Charge Separation in P3HT:SWCNT Blends Studied by EPR: Spin

Rachelle Ihly , Kevin S. Mistry , Andrew J. Ferguson , Tyler T. Clikeman , Bryon W. Larson , Obadiah Reid , Olga V. Boltalina , Steven H. Strauss , Ga...
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Charge Separation in P3HT:SWCNT Blends Studied by EPR: Spin Signature of the Photoinduced Charged State in SWCNT Jens Niklas,† Josh M. Holt,‡ Kevin Mistry,‡ Garry Rumbles,‡ Jeffrey L. Blackburn,*,‡ and Oleg G. Poluektov*,† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ Chemical and Materials Science Center, National Renewable Energy Laboratory, 15013 Denver West Pkwy, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: Single-wall carbon nanotubes (SWCNTs) could be employed in organic photovoltaic (OPV) devices as a replacement or additive for currently used fullerene derivatives, but significant research remains to explain fundamental aspects of charge generation. Electron paramagnetic resonance (EPR) spectroscopy, which is sensitive only to unpaired electrons, was applied to explore charge separation in P3HT:SWCNT blends. The EPR signal of the P3HT positive polaron increases as the concentration of SWCNT acceptors in a photoexcited P3HT:SWCNT blend is increased, demonstrating long-lived charge separation induced by electron transfer from P3HT to SWCNTs. An EPR signal from reduced SWCNTs was not identified in blends due to the free and fast-relaxing nature of unpaired SWCNT electrons as well as spectral overlap of this EPR signal with the signal from positive P3HT polarons. However, a weak EPR signal was observed in chemically reduced SWNTs, and the g values of this signal are close to those of C70-PCBM anion radical. The anisotropic line shape indicates that these unpaired electrons are not free but instead localized. SECTION: Physical Processes in Nanomaterials and Nanostructures

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has led to rather moderate performance enhancements in blends with low-band-gap copolymers. Single-wall carbon nanotubes (SWCNTs; Figure 1) could be used in organic photovoltaic (OPV) blends as a replacement or additive component for the currently used fullerene derivatives C60-PCBM and C70-PCBM because they possess unique structural and electronic properties that could enhance the efficiency of OPV cells.12−14 For example, the quasi 1-D structure of SWCNTs could potentially improve chargetransport processes, and the optical absorbance of SWCNTs can be tuned by synthesizing different types of SWCNTs to further optimize broadband solar capture. In BHJ OPV materials, light absorption and exciton diffusion are followed by charge separation, creating two charged counterparts, radical cations and anions, also called by analogy to semiconductor PV materials positive and negative polarons.15−18 The EPR-spectroscopic signatures of radical cations, localized on polymer, and radical anions, localized on the fullerene, have been known for several years in conventional BHJ OPV materials. Examples are poly(phenylenevinylene)

he increasing need for sustainable energy is driving efforts to develop new and cost-effective technologies to convert sunlight into electricity.1,2 Organic photovoltaic (OPV) devices are considered to be a cost-competitive and moderately efficient approach for utilization of solar energy and have the potential to help alleviate our global dependence on fossil fuels.2−5 While the conversion efficiency of OPV cells is gradually increasing (http://www.nrel.gov/ncpv/images/efficiency_chart.jpg),6 donor and acceptor materials that enable increased solar capture, higher mobility, and industrial-scale deposition are still being actively pursued. Significant efforts have been directed to the improvement of the electron donating (polymer) part of the bulk heterojunction (BHJ) solar cells, resulting in substantial progress with the design of novel low-band-gap push−pull polymers.7,8 Efficiency gains in these BHJ systems relative to reference systems, like P3HT:C60-PCBM (P3HT, poly(3-hexylthiophene-2,5-diyl); C60-PCBM, [6,6]-phenyl-C61butyric acid methyl ester; Figure 1), seem not to be due to improved charge-transport properties but instead to superior optical properties, that is, extended spectral absorption of the polymers used.9 Less progress has been made on the acceptor side. After the discovery that soluble derivatives of C60, and later of C70-fullerenes,10,11 greatly improved the efficiency of OPV cells, further chemical modification of fullerene acceptors © XXXX American Chemical Society

Received: December 9, 2013 Accepted: January 22, 2014

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Figure 1. Chemical structures of the polymer P3HT, the fullerene derivative C60-PCBM, and single-wall carbon nanotubes (SWCNT).

