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Charge Trapping in Organic Photovoltaic Materials Examined with Time-Resolved Vibrational Spectroscopy† Ryan D. Pensack, Kyle M. Banyas, and John B. Asbury* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: May 29, 2009; ReVised Manuscript ReceiVed: July 13, 2009
Bimolecular charge recombination is examined in a polymer blend photovoltaic material using time-resolved vibrational spectroscopy on the 100 fs to millisecond time scales. The carbonyl (CdO) stretch of the functionalized fullerene, PCBM, is probed as a local vibrational reporter of electron transfer and subsequent bimolecular charge recombination in a blend of the conjugated polymer, CN-MEH-PPV, with PCBM. Electron transfer from CN-MEH-PPV to PCBM occurs on the sub-100 fs to 1 ps time scale following ultrafast excitation of the conjugated polymer. Bimolecular charge recombination occurs much more slowly on the 100 µs time scale, in accord with recent optical measurements of charge recombination in similar MDMO-PPV-based polymer blends. However, a new vibrational feature that is assigned to the negative polaron of PCBM appears in the transient spectra on the few microsecond time scale. The appearance of this feature on time scales that are slow in comparison to electron transfer but fast with respect to bimolecular charge recombination suggests that it arises from an intermediate relaxation process, perhaps resulting from the formation of trapped electrons. The time scale for the appearance of the negative polaron absorption coincides with the bimolecular charge recombination lifetime of similar MDMO-PPV-based polymer blends that have been reported in the literature from transient photocurrent measurements. I. Introduction Organic solar cells offer the possibility of inexpensive solar energy because they can be processed from solution.1-4 Significant progress has been made toward enhancing the efficiency of organic solar cells.5-10 However, bimolecular charge recombination remains an important loss mechanism in the devices, particularly at their maximum power point.11 At this point, the electric field that drives charge carrier drift in the organic photovoltaic (OPV) layer is suppressed by application of an external load in comparison to the short-circuit condition of the device.12 As a result, the transit time of charge carriers approaches the time scale for bimolecular charge recombination, leading to reduced fill factors and lower power conversion efficiency. For example, approximately 25% of photogenerated charge carriers recombine in optimized solar cells composed of polymer blends of MDMO-PPV with PCBM (see Figure 1) that are operated at their maximum power point.12 The importance of bimolecular charge recombination as a loss mechanism depends on the type of OPV material that is examined. Organic solar cells based on polymer blends of RRP3HT (Figure 1) with PCBM are capable of collecting over 90% of photogenerated charge carriers under short-circuit conditions.13,14 Consistent with this capability, RR-P3HT-based organic solar cells exhibit bimolecular charge recombination on the several hundred microsecond time scale in transient photocurrent studies.15-19 This time scale is orders of magnitude longer than the time scale predicted from Langevin’s meanfield theory of bimolecular charge recombination in which the recombination coefficient is proportional to the charge carrier mobility divided by the dielectric constant.20 In principle, Langevin’s theory should describe the longest time scale on which bimolecular recombination occurs, so the observation of †
Part of the “Barbara J. Garrison Festschrift”.
Figure 1. Molecular structures of two conjugated polymers and the electron accepting fullerene, PCBM, that have been extensively studied in organic photovoltaic devices.
recombination on longer time scales is quite notable. In contrast, organic solar cells composed of polymer blends of MDMOPPV with PCBM appear to undergo Langevin bimolecular charge recombination in transient photocurrent measurements on time scales that are 2 orders of magnitude faster than time scales observed in RR-P3HT-based devices.18,21,22 These observations are consistent with the relative importance of bimolecular charge recombination in organic solar cells fabricated from MDMO-PPV:PCBM polymer blends.12 However, the origin of these qualitatively different recombination processes remains unclear. Recent measurements of bimolecular charge recombination by Durrant and co-workers may provide insight into the mechanistic origin of the distinct recombination in MDMOPPV:PCBM versus RR-P3HT:PCBM polymer blends. In particular, these authors observed pump-energy-independent bimolecular charge recombination on the 100-400 µs time scale inMDMO-PPV:PCBMpolymerblendsusingopticalspectroscopy.23,24 The blends that were examined were of similar composition to polymer blends in which recombination was observed on the few microsecond time scale using transient photocurrent measurements.21 While insufficient information is available to determine exactly why the measured recombination lifetimes
10.1021/jp905061y 2010 American Chemical Society Published on Web 10/09/2009
Charge Trapping in Organic Photovoltaic Materials
Figure 2. (top) Structure of the conjugated polymer CN-MEH-PPV examined in this study. (bottom) Linear IR spectra of a 1:1 polymer blend of CN-MEH-PPV with PCBM compared with a spectrum of the pure polymer. The polymer does not have an IR-active vibrational mode at 1740 cm-1, indicating that all transient vibrational features arise from transferring electrons to PCBM.
