Letter pubs.acs.org/JPCL
Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions Dominick J. Bindl,† Andrew J. Ferguson,‡ Meng-Yin Wu,§ Nikos Kopidakis,‡ Jeffrey L. Blackburn,*,‡ and Michael S. Arnold*,† †
Department of Materials Science and Engineering and §Department of Electrical Engineering, University of WisconsinMadison, Madison, Wisconsin, 53706, United States ‡ Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising for solution-processed, thin film photovoltaics due to their strong nearinfrared absorptivity and excellent transport properties. We report on the generation yield and recombination kinetics of free charge carriers in photoexcited thin films of polymer-wrapped s-SWCNTs with and without an overlying electron-accepting C60 layer, using time-resolved microwave photoconductivity (TRMC). Free carriers are generated in neat s-SWCNT films, even without an obvious driving force for exciton dissociation. However, most carriers recombine in 1.3 cm2 V−1 s−1. These studies improve understanding of sSWCNT photoresponses in solar cells and photodetectors. SECTION: Spectroscopy, Photochemistry, and Excited States
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generated excitons and the collection of resulting free carriers with efficiencies approaching unity.22,23 We have exploited this high charge generation yield at the s-SWCNT−C60 heterointerface to demonstrate a bilayer solar cell with a 1% power conversion efficiency, in which most of the response is driven by absorption from an ultrathin s-SWCNT layer 1205 nm. To better elucidate the s-SWCNT chirality dependence on photogeneration yield, we analyze the spectral dependence of the EOP ϕ∑ μ at constant fluence in Figure 3B. In the sSWCNT−C60 bilayers, the ϕ∑ μ corresponding to the smaller band gap (8,7) and (9,7) nanotubes are less than the ϕ∑ μ corresponding to the larger band gap (7,5), (7,6), and (8,6) nanotubes. Specifically, ϕ∑ μ falls from 0.33, 0.37, and 0.33 cm2 V−1 s−1 for the (7,5), (7,6), and (8,6) chiralities to 0.13 and 0.11 cm2 V−1 s−1 for the (8,7) and (9,7) chiralities, respectively. Because the TRMC measurement is incapable of directly decoupling the contributions of ϕ and μ to the decrease in ϕ∑ μ, at first glance, this decrease could potentially be explained by a reduced carrier yield or reduced high-frequency mobility of holes on the (8,7) and (9,7) s-SWCNTs. However, field effect transistor measurements demonstrate an increase in mobility with increasing diameter (decreasing band gap),43 and semiclassical carrier transport theory also suggests that the low-
Figure 3. (A) Spectral dependence of EOP photoconductance (ΔGEOP), normalized to the incident photon fluence (I0), for neat PFO-wrapped s-SWCNTs (neat, red diamonds) and a PFO-wrapped s-SWCNT film in bilayers with C60 (bilayer, blue circles) compared to the absolute absorptance (1 − transmittance) for the same samples. (B) Near-infrared spectral dependence of the yield-mobility product (ϕ∑ μ) at EOP for neat and bilayer films. Vertical gray bars indicate wavelengths in resonance with the S1 transition of the s-SWCNT chirality indicated.
field carrier mobility increases with decreasing s-SWCNT band gap (by a factor of 2 going from the (7,5) to the (9,7) nanotube).44 Thus, it is more likely that the decrease in ϕ∑ μ indicates a reduction in free-carrier generation yield as the nanotube band gap decreases. Further support for this interpretation comes from device studies in which the IQE for exciton dissociation and charge collection from photoexcited (8,7) and (9,7) SWCNTs is suppressed with respect to the larger band gap species due to an insufficient energy offset between the conduction band of these nanotubes and the lowest unoccupied molecular orbital (LUMO) of C60.23,44 In this case, free carrier generation is driven by electron transfer from the SWCNT film to the C60 film, but only for s-SWCNTs in which the electron affinity (EA) provides a sufficient thermodynamic driving force. The vanishing free carrier generation, observed by TRMC for the (9,7) chirality in the bilayer film, provides an opportunity to estimate the EA of this s-SWCNT. Previous device-scale 3554
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matrix.31 Another recent pump−probe study observed that the probe pulses formed trions (bound exciton−charge complexes) following excitation of (6,5) SWCNTs with pump pulses of fluences ranging from 1012 to 1014 photons cm−2 per pulse.28 The results of this study suggested a direct production of free charge carriers from the photoexcited exciton population resulting from AI at high exciton densities. Furthermore, the charge carrier yield (carriers per incident photon) decreased with increasing fluence due to a saturation of the exciton density created by pump photons. Our current study indicates a low-fluence free carrier yield on the order of ∼6% (see below) that decreases with increasing pump fluence, both of which are consistent with the results of the TA studies discussed above.28,31 Finally, in an effort to deconstruct the yield-mobility product (ϕ∑ μ), we estimate ϕ for s-SWCNT films with a C60 overlayer using PLQ data (Figure 4A). Photoluminescence was measured from films equivalent in thickness and of identical composition to those studied by TRMC, using a
