pubs.acs.org/NanoLett
Prolonging Charge Separation in P3HT-SWNT Composites Using Highly Enriched Semiconducting Nanotubes Josh M. Holt, Andrew J. Ferguson, Nikos Kopidakis, Brian A. Larsen, Justin Bult, Garry Rumbles, and Jeffrey L. Blackburn* Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, United States ABSTRACT Single-walled carbon nanotubes (SWNTs) have potential as electron acceptors in organic photovoltaics (OPVs), but the currently low-power conversion efficiencies of devices remain largely unexplained. We demonstrate effective redispersion of isolated, highly enriched semiconducting and metallic SWNTs into poly(3-hexylthiophene) (P3HT). We use these enriched blends to provide the first experimental evidence of the negative impact of metallic nanotubes. Time-resolved microwave conductivity reveals that the long-lived carrier population can be significantly increased by incorporating highly enriched semiconducting SWNTs into semiconducting polymer composites. KEYWORDS Single-walled carbon nanotubes, P3HT, charge generation, charge recombination, time-resolved microwave conductivity, organic photovoltaic, polythiophene
solar cells has been reported to be only ∼0.5%.19 Several factors may contribute to such poor efficiencies, including short-circuiting and shunting due to extremely high SWNT aspect ratios, a lack of control over the intrinsic carrier density for SWNTs due to p-type doping in ambient conditions, and the presence of metallic SWNTs within the composites. Typically, as-prepared (bulk) SWNTs fractionally consist of one-third metallic (m-) and two-thirds semiconducting (s-) nanotubes. As m-SWNTs lack a true band gap, one would naturally suspect these species to act as charge carrier or exciton recombination centers, lowering the efficiency of charge separation, and, by extension, the efficiency of devices. Another hypothesis outlined by Kanai et al. proposes that the ground state interaction between poly(3-hexylthiophene) (P3HT) and m-SWNTs substantially redistributes charge density, resulting in a potential well for holes on the m-SWNT and electrons on the polymer, thus seriously hindering charge separation efficiency.20 We present in this letter a conclusive experimental demonstration that charge separation is significantly enhanced as the concentration of metallic species is reduced within a prototypical SWNT/semiconducting polymer blend. Utilizing time-resolved microwave conductivity (TRMC), a contactless pump-probe technique sensitive only to free mobile carriers and not charge-neutral excitons, we study the yield of free carriers generated under illumination and their mobility and decay dynamics in composite films of highly enriched semiconducting and metallic SWNTs dispersed in regioregular P3HT. We have found that the longlived population of charge carriers produced by charge separation significantly increases when using samples highly
T
he most efficient organic photovoltaic (OPV) solar cells to date utilize a bulk-heterojunction (BHJ) of conjugated polymer electron donor and fullerene derivative electron acceptor with solar power conversion efficiencies now approaching 8%.1,2 Electron mobility in these materials, however, depends strongly on fullerene clustering and hopping transport.3 Single-walled carbon nanotubes (SWNTs) tout several unique properties that nominate them as intuitive and possibly superior candidates to replace the electron accepting and transport fullerene phase in BHJ OPV systems.4 First, recent studies on blends of conjugated polymers and SWNTs5-11 suggest that efficient exciton quenching occurs at these interfaces. As was recognized recently, exciton dissociation is a necessary prerequisite, although not solely sufficient condition, for free carrier generation,12-14 and only a few studies have shown direct evidence for the generation of mobile carriers in polymer-SWNT composites.5,11 Once carriers are separated, ballistic carrier transport in single tubes15 and extremely high conductivity in networks naturally suggest that SWNTs may yield higher mobilities and form more conductive percolation networks than fullerenes and at lower loadings.16 Moreover, the observations that several conjugated polymers effectively wrap and isolate individual SWNTs17,18 guarantees intimate heterojunction contact while increasing interfacial area. So far, however, the maximum power conversion efficiency achieved by SWNT-composite * To whom correspondence should be addressed. Tel: +1 (303) 384-6649. Fax: +1 (303) 384-6655. E-mail:
