Optically Generated Free-Carrier Collection from an All Single-Walled

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Optically-generated Free-carrier Collection From an All Single-walled Carbon Nanotube Active Layer Lenore Kubie, Kevin J. Watkins, Rachelle Ihly, Henry V. Wladkowski, Jeffrey L. Blackburn, William Douglas Rice, and Bruce A Parkinson J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01850 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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The Journal of Physical Chemistry Letters

Optically-generated Free-carrier Collection From an All Single-walled Carbon Nanotube Active Layer Lenore Kubie [1][2], Kevin J. Watkins [1], Rachelle Ihly [2], Henry V. Wladkowski [3], Jeffrey L. Blackburn [2], William D. Rice [3], Bruce A. Parkinson [1]* [1] Department of Chemistry, University of Wyoming, Laramie, WY 82071 [2] Energy Sciences Division, National Renewable Energy Laboratory, Golden, CO 80401 [3] Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071 AUTHOR INFORMATION Corresponding Author *Email: [email protected]

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Abstract

Semiconducting single-walled carbon nanotubes’ (SWCNTs) broad absorption range and allcarbon composition make them attractive materials for light harvesting. We report photoinduced charge transfer from both multi-chiral and single-chirality SWCNT films into atomically-flat SnO2 and TiO2 crystals. Higher energy second transitions excitonic SWCNT transitions produce more photocurrent demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes. A logarithmic relationship between photoinduced electron transfer driving force and photocarrier collection efficiency especially with smaller diameter SWCNTs. Photocurrents are generated from both conventional sensitization and in the opposite direction with the semiconductor under accumulation and acting as an ohmic contact with only the p-type nanotubes. Finally, we demonstrate that SWCNT surfactant choice and concentration play a large role in photon conversion efficiency and present methods of maximizing photocurrent yields.

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Single-walled carbon nanotubes (SWCNTs) are quasi-one-dimensional materials that, depending on chirality, can be semiconducting (s-SWCNT) or metallic (m-SWCNT).1 As predicted for a 1D quantum-confined system, the density of states (DOS) for electrons and holes in SWCNTs are characterized by sharp van Hove singularities. Optical transitions for s-


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SWCNTs produce strong absorption bands in the near-infrared (NIR), visible, and ultraviolet (UV) regions of the electromagnetic spectrum and are denoted as S11, S22, S33, etc. for transitions of increasing energy. Excitation at these transitions generates bound excitons where the relaxation of highly-excited excitons (i.e. S22, S33) to the fundamental band (S11) edge occurs within ca. 100 fs.1,2 The wide range of energies over which SWCNTs absorb, paired with their inexpensive all-carbon composition, make these materials attractive for photoconversion devices.3-6 However, it has been shown that excitons in SWCNTs have large exciton binding energies (EBEs) (up to ~1 eV in s-SWCNTs and ~100 meV in m-SWCNTs) that can be as large as half the optical bandgap or 40% of the tight-binding (electronic) bandgap.1 The binding energies have been shown to depend on several factors such as SWCNT chirality, local dielectric environment and the degree of tube-tube interaction (bundling).2,7-12 Previous work in the field of SWCNT-PV has reported various photoelectric responses from SWCNTs between two electrodes with a bias along or across the tube axis.13-20 Additionally, previous work demonstrates the generation and collection of photogenerated carriers from SWCNTs by creating a heterojunction with polymers and/or fullerenes.21-28 Other work has shown both photoconductivity and excited state emission from SWCNTs.15,29-31 Spectroscopic studies show that upon photoexcitation, aggregates and films of both metallic and semiconducting SWCNTs can efficiently (> 60%) produce free-carriers.32,33 Moreover, it has been shown that despite large EBEs, free-carriers are generated spontaneously within SWCNTs in solution without exciton-exciton interactions or interaction between SWCNTs and electrical contacts.34 While work on fullerenes and/or polymers has shown the successful dissociation of SWCNT excitons and collection of free charge carriers,18-21,23,24,35-37 the low dielectric constant and


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hopping transport within these phases enhances the probability of geminate and/or non-geminate recombination. In contrast, the high dielectric constant of metal oxides should effectively screen the separated charges and single crystals enhance charge transport and collection. While metal oxide charge extraction electrodes are ubiquitous in the literature for both fundamental and device studies of various photovoltaic systems, little work has been done to explore charge transfer across well characterized SWCNT/oxide heterojunctions.38-41 Such systems also provide the ability to create heterojunctions between absorbing species (SWCNTs in this case) and single crystals with defined crystal terminations, enabling well-controlled fundamental studies of interfacial charge transfer. In this Letter, we demonstrate the collection of optically generated free-carriers from sSWCNT films and report the incident photon conversion efficiencies (IPCE) for both mixed- and single-chirality SWCNTs adsorbed onto single crystal SnO2 and TiO2 surfaces. Using submonolayer coverages of SWCNTs on single crystal electrodes, we are able to perform highly controlled and precise photoelectrical measurements and determine how SWNCT coverage, surfactants, electron transfer driving force and applied bias all affect photocurrent collection from various SWCNT species.42-45 Using this model system, we demonstrate a logarithmic relationship between the driving-force for photoinduced electron-transfer (PET) and the efficiency