measurements of charge transfer across this interface required the simultaneous measurement of photocarrier generation and collection, potentially convoluting diametric/chirality trends in the charge-transfer process with diametric/chirality trends in the carrier collection process.9 This is nontrivial as it is expected that free carriers will pool on larger-diameter s-SWCNT species, which exist in lower relative abundance and potentially do not form percolating networks for charge extraction. Here, we uniquely probe the photogeneration process and are free from any concerns regarding collection. The observation that the measured yield-mobility product is nearly equivalent with and without the C60 interface when optically exciting the (9,7) chirality (Figure 3B) allows us to estimate this SWCNT’s EA and ionization potential (IP), related to the energies of the LUMO and highest occupied molecular orbitals (HOMO), respectively. Because we see no significant photoconductivity gains with the addition of the C60 interface, we assert that the driving force for photoexcited electron transfer to C60 is approximately 0 eV. If we take the EA of C60 to be 4.28 eV45 and use the exciton binding energy calculations of Capaz et al. with a relative dielectric constant of 4 to estimate the exciton binding energy on the (9,7) chirality as 0.2 eV,46 we can estimate the (9,7) EA to reside 4.08 eV below vacuum. Assuming that the electronic band gap is given by the sum of the optical band gap (with an optical transition of ∼1375 nm) and the exciton binding energy, we can then estimate the (9,7) IP to reside 5.18 eV below vacuum. In contrast to the bilayer samples, photoconductivity measurements in the neat s-SWCNT films reveal that ϕ∑ μ is relatively independent of nanotube diameter (chirality). In fact, the dependence of ϕ∑ μ on nanotube chirality appears to exhibit an opposite trend to the bilayer sample, increasing slightly with decreasing s-SWCNT band gap. This trend reversal suggests that photocarriers are produced via a different mechanism in neat versus bilayer samples. The observed diametric trend in neat s-SWCNT films, specifically the increase in ϕ∑ μ for the (8,7) and (9,7) SWCNTs, may simply be due to an increase in the carrier mobility, as mentioned above, and an equivalent photogeneration yield across diameters. The fact that free carriers are being photogenerated in neat polymer-wrapped s-SWCNT films at all, let alone with yields up to 6% (see below), is rather surprising but consistent with our previous THz26 and GHz30 spectroscopic studies, as well as several recent transient absorbance or PL studies.27−29,31 In the absence of the C60 electron-accepting layer, there is no intentional driving force for overcoming the exciton binding energy, which is expected to be between 200 and 250 meV for the s-SWCNTs studied here, in a medium expected to exhibit a spatially averaged relative dielectric permittivity of 4.46 Thermally driven “spontaneous” exciton dissociation cannot explain the relatively large charge generation yield because of the large binding energy with respect to kBT. Furthermore, the dispersing polymer, PFO, and all chiralities present are expected to form type-I heterojunction(s), and the work function across these chiralities is not expected to vary significantly. Therefore, it would seem unlikely that free carriers are being generated via charge transfer from one material component to another.9 As discussed above, it is also possible that free carriers are being generated via AI. A recent transient absorption (TA) measurement suggested that free carriers were generated with yields on the order of a few percent for uncoupled (6,5) SWCNTs isolated in a gelatin
Figure 4. (A) Photoluminescence emission of PFO-wrapped sSWCNT films before (red diamonds) and after (blue circles) deposition of C60. Complete quenching of S1 emission is not observed because the films utilized here are much thicker than the exciton diffusion length, estimated to be on the order of 3 nm elsewhere.23 (B) Fluence dependence of the calculated free carrier generation yield (ϕ) for neat films (red diamonds) and bilayers with C60 (blue circles). 3555