(PPV) and poly(3-hexyl-thiophene) (P3HT) polymers as well as C60-fullerenes derivatives.19−22 Recently, the spectroscopic signatures of low-band-gap push−pull polymers and C70fullerenes derivatives have also been reported.21,22 Here we present a study of electronic properties of radical cation and anion in blends of the polymer P3HT with differently prepared SWCNTs using EPR methods. The major goal was to detect the EPR signatures of reduced SWCNTs, as this paramagnetic species should be generated as a consequence of light-driven charge separation in OPV materials containing SWCNTs. Previous studies have attempted to record EPR signals of SWCNTs, but the results have been inconsistent. Zaka et al. reported that they could not detect any EPR signal in pure SWCNTs.23 However, they did not use a blend with an electron-donating polymer or check for a light-induced (chargeseparated) signal. Konkin et al. reported a variety of different signals, which they attributed to relatively fast electron exchange between SWCNT and donor/acceptor of BHJ composites as well as orientation effects of polymer in the film.24 On the basis of these partially contradicting results, we decided to investigate and compare the EPR signatures of SWCNTs reduced by two different methods: chemically reduced and reduced by light-induced electron transfer from polymer donor to SWCNT acceptor in P3HT:SWCNT blends. To record the charge-separated (CS) states in polymer:SWCNT blend, we began with concentrated toluene solutions of P3HT blended with small concentrations of SWCNTs prepared by several different methods: laser vaporization (LV, prepared at NREL), CoMoCAT (SG76 and CG100 - commercial SWCNTs from SWeNT), and HiPCO (commercial SWCNTs from Unidym). Motivation for the use of concentrated toluene solution is as follows. In solution, the separation between donors (e.g., polymers) and acceptors (e.g., fullerenes, SWCNTs) is larger than that found in films. This significantly slows down charge recombination kinetics, and as a consequence the light-induced EPR signal of CS states on polymer and fullerene/SWCNTs is stronger. Moreover, concentrated solutions are free of the effects related to the orientation of the polymer film on the walls of the EPR sample tube,24 which might obscure interpretation of the data for solid-state samples.24 Note that under our experimental conditions (continuous illumination, T = 20−50 K) we observe CS states that live milliseconds or longer. After illumination is ceased, a fraction that recombines within minutes is referred to as reversible. CS states, which do not recombine within1 h, are referred to as irreversible. Figure 2 depicts continuous (cw) X-band EPR spectra of multiple P3HT:SWCNTs blends in concentrated toluene solution after a 20 min period of illumination (traces A−D). The signals prior to illumination are weak as compared with the light-induced signals. The signals during and after illumination

Figure 2. EPR spectra of concentrated toluene solution of P3HT:SWCNT (100:1 wt %) blends in which SWCNTs were prepared by different methods: (A) P3HT:LV SWCNT; (B) P3HT:SG76 SWCNT; (C) P3HT:CG100 SWCNT; and (D) P3HT:HiPCO SWCNT. The cw EPR spectrum of a P3HT:C60PCBM (100:1 wt %) prepared and recorded under similar conditions (E) is shown for comparison. Black, experiment; color, computer simulations. Red, P3HT+; green, C60-PCBM−. Spectra were recorded after prolonged (10−20 min) illumination at 50 K. Intensities of the spectra were normalized for easy comparison.