of

carrier

collection,

supporting

previous

work

on

SWCNT/fullerene

heterojunctions.46 In contrast to SWCNT/fullerene heterojunctions,23,37 we observe that charge collection from higher energy SWCNT states (e.g. S22) is more efficient than that from lower energy states (i.e. S11), indicating hot electron injection from high energy charge transfer states is competitive with exciton relaxation to the band edge.34,47 Taken together these results highlight


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the advantages of using bulk (as opposed to molecule-like) acceptor/donor materials in practical design aspects of SWCNT photodetectors and PV devices. Highly-enriched [(6,5), (7,5)], and ensemble CoMoCAT and HiPco SWCNT solutions were used to sensitize the (110)-faces of SnO2 and TiO2 crystals (see supporting information sections for details of sensitization, solution preparation, and characterization). In order to produce submonolayer coverages of SWCNTs and thus reduce the effects of tube-tube interactions, we chose to use a dip-coating method. Briefly, clean, atomically flat synthetic SnO2 and TiO2 singlecrystals were sensitized by submerging the crystals in SWCNT solutions (absorbance at S11,max ≈ 1 in a 1 cm cuvette). Sodium cholate-wrapped SWCNT samples were rinsed with DI water, and poly(9,9-di-n-octylfluorenyl-2,7-diyl (PFO) -wrapped SWCNT samples were rinsed with toluene; all samples were then dried under nitrogen. The unpolished backsides of the crystals were gently abraded with sandpaper to remove any SWCNTs from the back contact. To evaluate the SWCNT coverage on the oxide surface, atomic force microscopy (AFM) imaging was performed on each sample. AFM images showed uniform coverage across a given sensitized metal oxide sample (not shown), but varying coverages between samples (Figure 1 and S1); this variance depended on both SWCNT surfactant and metal oxide substrate and adsorption time.


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Figure 1. AFM images showing the large variation in nanotube coverage depending on the surfactant; to emphasize this, shown here are the two most drastically-different samples. While coverage varied highly depending on the combination of metal oxide and surfactant, coverage was fairly uniform across a single sample. (a) Sodium cholate wrapped (6,5)-enriched SWCNTs on SnO2 (110). (b) PFO-wrapped (7,5)enriched SWCNTs on SnO2 (110).


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Importantly, care had to be taken to remove excess surfactant or polymer wrapping in the SWCNT suspensions before sensitizing the electrodes. If this was not performed, SWCNTs would cover the electrode (confirmed via AFM), however no or very small photocurrents were measured (Figure S2). This effect is analogous to the ligand effects seen in quantum dot sensitized solar cells, where excess ligands, and long-chain ligands act as insulators in the system and hinder photocurrent collection.44 CoMoCAT SWCNT suspensions with sodium dodecyl sulfate (SDS) or sodium dodecylbenzene sulfonate (SDBS) surfactants were also prepared using a method analogous to that used for the SC-wrapped CoMoCAT SWCNT suspensions. However, no photocurrent could be detected from the SDS- or SDBS-wrapped SWCNTs (not shown), and thus we surmise these surfactants form a highly insulating a layer around the SWCNTs inhibiting photoinduced charge transfer. Unfortunately, due to the doping and therefore coloration of the sensitized crystals, it was not possible to perform optical absorbance measurements directly on the sensitized crystals, and therefore absorbed photon to electron conversion efficiencies cannot be reported. Because of this, and the variance in SWNCT coverage between samples, quantitative conclusions could not be drawn about IPCE differences between samples, but instead only between different SWCNT chiralities (and thus different SWCNT bandgaps and electron-injection driving forces) on the same sensitized metal oxide sample. A standard three-electrode potentiostat was used to measure photocurrents from SWCNT films. In this setup, the sensitized metal oxide substrate was mounted in a custom electrochemical cell using either a flattened gallium-pellet or indium-gallium paste as an ohmic back-contact (Figure S3). The sensitized oxide crystal substrate served as the working electrode, a platinum wire was used as the counter electrode, and a silver wire was used as the pseudo-


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reference electrode. Measurements were performed in an acetonitrile electrolyte solution consisting of 50 mM tetrabutylammonium hexafluorophosphate (TBA-HFP) as a supporting electrolyte and 10 mM hydroquinone as a hole scavenger. Hydroquinone was chosen as the regenerator for this system because of its optical transparency and well-situated energy-level alignment (Figure 2a). Light from a tungsten lamp, dispersed through a monochromator and then focused on the sample, was used to collect photoaction spectra. Long-pass filters were used to block any second harmonic illumination. Front-side illumination through a quartz window was employed to produce photocurrents.