varied so much between the optical and photocurrent techniques, one likely explanation is that the optical measurements report the recombination kinetics of the holes in the polymer blend regardless of their mobility, while the photocurrent measurements report the kinetics of the most mobile charge carriers. If electrons or holes are deeply trapped in the MDMO-PPV:PCBM polymer blends, then the photocurrent measurements would be morestronglyinfluencedincomparisontotheopticalmeasurements. In an effort to further explore bimolecular charge recombination in OPV materials, we examined the charge recombination lifetime in a polymer blend of CN-MEH-PPV (Figure 2, top) with PCBM using time-resolved infrared (TRIR) spectroscopy. A linear infrared (IR) spectrum of a 3 µm thick film of a 1:1 (by mass) blend of CN-MEH-PPV with PCBM is displayed in Figure 2 (bottom) in the region of the carbonyl (CdO) stretch of PCBM around 1740 cm-1. Included in Figure 2 is a linear IR spectrum of CN-MEH-PPV without PCBM, demonstrating that the polymer has no IR active vibrational modes in this spectral region. We chose this particular conjugated polymer because the absorption cross section of polarons in the 1740 cm-1 region is smaller in comparison to the polaron cross section in MDMO-PPV, permitting us to examine the charge carrier dynamics in the blend through the carbonyl vibrational features of PCBM.25-30 We find that the frequency of the carbonyl stretch of PCBM is sensitive to changes in the charge distribution following electron transfer from CN-MEH-PPV. This sensitivity is used to examine the bimolecular charge recombination kinetics in the blend. In particular, we observe bimolecular charge recombination on the ∼100 µs time scale similar to Durrant and co-workers.23,24 However, we also observe a new vibrational feature assigned to the negative polaron of PCBM that appears on the few microsecond time scale similar to the transient photocurrent studies.21 Our data in conjunction with previous reports from the literature suggest the reaction scheme displayed in the top panel of Figure 3 may describe charge carrier dynamics in organic photovoltaic materials. We propose that, following mobile carrier generation, PPV-based polymer blends with PCBM undergo charge trapping on the few microsecond time scale, dramatically decreasing the mobility, µ, of the charge carriers and inhibiting their detection with transient photocurrent techniques. Because the density of charge carriers, n(t), is still
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Figure 3. (top) Reaction scheme describing charge carrier dynamics in PPV-based organic solar cells following optical absorption. Mobile charges become trapped before they recombine. Charge trapping causes the mobility of charge carriers to approach zero, which inhibits their detection with transient photocurrent techniques. (bottom) Timeresolved vibrational spectroscopy enables these processes to be examined through their influence on the line shapes of the vibrational modes of the materials.
significant, trapped charges can be detected through the spectroscopic signatures of polarons using optical or infrared spectroscopy. Time-resolved vibrational spectroscopy permits direct measurement of this charge trapping process because the line shapes of certain vibrational modes are sensitive to fluctuations in the local electric fields arising from the motion of mobile electrons in the materials (Figure 3, bottom). Thus, we assign the new vibrational features observed in the transient vibrational spectra to the formation of trapped electrons. II. Experimental Procedures The procedures for the ultrafast visible pump-infrared probe (vis-IR) and time-resolved IR (TRIR) experiments have been described in detail.26,27 Briefly, the ultrafast instrumentation is based on a Ti:sapphire laser system (Quantronix) that is used to pump two optical parametric amplifiers (OPAs, Light Conversion). One OPA is used to generate mid-infrared (IR) pulses that serve as the probe source. For the experiments reported here, the probe is tuned to the 5.8 µm region to examine the carbonyl stretch of PCBM. The second OPA is used to generate tunable visible pump pulses that can be adjusted to specifically excite CN-MEH-PPV at 550 nm. The cross correlation time for the IR and visible pulses is less than 200 fs. The ultrafast probe beam is dispersed in a monochromator and detected with a 64-element dual array mercury cadmium telluride (MCT) detector (Infrared Associates/Infrared Systems Development). The second harmonic (532 nm) of a pulsed Nd: YAG laser is used as the excitation source for the TRIR experiments. A compact ceramic globar light source is used to generate an IR continuum probe via blackbody radiation. After interacting with the region of the sample that is excited by the excitation source, the IR probe light is dispersed in a monochromator and detected by a single element MCT detector (Infrared Associates/Infrared Systems Development). The optical excitation density in both the ultrafast vis-IR and TRIR experiments is 2-3 mJ/cm2. For comparison, this excitation density corresponds to a 1 µJ/pulse laser beam focused to a spot size of 200 µm diameter at the sample. We note that all experiments were conducted at the same excitation density such that the observation of two characteristic time scales in the experiments described below is not a result of different excitation conditions. The OPV samples were prepared as previously described.26,27 Briefly, the functionalized fullerene (PCBM, American Dye Source) and the polymer (CN-MEH-PPV, American Dye
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Figure 4. Comparison of the visible absorption spectrum of the 1:1 CN-MEH-PPV:PCBM polymer blend with the absorption spectrum of the same amount of pure PCBM present in the polymer blend. The absorption of CN-MEH-PPV dominates the optical density of the sample at the excitation wavelength of 550 nm.
