separation of timescales (30 fs vs. 150 fs), which is observed as a clear kink at ~50 fs delay in the PIA transients for the samples with a high PCBM content (fig. 7-2a). An alternative explanation for the ~150 fs growth component is the presence of an intermediate process which precedes charge separation, such as for instance thermalization of the photo-excited molecule or/and energy transport within the PCBM nanoclusters.4 In the latter case, the slow component originates from excitation of PCBM molecules away from the PCBM-polymer interface that need time to reach the interface itself in order to contribute to the charge separation process. This hypothesis appears the most probable to us and will be discussed in more detail later.

The assignment of the 30 fs delay to the HT time relies on the assumption that blends with low PCBM content generate charges mostly as a result of polymer sub-bandgap excitation at 630 nm, while in PCBM-rich blends charge generation occurs mostly through the HT process. The validity of this assumption can be confirmed by transient anisotropy measurements 49 in the following way. Right after direct polymer excitation, the anisotropy of the polaron-band response should be

Figure 7-3. Transient anisotropy at 100 fs delay as a function of PCBM content in the blend. The

solid curve presents the expected anisotropy values calculated on basis of the two-component

model.

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

173 173

relatively high 23, 50, 51. In contrast, the anisotropy of the polymer response after PCBM excitation should be low since there is no reason for the orientation of the dipole moment of the polaron band to be correlated with the dipole moment orientation of acceptor excitation. This consideration directly links the early time anisotropy to the particular pathway of the charge-separation process.

Figure 7-3 presents the transient anisotropy of the observed pump-probe signal at 0.1 ps delay. When the polymer absorption in the blend dominates (1-5% of PCBM), the anisotropy stays relatively high, consistent with the interpretation that most of the charges are generated directly by polymer excitation. Small deviations from the maximal anisotropy value of 0.4 are caused by depolarization due to the energy or charge transfer processes in the blend even at such a short time scale. As the PCBM absorption takes over, the anisotropy rapidly diminishes, which confirms that more and more charges on the polymer are generated through the HT process. Such interpretation is supported by the clear correlation between the anisotropy at 0.1 ps and relative input of PCBM excitation into the charge photogeneration as presented by the solid curve in figure 7-3. Similarly to absorption (fig. 7-1, right) and delay (fig. 7-2c) data, the anisotropy clearly correlates with the PCBM optical contribution to the absorption, with the same ratio of PCBM/polymer extinction coefficients of s=10. Minute but apparent deviation of experimental values from the model curve at the highest PCBM contents will be explained later.

To solidify the phenomenological two-component model utilized before, we made global fit of all PIA transients with the time constants being identical for all experiments (see the Appendix 7.6.). The results of the fitting procedure are shown as solid curves in the Figure 7-2a. This allows us to separate different contributions to the charge generation and more importantly, to determine more accurately the time constants THT=30±10 fs for HT and T2=150 ± 30 fs for the additional growing component.

7.4. Photon-to-charge conversion

The relative efficiency of exciton-to-charge conversion in different blends can be estimated by comparing the maximal amplitudes of PIA responses (fig. 7-4). The response (and, thus, the number of holes generated on the polymer) scales linearly

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up to 30% of PCBM content, in a perfect agreement with the linear increase of blend absorption (fig. 7-1, right). However, for higher PCBM concentrations the PIA amplitude suddenly begins to decrease with increasing of acceptor amount. Therefore, only a minor fraction of excited PCBM molecules provides holes on the polymer chains. This, most likely, is related to the sample morphology as blends with a high PCBM content are known to develop phase separation on a length scale of tens of nm, forming rather pure PCBM domains.4, 52 Excitons generated away from immediate neighborhood of the acceptor-donor interface can not contribute to the charge generation on a sub-ps time scale, since there is not enough time to diffuse to the interface. As the PCBM clusters grow, the fraction of such excitons increases, thereby leading to a reduction of the observed signal.