Figure 2. Experimental energetics. (a) Energy-level diagram of the system. (7,5) SWCNT levels (calculated using methods described by Bindl et al.),48 and SnO2 band-levels (from Xu and Schoonen) are shown.49 (b) Energy level diagram of the optical bandgap of a (6,5) SWCNT compared to the valance and conduction bands of SnO2 and TiO2. Note that the conduction band of SnO2 is lower in energy than that of TiO2, and that PET driving force into SnO2 will therefore always be larger. Also note that different SWCNT chiralities will have different bandgaps due to their different confinement. (c) Energy flow diagram of both electron injection (positive bias/depletion) and hole injection (negative bias/ accumulation) where M = Ti, Sn; HQN is hydroquinone and SQN+ is semiquinone. At potentials above than the flat-band potential (top), PET from SWNCTs into the MO2 occurs. Whereas at potentials below


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the flat-band potential (bottom), filled surface states on the MO2 (green) promote photoinduced hole injection from the SWCNTs. (not to scale).

The large bandgap of SnO2 (3.8 eV) allows for the exclusive measurement of SWCNT photocurrent without excitation of the substrate at both the S11 and S22 transitions, whereas all higher energy S22 transitions cannot be measured on TiO2 due to excitation of the TiO2 itself (Eg = 3.0 eV). Sub-bandgap photocurrents from SnO2 appear at ~3.0 eV and on TiO2 at ~2.1 eV (Figure S4). Photoaction spectra were measured for both mixed- and single-chirality SWCNT films on both TiO2 and SnO2. IPCE measurements of all samples produced photoaction spectra with clear peaks at energies matching S11. When possible, as dictated by the substrate absorbance, S22 absorption features of the SWCNTs were also examined (Figure 3). Applied biases were chosen to produce the best signal to noise while staying in the electron-injection regime where possible (explained in more detail below).


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Figure 3. UV-vis spectra of sensitization solutions. (black dashed lines), and normalized IPCE Spectra of various SWCNTs on SnO2 (red) and TiO2 (blue) where applicable. All spectra are normalized to the largest S11 peak and are collected in the electron injection regime unless otherwise noted. Peaks are labeled with their corresponding chirality, (n,m), and transition index (i.e. 11 or 22). Phonon sidebands are denoted S(11+k). (a) Sodium cholate-wrapped CoMoCAT SWCNTs. SnO2 spectra collected at +300 mV vs Ag-wire. TiO2 spectra collected at +100 mV vs Ag-wire. (b) PFO-wrapped HiPco SWCNTs SnO2 spectra collected at 0.0 V vs Agwire. TiO2 spectra collected at -600 mV vs Ag-wire (hole injection). (c) Sodium cholate-wrapped (6,5)enriched SWCNTs. SnO2 spectra collected at 0.0 V vs Ag-wire. (d) PFO-wrapped (7,5)-enriched SWCNTs. SnO2 spectra collected at 0.0 V vs Ag-wire.


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Interestingly, in all cases, excitation at the second excitonic transition (S22) of the SWCNTs produces more photocurrent than that obtained by excitation of the S11 transition or lowest energy phonon sideband,50 relative to the absorbance of the sensitization solution at each transition. Because the S22 transition does not absorb at an energy ≥ 2 Eg, we conclude that this is not due to multiple exciton generation (MEG), but is likely due to a process that is 1) a hotinjection process producing electrons that are more deeply-injected into the metal oxide, and are therefore less likely to recombine with the hole remaining on the SWCNT, 2) a hot-injection process that is faster than band edge injection due to the increased DOS in the electrode at higher energies or 3) a combination of both 1 and 2. Previously, Park et. al demonstrated increased free-carrier generation in solutions of (7,5) enriched SWCNTs at the S22 transition compared to the S11 transition relative to absorbance at those transitions, and Aspitarte et. al observed increased photocurrent quantum yields at S55 relative to S44.34,35 We considered that this increased free-carrier generation, with subsequent fast relaxation to the band edge before injection into the oxide, explain our observation. However, we note that studies on several of the same SWCNTs species have not demonstrated increased free-carrier generation or collection at S22 versus S11 in SWCNT/fullerene systems, and we therefore conclude that this is not due to an autoionization process.37,51 The absolute IPCE of these sub-monolayer SWCNT systems at the S11-transition was on the order-of-magnitude of 10-6, compared to 10-4 for monolayer PbS QDs on TiO2.42 The low IPCE value is not surprising since the SWCNT coverage on the metal oxide was sub-monolayer, and thus the films have very low absorptivity (Figures 1 and S1). Another interesting observation is the very small (0 to ≤ 10 meV) spectral red shift between the absorbance maximum of the SWCNTs in solution and the photoaction spectra, much less than