[email protected]. Received for review: 08/4/2010 Published on Web: 10/12/2010 © 2010 American Chemical Society
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enriched in semiconducting SWNTs compared to metallicenriched composites. SWNTs were produced in-house by laser vaporization (LV) of a graphite target containing 0.3 atom % each of cobalt and nickel in a nitrogen atmosphere. The LV SWNTs used in this study have an average diameter of ∼1.35 nm, as determined by Raman spectroscopy and atomic force microscopy (AFM).16 Raw SWNTs were dispersed in a cosurfactant solution of sodium dodecyl sulfate (SDS) and sodium cholate (SC) at a 3:2 or 1:4 SDS-SC ratio optimized to retrieve metallic or semiconducting species, respectively, and subsequently centrifuged in an iodixonal density gradient at 207 000 g. The separated bands were drawn off and aliquots were characterized using photoabsorption spectroscopy to determine the relative ratios of s- and m-SWNTs.21 Following separation, enriched aliquots were combined to yield large volumes (20-50 mL) of dispersions with varying ratios of s- and m-SWNTs. Producing uniform blends of enriched SWNTs in a P3HT matrix is not a straightforward process. The enriched SWNTs, separated by density gradient ultracentrifugation (DGU), are in aqueous solution with high concentrations of two surfactants and iodixanol density gradient medium, all of which must be removed to ensure that intimate contact is achieved between SWNTs and P3HT in an organic solvent. To address this issue, we developed the following procedure. First, an equal volume of methanol was added to the separated SWNT dispersion to precipitate the SWNTs. This solution was centrifuged for 20 min (24 000 rpm Ti45 rotor), and the supernatant was removed. The precipitated SWNTs were washed 2 times with deionized water, followed each time by centrifugation (24 000 rpm, 10 min, Ti45 rotor), to remove excess surfactant and iodixanol. Water washing was followed by two rounds of acetone washing: 25 mL of acetone, 35 000 rpm, 15 min, Ti70 rotor, stainless steel centrifuge tubes. Finally, the SWNTs were washed with of toluene: 25 mL of toluene, 35 000 rpm, 15 min, Ti70 rotor, stainless steel centrifuge tubes. After removing the toluene supernatant, the wet SWNT precipitate was dispersed in a toluene solution of P3HT via tip sonication for 30 min. For the dispersions studied here, ∼600 µg of enriched SWNTs were dispersed in 4 mL of a 5 mg/mL P3HT solution, yielding a SWNT loading of ∼3% weight ratio relative to P3HT. Solutions were spin-coated onto Z-cut quartz substrates at 600 rpm to form films for measurement by TRMC. Figure 1 shows the absorbance of P3HT blended with SWNTs in solution at various semiconducting/metallic enrichments normalized to the first absorption feature of P3HT at 2.05 eV. The absorption bands corresponding to the first (S11) and second (S22) excited states of the semiconducting SWNTs are seen in the ranges of 0.6-0.9 eV and 1.0-1.5 eV, respectively. The structure observed for the S11 envelope suggests that P3HT acts as a reasonable dispersant for the LV SWNTs, since the absorbance of dispersions made with poorly dispersing surfactants typically display broad, fea© 2010 American Chemical Society
FIGURE 1. Normalized absorbance spectra of P3HT solutions blended with a few wt % SWNTs of varying semiconducting nanotube content; absorption of toluene has been subtracted out. Dotted lines represent absorbance of enriched SWNTs in aqueous solution before redispersion in P3HT with scaling adjusted to match the S22 peak in the P3HT blends. Inset depicts material structures.