To verify this hypothesis, we performed an AFM microscopy study of the morphology of the blends used in our PIA experiments. AFM images of the blend surfaces (for the very same samples which were used in optical experiments, prepared from chlorobenzene solutions) are shown in Fig.7-5. As long as the concentration of PCBM in the film does not exceed 15%, the AFM measurements indicate a flat surface with a typical roughness of ~1 nm and no signs of phase separation, within 4 nm spatial resolution. However, between 15 and 30% of

Figure 7-4. Peak amplitudes of the PIA response during the first ps after excitation at 630 nm

(red circles) and a characteristic scale of phase separation in the blend (diamonds) as a function of

PCBM content. The line on the left shows the linear growth of PIA amplitude with PCBM

concentration as follows from linear absorption (Fig.7-1, right).

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

175 175

PCBM content the morphology dramatically changes, showing phase separation in the blends with a typical length scale of ~20 nm. In previous studies such segregation was attributed to the formation of PCBM domains in the blend.52 Upon further increase of the PCBM fraction, the domain size rapidly grows up to ~400 nm. While the spatial scale of phase separation is similar to the one reported in previous studies 52-54, segregation in our films begins at lower PCBM concentrations, due to the drop-cast method employed here, as opposed to conventional spin-coating.4, 53

In order to establish at which phase separation PCBM excitations are efficiently harvested, we followed the approach of Refs.55, 56 and correlated the results of AFM measurements with the outcome of PIA experiments. The morphology of each sample was characterized by the average PCBM domain size obtained from an autocorrelation analysis (see experimental section 7.2. for details). The evaluated domain sizes as a function of PCBM concentration are shown in figure 7-4 and can be directly correlated with the PIA results. Although for PCBM concentrations lower than 30% it is difficult to reliably quantify the phase separation, the size of the largest domains does not clearly exceed 20 nm. For these concentrations, the PIA amplitude increases linearly with PCBM content, indicating that most of the excitations created in PCBM have reached the polymer/PCBM interface and dissociated into charges. When the PCBM domains become larger, the amplitudes of the PIA start to decrease, even though the linear absorption keeps on increasing (see figure 7-1, right). The observed correlation between the decrease of the amplitude of the PIA transients and increasing domain size clearly confirms the suggestion that for large domain sizes the PCBM excitations do not reach the acceptor-donor interface, and therefore can not contribute to the charge generation (at ps timescale).

A qualitatively similar conclusion about PCBM exciton dynamics was previously made on the basis of time-resolved fluorescence of the methanofullerene in a MDMO-PPV:PCBM bulk heterojunction.57 The presented picture is also in agreement with the recent results of Yamamoto et al., who observed that in the MDMO-PPV/PCBM blends with strong phase separation a hole (called cation in the paper) is formed on the methanofullerene 44. Apparently, when the phase separation in the blend is pronounced, a number of PCBM excitons are generated inside the acceptor domains. Due to spontaneous

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dissociation, a fraction of such excitons can produce methanofullerene-based holes. These holes, observed by Yamamoto et al.,44 cannot reach the polymer at ps timescales. This effect is observed in our measurements as low PIA response at high acceptor concentrations.

Inspired by the clear correlation between the PCBM domain sizes and exciton harvesting, we considered a simple domain model (i.e. with both phases fully separated) in order to extract the exciton diffusion length. Within this model, the excitons created at the interface dissociate into charges with the HT time of 30 fs. The excitons generated within a certain distance from the interface, reach the interface with a delay thereby causing the previously discussed additional growing component with the time constant of T2=150 ± 30 fs (similarly to the recently reported observation of exciton diffusion in the phase-separated co-polymer mixtures 56). Although the model was quite successful in describing the experimental data (fig.7-4 in particular) quantitatively, the extracted exciton diffusion coefficient in the PCBM domain was unrealistically high (~1 cm2/s). Another possibility is that the initial excitation is strongly delocalized in a PCBM crystalline structure and it moves toward the interface in a ballistic rather than diffusional manner because of electric field gradients. However, it is much more likely that the PCBM phase is not pure (as was explicitly assumed) but strongly percolated with short polymer segments.52, 55 These segments serve as charge separation centers and, at the same time, as probe molecules for ultrafast spectroscopy. Apparently, for larger phase separation spatial scales, the PCBM domains become more pure. We therefore conclude that sub-ps PCBM exciton harvesting is only efficient as long as the phase separation in the blend does not exceed 20 nm.