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continuous wave (CW), λ = 655 nm laser diode as an excitation source with an intensity of 15 mW cm−2, equivalent to a photon flux of 4.8 × 1016 photons cm−2 s−1 or 1.9 × 108 photons cm−2 per 4 ns. The PLQ efficiency was 0.65, consistent with previous measurements of an IQE of 0.72 for photogenerated exciton to collected charge conversion in sSWCNT−C60 bilayer devices fabricated from films with the same PFO/s-SWCNT ratio at a s-SWCNT thickness of 9.5 nm.56 This observation indicates that the product of exciton quenching is free carriers resulting from photoinduced interfacial charge transfer. We use ϕ = 0.65 measured here via PLQ for the initial free carrier yield that should be measured by TRMC. Specifically, we couple this estimate (ϕ = 0.65) with the low-fluence yield-mobility product extrapolation of ϕ∑ μ = 0.84 cm2 V−1 s−1 for excitation at λ = 1205 nm, estimated earlier from the empirical fit using eq 2. This results in a lower limit for the high-frequency (@ ∼9 GHz) mobility sum in the bilayer of ∑ μ > 1.29 cm2 V−1 s−1 because it is possible that a fraction of the photoinduced carriers decays faster than can be resolved in our low-field TRMC measurement. This carrier mobility sum represents the sum of the electron mobility in C60 with the hole mobility in the s-SWCNT phase; therefore, extracting the SWCNT hole mobility requires an estimate of the C60 electron mobility. Recent time-resolved terahertz spectroscopy measurements indicate that the high-frequency (∼1 THz) electron mobility in a thin, thermally evaporated C60 film is on the order of 50 cm2 V−1 s−1, decaying to around 25 cm2 V−1 s−1 in the first several tens of picoseconds.47 Previous pulse radiolysis TRMC measurements, carried out at ∼32 GHz, of carrier transport in C60 powders estimated ∑ μ to have a minimum value of ∼0.3 cm2 V−1 s−1 and, assuming an equal contribution of electrons and holes to the measured radiation-induced conductivity, a value for μe,min of ∼0.15 cm2 V−1 s−1.48,49 In Figure S2 (Supporting Information), we estimate the measured high-frequency mobility as a function of probe frequency, calculated by solving the three-dimensional diffusion equation inside of a cube of edge-length a, with reflecting boundary conditions at the sides of the cube.50,51 If one assumes a cube with edge-length a = 50 nm, corresponding roughly to the crystallite sizes observed for thermally evaporated thin films52 such as those deposited here and in the TRTS study,47 the high-frequency electron mobility detected at 32 and 9 GHz would be μe,32 GHz ≈ 21.7 cm2 V−1 s−1 and μe,9 GHz ≈ 5.1 cm2 V−1 s−1, respectively. These values appear to be unrealistically large, suggesting that the true scattering length is smaller than the crystallite dimensions. In fact, a value of a = 10 nm successfully reproduces the values obtained at 1 THz (μe,1THz ≈ 25 cm2 V−1 s−1) and at 32 GHz (μe,32 GHz ≈ 0.15 cm2 V−1 s−1) and results in a high-frequency electron mobility detected at 9 GHz of μe,9 GHz ≈ 0.01 cm2 V−1 s−1. It should be noted that this value is on the order of that measured for the high-frequency electron mobility in domains of the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blended with poly(3-hexylthiophene) (P3HT).37 We now use the estimate for the electron mobility in the C60 layer at 9 GHz (μe,9 GHz ≈ 0.01 cm2 V−1 s−1) to predict the contribution of mobile holes in the SWCNT layer. Because the high-frequency (@ ∼9 GHz) mobility sum in the bilayer is ∑ μ = 1.29 cm2 V−1 s−1 and we estimate μe,9 GHz ≈ 0.01 cm2 V−1 s−1, the high-frequency mobility of holes in the SWCNT layer is estimated to be μh,9 GHz ≈ 1.28 cm2 V−1 s−1. This suggests that even if the scattering length and therefore electron mobility in
the C60 domains are larger that the photoconductance in the SWCNT−C60 bilayer is dominated by mobile holes in the SWCNT layer. In the case of the neat s-SWCNT film, we assume that mobile holes and electrons contribute equally to the measured photoconductance, which results in a highfrequency mobility sum of ∑ μ ≈ 2.6 cm2 V−1 s−1. Similar to the process described above for the s-SWCNT−C60 bilayer, we use the low-fluence yield-mobility product extrapolation of ϕ∑ μ = 0.17 cm2 V−1 s−1 for excitation of the neat s-SWCNT film at λ = 1205 nm, estimated from the empirical fit using eq 2, and estimate the low-fluence saturation photogenerated free carrier yield (ϕ) in the neat s-SWCNT film to be ϕ ≈ 6%, with experimental fluence-dependent values shown in Figure 4B. It is important to keep in mind that this estimate is informed by several assumptions, each of which comes with associated uncertainties that are discussed in more detail in the Supporting Information. However, we note that the values obtained from our analysis are consistent with recent estimates of free carrier yield in neat s-SWCNT films,31 as well as with the GHz mobility of electrons in PCBM domains,37 lending credibility to our analysis. While field effect transistor studies on identically prepared s-SWCNT films report FET hole mobilities of ∼2 cm2 V−1 s−1,53 the high-f requency carrier mobility in s-SWCNTs is still largely unexplored. While a free carrier photogeneration yield of ∼6% is not promising in-and-of