Evidence for High-Efficiency Exciton Dissociation at Poly(3-hexylthiophene)/Single-Walled Carbon Nanotube Interfaces in Planar Nano-Heterojunction Photovoltaics
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There is significant interest in combining carbon nanotubes with semiconducting polymers for photovoltaic applications due to potential advantages from smaller exciton transport lengths and enhanced charge separation. However, to date, bulk heterojunction (BHJ) devices have demonstrated relatively poor efficiencies, and little is understood about the polymer/nanotube junction. To investigate this interface, we fabricate a planar nano-heterojunction comprised of well-isolated millimeter-long single-walled carbon nanotubes underneath a poly(3-hexylthiophene) (P3HT) layer. The resulting junctions display photovoltaic efficiencies per nanotube ranging from 3% to 3.82%, which exceeds those of polymer/nanotube BHJ by a factor of 50–100. The increase is attributed to the absence of aggregate formation in this planar device geometry. It is shown that the polymer/nanotube interface itself is responsible for exciton dissociation. Typical open-circuit voltages are near 0.5 V with fill factors of 0.25–0.3, which are largely invariant with the number of nanotubes per device and P3HT thickness. A maximum efficiency is obtained for a 60 nm-thick P3HT layer, which is predicted by a Monte Carlo simulation that takes into account exciton generation, transport, recombination and dissociation. This platform is promising for further understanding the potential role of polymer/nanotube interfaces for photovoltaic applications.