tureless peak envelopes for LV SWNTs. The metallic absorption band, M11, typically lies between 1.8-2.1 eV for LV SWNTs but is unobservable in the composites since the absorbance of the P3HT polymer dominates the range of 1.9-3.1 eV, peaking at 2.6 eV. Also shown in Figure 1, as dotted lines, are the absorbance spectra of the separated SWNTs (in water) before precipitation and redispersion with P3HT. Note that the spectra are scaled so the S22 peaks are comparable to the corresponding spectra for the P3HT blends. The M11 transitions are apparent in these spectra with a peak at 1.9 eV, roughly at the absorption edge of P3HT, and allow visualization of metal/semi enrichment for each blend. Nanotube loading (referring to the total weight of both metallic and semiconducting species) was maintained near 3 wt % SWNT/P3HT for all composite samples, although we note that the processing steps required to successfully disperse enriched SWNTs into the polymer/organic solvent phase introduce some uncertainty to the loading ratio. By varying the proportion of semiconducting-to-metallic nanotubes, while maintaining constant SWNT loading, one would expect a direct correlation in the magnitude of the Siiabsorption bands. Figure 1 shows that the expected trend is generally followed, as S11 increases in normalized absorbance (solid lines) from 0.5% for the 12% sample to 1.2% for the 59% sample, although we note that the sample loading ratio for the 90% semiconducting-rich hybrid falls below the trend, indicating the loading for this sample may be lower than the target value. The normalization in Figure 1 is arbitrary and provided to qualitatively illustrate the relative SWNT absorption to that of the polymer; it cannot be used to deduce absolute loading ratios. For a better comparison of semiconductor/metallic SWNT absorption profiles, see Figure S1 in Supporting Information. Additionally, it has been shown that the S11 absorption is quenched by either p- or n-doping SWNTs.21,22 The relative magnitude 4628
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solvents with P3HT effectively isolates SWNTs primarily as individual SWNTs. It is known that the transition energies of SWNTs depend on the dielectric response of their environment that alters the Coulomb energies of the exciton.9 In particular, the S11 and S22 transition energies tend to follow systematic diameterdependent trends that are sensitive to the specific environment (surfactant and solvent) in which they are dispersed. The significant global bathochromic (red) shifts in both excitation and emission energy observed in Figure 2a are attributed to the polarizability of the surrounding P3HT polymer and, to some extent, the toluene. It has also been suggested that the bathochromic shift results from the formation of a type II heterojunction formed at the SWNT/ polymer interface.9 Figure 2b compares the measured emission red shifts versus tube diameter of three P3HT-dispersed SWNT samples: bulk HiPCo (smaller diameter), bulk LV, and the enriched LV nanotubes used in this study. Bathochromic shifts were calculated relative to the published values of Weisman et al.24 The shifts of the enriched LV nanotubes are identical within experimental error to bulk LV SWNTs dispersed in P3HT and follow the trend of smaller diameter HiPCo SWNTs dispersed in P3HT. The results shown for bulk HiPCO SWNTs also correlate well with the results of Schuettfort et al.9 who measured the emission red shifts for HiPCO SWNTs wrapped with P3HT. The statistically identical red shifts for the redispersed, enriched-LV SWNTs relative to unseparated LV-SWNTs provide good evidence that the interfacial properties for the redispersed SWNTs are controlled by the P3HT polymer and not by any residual species remaining from the DGU separation (viz. SDS, SC, or iodixanol). Thus, the redispersion process developed here allows for successful phase transfer of SWNTs into semiconducting polymer matrices, a critical step for producing enriched SWNT/polymer blends for organic photovoltaics. With the SWNTs at various semiconductor/metal enrichment ratios readily dispersed in P3HT, we can study the mobile charge dynamics of the nanohybrid architecture by TRMC. In the TRMC experiment, film samples within a resonant microwave cavity are excited by wavelength-tunable laser pulses. If mobile charges are created as a result of photoexcitation, for example, by autodissociation of excitons or charge transfer at an interface, the microwave signal is attenuated by free carrier absorbance. The amount of microwave energy absorbed by the photogenerated mobile carriers is detected as a function of time to glean information on the kinetics of free carrier decay.5,25 It is important to note that the TRMC signal is proportional to the product of free carrier yield and carrier mobility of both free carrier types, that is
FIGURE 2. (a) PLE map of 90% semiconducting enriched SWNT/P3HT dispersion. Expected positions of semiconducting SWNTs in SDS/ H2O24 are indicated by their corresponding (n,m) index; arrows indicate selected red shifts. (b) Energy red shifts (∆E11) of E11 transition peaks, relative to SDS/H2O plotted as a function of tube diameter for HiPCo (black squares), bulk LV (red triangles) and the enriched LV nanotubes used in this study (green circles) dispersed in P3HT.
