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J. Phys. Chem. C 2008, 112, 3926-3934
Excitation Transport and Charge Separation in an Organic Photovoltaic Material: Watching Excitations Diffuse to Interfaces Larry W. Barbour, Ryan D. Pensack, Maureen Hegadorn, Sergei Arzhantsev, and John B. Asbury* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: July 27, 2007; In Final Form: December 14, 2007
Photoinduced charge separation and excitation transport to the interfaces of an organic photovoltaic material are examined using ultrafast visible pump-infrared probe spectroscopy. The carbonyl (CdO) stretch of the butyric acid methyl ester group of a functionalized fullerene, PCBM, is probed as a local vibrational reporter of the charge-transfer dynamics in a blend of PCBM with a conjugated polymer, CN-MEH-PPV. Following ultrafast excitation of the polymer, charge transfer to the fullerene occurs at the interfaces of the materials over many time scales ranging from sub-100 femtoseconds to nanoseconds. A fast charge-transfer component arises from excited states in the polymer that form near interfaces with PCBM aggregates where little or no excitation transport is required to affect charge separation. A slower component occurs because excited states that are created throughout the polymer phase must diffuse toward the interfaces to affect charge separation, giving rise to an induction period in the charge-transfer dynamics. Polymer segments are excited at a distribution of distances from the interfaces with a corresponding range of induction periods that collectively cause the slow component to appear with a roughly 3 ns time constant. The time scale and amplitude of the slow component is directly related to the spatial transport of excitations in the polymer, which suggests that this approach may prove useful for studying excitation transport in the complex phase-separated environments of organic solar cells.
I. Introduction Solar cells function by absorbing photons, converting the excitations into separated electrons and holes, and transporting the charge carriers to the electrodes to generate photocurrent.1,2 Organic solar cells convert excitations into separated electrons and holes primarily at interfaces of electron-donating and -accepting materials because the organic semiconductors on which the cells are based are excitonic in nature.3,4 The excitonic nature causes electron and hole pairs that form by absorption of light to be bound together by several tenths of an electron volt,1 a binding energy that is much greater than ambient thermal energy. At interfaces between electron-donating and -accepting materials, the driving force to separate the charge carriers is great enough to overcome this binding energy.1,2,4-16 To achieve efficient charge separation, organic solar cells must possess a high density of interfaces because excitations in the materials have short diffusion lengthsson the order of 10 nm.1,17-23 The need for high interfacial density causes the organic photovoltaic (PV) materials that are used in solar cells to have low charge carrier mobility,14 in part, because the interfaces impede the transport of charges through the materials.24 The low carrier mobility causes high series resistance, which reduces the efficiency of organic solar cells.7,13 A logical approach to overcome the need for high interfacial density is to develop organic PV materials that support longer diffusion lengths for photoinduced excitations.19-21 Longer diffusion lengths would permit larger interpenetrating networks of electron-donating and -accepting materials, which, in turn, would reduce the number of interfaces encountered by charge * To whom correspondence should be addressed.
carriers on their migration paths. A number of investigators have developed approaches to measure the exciton diffusion length in pure conjugated polymer films in an effort to understand how the diffusion length depends on the polymer characteristics.1,17-21 Others have addressed fundamental properties and dynamics of excitations in pure conjugated polymer films or nanoparticles.25-37 For example, Barbara25-29 and Huser30,31 developed single-molecule spectroscopy approaches to study nanoparticles consisting of individual polymer molecules. They directly observed dispersive excitation transport to the lowest-energy states within the nanoparticles.27-31 Schwartz and co-workers studied the influence of interchain excitations on the photophysics of conjugated polymers using a variety of optical and microscopy techniques.32-35 Vardeny and co-workers used ultrafast spectroscopy to identify the primary excited-state species in conjugated polymers and to study the properties of the materials that determine the probability of each excitation type.36,37 Ideally, the diffusion of excitations could be studied in the microphase-separated environments of organic PV polymer blends or small-molecule heterojunction materials. Recent work on excitation transport in polymers has demonstrated that the detailed conformation of polymer segments strongly influences their transport properties.29,34 The distribution of conformations depends on the morphology of the polymer films32 and on the deposition conditions.33 In light of these realizations, it is useful to study excitation transport in situ, under the morphology and deposition conditions that exist in organic solar cells. Ultrafast visible pump-infrared probe (vis-IR) spectroscopy is a promising approach to study the diffusion of excitations in organic PV materials through the influence of the transport process on
10.1021/jp0759628 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/20/2008
Watching Excitations Diffuse to Interfaces
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3927 slower component occurs because photons are absorbed throughout the polymer phase. Excitations that do not form near an interface must either diffuse for interfacial charge transfer to occur or undergo long-range electron transfer.57-62 Consideration of the morphology of the polymer blend indicates that the slow component of the bleach area kinetics results primarily from the diffusion of excitations to the interfaces. II. Experimental Procedures
Figure 1. (A) Structures of CN-MEH-PPV and PCBM. (B) SEM image of the morphology of the polymer blend. PCBM domains are roughly 50 nm spheres (light regions) surrounded by the polymer (dark regions).
