Photoinduced Energy and Charge Transfer in P3HT: SWNT Composites

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Photoinduced Energy and Charge Transfer in P3HT:SWNT Composites Andrew J. Ferguson,*,† Jeffrey L. Blackburn,*,† Josh M. Holt,† Nikos Kopidakis,† Robert C. Tenent,† Teresa M. Barnes,‡ Michael J. Heben,§,† and Garry Rumbles† †

Chemical & Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, National Center for Photovoltaics, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, and § Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio 43606 ‡

ABSTRACT Using steady-state photoluminescence and transient microwave conductivity (TRMC) spectroscopies, photoinduced energy and charge transfer from poly(3-hexylthiophene) (P3HT) to single-walled carbon nanotubes (SWNTs) are reported. Long-lived charge carriers are observed for excitons generated in the polymer due to interfacial electron transfer, while excitation of the SWNTs results in short-lived carriers confined to the nanotubes. The TRMC-measured mobility of electrons injected into the SWNTs exhibits a surprisingly small lower limit of 0.057 cm2/(V s), which we attribute to carrier scattering within the nanotube that inhibits resonance of the microwave electric field with the confined carriers. The observation of charge transfer and the lifetime of the separated carriers suggest that the primary photoinduced carrier generation process does not limit the performance of organic photovoltaic (OPV) devices based on P3HT:SWNTcomposites. With optimization, blends of P3HT with semiconducting SWNTs (s-SWNTs) may exhibit promise as an OPV active layer and could provide good solar photoconversion power efficiencies. SECTION Energy Conversion and Storage

photoinduced electron transfer,11,14 without any conclusive evidence for the formation of charged species. Finally, SWNTs have been shown to alter the ordering, and hence optical properties, of several conjugated polymers,11,16-18 which has been used as justification for an observed increase in the effective carrier mobility within a polythiophene:SWNT composite.19 Despite these promising findings, replacement of the ubiquitous fullerene acceptors by SWNTs in OPV devices has shown very limited success thus far. Devices typically exhibit poor rectification behavior,17,20-22 and the highest reported power conversion efficiency for a simple polythiophene:SWNTactive-layer is only 0.5%,23 which is an order of magnitude lower than the typical polythiophene:fullerene device.24,25 However, direct comparison of devices employing either SWNTs or fullerenes as the electron acceptor may be misleading at this point, since the number of fundamental investigations of charge transfer between SWNTs and conjugated polymers is rather low. As-produced SWNT samples represent an extremely heterogeneous system with large electronic polydispersity, meaning there are a number of factors that can affect their optical, electronic and morphological properties, which could in turn influence device performance.

S

ince the discovery of highly efficient photoinduced electron transfer in an interpenetrating network of two different organic semiconducting materials over a decade ago,1,2 organic photovoltaic (OPV) devices based on a bulk heterojunction (BHJ) architecture have been extensively investigated.3 To date, the best performance has been achieved using a blend of an electron-donating conjugated polymer and an electron-accepting fullerene,3 although much recent focus has turned to optimization of the optical and electronic properties of both components.4-6 Several unique properties of single-walled carbon nanotubes (SWNTs) have motivated their investigation as potential replacements for fullerene derivatives as the acceptor phase of BHJ OPV devices.7 Excellent charge transport properties arising from high aspect ratios and extremely high conductivity at the level of both single tubes8 and networks9 suggest that SWNTs should yield a more efficient percolation network and potentially higher mobility than fullerenes. Although there is still some uncertainty regarding precise values, estimates of the ionization potential (IP) and electron affinity (EA) for P3HT and various SWNTs (Figure S1) suggest that a type II band offset should be formed at the interface between the two materials. Indeed, several spectroscopic studies on polymer: SWNT systems have concluded that photoinduced interfacial charge separation occurs in these systems.10-15 However, it should be noted that quenching of the polymer photoluminescence (PL) or a faster PL decay is often cited as an indication of

r 2010 American Chemical Society

Received Date: June 5, 2010 Accepted Date: July 16, 2010 Published on Web Date: July 26, 2010

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DOI: 10.1021/jz100768f |J. Phys. Chem. Lett. 2010, 1, 2406–2411