are almost identical in spectral position (electronic g value) and line shape; the minor differences might be explained by generation of a small amount of irreversible photoproduct (sample degradation). The spectra in Figure 2 are obtained by subtracting the “dark before light” from “dark after light” spectra. This subtraction removes the potential background signals of resonator and the samples, and only light-induced signals are visible. These signals are centered at g ≈ ge (2.0023), with a line width of 0.35 ± 0.03 mT. This is typical for the positive polaron, P+, located on the polymer P3HT.21,22,25 A comparison with a P3HT:C60-PCBM blend is shown in Figure 2E. In contrast with conventional P3HT:C60-PCBM blends, which contain a P3HT:C60-PCBM weight ratio of 1:1 to 1:2, these blends contain a ratio of 100:1 to be better comparable to the weight ratios in P3HT:SWCNT blends. In the P3HT:C60-PCBM blends, two signals contribute to the observed spectrum: at lower magnetic field the signal of the cation on P3HT (red simulation trace) and at higher magnetic field the anion on the fullerene (green simulation trace). In contrast, we did not observe a second signal for the P3HT:SWCNT blends that could be assigned to the SWCNT radical anion. The comparable EPR signal intensity during illumination and after illumination suggests that the majority of 602

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of P3HT+ within these blends results from light-induced electron transfer from P3HT to SWCNT acceptors (photoinduced exciton dissociation), in agreement with our previous studies.26,27 It is noteworthy that the signal is significantly larger in P3HT:C60-PCBM blends relative to P3HT:SWCNT blends if the same weight percentage of electron acceptor is present. The 1-D type of charge transport in SWCNTs could be one of the reasons, as this could result in faster recombination rate. Another important consideration for the SWCNT blends is the presence of a fraction of metallic SWCNTs in the samples (in addition to the semiconducting fraction). Our recent studies demonstrated that metallic SWCNTs act as recombination centers and reduce the fraction of long-lived carriers in P3HT:SWCNT blends.27 In a sample with the statistical 2:1 ratio of semiconducting to metallic SWCNTs,27 the metallic SWCNTs should reduce the amount of long-lived carriers by a minimum of ∼33%, with this percentage rapidly increasing if the sample contains bundled SWCNTs in addition to (or instead of) isolated individual nanotubes. Thus, in future studies, we plan to systematically study the effect of changing the concentration of metallic SWCNTs on the magnitude of the P3HT+ EPR signal. The absence of a clearly defined EPR signal for the SWCNT radical anion (SWCNT−) in the P3HT:SWCNT blends (Figure 2) is in agreement with the common idea that “free” electrons are generated in isolated SWCNTs upon lightinduced electron-transfer process. It could be envisioned that these electrons are not trapped even at low temperature and thus would not show up in EPR spectra because their high mobility and fast relaxation rates would lead to large homogeneous broadening. Another possible explanation may be that the EPR signal of SWCNT is hidden under the P3HT signal. In an attempt to test these hypotheses and observe the elusive EPR signal of the SWCNTs, we used SWCNT buckypaper chemically reduced with hydrazine.28 This sample induced a deterioration of the quality factor of our resonator, a behavior that is typical for conductive samples with highly mobile electrons. This behavior is exactly what is expected for reduced SWCNTs where the “excess electrons” can move relatively freely. At temperatures below 50 K, we observed a weak EPR signal, which we assign to shallow-trapped electrons (Figure 4). The computer simulation gives the following g-tensor parameters for this signal: gx = 2.0053; gy = 2.0027; gz = 2.0022. The anisotropic EPR line shape indicates that these unpaired electrons are not free but rather trapped in the lattice, and the disappearance of the signal at elevated temperatures (T > 50 K ≈ 5 meV) confirms a shallow trapping energy. The structural identity of these traps remains unclear. Isolated and pristine SWCNTs should not have any trapping sites along the SWCNT sidewall, although SWCNT ends, sidewall defects induced by sonication or purification, and tube−tube junctions within bundles may all result in sites that can localize/trap electrons. The relatively weak EPR signal suggests that the overall density of these traps is relatively low for the LV sample studied here. The g-tensor and line width of this signal are similar to those observed for C70-PCBM radical anions.21,22 This might be an indication that some unpaired electrons become trapped near the capped end of the SWCNT, which has a geometry similar to C70. For comparison, the signals recorded in P3HT:C70-PCBM blends and P3HT:LV SWCNT blends are shown in Figure 4. This Figure clearly demonstrates