Figure 5. Comparison of the vis-IR spectra of the CN-MEH-PPV: PCBM polymer blend following optical excitation at 550 nm with the best fit spectra used to extract the CdO stretch bleach. The absorption of polarons in the polymer gives rise to the offsets between the spectra.
Source) are dissolved in chlorobenzene (1.0% by mass, each). The solution is either drop cast onto CaF2 optical flats (for visIR experiments) or silver mirrors (for TRIR experiments). The samples are either held under inert nitrogen atmosphere or in a vacuum.
no IR active vibrational modes in the 1740 cm-1 spectral region, the transient vibrational feature observed in Figure 5 arises exclusively from PCBM. The inner-filter calculation described above demonstrates that most of the pump photons at 550 nm are absorbed by CN-MEH-PPV in the 1:1 blend with PCBM. Therefore, the transient vibrational features in Figure 5 arise predominantly from electron transfer from photoexcited CNMEH-PPV to PCBM which reduces the absorption of the carbonyl stretch of neutral PCBM. The amplitudes of the carbonyl bleach in the transient spectra demonstrate that the ultrafast electron transfer process is largely complete within the first few picoseconds. We have determined that a second electron transfer process occurs on the 2-3 ns time scale that results from excitons that form far from CN-MEH-PPV:PCBM interfaces.27 These excitons must diffuse to an interface to transfer an electron. The time scale for this process is limited by the excited state lifetime in CN-MEH-PPV, which is about 3 ns.27 The area of the carbonyl bleach at a particular time delay is proportional to the concentration of electrons that have transferred to PCBM molecules at that time. Because the electron transfer process randomizes the polarization memory of the 550 nm optical excitation pulse,39 the carbonyl bleach amplitude is isotropic and does not need to be corrected for orientational relaxation dynamics. A fitting procedure was developed to quantify the time dependence of the bleach area. In this procedure, the polaron absorption is modeled with a third-order polynomial to fit the shape. The carbonyl bleach spectrum is described by a Gaussian line shape. A Gaussian line shape is extracted from each transient spectrum using a least-squares fitting procedure in which the sum of the third-order polynomial and the Gaussian function is compared to the data. This procedure is repeated independently for each time delay recorded in the experiment. Examples of the results of the fitting procedure are overlaid on the transient IR spectra in Figure 5. The Gaussian line shapes are visible by comparing the data (and fit curves) with the polynomial functions that appear as the lines under the bleach features. Transient spectra are fit using this procedure starting at 50 fs time delays. Before this time delay, the bleach feature is too small for the algorithm to converge. We are able to fit the transient IR spectra near the time origin of the experiment because the nonresonant signal that results during the pulse overlap is negligible. We coat the polymer blend films onto CaF2 optical flats and orient them such that the visible and IR beams encounter the strongly absorbing
III. Results The visible absorption spectrum of a 1:1 (by mass) polymer blend of CN-MEH-PPV with PCBM is displayed in Figure 4. The optical density at the absorption maximum around 500 nm is approximately 4, corresponding to a polymer blend film thickness of 3 µm. This thickness is required to achieve an optical density of 0.3 for the carbonyl (CdO) stretch of PCBM at 1740 cm-1 (see Figure 2). The absorption of the sample at 550 nm is 2.8 and is dominated by the polymer. The visible absorption spectrum of PCBM is represented in Figure 4 for comparison. The spectrum is scaled to match the long wavelength tail of PCBM in the polymer blend spectrum where the polymer has negligible absorption. The comparison demonstrates that PCBM molecules in the polymer blend contribute only about 0.3 to the optical density at 550 nm. The fraction of 550 nm photons that are absorbed by PCBM as the pump beam propagates through the sample was determined by numerically dividing the 3 µm thick film into 1000 layers of 3 nm thickness. These layers are optically thin such that the photon flux through each layer can be considered constant. The fraction of photons absorbed by each component in the blend was calculated for each layer, and the total transmission of each layer was computed. The partition of photons into the CN-MEH-PPV and PCBM layers was determined by summing over all 1000 layers. From this inner-filter calculation, we find that only about 10% of the pump photons are absorbed by PCBM, and the remaining 90% are absorbed by the polymer. Transient IR spectra of a 1:1 CN-MEH-PPV:PCBM polymer blend are represented in Figure 5 at several time delays following excitation at 550 nm. The transient spectra were collected with the ultrafast vis-IR instrumentation and are centered on the 1740 cm-1 region where the carbonyl stretch of PCBM absorbs. The transient spectra result from the superposition of two features: a broad featureless absorption of polarons in the polymer31-38 and a reduced absorption of the carbonyl stretch of neutral ground state PCBM molecules (termed the carbonyl bleach). The absorption of polarons results in the apparent offsets between spectra; no offsets were added to displace the transient spectra. Because CN-MEH-PPV has
Charge Trapping in Organic Photovoltaic Materials
Figure 6. Integrated area of the carbonyl bleach of PCBM versus the corresponding time delay for the 1:1 CN-MEH-PPV:PCBM polymer blend plotted on a linear time axis. The dynamics within the first 5 ps show the gradual rise resulting from interfacial electron transfer on the ∼90 fs and ∼0.9 ps time scales. (Inset) The same data plotted on a longer time axis. After the ultrafast electron transfer process, a small fraction of the electrons transfer back to the polymer, resulting in a decrease in the integrated bleach area.