the interfacial charge-transfer kinetics. Vis-IR spectroscopy has been used to study charge transfer in a variety of systems, including molecules adsorbed onto semiconductor nanoparticles38-50 and intermolecular51 and bridge-mediated intramolecular52-55 donor/acceptor pairs. As we demonstrate below, charge transfer in organic PV materials can be detected through the vibrational modes of the electron-donor or -acceptor materials using vis-IR spectroscopy. As excitations diffuse to the interfaces, the time dependence of the resulting chargetransfer reflects the time scale of excitation transport. The vibrational modes also provide structural information about the molecular environments where charge transfer occurs.24,56 In this contribution, we report the examination of charge separation in an organic photovoltaic material composed of a 1:1 mixture (by mass) of the conjugated polymer CN-MEHPPV and the electron-accepting functionalized fullerene, PCBM, (Figure 1A) using vis-IR spectroscopy. A SEM micrograph of a cross section of the polymer blend (Figure 1B) demonstrates that the materials phase separate into roughly 50 nm diameter spheres of the fullerene6,8,9 (lighter regions) that are surrounded by the polymer (darker regions). The carbonyl (CdO) stretch of the fullerene molecules, which appears at a frequency of 1740 cm-1 was studied as the reporting group to measure the chargetransfer dynamics of the system. The frequency of the carbonyl group has been found to be sensitive to the local structural environment24,56 and to the presence of electrons.24,38,53,54 The sensitivity of the carbonyl group to the presence of electrons allows us to examine charge transfer directly through the bleach of the carbonyl mode of the fullerene.24 The polymer is selectively excited at 550 nm, which induces charge transfer to the fullerenes across the interfaces between the materials. As electrons transfer, the ground-state absorption of the carbonyl mode of the fullerene is reduced, which results in a bleach in the vis-IR spectra. The time-dependent area of the bleach displays the extent of charge transfer at a given time and indicates that the process occurs over many time scales ranging from less than 100 fs to a few nanoseconds. The fast chargetransfer component arises from excitations in the polymer that form near an interface with the fullerene where little or no excitation transport is required to affect charge separation. The
An ultrafast Ti:sapphire laser (Quantronix) is used to pump two optical parametric amplifiers (OPAs, Light Conversion). One OPA is used to generate mid-IR pulses at 5.8 µm with 100 fs duration for the probe in the vis-IR experiment. A second OPA generates pump pulses at 550 nm with 1 µJ pulse energy and 100 fs duration. The infrared probe pulse is focused at the sample with a 200 µm diameter spot size. The visible pump pulse has a spot size of 250 µm at the sample. The time delay for the vis-IR experiment is adjusted with a computercontrolled 0.6 m linear translation stage. All experiments utilize a 64 element mercury cadmium telluride (MCT) dual array detector (Infrared Systems/Infrared Associates) to capture 32 probe frequencies simultaneously through a spectrograph (JY Horiba) while facilitating single-shot normalization. The organic PV material is prepared by combining the functionalized fullerene (PCBM, American Dye Source) and the polymer (CN-MEH-PPV, H.W. Sands) in chlorobenzene (1.2 and 1.1% by mass, respectively). The solution is drop cast onto a CaF2 window and is spun at 80 RPM to ensure that the film dries uniformly. For the microsecond time scale experiments, the mixture is drop cast onto a silver mirror. The sample is mounted on a two-dimensional computer-controlled translation stage to allow for automated raster scanning during the experiment, which ensures that a fresh spot on the sample is examined for each data point collected. For the ultrafast experiments, the sample on a CaF2 window is oriented to allow the visible pump pulse to interact with the sample before encountering the supporting window in the beam overlap region. Because the sample optical density is ∼2 at 550 nm, this geometry virtually eliminates any nonresonant signal in the experiment. Parallel and perpendicular pump and probe polarizations are used in the experiment where appropriate. Unless stated otherwise, the pump and probe polarizations are parallel to each other. The entire beam path and sample area in the ultrafast experiment are encompassed by a dry air-purged enclosure to remove ambient H2O. All experiments are conducted at room temperature. For the microsecond time scale experiments, the second harmonic of a pulsed Nd:YAG laser (Continuum) that is customized to operate at a repetition rate of 50 Hz is used to excite the polymer blend. The pulse energy used for the experiments is 1.5 ( 0.25 mJ, with a corresponding beam diameter of 6 mm. A compact ceramic Globar Light Source (HORIBA Jobin Yvon) is used to generate the infrared probe. The continuous-wave infrared probe light is focused on the sample, overlapped with the laser pulse, and subsequently dispersed in a monochromator (HORIBA Jobin Yvon). Transient absorption measurements are performed with a 1 × 1 mm2 MCT single-element detector, which is positioned at the exit slit of the monochromator and is doped for a cutoff wavelength of 16 µm (Infrared Associates, Inc.). A preamplifier with band-pass frequencies of 1.5 Hz to 1.0 MHz (Infrared Systems Development) is used to amplify the detector signal prior to digitization with either a computer-mounted 1 MHz analog/digital converter card (National Instruments) or a 350 MHz digital oscilloscope
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Figure 3. Two-dimensional surface plot of the carbonyl bleach spectra versus the corresponding time delay. Figure 2. Upper panel: Comparison of vis-IR spectra of the polymer blend following ultrafast optical excitation at 550 nm, with best-fit spectra used to extract the CdO stretch bleach. Lower panel: Linear IR spectra of the polymer blend compared with that of the pure polymer. The polymer does not have an IR-active vibrational mode at 1740 cm-1. All transient vibrational features arise from transferring electrons to PCBM.