of S11 compared to S22 in solution attests that the SWNTs remain primarily intrinsic in the P3HT dispersion. The absorbance spectrum is a convolution of the overlapping absorption profiles of all the nanotube species present and cannot distinguish specific (n,m) species. Much more information can be derived by photoluminescence excitation (PLE) spectroscopy. PLE maps were measured with a customized Thermo-Nicolet FT960 Raman spectrometer equipped with a liquid nitrogen-cooled Ge detector, as described previously.23 Since PL from semiconducting SWNTs is quenched by the presence of nanotubes with metallic or quasi-metallic density of electronic midgap states, the observation of luminescence requires that s-SWNTs are sufficiently debundled and isolated from metallic species. Figure 2a shows the PLE map of 90% semiconducting SWNTs dispersed in P3HT solution. Each peak represents a different s-SWNT species with a unique set of (n,m) chiral indices; following the empirical formulation of Weisman et al.,24 anticipated locations of semiconducting SWNT peaks in SDS-water have been indicated by their (n,m) chiral indices. The observation of luminescence in the SWNT/P3HT dispersion demonstrates that the technique developed here for phase transfer of DGU-enriched SWNTs into organic © 2010 American Chemical Society
∆G(t) ) βqeI0FAφΣµ ) βqeI0FAφ[µe + µh]
(1)
where β ) 2.2 is the ratio of the long and short dimension of the X-band waveguide used,26 qe is the elementary charge, I0 4629
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ated electron becomes highly mobile along the nanotube and is detected as additional TRMC signal. The photoconductance transients provide information about the kinetics of free carrier decay by recombination, trapping, etc. Time sweeps shown in Figure 3a were taken at or below a moderate absorbed photon flux I0FA e 1014 photons/cm2 to limit nonlinear carrier decay processes such as geminate recombination, exciton-exciton annihilation, or exciton quenching by charge carriers.27,28 The instrument response function (IRF) is compared on the same graph to show that the immediate decay is not limited by system response. It is immediately apparent that the photoconductance decay kinetics depend strongly on the concentration of semiconducting species within the composite. The dynamics of the composite enriched with m-SWNTs show an initial decay that is faster than that of pristine P3HT. In the highly enriched semiconducting case, however, the decay dynamics are significantly slower than both the pristine P3HT and the metallic-enriched P3HT composite. Accordingly, carrier survival extends to longer times, an important feature for charge extraction in OPV devices. Transients taken at lower excitation intensities (I0FA ) 1013 cm-2), closer to solar fluences, show that the carrier lifetime of the 90% s-SWNT increases even further (see Figure 4). In a recent work from our group,5 we proposed that the long-lived TRMC signal in P3HT/SWNT composites arises from electron transfer from photoexcited P3HT to semiconducting SWNTs, resulting in a sustained charge-separated state at the P3HT/s-SWNT interface with some “preexisting” contribution from intrinsic carrier generation in the polymer phase that also manifests itself in the pristine polymer.27,28 It follows that lower signals and faster decay times imply a reduced yield of the chargeseparated state. The results in Figure 3a further support this hypothesis, as they demonstrate that composites enriched with s-SWNTs show dramatic lengthening of free carrier decay, indicative of a long-lived charge separated state. In contrast, composites enriched with m-SWNTs do not enable such long-lived charge separation, as expected from their midgap density of states and previous theoretical predictions.20 Since long-lived mobile photocarriers are paramount to charge extraction in a device architecture, we will devote some attention to photoconductance at longer times. The persistence of long-lived carriers can be easily compared for all of the composites by plotting the photoconductance as a function of excitation intensity measured at long delay times. The normalized photoconductance shown in Figure 3b, which is the yield-mobility product, represents the combined contribution of both population and mobility of charge carriers in pristine P3HT and the P3HT/SWNT blends measured at t ) 400 ns (i.e., 400 ns after the cessation of the excitation pulse) versus absorbed photon flux. The sublinear dependence of photoconductance on incident light intensity, has been explained for P3HT by fast bimolecular recombination,25 exciton-exciton annihilation,27 or more
FIGURE 3. (a) Time sweeps of the photoconductance normalized to the peak signal measured at an absorbed photon flux of 1 × 1014 (solid lines) and 1 × 1013 (dotted lines) photons/cm2. (b) Photoconductance signal normalized to absorbed photons measured at t ) 400 ns as a function of absorbed photon flux for P3HT and P3HT/ SWNTs. Samples were photoexcited at 520 nm. Solid lines are fits to eq 3 from text.
