Letter pubs.acs.org/NanoLett
Photoexcitation Dynamics of Coupled Semiconducting Carbon Nanotube Thin Films Randy D. Mehlenbacher,† Meng-Yin Wu,‡ Maksim Grechko,† Jennifer E. Laaser,† Michael S. Arnold,*,§ and Martin T. Zanni*,† †
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States Department of Electrical and Computer Engineering, §Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Avenue, Madison, Wisconsin 53706, United States
‡
S Supporting Information *
ABSTRACT: Carbon nanotubes are a promising means of capturing photons for use in solar cell devices. We time-resolved the photoexcitation dynamics of coupled, bandgap-selected, semiconducting carbon nanotubes in thin films tailored for photovoltaics. Using transient absorption spectroscopy and anisotropy measurements, we found that the photoexcitation evolves by two mechanisms with a fast and long-range component followed by a slow and short-range component. Within 300 fs of optical excitation, 20% of nanotubes transfer their photoexcitation over 5−10 nm into nearby nanotube fibers. After 3 ps, 70% of the photoexcitation resides on the smallest bandgap nanotubes. After this ultrafast process, the photoexcitation continues to transfer on a ∼10 ps time scale but to predominantly aligned tubes. Ultimately the photoexcitation hops twice on average between fibers. These results are important for understanding the flow of energy and charge in coupled nanotube materials and light-harvesting devices. KEYWORDS: Carbon nanotube, exciton, transient absorption, anisotropy, photovoltaic ight harvesting is the first step in harnessing solar radiation in photovoltaics. Semiconducting quantum dots are receiving a lot of attention as next-generation photoabsorbers because their bandgaps can be tuned across the solar spectrum and they can be tethered or patterned in arrays and films to funnel the generated excitons to the electrode.1−4 Because of recent developments in processing, semiconducting carbon nanotubes can be isolated from their metallic counterparts.5 This facilitates the use of carbon nanotubes in the active layer of photovoltaic devices in a similar manner to quantum dots, albeit with potentially more attractive characteristics for light harvesting. Carbon nanotubes have strong, tunable optical absorptivity, ultrafast exciton and charge transport, excellent chemical stability, and economical solution-processability.6−13 Bilayer donor/acceptor heterojunction devices based on carbon nanotube photoabsorbers as the donors have already been demonstrated with internal and external quantum efficiency of 80 and 35%, respectively, at the optical bandgap of the nanotubes in the near-infrared.13,14 While these devices are a promising proof-of-principle, there is still a need to improve their efficiency if nanotubes are to be exploited as photoabsorbers in future solar cells. The improvement in efficiency might be accomplished, in part, by better understanding the photoexcitation dynamics of carbon nanotubes that are coupled to one-another in thin films similar to those used in devices. By characterizing the bottlenecks constraining the dynamics and
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© XXXX American Chemical Society
efficiency of photoexcitation transfer between coupled carbon nanotubes, one might engineer future nanotube-based materials and device architectures with improved performance. Thus, in analogy to experiments that ascertained the photophysics associated with coupled quantum dots,4,15,16 we seek to understand the photophysics associated with exciton and charge transport in films of coupled carbon nanotubes. Over the past decade, the photophysics of individual,17 pairs,18−20 and small bundles21−26 of nanotubes have been studied in detail. It is well-known that the absorption of light by a semiconducting carbon nanotube at its Eii transitions generates an exciton (bound by ∼0.25 eV for an 8 Å nanotube in a medium of εr = 4) rather than a free electron−hole pair.27 Photoluminescence spectroscopy20−24 and near-field microscopy18,19 experiments have shown that excitons can hop from tube to tube in binary pairs and within small bundles of nanotubes. The hopping times are still not fully established but may potentially be as fast as 10 fs.28 While “hot” excitons can autoionize in bundles of bare nanotubes, autoionization is generally thought to be inefficient due to the large exciton binding energy.26 In carbon nanotube photovoltaic devices Received: December 12, 2012 Revised: February 26, 2013
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dx.doi.org/10.1021/nl304591w | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
utilizing a donor/acceptor heterojunction, the hopping of excitons between tubes is important because it provides a means for excitons to reach the nanotube/acceptor heterointerface. At the interface, there is an energy offset that drives electron transfer from the exciton on the nanotube to the acceptor with >80% internal quantum efficiency.14,29 However, there is still relatively little known about how excitons behave in thin films of semiconducting nanotubes. Koyama et al. have reported on time-resolved photoluminescence from thin films of nanotubes similar in bandgap polydispersity to the ones used in proof-of-principle photovoltaic devices.30 Koyama et al. nonresonantly excited all of the species of nanotubes and observed that the photoluminescence lifetime of the different species decreased with increasing bandgap, suggesting a pathway for exciton relaxation from large to small bandgap tubes similar to what has been observed in bundles.30 In this paper, we resonantly excite specific (n, m) chiralities of semiconducting nanotubes and then probe the resulting transient absorption for all five chiralities of nanotube from which the sample is made. Because the chirality is related to the bandgap, the polarization, energy, and time-dependence of the transient absorption provides information about the lifetime of the photoexcitation, the internanotube transfer times, and the extent of how far the photoexcitation spreads, all of which are ultimately tied to device performance. We prepared thin films of type-controlled semiconducting carbon nanotubes for this study to match those previously used in proof-of-principle bilayer carbon nanotube/acceptor heterojunction devices. Specifically, semiconducting nanotubes are prepared using poly(9,9-dioctylfluorene) (PFO) to selectively disperse the (7, 5), (7, 6), (8, 6), (8, 7), and (9, 7) chiralities with optical bandgaps of 1044, 1144, 1194, 1290, and 1356 nm, respectively, in toluene, based on the original procedures of Nish et al.31 The E11 optical transitions are spectrally narrow (15 to 20 nm line width), enabling each of these chiralities to be separately probed and thereby enabling us to measure how the photoexcitation relaxes from species to species as a function of time. We created two samples from these nanotubes: (1) a coupled thin film and (2) an uncoupled control sample in which the nanotubes are diluted into a polymer matrix. Comparing the two samples allows for differentiation of phenomena intrinsic to the nanotubes versus behaviors that arise due to their coupling. Prior to film casting, excess PFO is removed from the nanotube solutions via repeated centrifugal sedimentation and redispersion following the procedures of Bindl et al. until the mass ratio of PFO:nanotubes is approximately 1:1, which we quantify by measuring the PFO absorption at 390 nm and the nanotube E22 absorption in the visible spectrum (Figure 1A) in solution.32 The coupled films are fabricated by doctor-blade casting the PFO wrapped nanotubes from chlorobenzene onto clean indium tin oxide (ITO) coated-glass on a hot-plate set to 110 °C in a nitrogen glovebox (