(LeCroy), depending on the desired sampling frequency. Throughout the experiment, the monochromator is purged with dry air, and the sample stage is raster scanned using computercontrolled translation stages to prevent photodegradation of the polymer blend film. III. Results Visible pump-infrared probe (vis-IR) spectra of a polymer blend made from a 1:1 mixture (by mass) of the materials depicted in Figure 1A are represented in Figure 2 (upper panel) at several time delays following excitation with an ultrafast 550 nm optical pulse from the ultrafast laser system. The spectra focus on frequencies surrounding the carbonyl mode of the methyl ester group of PCBM. The transient spectra are characterized by an increase in transmission of the carbonyl mode (termed a ground-state bleach) that is superimposed on a broad absorption offset. For reference, the baseline corresponding to no change in transmission of the infrared probe appears at the top of the graph. No offsets were introduced to displace the spectra. Excitation of the blend at 550 nm selectively excites the polymer24 because this wavelength falls in a local minimum between two forbidden optical transitions of the fullerene63 and overlaps the rising edge of the polymer absorption. The photogenerated excitations in the polymer cause the broad featureless absorption offset that is observed in the spectra.36,64 The absorption offset is largely featureless because the polymer does not have infrared-active modes in this spectral region, as indicated by the linear infrared absorption spectrum of a pure CN-MEH-PPV film, which is depicted in the lower panel of Figure 2. The bleach of the carbonyl mode, which appears as the positive-going vibrational feature superimposed on the broad absorption offset, results directly from interfacial charge transfer from the polymer to the fullerene. Photoexcited conjugated polymers that are blended with fullerenes are known to transfer electrons to the fullerenes on ultrafast time scales.65-69 Charge transfer creates a new anionic species and reduces the carbonyl absorption of ground-state PCBM molecules. We specifically excite the polymer which has no infrared-active vibrational modes around 1740 cm-1. Consequently, the vibrational features in the vis-IR spectra arise exclusively from the reduced groundstate absorption of the carbonyl mode of PCBM as a result of
interfacial charge transfer.24 For comparison, the linear absorption spectrum of the polymer blend which contains PCBM molecules is represented in the lower panel of Figure 2. The vibrational transition in the linear spectrum corresponds to the carbonyl mode of the methyl ester group. The carbonyl bleach in the vis-IR spectra shows negligible dependence on the probe polarization relative to the pump pulse because interfacial charge transfer does not preserve polarization memory of the excitations from which electrons are transferred. The area of the carbonyl bleach in a vis-IR spectrum at a particular time delay is proportional to the concentration of electrons that have transferred to the fullerene domains. Therefore, the time dependence of the bleach area is a direct measure of the charge-transfer dynamics of the system. To quantify the time-dependent area, we developed a fitting procedure to extract the bleach spectrum from the absorption offset. The absorption offset is modeled with a third-order polynomial, and the bleach is characterized with a Gaussian line shape. The fit curve is the sum of the polynomial and Gaussian functions. We vary the parameters of the polynomial function while holding the Gaussian function constant and then vary the Gaussian function while holding the polynomial function constant. These steps are repeated to minimize the sum of the squares of the residuals from the comparison of the fit curve to the data. The entire procedure is repeated independently for each time delay that is recorded in the data starting from 50 fs. Examples of the fitting results are overlaid on the vis-IR spectra in Figure 2 for comparison. The polynomial function that describes the absorption offset for a given spectrum is visible as the line under the bleach feature in each spectrum. The comparison demonstrates that the fitting procedure provides a high-fidelity description of both the absorption offset and the carbonyl bleach spectrum. We note that we can fit the vis-IR spectra near the time origin because the nonresonant signal is negligible in the experiment. We coat the polymer blend film on CaF2 optical windows and orient the samples so that the optical pump pulse encounters the highly absorbing polymer blend before it passes through the supporting optical window. The intensity of the optical pump pulse in the CaF2 window is about 1% of its intensity before the sample, which virtually eliminates the nonresonant signal that typically occurs in pumpprobe experiments. Figure 3 displays the best-fit Gaussian functions for each time delay recorded in the experiment as a two-dimensional surface plot, where the horizontal axis is the time delay and the vertical axis represents the frequency. The amplitudes of the fit functions are represented by gradations in color, with the corresponding normalized values indicated in the color bar. The time axis is
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Figure 4. Carbonyl bleach area versus time delay. Overlaid on the data is a fit curve based on a detailed kinetic model (see text). The numbers correlate processes in the kinetic model with the observed features of the bleach area kinetics.