charge separation is irreversible; that is, the lifetime is more than 1 h at cryogenic temperature (20−50 K). The observed extremely slow recombination at low temperature (T < 100 K) in frozen solution is distinctly different from liquid solution at room temperature, where recombination is mediated by diffusion of the positively charged P3HT and negatively charged SWCNTs. This diffusion is effectively suppressed at cryogenic temperatures in frozen solution. Similar irreversibility is found for 100:1 P3HT:C60-PCBM solutions at low temperatures. In contrast, similar P3HT:SWCNT samples prepared as films demonstrated recombination times on the order of microseconds following charge separation across the P3HT:SWCNT interface, as revealed by microwave conductivity measurements.26 To provide evidence that the EPR signal of P3HT cation observed in the polymer:SWCNT blends is formed due to the charge separation between P3HT and SWCNT, we carried out a comparative study of the intensity and recombination rate dependence on the concentration ratio of SWCNT and PCBM in the respective blends with P3HT. For the P3HT:C60-PCBM 100:1 ratio blends, we observed that a large fraction does irreversible charge separation (Figure S1, Supporting Information) that is similar to the behavior of the P3HT:SWCNT blends with the same weight ratio 100:1. Note that in the blends of P3HT:C60-PCBM with weight ration 1:1, most of the CS states demonstrate high reversibility. This difference is expected if we take into account that charge recombination depends on the average distance between positive and negative polarons and thus gets faster with an increase in the relative acceptor concentration. The match of CS and charge recombination kinetics in P3HT:SWCNT and P3HT:fullerene blends with equal weight ratio suggests that the observed lightinduced EPR signal in the P3HT:SWCNT blends is due to charge-separation processes between P3HT and SWCNTs. Figure 3 shows a comparison of positive polaron signal intensity generated in blends with different ratios of the P3HT to the acceptor. The P3HT+ signal increases systematically with an increase in the weight percentage of LV-SWCNTs within the blend. This correlation again proves that the primary source

Figure 3. Intensity of the light-induced cw EPR signal of the positive polaron on P3HT of blends with different P3HT:acceptor ratios. The SWCNTs used in this experiment were LV SWCNTs. 603

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type of experiment due to insufficient steady-state population. To check if a certain amount of the sample also shows very short-lived charge separation (≪100 ms), too short to be visible in the experiments previously described, we performed time-resolved pulse EPR experiments in which we recorded only the time dependent EPR signal right after the laser flash. In this experiment, we did not observe any light-induced signal within 100 ns to 100 ms lifetime range. Only relatively weak signals of an excited triplet state were recorded (Figure S2, Supporting Information). A comparison with excited triplet signals in P3HT solutions without acceptors (as C60-PCBM or C70-PCBM) demonstrates that the excited triplet state signal arises from the polymer P3HT because the magnetic zero-field splitting parameters are clearly different from those of fullerenes C60-PCBM, C70-PCBM, and SWCNTs. (See Supporting Information and refs 29 and 30.) The excited triplet state can be generated by two different pathways: radical pair mechanism (from donor−acceptor charge recombination) or by intersystem crossing mechanism. The comparison of the polarization patterns reveals that this species is created by intersystem crossing from the light-induced excited singlet state of P3HT and does not result from electron transfer to the SWCNTs and followed back recombination.31,32 Hence, we attribute this signal to the excited P3HT molecules, which are in such a large distance to SWCNTs that charge transfer cannot occur. In this case, singlet excitons convert to triplet excitons, which have lifetimes long enough to be recorded by timeresolved EPR. Because the lower limit for the time-resolution of our experiment is 100 ns, we have no indication for significant reversible charge separation with charge recombination in the time range 100 ns to 100 ms. If there is a very small fraction of the sample ( 50 K ≈ 5 meV), which points to a shallow trap depth for the observed electrons. The EPR signature of light-reduced SWCNT in polymer:SWCNT blends was not observed for two reasons. First, the unpaired electrons in light-reduced SWCNTs are free and fast-relaxing. This is certainly valid for metallic SWCNTs, but our data suggest free, fast-relaxing electrons within reduced semiconducting SWCNTs as well because the long-lived charge-separated signal will be dominated by the semiconducting SWCNTs.27 As