itself for high-efficiency photovoltaics, it is significant. More problematic is that without a mechanism for rapid spatial separation of these free carriers, they rapidly recombine, as shown in Figure 1C. It is interesting to note that by utilizing very similar polymer-wrapped s-SWCNT samples and dispersions, studies designed specifically to look for free carrier photogeneration and collection failed to measure photocurrent from the excitation of neat, polymer-wrapped sSWCNT films,9 likely due to the fast recombination for neat sSWCNT films, ∼10 ns. However, Soavi et al. recently demonstrated photocurrent generation from a neat sSWCNT film sandwiched between ITO and an aluminum cathode.31 The predominant differences between our previous device studies and the Soavi study include (1) the absence of noncovalent surface modification by PFO in the Soavi study and (2) the additional presence of a s-SWCNT−Al interface. It is possible that free carrier mobilities are much greater in sSWCNT films without PFO, enabling the rapid extraction of photocurrent. It is also possible that s-SWCNT exciton dissociation is occurring at the Al interface, effectively accomplishing exciton dissociation and charge extraction through a single photoexcited electron- or hole-transfer event. Before ending with a discussion of the conclusions that we can firmly make, we briefly discuss effects that cannot be directly resolved in our TRMC experiment but that may be relevant to our results and future studies aimed at probing carrier generation in neat s-SWCNT films. The mechanism for free carrier generation in neat s-SWCNT films is still poorly understood; whereas Yuma et al. ascribe charge generation in dispersed SWCNTs primarily to AI,28 another possible source for carriers within our thin films is exciton dissociation at defects and/or traps. In addition to free charge carriers, trions (bound exciton−charge pairs) are understood to be readily created in s-SWCNTs optically excited at high fluence when there is an overlap between the optically excited exciton and charge populations.27,28,54,55 The relatively long duration of our pump pulse, relative to exciton lifetimes and collision rates, coupled with the finite free carrier yield, results in a population 3556
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toluene using a horn tip ultrasonicator for 1 h, utilizing a water bath to cool the solution. The resulting suspension was then centrifuged for 15 min at 50 000g over an 11 cm path length in a swing-bucket rotor, the supernatant was collected, and the pellet was discarded. The supernatant was then filtered through a 5 μm syringe filter. The resulting dilute solution was concentrated while simultaneously removing excess PFO by pelleting the s-SWCNTs out of solution at 50 000g in 11 cm long fluoropolymer centrifuge tubes in a 30° fixed angle rotor held at a temperature of 4 °C over a period of time approaching 90 h for high extraction yields. The pellet was then redispersed and dissolved in fresh tetrahydrofuran (THF) by heating on a hot plate set to 90 °C for iterative pelleting or redispersed into chlorobenzene to yield a stable solution. The resulting solutions consisted of primarily the (7,5), (7,6), (8,6), (8,7), and (9,7) chiralities of semiconducting nanotubes, typical of the HiPco PFO/toluene system, wrapped by tunable amounts of PFO, with minimal quantities of metallic nanotubes, amorphous carbon, aggregates, or residual catalyst. The solutions utilized here had negligible amounts of free solution-phase PFO (i.e., not adsorbed to the s-SWCNT surface), and the PFO/sSWCNT weight ratio was roughly 0.65:0.35, determined from solution absorption spectra using known and measured optical cross sections, as described in our previous work.56 Thin films of s-SWCNTs were deposited on quartz substrates. Before s-SWCNT deposition, the quartz substrates were cleaned in a typical solvent degreasing process involving acetone and isopropanol; following solvent baths, the substrates were cleaned with oxygen plasma for 10 min. The s-SWCNTs were deposited by doctorblade casting on a hot plate with a surface temperature of 100 °C in a dry argon glovebox. Droplets (5−20 μL) of a s-SWCNT-containing solution were placed at one end of the heated substrate and immediately drawn across the substrate using a doctorblade (casting knife) with a substrate clearance between 0.1 and 0.25 mm. Films of increasing thickness were built up by iterative casting. This method generates films that are visually uniform (optically and via scanning electron microscopy) on the micrometer to centimeter scale. Slowly varying gradients in thickness (as determined from optical density using a 5 mm probe) are observed over a several centimeter scale; however, the variation is less than 30%. Previous measurements56 of thicker (20−30 nm) calibration films via profilometry indicate that the thickness varies