sample before they encounter the CaF2 substrate. The intensity of the optical pump pulse is attenuated by nearly 3 orders of magnitude before it encounters the substrate. Consequently, the nonresonant response that would normally result from pulse overlap in the substrate optical material is strongly suppressed. The sample itself produces some nonresonant signal, but we find this contribution to be overwhelmed by the resonant response, which is the signal we desire to measure. Figure 6 represents the time dependence of the carbonyl bleach area extracted from the transient IR spectra using the fitting procedure described above. Each data point at a particular time delay in the figure is obtained by integrating the corresponding bleach spectrum to obtain its area. Because the bleach area is proportional to the concentration of electrons, the time dependence of the bleach area describes the electron transfer kinetics from CN-MEH-PPV to PCBM. A multiexponential fit curve that is convolved with the instrument response function is overlaid on the data. The fit curve represents electron transfer processes to PCBM occurring on the 90 fs (∼30%) and 0.9 ps (∼70%) time scales. These dynamics are similar to previous measurements of electron transfer in other conjugated polymer: fullerene blends.40-45 The inset of Figure 6 diplays the same data on a longer time axis. The data indicate that a small fraction of electrons transfer back to the polymer on the 10-100 ps time scale. Transient IR spectra of the carbonyl bleach of PCBM measured at 100 ps, 3 ns, 2 µs, 10 µs, and 100 µs after optical excitation are displayed in Figure 7. The broad polaron absorption in each spectrum has been removed by subtraction of a linear offset such that the spectra have the same amplitudes at 1710 and 1790 cm-1. The spectra have also been scaled to have unit maximum positive signal for comparison. The positive-going peak around 1740 cm-1 corresponds to the carbonyl bleach that results from electron transfer from the polymer (cross reference the bleach in Figure 5). The shift in frequency of the bleach peak over time has been discussed in detail.26 Briefly, the shift arises from the diffusion of electrons from the interfaces toward the centers of PCBM domains in the polymer blend. The diffusion of electrons results in a frequency shift because PCBM molecules at the interfaces of the domains have higher frequency carbonyl stretch modes than do molecules in the interiors of the domains.26 The transient spectra reveal the growth of a new absorption around 1760 cm-1
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Figure 7. Comparison of transient IR spectra collected of the carbonyl stretch of PCBM at several time delays. The spectra at 100 ps and 3 ns time delays were collected with the ultrafast vis-IR instrument. Spectra at longer delay times were collected with the TRIR instrument. The spectra display the formation of the negative polaron absorption at 1760 cm-1 on the microsecond time scale. The negative polaron peak has little or no amplitude on ultrafast time scales following the photoexcitation event.
Figure 8. Comparison of kinetics traces measured at the bleach (1740 cm-1) and negative polaron (1760 cm-1) vibrational features of PCBM following photoexcitation of the 1:1 CN-MEH-PPV:PCBM polymer blend at 532 nm. The fit curves through the data are described in the text. (Inset) Comparison of the same kinetics traces plotted on a longer time axis. The kinetics traces in both panels have been scaled to demonstrate that the negative polaron decays synchronously with the bleach on the 100 µs and longer time scale after it has completed its growth in proportion to the bleach.
that appears on the microsecond time scale. The new absorption has little or no amplitude in the 100 ps and 3 ns transient spectra, indicating that its growth occurs on time scales much longer than a few nanoseconds. Kinetics traces of the carbonyl bleach and new absorption that were measured with the TRIR instrumentation at 1740 and 1760 cm-1, respectively, are displayed in Figure 8. The bleach kinetics trace displays an instrument-limited rise (