Figure 4. Experimental EPR spectra of: (A) chemically reduced SWCNT buckypaper; (B) light-induced charge-separated state of P3HT:C70-PCBM blend; and (C) light-induced charge-separated state of P3HT:SWCNT (100:3 wt %) blend. The SWCNTs used in this experiment were LV SWCNTs containing 2/3 semiconducting SWCNT. Black, experiment; color, computer simulations. Green, SWCNT− and C70-PCBM−; red, P3HT+. Theoretical spectra were simulated with the following parameters: SWCNT− g values: 2.0053, 2.0027, 2.0022; C70-PCBM− g values: 2.0060, 2.0028, 2.0021; P3HT+ g values: 2.0038, 2.0023, 2.0011.21,22

that, like in P3HT:C70-PCBM, signals of radical cations and anions are strongly overlapped in P3HT:LV SWCNT blend as well. Our attempt to overcome the signal overlap, by going to high-frequency EPR (D-band, 130 GHz) and thus increasing spectral resolution, failed due to the weakness of the lightinduced EPR signal. Thus, the overlap between cation/anion signals and low concentration of the trapping sites in SWCNTs prevented us from identification of this signal in photoexcited P3HT:SWCNT blends. To further corroborate the origin of the observed EPR signal in chemically reduced SWCNTs, we flushed the EPR tube with oxygen gas. This treatment resulted in the loss of the EPR signal, as expected for the reduced sample,28 and excludes the possibility that the signal was a background signal. The observation of an EPR signal for SWCNTs that is undoubtedly correlated to the presence of “free” electrons is an important demonstration that helps to clear up inconsistencies found in other recent SWCNT EPR studies. Namely, we confirm the absence of EPR signal in pure SWCNTs isolated in solutions or in the buckypaper. EPR signal can be observed only in reduced SWCNT, and, in our case, we recorded an EPR signal only in chemically reduced SWCNTs at sufficiently low temperatures to trap electrons into the potential wells of currently unidentified trap sites. The light-induced EPR signal of negatively charged SWCNTs in donor−acceptor blends is very weak for the reasons previously discussed. Identification of this signal is masked by the overlap with EPR signal from the radical cation formed on polymer, P3HT+, as well as orientation effects in the donor−acceptor films.24 Finally, we note that there is a small decay of the P3HT EPR signal after the end of illumination in P3HT:LV SWCNT blends, which we attribute to a small, reversible fraction, that persists longer than 100 ms before charge recombination. If the donor−acceptor pairs of radical cations/anions would be much shorter-lived than the inverse of the repetition rate of the laser (1/(10 Hz) = 100 ms), we would not detect them with this 604

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such, the concept of a “polaron”, in which the injected carriers perturb the lattice and move relatively slowly, is not entirely appropriate for SWCNTs. Future EPR measurements on highly enriched semiconducting SWCNT OPV blends will enable an even deeper understanding of the charge-separated state. Second, the signal(s) of shallow trapped electrons in lightreduced SWCNT in OPV blends are strongly overlapped with the signal of oxidized polymer, thus preventing identification. These results establish the utility of EPR for measuring lightinduced charge separation at SWCNT heterojunction interfaces and can be expanded to a number of other technologically relevant OPV materials.