depicted on a logarithmic scale for display purposes. We will refer to the best-fit Gaussian function of a particular spectrum as the carbonyl bleach in the subsequent discussion. In this contribution, we focus on the time-dependent area of the bleach as a measure of the interfacial charge-transfer dynamics. In a separate publication,24 we focus on the time-dependent frequency shift of the bleach (highlighted by the dashed curve) as a measure of the motion of electrons in the PCBM domains. The interfacial charge-transfer kinetics are obtained by integrating the carbonyl bleach spectra to obtain their area at each delay time recorded in the experiment. Figure 4 depicts the bleach area plotted versus the corresponding delay time on a logarithmic scale. Three distinct processes are evident from the time dependence of the data: a fast rise on the ∼1 ps time scale, a slight decrease in area between 5 and 100 ps, and a second rise on the few nanosecond time scale. The processes are marked sequentially according to a kinetic model that is discussed below. We should point out that we have tested whether the second rise in the bleach area could result from an artifact in the data collection or analysis procedures. We repeated the experiment numerous times on many different samples and investigated the pump beam alignment, collimation characteristics, and spot size at the sample, only to find that the feature remained quantitatively the same. We also investigated whether the procedure to extract the bleach from the vis-IR spectra might have some systematic error that would cause the second rise. We varied the fitting procedures, starting parameters for the fitting algorithm, polynomial order, and spectral width over which the sum of the squares of the residuals are calculated. Again, the second rise consistently appeared in the results. We finally concluded that the second rise must represent a real process because its amplitude is significantsover 13% of the total bleach areasand the signal-to-noise ratio in the experiment is more than sufficient to resolve the process. Also, the time scale for the second rise is clearly distinguished from the other processes by at least 2 orders of magnitude. Vis-IR spectra of the carbonyl bleach (the data not their Gaussian fits) measured at 100 ps, 3 ns and 10 µs following the optical excitation pulse are compared with the linear IR absorption spectrum of the sample in Figure 5A. The spectra have been shifted by linear offsets to ensure that they have the same amplitudes at 1710 and 1780 cm-1. The purpose of this data-processing step is to remove the time-dependent absorption offset and to compare the vibrational features without needing to assume a particular line shape. The spectra have also been normalized to their maximum positive signal. The comparison shows that the bleach shifts toward lower frequency and closely matches the line shape and frequency of the linear spectrum by
Figure 5. (A) Transient vis-IR spectra collected at several time delays. The comparison indicates the growth of the PCBM anion peak on the microsecond time scale. The linear FTIR spectrum is overlaid on the vis-IR spectra for comparison. (B) Comparison of kinetics traces of the PCBM bleach and anion peaks recorded at 1740 and 1760 cm-1, respectively. The anion trace appears to grow in on the few microsecond time scale and decays synchronously with the bleach trace.
10 µs.24 The comparison also reveals the growth of a new absorption (negative ∆T/T) around 1760 cm-1 that forms on the microsecond time scale. Figure 5B represents kinetics traces of the PCBM bleach and the new absorption measured with the microsecond transient absorption instrument at 1740 and 1760 cm-1, respectively. The bleach trace displays an instrument-limited rise that is consistent with the immediate formation of the bleach peak following electron transfer from the polymer. The trace corresponding to the new absorption exhibits a biphasic rise consisting of a fast component that appears as an instrument-limited rise comprising 75% of the amplitude followed by a slower component with a ∼20 µs time constant. Both traces decay with identical kinetics on the ∼300 µs and few millisecond time scales. Multiexponential fit curves are overlaid on the kinetics traces that highlight the difference in the formation processes and the similarity in the decay dynamics. The features do decay completely, indicating that the new absorption at 1760 cm-1 does not result from photo-oxidation of the polymer.70,71 IV. Discussion IV.A. Assignment of the Second Rise of the Bleach Area. The observation of a recurrence in the signal from a nonequilibrium jump experiment is remarkable and generally indicates that the system undergoes complex dynamics. We wish to assign the origin of the second rise observed in the bleach area (Figure 4, process 3) as a means to understand the charge-transfer processes occurring in the blend. We begin by examining the bleach spectra represented in Figure 5A. First, the bleach adopts
3930 J. Phys. Chem. C, Vol. 112, No. 10, 2008 the equilibrium carbonyl spectrum of the sample in less than 10 µs. We recently assigned the time evolution of the bleach to the motion of electrons in the PCBM domains.24 Electrons initially transfer at the interfaces of the domains where the carbonyl frequencies are higher due to interactions with the polymer.24 The electrons then diffuse or drift into the domains and sample the equilibrium distribution of environments. As this process occurs, the bleach, which results from the formation of PCBM anions, shifts toward the equilibrium spectrum. Second, an absorption resulting from PCBM anions is conspicuously missing in the vis-IR spectra corresponding to 100 ps and 3 ns time delays. However, by 10 µs, a new absorption grows in at 1760 cm-1, which is assigned to the carbonyl absorption of the anions on the basis of its decay kinetics which match the recovery kinetics of the bleach (see Figure 5B). Because we selectively excite the polymer at 550 nm,24 the carbonyl bleach results from the transfer of electrons from the polymer to PCBM. The recovery of the bleach corresponds to back electron transfer from PCBM to the polymer. Since the new absorption at 1760 cm-1 decays on the same time scale as that of the bleach, we assign it to the carbonyl stretch of PCBM anions. We should point out that we have examined the temperature dependence of the carbonyl stretch and found it to be extremely insensitive to temperature changes24 