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For example, it has been suggested that the continuous density of “mid-gap” electronic states of metallic SWNTs (m-SWNTs) can act as an efficient recombination pathway for excitons in polymer:SWNT blends.26 Clearly, fundamental studies of conjugated polymer-SWNT blends designed to both demonstrate and optimize charge separation will facilitate the rational design and improved efficiencies of OPV device architectures based on these systems. Here we show, using a combination of steady-state and time-resolved spectroscopic techniques, that the excited state of poly(3-hexylthiophene) (P3HT) is indeed quenched by the presence of SWNTs. Importantly, we demonstrate conclusively that charge transfer occurs in addition to the previously observed energy transfer mechanism that is easily discernible by steady-state PL spectroscopy.18 We selectively excite either P3HT or SWNTs in composite films and probe free carrier generation with time-resolved microwave conductivity (TRMC), and demonstrate that photoexcitation of the polymer gives rise to long-lived carriers, due to spatial separation of the charges across the donor-acceptor interface. In contrast, no charge transfer to the polymer is observed after photon absorption by the SWNTs, and the carriers are short-lived as a result of confinement to isolated nanotubes. Figure 1a shows the absorbance of SWNTs, prepared by laser vaporization (LV), dispersed in tetrachloroethylene (TCE) by P3HT (blue curve). Broad envelopes of peaks are observed in the ranges of 0.5-0.9 eV and 1.0-1.5 eV, corresponding to the first and second excited states of the s-SWNTs (S11 and S22), respectively. A small shoulder is also present around 1.8 eV, corresponding to the first excited states (M11) of m-SWNTs. The significant structure observed for the S11 and S22 peak envelopes indicates that P3HT acts as a reasonable dispersant for the LV SWNTs, since the absorbance of dispersions made with poorly dispersing surfactants typically display broad, featureless peak envelopes for LV SWNTs.27 The absorption features due to SWNTs are red-shifted in the P3HT dispersion relative to those observed for SWNTs dispersed by sodium carboxymethyl cellulose (CMC) in D2O (black trace). CMC is a useful inert polymer dispersant that allows us to deposit uniform SWNT films9 for comparison to P3HT:SWNT composites. In addition to the peak envelopes for SWNTs, the absorbance of the P3HT polymer is seen in the range of 1.9-3.5 eV. Interestingly, superimposed on the main polymer absorbance centered at 2.7 eV is a sideband centered at ∼2.05 eV. This sideband is not observed in the absorption spectrum of the neat polymer in a good solvent such as TCE (red curve) but can be induced by the addition of a poor solvent, such as methanol, to the P3HT solution (green curve) or if the P3HT is deposited as a film (red curve in Figure 1b).28 With regards to the neat polymer, such a sideband arises when the P3HT chains exhibit enhanced intramolecular ordering as a result of the formation of weakly interacting H-aggregates, either in concentrated solution or as a result of film formation.29 This sideband has been observed in previous studies of P3HT: SWNT nanohybrids in various solvents,16,18 and has been attributed to either the polymer ordering directly on the SWNT surface, SWNT-induced polymer/polymer ordering, or a combination of both mechanisms.


Figure 1. (a) Normalized absorbance spectra of P3HT in TCE (red), P3HT in TCE upon addition of MeOH (green), and SWNTs suspended in TCE by P3HT (blue); also shown is the spectrum of SWNTs suspended in D2O by CMC (black). (b) Attenuation spectra of a P3HT film (red), a P3HT:SWNT composite (blue), and the absorption features due to the SWNTs in the P3HT:SWNT composite multiplied by a factor of 10 (blue, dashed); also shown is the spectrum of a SWNT:CMC composite (black). The arrows denote the pump energies used in the TRMC measurements.

Figure S2a,b (Supporting Information) contain Fourier transform photoluminescence excitation (PLE) maps for LV SWNTs dispersed in TCE with P3HT, showing excitation and emission peaks that are strongly red-shifted relative to their positions in traditional aqueous dispersions.15 Furthermore, an excitation scan of the SWNT emission at 0.775 eV (1600 nm) (Figure S2d) strongly suggests that photoexcited P3HT, when bound to the SWNT surface, can undergo energy transfer to SWNTs, in accordance with a previous study of bound P3HT:SWNT nanohybrids obtained via solvent extraction.18 These results indicate that ordering of P3HT on the surfaces of SWNTs and SWNT bundles contributes, at least in part, to the appearance of the ∼600 nm shoulder in the attenuation spectra for these dispersions shown in Figure 1. In contrast, photoexcitation of P3HT between 450 and 500 nm does not lead to appreciable SWNT luminescence (Figure S2c), indicating a much reduced dipole interaction between SWNTs and unbound solution-phase P3HT.