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generated Charge Carriers in Organic Solar Cells Under Real Operating Conditions. Adv. Mater. 2012, 24, 4381−4386. (10) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells - Enhanced Efficiencies Via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789− 1791. (11) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Efficient Methano[70]fullerene/MDMO-PPV Bulk Heterojunction Photovoltaic Cells. Angew. Chem., Int. Ed. 2003, 42, 3371−3375. (12) Dillon, A. C. Carbon Nanotubes for Photoconversion and Electrical Energy Storage. Chem. Rev. 2010, 110, 6856−6872. (13) Bounioux, C.; Katz, E. A.; Yerushalmi-Rozen, R. Conjugated Polymers - Carbon Nanotubes-Based functional Materials for Organic Photovoltaics: A Critical Review. Polym. Adv. Technol. 2012, 23, 1129− 1140. (14) Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Carbon Nanomaterials for Electronics, Optoelectronics, Photovoltaics, And Sensing. Chem. Soc. Rev. 2013, 42, 2824−2860. (15) Deibel, C.; Dyakonov, V. Polymer-Fullerene Bulk Heterojunction Solar Cells. Rep. Prog. Phys. 2010, 73, 096401. (16) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736−6767. (17) Hains, A. W.; Liang, Z. Q.; Woodhouse, M. A.; Gregg, B. A. Molecular Semiconductors in Organic Photovoltaic Cells. Chem. Rev. 2010, 110, 6689−6735. (18) Bredas, J. L.; Street, G. B. Polarons, Bipolarons, and Solitons in Conducting Polymers. Acc. Chem. Res. 1985, 18, 309−315. (19) De Ceuster, J.; Goovaerts, E.; Bouwen, A.; Hummelen, J. C.; Dyakonov, V. High-frequency (95 GHz) Electron Paramagnetic Resonance Study of the Photoinduced Charge Transfer in conjugated Polymer-Fullerene Composites. Phys. Rev. B 2001, 64, 195206. (20) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron-Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (21) Niklas, J.; Mardis, K. L.; Banks, B. P.; Grooms, G. M.; Sperlich, A.; Dyakonov, V.; Beaupre, S.; Leclerc, M.; Xu, T.; Yu, L.; Poluektov, O. G. Highly-Efficient Charge Separation and Polaron Delocalization in Polymer-Fullerene Bulk-Heterojunctions: A Comparative MultiFrequency EPR and DFT Study. Phys. Chem. Chem. Phys. 2013, 15, 9562−9574. (22) Poluektov, O. G.; Filippone, S.; Martin, N.; Sperlich, A.; Deibel, C.; Dyakonov, V. Spin Signatures of Photogenerated Radical Anions in Polymer-[70]Fullerene Bulk Heterojunctions High Frequency Pulsed EPR Spectroscopy. J. Phys. Chem. B 2010, 114, 14426−14429. (23) Zaka, M.; Ito, Y.; Wang, H. L.; Yan, W. J.; Robertson, A.; Wu, Y. M. A.; Rummeli, M. H.; Staunton, D.; Hashimoto, T.; Morton, J. J. L.; Ardavan, A.; Briggs, G. A. D.; Warner, J. H. Electron Paramagnetic Resonance Investigation of Purified Catalyst-Free Single-Walled Carbon Nanotubes. ACS Nano 2010, 4, 7708−7716. (24) Konkin, A.; Bounioux, C.; Ritter, U.; Scharff, P.; Katz, E. A.; Aganov, A.; Gobsch, G.; Hoppe, H.; Ecke, G.; Roth, H. K. ESR and LESR X-Band Study of Morphology and Charge Carrier Interaction in Blended P3HT-SWCNT and P3HT-PCBM-SWCNT Solid Thin Films. Synth. Met. 2011, 161, 2241−2248. (25) Aguirre, A.; Gast, P.; Orlinskii, S.; Akimoto, I.; Groenen, E. J. J.; El Mkami, H.; Goovaerts, E.; Van Doorslaer, S. Multifrequency EPR Analysis of the Positive Polaron in I(2)-Doped Poly(3-hexylthiophene) and in Poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene. Phys. Chem. Chem. Phys. 2008, 10, 7129−7138. (26) Ferguson, A. J.; Blackburn, J. L.; Holt, J. M.; Kopidakis, N.; Tenent, R. C.; Barnes, T. M.; Heben, M. J.; Rumbles, G. Photoinduced Energy and Charge Transfer in P3HT:SWNT Composites. J. Phys. Chem. Lett. 2010, 1, 2406−2411. (27) Holt, J. M.; Ferguson, A. J.; Kopidakis, N.; Larsen, B. A.; Bult, J.; Rumbles, G.; Blackburn, J. L. Prolonging Charge Separation in P3HTSWNT Composites Using Highly Enriched Semiconducting Nanotubes. Nano Lett. 2010, 10, 4627−4633.