is the incident photon flux, FA is the fraction of light absorbed in the sample at the excitation wavelength, φ is the yield for free carrier generation, and µe (µh) is the high-frequency electron (hole) mobility. The figure of merit extracted from this equation is the product of free-carrier yield and mobility, which is the photoconductance normalized by the absorbed photon flux
φΣµ ) ∆G/(βqeI0FA)
(2)
Normalized photoinduced conductance transients of P3HT/SWNT composite films containing the lowest and highest semiconducting contents are compared in Figure 3a to photoexcited pristine P3HT (no SWNTs). The excitation energy, 2.38 eV (520 nm), was chosen to photoexcite primarily P3HT; as shown in Figure 1a, P3HT absorbs ∼99% of the photons in the dispersions at this energy. The photoconductivity signals shown in Figure 3 convey information about the number density and mobility of free charge carriers produced by exciton dissociation events following photoexcitation. In most cases, these exciton dissociation events occur on time scales faster than the instrument response function of the TRMC experiment. Thus, the endof-pulse (EOP) signal, that is, ∆G at t ) 0, conveys information about the efficiency of free charge carrier production within the time frame of the laser pulse. After photo-induced electron transfer from the P3HT to the SWNT, the dissoci© 2010 American Chemical Society
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still be concluded from Figure 3b that as the ratio of semiconducting SWNTs is increased, maintaining approximately the same loading ratio, the conductance increases at long times by over an order of magnitude above the polymer-only baseline. The TRMC data presented here can be interpreted by the following model. Charge separation occurs at P3HT/s-SWNT interfaces when photoexcited P3HT injects an electron into the s-SWNT conduction band, leaving a hole resident in P3HT. This charge-separated state is long-lived and the mobile carriers produce a persistent photoinduced microwave absorbance that lasts hundreds of nanoseconds. In contrast, ∆G of the m-SWNT samples decays to half its peak value within about 50 ns, indicating that charge separation either does not occur, or is much less efficient, at photoexcited P3HT/m-SWNT interfaces. Furthermore, high concentrations of m-SWNTs apparently decrease the initial native free carrier decay times in P3HT, presumably because of fast charge transfer and/or recombination processes which will be discussed in detail later on. Therefore the profile of the metallic-loaded TRMC transient closely resembles that of pristine P3HT. To summarize the results from Figure 3 and to elucidate the most significant trends, we map long-lived photoconductance amplitude and average photoconductance lifetime to s-SWNT concentration in Figure 4. Figure 4b displays the long-lived photoconductance (at t ) 400 ns) as a percentage of the EOP signal for several absorbed fluences. For all excitation intensities, at low semiconducting SWNT content, this ratio is near that of pristine P3HT and increases approximately linearly with increasing semiconducting content. As discussed above, high intensity excitation provides greater exciton density, which results in significant annihilation at the outset, thus reducing the overall charge density.28 Therefore low laser intensities represent the best-case scenario for charge carrier survival with over three times the likelihood of charge preservation at 400 ns, from ∼10% in pristine P3HT and in m-SWNT/P3HT to ∼35% in s-SWNT/ P3HT (blue squares). Characteristic free carrier decay times at various semiconducting SWNT loadings were extracted by globally fitting transients to a triple exponential function (for detailed results, see Table 1 in Supporting Information) and the average decay time, τ, was calculated according to29
FIGURE 4. (a) Maximum photoconductance at low excitation intensity as a function of percent semiconducting SWNTs as extracted from the empirical fit in Figure 3b. (b) Extant photoconductivity after 400 ns relative to end-of-pulse signal versus semiconducting content at the various absorbed photon fluxes quantified in the lower-right legend. (c) Average characteristic decay time of free charge carriers, calculated by eq 4, measured at the absorbed fluxes indicated.