probably because of the lack of hydrogen bonding in this system. Consequently, the bleach recovery and absorption decay dynamics that appear in Figure 5B are not a result of thermal redistribution in the film. The decay of the bleach and absorption features also demonstrates that they do not arise from photooxidation of the polymer.70,71 The kinetics trace measured at 1760 cm-1 also displays a slow growth component which is consistent with the absence of the 1760 cm-1 peak in the 3 ns vis-IR spectrum. We therefore conclude that the carbonyl stretch of PCBM anions appears at 1760 cm-1 on the microsecond time scale. On the few nanosecond time scale (3 orders of magnitude earlier), the anions contribute negligibly to the vis-IR spectra. Consequently, PCBM anions do not interfere with the measurement of the carbonyl bleach area on the few nanosecond time scale and cannot be the cause of the observed second rise in the bleach area (Figure 4, process 3). The slow appearance of the PCBM anion carbonyl absorption at 1760 cm-1 raises the obvious questions: What is the primary species that forms following electron transfer from the polymer to PCBM? What changes cause PCBM anions to exhibit a distinct carbonyl absorption microseconds after the electrontransfer event? Answers to these questions will provide insight into the charge-transfer and charge-trapping processes in organic photovoltaic materials. For now, we note that we have investigated a 200 cm-1 wide spectral window around 1740 cm-1 on the femtosecond to nanosecond time scales and have not found a distinct absorption that can be assigned to PCBM anions. The anions either absorb in a separate spectral window or their absorption is so broad as to be indistinguishable from the absorption offset resulting from excitations in the polymer.36,64 The latter case is possible if the carbonyl modes of PCBM are coupled to free electrons in the fullerene domains (carbonyl frequencies are known to be sensitive to electric fields).72-74 If this coupling exists, then small fluctuations in the charge distribution arising from diffusive or dispersive motion of the electrons or orientational motion of the carbonyl bond24 would change the carbonyl frequency, leading to fast dephasing and a corresponding broad vibrational line shape of the anions. On the basis of this reasoning, we speculate that the distinct absorption of PCBM anions forms as electrons in the fullerene
Barbour et al. domains become trapped, which reduces their mobility and possibly mitigates their dephasing influence on the carbonyl vibrations. We are in the process of examining the absorption of PCBM anions in polymer blends at various temperatures in order to gain insight into the formation of the distinct anion peak. Having established that PCBM anions do not interfere with the measurement of the carbonyl bleach area on the nanosecond time scale, we conclude that process 3 in Figure 4 must be related to changes in the concentration of electrons occupying PCBM domains. Two mechanisms can be envisioned to explain the observation of nanosecond time scale charge transfer: (1) long-range electron transfer or (2) excitation transfer to the interfaces followed by either short- or long-range electron transfer. Consideration of the morphology of the material provides a means to identify which mechanism is most important. From SEM imaging, we find approximately 50 nm spherical domains of PCBM imbedded in a matrix of the polymer (Figure 1B). We have determined that the film morphology is consistent throughout its 3 µm thickness. From the mass ratio of the polymer and PCBM components (1:1) and their densities, we can calculate their volume fractions. The density of most polymers is ∼1 g cm-3,75 while that of PCBM is estimated from the density of crystalline C60 (1.6 g cm-3).76 From these values, the volume fractions of the polymer and PCBM are 0.61 and 0.39, respectively. If we assume that the domains adopt ordered-packing geometries, such as the cubic geometry, then we can estimate the minimum average polymer thickness between the PCBM domains to be 12 nm. The maximum thickness between domains is estimated to be 25 nm. These average thicknesses are not sensitive to the specific packing arrangement of the domains because the volume fraction of the domains is relatively small in comparison to volume fraction of close-packed geometries (∼0.74). Photons are assumed to be absorbed randomly throughout the polymer phase, indicating that excitations are created at a distribution of distances ranging from 0-12 nm from the interfaces of the PCBM domains. Long-range electron transfer is of great importance in photosynthetic systems,59,77-79 molecular electronics,61,80,81 proteins,57,82 and DNA83-85 where electron-donating and -accepting species are separated by various bridging materials. In this case, the donor is the excited state in the polymer, and the acceptor is an individual or a collection of PCBM molecules in a domain. The material between these two species is assumed to be layers of π-π-stacked polymer chains. The tunneling probability decreases with distance according to P(r) ) exp[-βr], where β is the distance decay factor.57 Electron tunneling through toluene glass at 77 K has been studied in detail, yielding a distance decay factor of 1.23 Å-1.58 This large distance decay factor is believed to result from the significant reduction of electronic coupling between the donor and acceptor species due to the van der Waals gaps between the solvent molecules.57,58 These gaps exist between the π-π-stacked chains in the polymer blend; therefore, we adopt the same value of β ) 1.23. Ultrafast electron transfer from conjugated polymers to fullerenes has been observed on the 45 fs time scale.66 We use ko ) (45 fs)-1 as the intrinsic electron-transfer rate at the minimum donoracceptor separation for this system. The apparent rise time of the slow rise of the bleach area (few nanoseconds) in combination with the distance-dependent decay factor sets an upper limit of ∼0.9 nm for long-range electron transfer. Comparison of the volume of 0.9 nm thick polymer shells surrounding the PCBM domains with the total volume of the polymer indicates that