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the shifts measured for dye molecule sensitization.52 The lack of an appreciable spectral shift suggests that there is not a large change in local dielectric environment for the SWCNTs (εPFO ≈ 4;46 εSC ≈ 7;53 εTiO2 ≈ 100;54 εSnO2 ≈ 10 55). However, some of the spectral broadening seen in the IPCE photoaction spectra may be due the inhomogeneous dielectric environment of the SWCNT’s. Because we can visualize and measure the height profiles of the SWCNTs on our electrodes via AFM (Figures 1 and S1), we can eliminate SWCNT-bundling as a cause for this broadening.56 While the results reported herein focused on sub-monolayer coverages of SWCNTs on TiO2 or SnO2 crystals, it is possible to make much thicker films of SWCNTs.23 In so doing, the amount of light absorbed by these films would increase dramatically and should lead to increased IPCE values if exciton transport within the SWCNT film is efficient. Photocurrent-voltage (P-V) curves were collected to determine the relationship between applied bias and photogenerated-carrier collection.57 In these measurements, photocurrent was collected through a lock-in amplifier by chopping the monochromatic excitation source and scanning the applied bias. All P-V curves were run in a solution containing hydroquinone with a silver wire as a psuedo-reference electrode and platinum wire as a counter electrode. We observe a strong dependence on applied bias on the photocurrent in all systems studied (Figure 4). Surprisingly, higher photocurrents are measured when the metal oxide is in accumulation (negative bias with respect to flatband) generating dark currents than in depletion (positive bias with respect to flatband) on these n-type semiconductors. In fact, in one case, PFO-wrapped HiPco SWCNTs on TiO2, no detectable photocurrent is produced at more positive biases (Figure 4a). This is also why we only report the hole-injection photoaction spectra in Figure 3 for PFO-wrapped HiPco SWCNTs on TiO2.


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Figure 4. Photocurrent-voltage curves for selected SWCNT/metal oxide systems. All scans are from positive to negative (purple) and then reversed from negative back to positive (green). Electrical-noise spikes can be seen at some apexes and are post prevalent in panels A and B. (a) PFO-wrapped HiPco SWCNTs on TiO2 excited at 1040 nm (S11). (b) PFO-wrapped HiPco SWCNTs on TiO2 excited at 650 nm (S22). (c) PFO-wrapped HiPco SWCNTs on SnO2 excited at 1040 nm (S11). (d) Sodium cholate-wrapped (6,5)-enriched SWCNTs on SnO2 excited at 1000 nm (S11).


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It is important to note that currents measured in depletion (more positive potentials) are anodic and accumulation (more negative potentials) are cathodic. At more positive potentials (metal oxide in depletion) electrons are being injected into the conduction band of TiO2 or SnO2—the normal process for sensitization (Figure 2c top). However, at more negative potentials (metal oxide in accumulation) holes are being injected into the substrate (Figure 2c bottom). In this second case, the electron is being accepted by semiquinone, as determined by an increase in negative current when the semiquinone concentration is increased (Figure S5). Notably, all the trends of relative efficiencies hold true when discussing hole injection (Figure 3b red curves). The energy level alignment between the SWCNTs and the TiO2 indicate that hole injection directly into the valence band is not possible (Figure 1a). However, it is possible that at sufficiently negative biases the SWCNTs are acting like p-type semiconductors and the accumulated TiO2 surface is acting as an ohmic contact (Figure 2c). This conclusion is supported by the fact that the majority of SWCNTs in our films will be p-type semiconductors due to their exposure to oxygen and wet solvents.58 If this hypothesis is correct the sensitization experiment provides a method for determining the ratio of n and p-type SWNTs in a given sample. Previous work in the field has demonstrated a Marcus relationship between photoinduced electron transfer (PET) driving-force and charge-separation.46 In order to determine the driving force relationship in our system, we plotted the relative photon conversion efficiency (vide infra) against the driving force. The driving force was calculated using a previously reported method.46,48 In short, the driving force (∆GPET) is defined by the difference between the ionization potential of the donor (IPD) and electron affinity of the acceptor (EAA) minus the energy of the SWCNT-exciton (Eexciton) for each chirality present (equation (1)).


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∆GPET = (IPD – EAA) – Eexciton

(1)

The SWCNT chiralities in each sample were determined by photoluminescence excitation (PLE) mapping (Figure S6). Figure 5 demonstrates a clear logarithmic relationship between IPCE and ∆GPET, with only a “normal” regime in which the injected electron density increases with increasing driving force. Tvrdy et al. previously observed, for semiconductor quantum dots adsorbed on metal oxide surfaces, that the electron transfer rate did not decrease at high driving forces.59 This lack of a Marcus-like inverted regime was attributed to the continuum of states within the oxide conduction band with a density that increases with increasing energy above the conduction band edge. This continuum of acceptor states causes the electron transfer rate to taper, but not decrease, above a driving force equal to the reorganization energy. We also surmise that the larger oxide DOS high above the fundamental band edge (i.e. higher DOS at S22 than at S11) may be one reason why hot electron injection (injection from the S22 states) is more efficient than electron injection from the relaxed SWCNT S11 state. Therefore, we conclude that both an increased DOS at higher energy-levels in the electrode and Marcus-like behavior dictate the photoinduced charge separation efficiency in our system. Thus, these results provide an intriguing contrast to the case of SWCNT/fullerene heterojunctions where the donor and acceptor orbitals are both molecule-like in nature.