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observed by time-resolved terahertz spectroscopy (TRTS) for SWNT mats.30 Given the large difference between the time scale for carrier decay and the TRMC instrument response time (τIRF ∼ 5 ns),31 the confinement of both carriers to isolated tubes in the inert CMC matrix results in transient profiles that are limited by the instrument response, as demonstrated in Figure 3a, and essentially independent of pump energy (Figure S3a,b). The sensitivity of TRMC allows the detection of a small long-lived photoconductance signal, possibly due to carrier separation at a very small number of junctions between two tubes, tube ends, or even trapping of one type of carrier at a defect. Infrared excitation (Epump = 1.165 eV, S22 excitation) of SWNTs dispersed in a P3HT matrix (red curve in Figure 2b) results in photoconductance transient profiles identical to those obtained in the CMC matrix, suggesting that hole transfer to P3HT does not occur from LV SWNTs, and that the carriers remain confined to the nanotube phase, despite the predicted type-II band offset. A comparison of the data presented here and preliminary TRMC studies of composites of P3HT with nanotubes prepared by a gas-phase chemical-vapor-deposition process (data not shown) suggests that the efficacy of hole transfer to P3HT may be dependent on the tube diameter, which will determine whether the offset between the IPs of the two components is sufficient to overcome the SWNT exciton binding energy (∼300-400 meV).32,33 Consideration should also be given to the rapid exciton and carrier decay in SWNTs,30,34 which will compete with hole transfer to the polymer, meaning there may be both thermodynamic and kinetic barriers to this hole transfer step for the LV SWNTs studied here. In contrast, excitation of P3HT in the composite (Epump = 2.33 eV) results in a long-lived signal (green curve in Figure 2b), implying that recombination is slowed down by spatial separation of charge carriers into different phases and a concomitant reduction in the overlap of the electron and hole wave functions. The predicted type-II band offset formed between P3HT and SWNTs (Figure S1) suggests that this charge-separated state involves an electron in the SWNT lowest unoccupied molecular orbital (LUMO) and a hole in the P3HT highest occupied molcular orbital (HOMO); we discuss this charge-separated state in more detail below. Interestingly, no short-lived component/ contribution is observed from direct optical absorption in SWNTs since ca. 97% of the 2.33 eV photons are absorbed by the P3HT in the blend due to the low loading of SWNTs. It should be noted that the transient profiles observed upon direct excitation of SWNTs in both CMC and P3HT (Figure S3a/ b,c, respectively) show little dependence on absorbed photon flux. In contrast, a strong dependence of the transient profiles on the absorbed photon flux is observed upon excitation of the polymer in the P3HT:SWNT composite (Figure S3d). A similar absorbed photon flux dependence has been observed for blends of P3HT with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a prototypical donor:acceptor system with well-documented interfacial charge separation, using either TRMC31 (Figure S3e) or transient absorption spectroscopy.35 Such behavior, indicative of second-order (bimolecular) recombination, lends further weight to the suggestion that the carriers occupy different phases in the P3HT:SWNTcomposite following photoexcitation of P3HT.

Figure 2. Photoconductance transients, normalized by the absorbed photon flux, in this case 8 1013 photons/cm2, for (a) SWNTs dispersed in an inert CMC matrix and (b) SWNTs dispersed in a P3HT film. Also shown for comparison is the transient measured for pristine P3HT multiplied by a factor of 100. The inset shows the enhancement of the signal due to long-lived carriers when the SWNTs are dispersed in P3HT.