Watching Excitations Diffuse to Interfaces about 7% of the absorbed pump photons create excitations within these shells. Even if a value of β ) 1.0 is adopted for the distance decay factor, the upper limit for the long-range electron-transfer distance increases to only 1.1 nm, with about 9% of the photons absorbed within this distance from the PCBM domains. The exciton diffusion length of PPV-based polymers is reported to be 5-7 nm.17,19-21 Taking the average of these measurements, we estimate that 60% of the pump photons that are absorbed by the polymer create excitations within 6 nm of the PCBM interfaces and are thus able to diffuse to an interface and undergo either short- or long-range electron transfer. We cannot unambiguously determine the average distance over which electron transfer occurs from the present data. Regardless of the precise distance, we can conclude that excitation transport is the dominant mechanism that allows at least 85% (9% compared to 60%) of the excitations that are capable of transferring electrons to do so. Thus, we will discuss and model the recurrence of the bleach area represented in Figure 4 exclusively in terms of excitation transport in the following section with the understanding that long-range electron transfer may be important for excitations within approximately 1 nm of the interfaces of the PCBM domains. IV.B. Kinetics of Excitation Transport. Three distinct processes appear in the bleach area kinetics represented in Figure 4 that are described by a kinetic model which is detailed below. The fast rise in the bleach area (process 1) corresponds to charge transfer from excitations that form in regions of the polymer that are near an interface with a PCBM domain. The process occurs on the approximately 100 fs to 1 ps time scales.65-69 The bleach area reaches a maximum around 5 ps and then decays on the tens of picosecond time scale (process 2). This decay of the bleach area can be rationalized by two distinct explanations. The decay may result from back electron transfer. The bleach area is proportional to the concentration of perturbed PCBM molecules. Therefore, a partial recovery of the bleach may indicate a decrease in the concentration of electrons associated with back electron transfer to the polymer. An alternate explanation is that the recovery of the bleach arises from the diffusion of holes in the polymer phase.24 This explanation is based on the idea that since the presence of electrons perturbs the carbonyl stretch of PCBM molecules, perhaps the presence of holes near the interfaces of the PCBM domains also perturbs the carbonyl stretch. Conjugated polymers are known to be hole-transport materials; therefore, it is conceivable that the holes formed by electron transfer may diffuse away from the interfaces of the domains. As the holes diffuse away from the interfaces, their perturbation of the carbonyl stretch will decrease, which might result in the apparent recovery of the bleach. We are unable to determine whether back electron transfer or hole diffusion is principally responsible for the bleach recovery. We are currently examining this system in other spectral regions in an effort to obtain an independent measure of the charge-transfer kinetics as a means to distinguish between these processes. For the purposes of clarity in the following description of the kinetic model, we arbitrarily decide to discuss process 2 in terms of back electron transfer, with the understanding that the exact assignment of the process remains to be established. In section IV.A, we assigned the second rise of the bleach area (Figure 4, process 3) to the diffusion of excitations in the polymer phase toward the interfaces of the PCBM domains followed by either short- or long-range electron transfer. The time required for a particular excitation to diffuse to an interface
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Figure 6. Kinetic model describing charge-transfer processes. The numbers correlate processes in the kinetic model with observed features of the bleach area kinetics (see Figure 4).
results in a delay between the arrival of the excitation pulse and the observation of electron transfer. This delay causes an induction period in the charge-transfer kinetics. Because we excite polymer segments at a distribution of distances from the PCBM domains, the charge-transfer kinetics are influenced by a distribution of induction periods that collectively cause the second rise of the bleach area to occur over time scales ranging from 100 ps to a few nanoseconds. We should point out that it is unlikely that the second rise in the bleach area results from the diffusion of holes back to the interfaces of the PCBM domains. The back-electron-transfer rate in this system is very low,86-88 which is not consistent with the formation of a high density of holes residing at the PCBM domain interfaces on the nanosecond time scale. In order to quantify the time dependence of the excitation transport dynamics, we developed a detailed kinetic model consisting of the following coupled differential equations. The model is depicted graphically in Figure 6.
dNe(t) ) kfN/INT(t) - kbNe(t) dt dN/INT(t) ) kdN/BLK(t) - (kf + kr)N/INT(t) dt dN/BLK(t) ) -(kd + kr)N/BLK(t) dt
(1)
Referring to Figure 6, the terms Ne(t), N/INT(t), and N/BLK(t) correspond to the number density of electrons in PCBM domains, PCBM-, the density of excitations in interfacial polymer environments, PPVINT, and the density of excitations in bulk polymer environments, PPVBLK, respectively. The rate constants kf, kb, kd, and kr refer to the rates of forward charge transfer to PCBM, back charge transfer to the polymer, diffusion of excitations to the interfaces, and excited-state relaxation within the polymer, respectively. We assume that excited-state relaxation is the same at the interfaces and in the bulk. Back charge transfer and excited-state relaxation refill the ground state of the polymer. Inclusion of these relaxation processes ensures that the model maintains a detailed balance and accurately describes the charge-transfer dynamics. To summarize the kinetic model in words, the time-dependent number density of electrons in the PCBM domains, Ne(t), is determined by the rates of forward charge transfer, kfN/INT(t), and back electron transfer, kbNe(t). The time dependence of the number density of excitations at the interfaces, N/INT(t), is determined by the rates of forward charge transfer, diffusion of excitations toward the interfaces, kdN/BLK(t), and excited-state relaxation, krN/INT(t). Finally, the time-dependent number density of excitations in the bulk of the polymer, N/BLK(t), is determined by the rates of diffusion of excitations toward the