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Figure 5. Logarithmic relationship between the relative photon conversion efficiency (RPCE) and driving force. HiPco (red) and CoMoCAT (black) samples are normalized to the (7,5) nanotubes for each oxide substrate, and standard errors are shown. All IPCE spectra used in these calculations were taken in the electron injection regime (above flat band). (a) Various SWCNTs chiralities on SnO2. (b)Various SWCNTs chiralities on TiO2. Note that the HiPco on TiO2 sample is not shown as no electron injection photocurrent could be collected with HiPco SWCNTs on TiO2.

Because the SWCNT coverage varied from sample to sample, and because it was not possible to collect absorbance spectra on the doped single-crystals, we can only make conclusions about the relative photon conversion efficiency (RPCE) values for an individual multi-chiral sample. In this analysis, we assume that the relative concentrations of chiralities in the sensitization solution are the same as the relative concentration of chiralities on the electrode surface. To determine the RPCE of each chirality present, the IPCE was adjusted, using the corresponding absorbance spectrum, by the number of photons absorbed and then normalized to IPCE = 1 at the (7,5) SWCNT point. Note that we only report the relative driving force of electron injection.


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In summary, we have shown both photoinduced electron and hole transfer are possible in SWCNT films on single crystal metal oxide substrates. Additionally, we have shown that there is a logarithmic relationship between the driving force and the relative photon to collected electron conversion efficiency. This is similar to the Marcus relationship previously observed in SWCNT/ C60-derivative heterojunctions.46 However, unlike the C60-derivative system, no Marcus inverted regime in is observed due to the bulk DOS found in the metal oxides used herein. Furthermore, we measure that, when adjusted for absorbance, photoinduced free-carrier collection from the S22 states is more efficient than that from S11 states. This indicates that photoinduced charge injection is competitive with the fast (~100 fs) relaxation of the S22 exciton to the fundamental band-edge, and that the excess energy is beneficial to charge separation. As a whole, these results demonstrate very practical ways of improving an all-SWCNT activelayer PV device; we have shown it is of critical importance to remove excess surfactants from the SWCNTs before incorporating them into a photoconversion system, and we have shown that SWCNTs with higher-energy excitonic states will perform exponentially more efficiently than SWCNTs with lower-energy excitonic states. Thus, using SWCNTs with smaller diameters (e.g. CoMoCAT SWCNTs) would be beneficial to a SWCNT PV device. Furthermore, we have shown that using bulk carrier acceptor layers (as opposed to molecular acceptors) allow for more efficient free-carrier collection from higher-energy excitonic transitions. This effect is not seen when SWCNTs are interfaced with molecular acceptor materials, and therefore cannot simply be attributed to an increase in autoionization processes from high-energy states.37,51 Thus, we conclude that the increased charge collection is due to the increased DOS above the band-edge in bulk acceptor materials.


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ASSOCIATED CONTENT AUTHOR INFORMATION Email: [email protected] Website: http://www.uwyo.edu/parkinson/ Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank the Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences for financial support through Grant DE-SC0007115, and Zbigniew Galazka for supplying the synthetic SnO2 crystals. HVW and WDR would like to thank the Wyoming NASA Space Grant Consortium under grants #NNX15AI08H and NNX15AK56A, and the University of Wyoming School of Energy Carbon Initiative. This work was authored in part by Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the Solar Photochemistry Program, Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.


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Supporting Information. Experimental Methods, additional AFM images of sensitized substrates, diagram of photoelectric cell design, IPCE spectra of unsensitized SnO2 and TiO2, PV curves with and without excess semiquinone, PLE maps of multichiral SWCNT samples, IPCE spectra demonstrating the effect excess surfactant has on the photon conversion efficiency. (PDF)

REFERENCES (1)

Smalley, R. E.; Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Springer Science & Business Media, 2003; Vol. 80.

(2)

Scholes, G. D.; Rumbles, G. Excitons in Nanoscale Systems. Nature materials 2006, 5 (9), 683–696.

(3)

Ajayan, P. M.; Zhou, O. Z. Applications of Carbon Nanotubes. In Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Dresselhaus, M. S., Dresselhaus, G., Avouris, P., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2001; pp 391–425.

(4)

Dai, H. Carbon Nanotubes: Synthesis, Integration, and Properties. Accounts of chemical research 2002, 35 (12), 1035–1044.