Figure 1b shows the attenuation spectrum of a pristine P3HT film (red curve) and a P3HT:SWNT composite film (blue curve). In the composite, the interchain interactions observed in solid-state P3HT overcome those imposed by the presence of SWNTs, and, as a result, both spectra exhibit a typical solidstate P3HT spectrum: a broad absorption band with clearly discernible vibrational features.29 In the spectrum of the composite, absorption features due to the presence of SWNTs are superimposed on the very low absorption background of a pristine P3HT film below 1.9 eV. Photoconductance transients, normalized by the absorbed photon flux, for SWNTs dispersed in either an inert CMC matrix or a P3HT film, are shown in Figures 2a and 2b respectively. The photoconductance signal, explained in detail below, is proportional to the product of the yield of free carrier generation and the carrier mobilities. The transient signal, therefore, provides information on the kinetics of free carrier decay by, e.g., recombination, trapping, and so forth. Very rapid free-carrier decay (τcarrier ∼ 1-4 ps) has previously been


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Figure 3. Yield-mobility product, jΣμ, at (a) end-of-pulse and (b) t = 150 ns as a function of absorbed photon flux for SWNTs dispersed in CMC (dashed line, circles) and P3HT (solid line, squares). Data obtained at a pump energy of 2.33 eV are shown in green, and those at 1.625 eV are shown in red. Also shown are the corresponding yield-mobility products for a pristine P3HT film (solid line, triangles).

product of these two quantities (eq 1) but not each one independently. At low excitation intensities, where the contribution of nonlinear loss processes to the photocarrier generation mechanism diminishes,31 the data of Figure 3 show a plateau at a value of jΣμ = 0.040 cm2/(V s), which is about 100 times greater than pristine P3HT. Steady-state PL measurements (Figure S4) indicate that the emission from pristine P3HT is quenched by ∼85% in the composite with SWNTs. Of these excitons, we assume that 33% will dissociate on interfaces with m-SWNTs and will therefore not produce long-lived free carriers because the carriers will rapidly recombine within the metallic nanotubes. Assuming quantitative quenching of the remaining excitons by charge transfer to s-SWNTs and generation of free carriers at the polymer-nanotube interface, we can estimate an upper limit for the yield for free carrier generation j ≈ 56%. This yield can be used to estimate the high-frequency electron mobility in the SWNT phase, μe g 0.057 cm2/(V s), assuming that the high-frequency hole mobility in P3HT remains 0.014 cm2/(V s).31,37 Furthermore, the SWNT:P3HT blends studied here are not centrifuged after sonication, implying that some proportion of the SWNTs studied here is likely bundled. The probability of finding a metallic tube in any given bundle is likely to be significantly higher than 33%,39 which would further lower j, resulting in an increase in the effective electron mobility. Also, PLE measurements demonstrate that energy transfer occurs from photoexcited P3HT to s-SWNTs (Figure S2). Such energy transfer imposes a further limitation to free carrier yield by charge transfer, although our measurements do not allow us to quantify the potential reduction in free carrier yield (and associated increase in effective mobility). The above considerations suggest that μe ∼ 0.057 cm2/(V s) may be considered a lower limit for the mobility of the electron transferred from P3HT to SWNTs in composite films. Nonetheless, this estimate is significantly lower than

The measured photoconductance signal contains contributions from both free carriers36,37 (eq 1). ΔG ¼ β 3 qe 3 I0 FA 3 jΣμ ¼ β 3 qe 3 I0 FA 3 j½μe, S þ μh, P

ð1Þ

where β = 2.2 is the ratio of the long and short dimension of the X-band waveguide used,38 qe is the elementary charge, I0 is the incident photon flux, FA is the fraction of light absorbed in the sample at the excitation wavelength, j is the yield for free carrier generation, μe,S is the high-frequency electron mobility in the SWNT phase, and μh,P is the high-frequency hole mobility in the P3HT phase. Equation 1 allows one to estimate the yield-mobility product jΣμ=ΔG/(βqe.I0FA), shown in Figure 3 as a function of the absorbed photon flux for pure SWNT (in CMC), pure P3HT, and SWNT:P3HT composite film samples. The values obtained from the peak (end-of-pulse) photoconductance signals are shown in Figure 3a, and those obtained at t = 150 ns, i.e., after all rapid, early time decay processes are complete, are shown in Figure 3b. Since it is the long-lived carriers that can be harvested in an OPV device, it is important to consider the magnitude of the signal at t = 150 ns with respect to the peak signal. After accounting for the rapid carrier recombination observed by TRTS,30 the signals observed for the isolated SWNTs suggest that the yield of longlived carriers is more than an order of magnitude smaller than the peak value. In contrast, excitation of the P3HT in the P3HT: SWNT composite results in only a factor of 2 decrease of the peak photoconductance signal to that measured at t = 150 ns. This result suggests that P3HT:SWNT blends show great promise as the active layer for solar photoconversion. To evaluate the effectiveness of SWNTs as an acceptor in BHJ structures with a polymer donor, two important parameters need to be known: (a) the efficiency for free carrier generation per absorbed photon in the blend (j in eq 1) and (b) the mobility of these free carriers (Σμ in eq 1). As mentioned above, TRMC provides a measure of the