In view of the observed morphology effect, it is instructive to return to the discussion on deviations of transient anisotropy values from the PCBM-contribution curve (Fig. 7-3). The efficiency of photon-to-charge conversion, extracted from the figure 7-4, readily explains the mismatch between calculated and experimental values. As in the heavily phase-segregated blend not all photons absorbed by PCBM generate charges on the polymer within 0.1 ps, even a minor contribution of polymer excitations (with anisotropic response) is still sufficient to give a nonzero net anisotropy value.

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

177 177

The last issue we address is the origin of the T2 =150 fs component in the PIA transients. As we have mentioned before, the most possible explanation for additional delay is an intermediate process preceding the HT. However, the observed 150 fs time is much shorter then the previously reported ~1 ns singlet exciton radiative lifetime in PCBM.19, 58 On the other hand, the T2 time is reminiscent to a sub-ps timescale of formation and relaxation of charge carriers observed in C60 films 59, 60 and in PCBM nanodomains.44 Because of clear contradiction between the PCBM exciton lifetimes deduced from fluorescence and transient absorption data we can not confidently assign the 150 fs component to a particular process. What is clear is the presence of a fast energy transfer and/or relaxation process in the PCBM-nanocluster shell which brings the excited fullerene molecule into the state favorable for the charge separation. PCBM excitons detached from the polymer-fullerene interface can, probably, still contribute to the cell performance after dissociation into charges 44 or even after triplet generation due to efficient intersystem crossing 59. Nonetheless, although triplet excitations can probably still contribute to the photocurrent 61 they should be avoided as main precursors for the solar cell degradation.62

Figure 7-5. AFM images of the MDMO-PPV:PCBM film surface at different PCBM concentrations.

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7.5. Conclusions

Presented ultrafast spectroscopy study of charge dynamics after excitation of

MDMO-PPV:PCBM bulk heterojunction films into the PCBM absorption band has

shown that injection of holes into MDMO-PPV is delayed with respect to the

excitation. The delay displayed two components: the fast component, assigned to

the PCBM/polymer HT process with the time constant of ~30±10 fs, and the

subsidiary ~150 fs component, probably associated with preceding energy

transfer/relaxation in the PCBM nanoclusters. For the MDMO-PPV:PCBM blends

with the maximal size of phase separation below 20 nm, we observed that the

ultrafast charge separation after PCBM excitation is highly efficient. When the

phase segregation is more pronounced, the majority of PCBM excitons can not

reach the acceptor/donor interface at the ultrafast (ps) timescale, which

substantially lowers their harvesting.

The more recent and efficient P3HT:PCBM cells display phase separation on a

typical scale of 60 nm 4, which is larger then the phase separation scale optimal for

the fast conversion of PCBM excitons to charges. This is particularly important for

application of highly-absorbing fullerene derivatives, like [70]PCBM, in solar cells.

Utilization of acceptor for light-harvesting calls for the careful optimization of the

acceptor domain size in active layers of bulk heterojunction photovoltaic cells in

order to maximize the HT component in photocurrent generation.

7.6. Appendix: Data Modeling

To describe the experimental data, the following model is proposed. There are two

charge-separation pathways contributing to the observed PIA to be considered: (i)

charge generation after polymer excitation (PE), and (ii) HT charge generation after

excitation of PCBM. In accord with experimental observations, the contribution

from the PE was approximated by an instantaneous response (corresponding to

the formation of inter- and intra-chain excitons in polymer) followed by a partial

exponential decay with amplitude A1 and characteristic time TPE (to account for

recombination or/and change in the transition cross-section):

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

179 179

0,exp)( 21 ≥+

−⋅= tA

T

tAtA

PEPE (7-2)

where stands for the asymptotic (saturation) amplitude of the PE contribution.