(5)

Ouyang, M.; Huang, J.-L.; Lieber, C. M. Fundamental Electronic Properties and Applications of Single-Walled Carbon Nanotubes. Accounts of Chemical Research 2002, 35 (12), 1018–1025.

(6)

Zhou, O.; Shimoda, H.; Gao, B.; Oh, S.; Fleming, L.; Yue, G. Materials Science of Carbon Nanotubes: Fabrication, Integration, and Properties of Macroscopic Structures of Carbon Nanotubes. Accounts of chemical research 2002, 35 (12), 1045–1053.


19


(7)

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Wang, F.; Dukovic, G.; Brus, L. E.; Heinz, T. F. The Optical Resonances in Carbon Nanotubes Arise From Excitons. Science 2005, 308 (5723), 838–841.

(8)

Ando, T. Excitons in Carbon Nanotubes. Journal of the Physical Society of Japan 1997, 66 (4), 1066–1073.

(9)

Kane, C. L.; Mele, E. J. Ratio Problem in Single Carbon Nanotube Fluorescence Spectroscopy. Physical review letters 2003, 90 (20), 207401.

(10)

Pedersen, T. G. Variational Approach to Excitons in Carbon Nanotubes. Physical Review B 2003, 67 (7), 073401.

(11)

Spataru, C. D.; Ismail-Beigi, S.; Benedict, L. X.; Louie, S. G. Excitonic Effects and Optical Spectra of Single-Walled Carbon Nanotubes. Physical Review Letters 2004, 92 (7), 077402.

(12)

Perebeinos, V.; Tersoff, J.; Avouris, P. Scaling of Excitons in Carbon Nanotubes. Physical review letters 2004, 92 (25), 257402.

(13)

Barkelid, M.; Zwiller, V. Photocurrent Generation in Semiconducting and Metallic Carbon Nanotubes. Nature Photonics 2014, 8 (1), 47–51.

(14)

Kumamoto, Y.; Yoshida, M.; Ishii, A.; Yokoyama, A.; Shimada, T.; Kato, Y. K. Spontaneous Exciton Dissociation in Carbon Nanotubes. Physical review letters 2014, 112 (11), 117401.

(15)

Freitag, M.; Martin, Y.; Misewich, J. A.; Martel, R.; Avouris, P. Photoconductivity of Single Carbon Nanotubes. Nano Letters 2003, 3 (8), 1067–1071.

(16)

Gabor, N. M.; Zhong, Z.; Bosnick, K.; Park, J.; McEuen, P. L. Extremely Efficient Multiple Electron-Hole Pair Generation in Carbon Nanotube Photodiodes. Science 2009, 325 (5946), 1367–1371.


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17)

Zhang, Y.; Iijima, S. Elastic Response of Carbon Nanotube Bundles to Visible Light. Physical Review Letters 1999, 82 (17), 3472.

(18)

Alam, A.; Dehm, S.; Hennrich, F.; Zakharko, Y.; Graf, A.; Pfohl, M.; Hossain, I. M.; Kappes, M. M.; Zaumseil, J.; Krupke, R.; et al. Photocurrent Spectroscopy of DyeSensitized Carbon Nanotubes. Nanoscale 2017, 9 (31), 11205–11213.

(19)

Engel, M.; Moore, K. E.; Alam, A.; Dehm, S.; Krupke, R.; Flavel, B. S. Photocurrent Spectroscopy of (N, M) Sorted Solution-Processed Single-Walled Carbon Nanotubes. ACS nano 2014, 8 (9), 9324–9331.

(20)

Aditya D Mohite; Prasanth Gopinath; Hemant M Shah, A.; Alphenaar, B. W. Exciton Dissociation and Stark Effect in the Carbon Nanotube Photocurrent Spectrum. Nano letters 2007, 8 (1), 142–146.

(21)

Ferguson, A. J.; Blackburn, J. L.; Kopidakis, N. Fullerenes and Carbon Nanotubes as Acceptor Materials in Organic Photovoltaics. Materials Letters 2013, 90, 115–125.

(22)

Dowgiallo, A.-M.; Mistry, K. S.; Johnson, J. C.; Blackburn, J. L. Ultrafast Spectroscopic Signature of Charge Transfer Between Single-Walled Carbon Nanotubes and C60. ACS nano 2014, 8 (8), 8573–8581.

(23)

Bindl, D. J.; Wu, M.-Y.; Prehn, F. C.; Arnold, M. S. Efficiently Harvesting Excitons From Electronic Type-Controlled Semiconducting Carbon Nanotube Films. Nano letters 2011, 11 (2), 455–460.

(24)

Shea, M. J.; Arnold, M. S. 1% Solar Cells Derived From Ultrathin Carbon Nanotube Photoabsorbing Films. Applied Physics Letters 2013, 102 (24), 243101.