ASSOCIATED CONTENT

* Supporting Information S

Experimental details and additional EPR spectra are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Jeffrey L. Blackburn: E-mail: Jeff[email protected]. *Oleg G. Poluektov: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract DE-AC0206CH11357. J.L.B., G.R., J.M.H., and K.M. were funded by the Solar Photochemistry Program, Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (DOE), Grant DE-AC3608GO28308.

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REFERENCES

(1) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (2) Lewis, N. S. Toward Cost-Effective Solar Energy Use. Science 2007, 315, 798−801. (3) Hoppe, H.; Sariciftci, N. Polymer Solar Cells. In Photoresponsive Polymers II; Marder, S. R., Lee, K.-S., Eds.; Springer: Berlin/ Heidelberg, 2008; Vol. 214, pp 1−86. (4) Schlenker, C.; Thompson, M. E. Current Challenges in Organic Photovoltaic Solar Energy Conversion - Unimolecular and Supramolecular Electronics I; Springer: Berlin/Heidelberg, 2012; Vol. 312, pp 175− 212. (5) Green, M. A. Third Generation Photovoltaics: Advanced Solar Energy Conversion, Kamiya, T., Monemar, B., Venghaus, H., Yamamoto, Y. (Ed.), Springer: Berlin, 2005; Vol. 12. (6) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (version 42). Prog. Photovoltaics 2013, 21, 827−837. (7) Boudreault, P. L. T.; Najari, A.; Leclerc, M. Processable LowBandgap Polymers for Photovoltaic Applications. Chem. Mater. 2011, 23, 456−469. (8) Duan, C. H.; Huang, F.; Cao, Y. Recent Development of PushPull conjugated Polymers for Bulk-Heterojunction Photovoltaics: Rational Design and Fine Tailoring of Molecular Structures. J. Mater. Chem. 2012, 22, 10416−10434. (9) Baumann, A.; Lorrmann, J.; Rauh, D.; Deibel, C.; Dyakonov, V. A New Approach for Probing the Mobility and Lifetime of Photo605

dx.doi.org/10.1021/jz402668h | J. Phys. Chem. Lett. 2014, 5, 601−606

The Journal of Physical Chemistry Letters

Letter

(28) Mistry, K. S.; Larsen, B. A.; Bergeson, J. D.; Barnes, T. M.; Teeter, G.; Engtrakul, C.; Blackburn, J. L. n-Type Transparent Conducting Films of Small Molecule and Polymer Amine Doped Single-Walled Carbon Nanotubes. ACS Nano 2011, 5, 3714−3723. (29) Poluektov, O. G.; Niklas, J.; Mardis, K. L.; Beaupre, S.; Leclerc, M.; Villegas, C.; Erten-Ela, S.; Delgado, J. L.; Martin, N.; Sperlich, A.; Dyakonov, V. Electronic Structure of Fullerene Heterodimer in BulkHeterojunction Blends. Adv. Energy Mater 2014, DOI: 10.1002/ aenm.201301517. (30) Stich, D.; Späth, F.; Kraus, H.; Sperlich, A.; Dyakonov, V.; Hertel, T. Triplet-Triplet Exciton Dynamics in Single-Walled Carbon Nanotubes. Nat. Photon 2013, DOI: 10.1038/nphoton.2013.316. (31) Budil, D. E.; Thurnauer, M. C. The Chlorophyll Triplet State as a Probe of Structure and Function in Photosynthesis. Biochim. Biophys. Acta 1991, 1057, 1−41. (32) Lubitz, W.; Lendzian, F.; Bittl, R. Radicals, Radical Pairs and Triplet States in Photosynthesis. Acc. Chem. Res. 2002, 35, 313−320.

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