To account for the HT delay and the subsequent growing component, the HT

response was approximated by a sum of two growing exponents

0,exp1exp1)(2

21 ≥

−−⋅+

−−⋅= t

T

tB

T

tBtB

HTHT

(7-3)

where and stand for the hole-transfer and subsidiary times (amplitudes),

respectively. The sum of the two contributions was convoluted with the Gaussian

apparatus function (with time width of at FWHM level) to yield PIA responses:

( )

−⊗+= 2ln4exp)()()(

2

2

res

HTPEPIAT

ttBtAtA (7-4)

The model also assumes that all time constants are concentration independent and

are, therefore, global fit parameters, while amplitudes Ai,Bi are characteristic for

each transient.

Table 7-1. The parameters of global fit to the experimental data by the model described in the text.

PCBM

mass content 1% 5% 15% 30 % 50 % 70 % Ge

)10( 31

−⋅A 1.8 1.2 1.4 1.2 1 0.6 n.a. PEA

)10( 32

−⋅A 2.4 2.9 3.6 4 0.6 0.3 2.2

)10( 31

−⋅B n.a. 1.6 5.3 11 9 5.6 n.a. HTB

)10( 32

−⋅B n.a. n.a. 2 6 3 2.2 n.a.

2121 BBAAA +++= 4.2 5.7 12.3 22.2 13.6 8.7 n.a.

PET 800 fs n.a.

HTT 30 ± 10 fs n.a.

2T 150 ± 30 fs n.a.

resT 80 fs !

Chapter 7

180 180

All isotropic transients in figure 7-2a (including the one from the Ge film) were

fitted simultaneously with this unified model. Solid curves in the figure 7-2a show

the outcome of the fitting procedure with the parameters presented in the Table 7-

1. The fit curves perfectly reproduce all essential features of the experimental

transients, indicating that the proposed model adequately describes the complete

set of experiential data. The resulted apparatus time fsTres 80= is in perfect

agreement with the response of the Ge film, as well as with the independently

measured autocorrelations for pump and probe pulses separately, and their cross-

correlation.

To verify the robustness of the fit procedure, we compared the relative

contributions of the PE and HT processes to the PIA response derived from the fit,

with those expected from the blend constituency. Figure 7-6 presents the results of

such comparison: diamonds and circles show the normalized amplitudes ascribed

to the PE ( ( ) AAA /21 + ) and HT ( ( ) ABB /21 + ) processes, respectively. The solid

curves present relative excitations of PCBM and MDMO-PPV calculated on basis

of their contributions to the net optical density, i.e. as

)1( NN

N

−+σ

σ and

)1()1(NN

N

−+

−

σ , respectively (where s is the ratio of extinction

coefficients of PCBM and MDMO-PPV at the excitation wavelength of 630 nm, and

stands for PCBM mass content). The amplitudes obtained from the global fit

Figure 7-6. Relative contribution to the PIA response from the PE (diamonds) and HT (circles)

components. The solid curves show relative excitations of PCBM and MDMO-PPV, calculated

on basis of their contributions to the net optical density.

Ultrafast Hole-Transfer Dynamics in the Polymer:PCBM Bulk Heterojunction

181 181

procedure convincingly coincide with the expected contributions of donor and

acceptor into the absorption of the samples, giving additional support to the

model. However, the points that belong to the highest concentration of 70%, are

slightly but nonetheless clearly off the trend. This signifies the breakdown of the

assumption of proportionality of the PIA response to sample optical density,

which is discussed in detail in Section 7-3.

The main output of the global fit procedure is accurate determination of HT time

as fsTHT 1030 ±= and the second (subsidiary) component rise time as

fsT 301502 ±= . The latter contribution is more pronounced in the blends with

high acceptor contents, where it accounts up to ~30% of the total signal amplitude.

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