(25)

Jain, R. M.; Howden, R.; Tvrdy, K.; Shimizu, S.; Hilmer, A. J.; McNicholas, T. P.; Gleason, K. K.; Strano, M. S. Polymer-Free Near-Infrared Photovoltaics with Single


21


Page 22 of 26

Chirality (6,5) Semiconducting Carbon Nanotube Active Layers. Adv. Mater. Weinheim 2012, 24 (32), 4436–4439. (26)

Ramuz, M. P.; Vosgueritchian, M.; Wei, P.; Wang, C.; Gao, Y.; Wu, Y.; Chen, Y.; Bao, Z. Evaluation of Solution-Processable Carbon-Based Electrodes for All-Carbon Solar Cells. ACS nano 2012, 6 (11), 10384–10395.

(27)

Bindl, D. J.; Arnold, M. S. Efficient Exciton Relaxation and Charge Generation in Nearly Monochiral (7,5) Carbon Nanotube/C60 Thin-Film Photovoltaics. J. Phys. Chem. C 2013, 117 (5), 2390–2395.

(28)

Pfohl, M.; Glaser, K.; Graf, A.; Mertens, A.; Tune, D. D.; Puerckhauer, T.; Alam, A.; Wei, L.; Chen, Y.; Zaumseil, J.; et al. Probing the Diameter Limit of Single Walled Carbon Nanotubes in SWCNT: Fullerene Solar Cells. Advanced Energy Materials 2016, 6 (21), 1600890.

(29)

Balasubramanian, K.; Fan, Y.; Burghard, M.; Kern, K.; Friedrich, M.; Wannek, U.; Mews, A. Photoelectronic Transport Imaging of Individual Semiconducting Carbon Nanotubes. Applied Physics Letters 2004, 84 (13), 2400–2402.

(30)

Misewich, J. A.; Martel, R.; Avouris, P.; Tsang, J. C.; Heinze, S.; Tersoff, J. Electrically Induced Optical Emission From a Carbon Nanotube FET. Science 2003, 300 (5620), 783–786.

(31)

Mohite, A.; Lin, J.-T.; Sumanasekera, G.; Alphenaar, B. W. Field-Enhanced Photocurrent Spectroscopy of Excitonic States in Single-Wall Carbon Nanotubes. Nano letters 2006, 6 (7), 1369–1373.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32)

Beard, M. C.; Blackburn, J. L.; Heben, M. J. Photogenerated Free Carrier Dynamics in Metal and Semiconductor Single-Walled Carbon Nanotube Films. Nano letters 2008, 8 (12), 4238–4242.

(33)

Crochet, J. J.; Hoseinkhani, S.; Lüer, L.; Hertel, T.; Doorn, S. K.; Lanzani, G. FreeCarrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime. Physical review letters 2011, 107 (25), 257402.

(34)

Park, J.; Reid, O. G.; Blackburn, J. L.; Rumbles, G. Photoinduced Spontaneous FreeCarrier Generation in Semiconducting Single-Walled Carbon Nanotubes. Nature communications 2015, 6.

(35)

Aspitarte, L.; McCulley, D. R.; Minot, E. D. Photocurrent Quantum Yield in Suspended Carbon Nanotube P-N Junctions. Nano letters 2016, 16 (9), 5589–5593.

(36)

Pfohl, M.; Glaser, K.; Ludwig, J.; Tune, D. D.; Dehm, S.; Kayser, C.; Colsmann, A.; Krupke, R.; Flavel, B. S. Performance Enhancement of Polymer-Free Carbon Nanotube Solar Cells via Transfer Matrix Modeling. Advanced Energy Materials 2015, 6 (1), 1501345.

(37)

Guillot, S. L.; Mistry, K. S.; Avery, A. D.; Richard, J.; Dowgiallo, A.-M.; Ndione, P. F.; van de Lagemaat, J.; Reese, M. O.; Blackburn, J. L. Precision Printing and Optical Modeling of Ultrathin SWCNT/C60 Heterojunction Solar Cells. Nanoscale 2015, 7 (15), 6556–6566.

(38)

Chang, J.; Najeeb, C. K.; Lee, J.-H.; Lee, M.; Kim, J.-H. High-Performance Photoresponse From Single-Walled Carbon Nanotube–Zinc Oxide Heterojunctions. J. Phys. D: Appl. Phys. 2011, 44 (9), 095101.


23


(39)

Page 24 of 26

Wang, F.; Kozawa, D.; Miyauchi, Y.; Hiraoka, K.; Mouri, S.; Ohno, Y.; Matsuda, K. Fabrication of Single-Walled Carbon Nanotube/Si Heterojunction Solar Cells with High Photovoltaic Performance. ACS Photonics 2014, 1 (4), 360–364.

(40)

Wang, F.; Kozawa, D.; Miyauchi, Y.; Hiraoka, K.; Mouri, S.; Ohno, Y.; Matsuda, K. Considerably Improved Photovoltaic Performance of Carbon Nanotube-Based Solar Cells Using Metal Oxide Layers. Nature communications 2015, 6 (1), 6305.