recently for both pristine P3HT and P3HT/PCBM, excitoncarrier quenching.28 The data points can be fitted and extrapolated to low excitation intensities quite well for these blended systems using an empirical expression modified from Dicker et al.25
φΣµ ) A/(1 + √BI0FA + CI0FA)
(3)
where A, B, and C are fitting coefficients. In particular, A represents the maximum normalized photoconductance [φΣµ]max and is plotted versus percentage s-SWNT content in Figure 4a. A consistent increase in yield-mobility signal is observed as the concentration of semiconducting SWNTs is increased. As previously discussed, absorption data indicate that the 90% semiconducting sample appears as a lower loading than the other samples and may explain why the signal for this sample does not increase significantly beyond the 59% semiconducting sample. However, it can © 2010 American Chemical Society
τ)
ΣinAiτi2 ΣinAiτi
(4)
where Ai and τi are the pre-factor and lifetime, respectively, of the ith exponential decay component. The resulting average characteristic decay time of free carriers is plotted as a function of semiconducting concentration in Figure 4b showing a 45% increase in carrier decay time from the 12 4631
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to 90% semiconducting sample. We also note that even if SWNT loading varies slightly between enriched samples (due to uncertainties introduced by the redispersion process), carrier dynamics remain for the most part unchanged (see Supporting Information). Thus, the demonstration of increasing carrier lifetime with s-SWNT concentration shown in Figure 4 provides compelling evidence that s-SWNTs are more desirable than m-SWNTs for stabilizing long-lived charge separated states in SWNT-conducting polymer blends. The results reported here, namely, enhanced photoconductivity at long times (400 ns) and increased carrier lifetimes for blends enriched in s-SWNTs, assert that metallic species are indeed detrimental to the existence of long-lived charge carriers in light-harvesting P3HT/SWNT active layers. It has been suggested that the continuous density of “midgap” electronic states of metallic SWNTs can act as an efficient recombination pathway for excitons in polymerSWNT blends. We assume that all processes involving singlet excitons, such as exciton decay and energy transfer to the SWNT, occur within the duration of the excitation pulse, and that the charge mobility of the detected (i.e., exclusively untrapped) carriers does not change appreciably with time. Besides these exciton processes, there remain several potential pathways for free carrier recombination involving the metallic SWNTs: (1) electron transfer from the P3HT LUMO to empty midgap states within the m-SWNT, (2) electron transfer from filled states at the m-SWNT Fermi level to the P3HT HOMO (“quenching” of the P3HT hole), or (3) a combination of both. These processes can lead to charge quenching at the interface within the excitation pulse duration, producing the fast decay measured, although the TRMC measurement does not allow us to distinguish between them. Another detrimental effect of m-SWNTs has been proposed by the density functional theory calculations of Kanai et al.,20 which predicts that ground-state electron transfer from P3HT to m-SWNTs may establish a dipole at this interface that enhances the local electron affinity on the polymer and opposes photoinduced electron transfer to the m-SWNT. In contrast, s-SWNT species favor electron transfer from the polymer to the SWNT and enhances charge preservation due to the ability of a true band gap to stabilize the charge-separated state. In conclusion, the difficulty of separating m- from s-SWNT and redispersing them in a semiconducting polymer matrix has been a persistent obstacle to systematically investigating the role of each species separately by experiment. We have demonstrated effective redispersion of isolated, highly enriched semiconducting and metallic SWNTs into the conjugated polymer P3HT, forming an intimate interfacial contact morphology optimal for charge separation. TRMC results on P3HT/SWNT blends with varying ratios of m- and s-SWNTs demonstrate that m-SWNTs blended with P3HT are detrimental to photovoltaic performance and their elimination can improve long-lived mobile carriers available for © 2010 American Chemical Society
photocurrent by at least 3-fold. Our results suggest that previous attempts aimed at replacing fullerene-derivative acceptors by SWNTs in OPV devices may have been critically limited by the presence of the metallic species. Effectively, what has often been considered as a twocomponent hybrid system (conjugated polymer and SWNT) should be treated at a minimum as a three-component system: conjugated polymer, s-SWNT, and m-SWNT. Moreover, since every species of SWNT with distinct chiral (n,m) indices expresses unique electronic properties, future work will likely afford valuable insight into specific electronic traits of even more well-defined blends of polymers with chiral-specific s-SWNTs. We hope these results encourage future advancement of semiconductingrich processing for the implementation of SWNTs in photovoltaic active layers. Acknowledgment. This work was funded by the Solar Photochemistry program of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract No. DE-AC36-08GO28308 to NREL. Supporting Information Available. A comparison of the normalized enriched SWNT absorption profiles before redispersing in P3HT, a more rigorous analysis of the photoluminescent red shifts, the photoconductance signal at t ) end-of-pulse, normalized to absorbed photons, similar to that shown in Figure 3, a table of fit parameters used to calculate the average characteristic decay times, and summarized TRMC results of varying the bulk SWNT/P3HT loading ratio. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
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