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interfaces and by excited-state relaxation. We have assumed that once an excitation reaches an interface, it will always transfer an electron because the rate of forward charge transfer is much faster than the rate of diffusion of excitations to the interfaces. Therefore, we have not included a term for excitations diffusing away from an interface. For the purpose of modeling the data, we assume that the fast rise (Figure 4, process 1) describes the time dependence of electron transfer for excitations that initially form near the interfaces and for excitations that must first diffuse to an interface. All electron-transfer processes are assumed to be fast and therefore short range. The solutions to the set of coupled differential equations (eq 1) are given below
Ne(t) ) C1 exp[-kbt] - C2(exp[-(kf + kr)t] - exp[-kbt]) C3{(kd - kf)exp[-kbt] + (kf - kb + kr)exp[-(kd + kr)t] + (kb - kd - kr)exp[-(kf + kr)t]} (2)
N/INT(t) ) C2 exp[-(kf + kr)t] C3(kd/(kf - kd))(exp[-(kf + kr)t] - exp[-(kd + kr)t]) (3) N/BLK(t) ) C3 exp[-(kd + kr)t]
(4)
The solution of relevance for fitting the bleach area kinetics is eq 2, which describes the number density of electrons in the PCBM domains as a function of time. Frequently, dynamical processes in micro-heterogeneous systems are not well characterized by single-exponential processes.89-91 To describe the charge-transfer dynamics in the polymer blend material, we introduced biexponential functions to replace the single-exponential functions that model the forward- and backward-charge-transfer and excited-state relaxation processes in eq 2. The substitutions are as follows
exp[-kft] f a exp[-kf1t] + (1 - a)exp[-kf2t] exp[-kbt] f b exp[-kb1t] + (1 - b)exp[-kb2t] exp[-krt] f c exp[-kr1t] + (1 - c)exp[-kr2t]
(5)
We found that a single-exponential function was sufficient to describe the diffusion of excitations. Substituting eq 5 into eq 2 and reorganizing yields the final kinetic model that was used to fit the bleach area kinetics
Ne(t) ) A{B(β(t) - φ(t)) + (1 - B)[(kd - Cf)β(t) + (Cf - Cb + Cr)δ(t) + (Cb - kd - Cr)φ(t)]} (6) β(t) ) b exp[-kb1t] + (1 - b)exp[-kb2t] δ(t) ) c exp[-(kd + kr1)t] + (1 - c)exp[-(kd + kr2)t] φ(t) ) ac exp[-(kf1 + kr1)t] + (1 - a)c exp[-(kf2 + kr1)t] + a(1 - c)exp[-(kf1 + kr2)t] + (1 - a)(1 - c)exp[-(kf2 + kr2)t] (6a)
Cf ) akf1 + (1 - a)kf2 Cb ) bkb1 + (1 - b)kb2 Cr ) ckr1 + (1 - c)kr2
(6b)
The time ordering of the visible and infrared pulses in the visIR experiment mandates that C1 in eq 2 is identically zero. Electrons do not transfer to PCBM domains until after the visible pulse excites the sample. We have replaced C2 and C3 in eq 2
Figure 7. Population dynamics of excitations in CN-MEH-PPV collected at 400 and 460 nm excitation wavelengths measured using wavelength-integrated TCSPC. Population relaxation is excitation wavelength independent after the first few hundred picoseconds.
TABLE 1 a parameter
value
parameter
value
kf1b kf2b ac kb1b kb2d be
(1/0.09 ps) (1/0.9 ps) 0.30 (1/42 ps) 0 0.12
kr1c kr2c cc kdb Ae Be
(1/0.93 ns) (1/3.1 ns) 0.45 (1/2.5 ns) 0.032 0.96
a Parameters of the kinetic model used to fit the data are represented in Figures 4 and 7. b Estimated uncertainty: (30%; see text. c Estimated uncertainty: (10%; see text. d Estimated uncertainty: assumed to be zero; see text. e Estimated uncertainty: (5%; see text.
with the constants A and B, where A represents the overall amplitude of the fit function and B partitions the amplitude between the first term, which describes forward and back charge transfer from excitations that form near an interface, and the second term, which involves charge transfer from excitations that must first diffuse to an interface. The time scales for the forward- and back-charge-transfer and excitation-transport processes can be determined from the bleach area kinetics because each process occurs on a distinct time scale (Figure 4, processes 1-3). However, excited-state relaxation occurs on a similar time scale as that of excitation transport. To decouple these processes in our fitting procedure, we independently determined the excited-state relaxation dynamics through the photoluminescence decay of a pure CNMEH-PPV film measured with a time-correlated single-photon counting (TCSPC) instrument.92 To isolate the excited-state relaxation dynamics, we measured photoluminescence decay traces at 525 nm and at 50 nm intervals from 550 to 750 nm. These decay traces were averaged to produce the wavelengthintegrated photoluminescence decay traces that are displayed in Figure 7. Decay traces corresponding to excitation wavelengths of 400 and 460 nm are compared to evaluate the dependence of the excited-state dynamics on the wavelength of excitation. Because a Ti:Sapphire oscillator in conjunction with a pulse picker are used as the pulsed excitation source in the TCSPC instrument, excitation wavelengths greater than 460 nm are not readily accessible. Fortunately, comparison of the 400 and 460 nm decay traces demonstrates that the excitedstate relaxation dynamics are independent of the excitation wavelength after the first 300 ps. Because excitation transport occurs on time scales much longer than 300 ps, we can use the photoluminescence decay to estimate the excited-state relaxation of CN-MEH-PPV in the polymer blend sample. A biexponential fit curve is overlaid on the photoluminescence decay traces in Figure 7. From the fit, we obtain the parameters kr1, kr2, and c of eqs 5 and 6. The values of these parameters are provided in Table 1. We have determined that the parameters describing the charge-transfer processes in the kinetic model are not