(41)

Park, J.; Kim, Y.; Kim, G.-T.; Ha, J. S. Facile Fabrication of SWCNT/SnO2 Nanowire Heterojunction Devices on Flexible Polyimide Substrate. Advanced Functional Materials 2011, 21 (21), 4159–4165.

(42)

Sambur, J. B.; Novet, T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System. Science 2010, 330 (6000), 63–66.

(43)

Sambur, J. B.; Parkinson, B. A. CdSe/ZnS Core/Shell Quantum Dot Sensitization of Low Index TiO2 Single Crystal Surfaces. J. Am. Chem. Soc. 2010, 132 (7), 2130–2131.

(44)

Sambur, J. B.; Riha, S. C.; Choi, D.; Parkinson, B. A. Influence of Surface Chemistry on the Binding and Electronic Coupling of CdSe Quantum Dots to Single Crystal TiO2 Surfaces. Langmuir 2010, 26 (7), 4839–4847.

(45)

Parkinson, B. A.; Spitler, M. T. Recent Advances in High Quantum Yield Dye Sensitization of Semiconductor Electrodes. Electrochimica Acta 1992, 37 (5), 943–948.

(46)

Ihly, R.; Mistry, K. S.; Ferguson, A. J.; Clikeman, T. T.; Larson, B. W.; Reid, O.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G.; Blackburn, J. L. Tuning the Driving Force for Exciton Dissociation in Single-Walled Carbon Nanotube Heterojunctions. Nature chemistry 2016.


24

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(47)

Scholes, G. D. Insights Into Excitons Confined to Nanoscale Systems: Electron--Hole Interaction, Binding Energy, and Photodissociation. ACS nano 2008, 2 (3), 523–537.

(48)

Bindl, D. J.; Ferguson, A. J.; Wu, M.-Y.; Kopidakis, N.; Blackburn, J. L.; Arnold, M. S. Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions. The Journal of Physical Chemistry Letters 2013, 4 (21), 3550–3559.

(49)

Xu, Y.; Schoonen, M. A. The Absolute Energy Positions of Conduction and Valence Bands of Selected Semiconducting Minerals. American Mineralogist 2000, 85 (3-4), 543–556.

(50)

Blackburn, J. L.; Holt, J. M.; Irurzun, V. M.; Resasco, D. E.; Rumbles, G. Confirmation of K-Momentum Dark Exciton Vibronic Sidebands Using 13C-Labeled, Highly Enriched (6,5) Single-Walled Carbon Nanotubes. Nano letters 2012, 12 (3), 1398–1403.

(51)

Bindl, D. J.; Ferguson, A. J.; Wu, M.-Y.; Kopidakis, N.; Blackburn, J. L.; Arnold, M. S. Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions. The Journal of Physical Chemistry Letters 2013, 4 (21), 3550–3559.

(52)

Parkinson, B. A. Dye Sensitization of Van Der Waals Surfaces of Tin Disulfide Photoanodes. Langmuir 1988, 4 (4), 967–976.

(53)

Matsuzaki, K.; Yokoyama, I.; Komatsu, H.; Handa, T.; Miyajima, K. A Fluorescent Probing Study on Microenvironments in Bile Salt Micelles and Bile Salt/Phosphatidylcholine Mixtures in the Absence or Presence of Cholesterol. Biochimica et Biophysica Acta (BBA) - Biomembranes 1989, 980 (3), 371–376.


25


(54)

Page 26 of 26

Hollander, L. E., Jr; Castro, P. L. Dielectric Properties of Single-Crystal Nonstoichiometric Rutile (TiO2). Journal of Applied Physics 1962, 33 (12), 3421–3426.

(55)

Van Daal, H. J. The Static Dielectric Constant of SnO2. Journal of Applied Physics 1968, 39 (9), 4467–4469.

(56)

O'Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Hároz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; et al. Band Gap Fluorescence From Individual Single-Walled Carbon Nanotubes. Science 2002, 297 (5581), 593–596.

(57)

Watkins, K. J.; Parkinson, B. A.; Spitler, M. T. Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces as Revealed by the Doping Density Dependence of the Collection Efficiency of Dye Sensitized Photocurrents. J. Phys. Chem. B 2015, 119 (24), 7579–7588.

(58)

Zahab, A.; Spina, L.; Poncharal, P.; Marlière, C. Water-Vapor Effect on the Electrical Conductivity of a Single-Walled Carbon Nanotube Mat. Physical Review B 2000, 62 (15), 10000–10003.

(59)

Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. Photoinduced Electron Transfer From Semiconductor Quantum Dots to Metal Oxide Nanoparticles. Proc. Natl. Acad. Sci. U.S.A. 2011, 108 (1), 29–34.


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Optically Generated Free-Carrier Collection from an All Single-Walled

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