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mobility values determined in single-tube transistor geometries8 and in TRTS measurements of SWNT mats.40 We infer that carrier scattering events prevent the electron from oscillating in resonance with the electric field of the microwave probe beam, resulting in a reduction of the high-frequency mobility.41 The sources of these scattering events are under investigation and are currently beyond the scope of this work, but may include tube ends, tube-tube interactions within a bundle, tube defects, localized electronic perturbations created by strong SWNT:P3HT interactions, and so on. To conclude, we observe photoinduced energy and charge transfer from P3HT to SWNT by steady-state luminescence and TRMC, respectively. Electron transfer from photoexcited P3HT to SWNTs results in long-lived free carriers in P3HT: SWNT BHJs. Estimates of the electron mobility in the nanotubes result in a surprisingly small lower limit (μe g 0.057 cm2/(V s)), indicating that the electron mobility in these solidstate BHJs may be limited by electronic and morphological perturbations. The observation of long-lived carriers suggests that the performance of OPV devices based on a P3HT:SWNT active layer is not limited by the photoinduced carrier generation process, and that composites of P3HT and s-SWNTs may exhibit promise as the active layer in OPV devices, although significant progress needs to be made in order to optimize their performance.

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SUPPORTING INFORMATION AVAILABLE Detailed experimental section, theoretical prediction of type-II heterojunction formation, two-dimensional PLE spectra, normalized photoconductance transients, and P3HT PL quenching data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author: (12)

*To whom correspondence should be addressed. (A.J.F.) Tel: þ1 (303) 384.6637; fax: þ1 (303) 384.6655; e-mail: andrew.ferguson@ nrel.gov. ( J.L.B.) Tel: þ1 (303) 384.6649; fax: þ1 (303) 384.6432; e-mail: [email protected].

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ACKNOWLEDGMENT We gratefully acknowledge Prof. Dr. L. D. A. Siebbeles and the Opto-electronic Materials group of the Delft University of Technology for useful discussions regarding TRMC measurements. We acknowledge Dr. R. E. Larsen (NREL) for provision of the P3HT and SWNT structures used in the Table of Contents graphic. The solar image used in the Table of Contents graphic is provided courtesy of the SOHO/EIT consortium. SOHO is a project of international cooperation between ESA and NASA. 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.

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Charges in P3HT/PCBM Blends. J. Phys. Chem. C 2009, 113, 14500–14506. Savenije, T. J.; Kroeze, J. E.; Wienk, M. M.; Kroon, J. M.; Warman, J. M. Mobility and Decay Kinetics of Charge Carriers in Photoexcited PCBM/PPV Blends. Phys. Rev. B 2004, 69, 155205. Dicker, G.; de Haas, M. P.; Siebbeles, L. D. A.; Warman, J. M. Electrodeless Time-Resolved Microwave Conductivity Study of Charge-Carrier Photogeneration in Regioregular Poly(3hexylthiophene) Thin Films. Phys. Rev. B 2004, 70, 045203. Savenije, T. J.; de Haas, M. P.; Warman, J. M. The Yield and Mobility of Charge Carriers in Smooth and Nanoporous TiO2 Films. Z. Phys. Chem. 1999, 212, 201–206. The probability of finding a metallic SWNT within a bundle of 2, 3, and 4 SWNTs rises dramatically to 56%, 71%, and 81%, respectively. The high-frequency mobility can be calculated using μ = qeτs/m*, where qe is the elementary charge, τs is the average time between scattering events, and m* is the effective carrier mass. Values of 0.052 e τs e 0.089 ps and 0.1 me e m* e 1 me give rise to mobilities between 90 and 1500 cm2/V s. Prins, P.; Grozema, F. C.; Schins, J. M.; Patil, S.; Scherf, U.; Siebbeles, L. D. A. High Intrachain Hole Mobility on Molecular Wires of Ladder-Type Poly(p-phenylenes). Phys. Rev. Lett. 2006, 96, 146601.


Photoinduced Energy and Charge Transfer in P3HT: SWNT Composites

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