Watching Excitations Diffuse to Interfaces sensitive to moderate variations in the parameters that define the excited-state relaxation dynamics. The fit of eq 6 to the bleach area kinetics appears in Figure 4 as the curve overlapping the experimental data. Having independently measured the excited-state relaxation dynamics, the data contain sufficient information to determine all of the parameters that appear in eq 6. In particular, the parameters describing forward charge transfer, kf1, kf2, and a, are determined by the shape of the data within the first 5 ps (process 1). The depth and time scale to approach the local minimum around 100 ps determines the parameters kb1 and b, which describe back charge transfer (process 2). The back-charge-transfer process contains a slow component whose time scale is long compared to the 10 ns window covered by the data86-88 (Figure 5B). Consequently, we let kb2 approach zero in the fitting procedure. Finally, the relative amplitudes of the fast and slow rise components (processes 1 and 3) determine the parameter B, and the time scale of the slow rise determines the rate of excitation transport, kd. When fitting the data, we convolved eq 6 with a 200 fs Gaussian function that represents the time response function of the ultrafast instrument. The results of the leastsquares fitting procedure are provided in Table 1. We note that the bleach area at 50 fs is greater than the fit curve in Figure 4 because a portion of the ultrafast charge-transfer process occurs on the 45 fs time scale.66 We did not include a 45 fs component because we wished to minimize the number of parameters in the model and because the time resolution of our experiment was about 200 fs. Our intention in constructing the kinetic model is not to propose a particular functional form that describes the underlying charge-transfer and excitation-transport processes or to report an exact time constant for a given process. Both of these properties are likely to depend on the detailed deposition conditions and phase-separation morphology of the film,29,32-34 which will be addressed in a future publication. Instead, we wish to construct a model that captures the general features of the dynamics with sufficient fidelity to approximate the amplitude and time scale of charge transfer arising from excitation transport relative to charge transfer from excitations that initially form at the interfaces of the polymer blend. The relative amplitudes of these processes are given by the ratio of the terms involving (1 - B) and B in eq 6, respectively. Substitution of the parameters that are listed in Table 1 into eq 6 shows that approximately 30% of the electrons that transfer to PCBM domains originate from excitations that initially form in the bulk of the polymer phase and must diffuse to an interface. This proportion remains fairly constant for a variety of fitting parameters. The remaining 70% of the transferred electrons come from excitations that form near an interface. The rate constant, kd, indicates that excitations diffuse over a variety of distances to the interfaces on the 2-3 ns time scale. Having studied one polymer blend composition thus far, we are unable at present to convert the measurement of the diffusion rate and amplitude into an equivalent diffusion length. However, the vis-IR method that we have utilized provides a pathway for this measurement. We will vary the thickness of the polymer phase and examine the associated change of the amplitude and time scale of the slow charge-transfer component. As the average thickness approaches the diffusion length, the time scale of the slow component should begin to decrease. By comparing the time constant of the slow component with the thickness of the polymer phase, an average transport velocity can be estimated with a corresponding diffusion length. The average thickness of the polymer phase can be determined using SEM imag-
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3933 ing,6,8,9,93 while variation of the thickness can be accomplished by judicious adjustment of the concentration ratio of polymer to PCBM, the solvent, and its evaporation rate.6,8,9,94,95 This approach is, in fact, very similar to methods that are used to measure the exciton diffusion length in pure polymer films.19-21 The principal advantage of our approach is that the measurements can be made under the morphology and deposition conditions that exist in organic solar cells. IV. Concluding Remarks The results of ultrafast visible pump-infrared probe spectroscopy were used to study photoinduced charge separation and excitation transport in a polymer blend organic PV material. Ultrafast excitation of the conjugated polymer, CN-MEH-PPV, results in charge transfer to the electron-accepting functionalized fullerene, PCBM. The time-dependent area of the ground-state bleach of the carbonyl mode of PCBM provides a direct measure of interfacial charge-transfer dynamics. Charge transfer occurs in two distinct phases. A fast component occurs on the picosecond and faster time scale, which corresponds to charge transfer from excitations in the polymer that form near an interface with PCBM domains. A slower component arises from excitations that form throughout the polymer phase, not just at the interfaces. These excitations must diffuse to an interface to affect charge separation, which results in an induction period in the charge-transfer dynamics. Because a distribution of polymer segments are excited, the corresponding distribution of induction periods gives rise to a slow charge-transfer component that occurs on the 100 ps to a few nanosecond time scales. A detailed kinetic model is developed to describe all of the relevant dynamical processes affecting the charge-transfer dynamics. From the model, excitations are found to diffuse to the interfaces on the 2-3 ns time scale. These excitations contribute about 30% of the total number of electrons that are ultimately transferred to the PCBM domains. Using the kinetic model, the time scale and amplitude of the slow component can be directly related to the spatial transport of excitations in the polymer. By varying the thickness of the polymer phase, the vis-IR spectroscopy approach provides a means to measure the diffusion length of excitations under the morphology and deposition conditions that are found in organic solar cells. Acknowledgment. We would like to thank the Camille and Henry Dreyfus New Faculty Awards Program, the Petroleum Research Fund (Grant No. 45293 -G 10), and the Pennsylvania State University for support of this research. J.B.A. would like to thank Mark Maroncelli for generous access to the TCSPC instrument. References and Notes (1) Halls, J. J. M.; Friend, R. H. Organic Photovoltaic Devices. In Clean Energy from PhotoVoltaics; Archer, M. D., Hill, R., Eds.; Imperial College Press: London, 2001; Vol. 1, p 377. (2) Organic PhotoVoltaics: Mechanisms, Materials, and DeVices; CRC Press: Boca Raton, FL, 2005; Vol. 99. (3) Gregg, B. A. Mater. Res. Soc. Bull. 2005, 30, 20. (4) Gledhill, S. E.; Scott, B.; Gregg, B. A. J. Mater. Res. 2005, 20, 3167. (5) Brabec, C.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (6) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. AdV. Funct. Mater. 2004, 14, 1005. (7) Janssen, R. A. J.; Hummelen, J. A.; Sariciftci, N. S. Mater. Res. Soc. Bull. 2005, 30, 33. (8) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45.
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