High-efficiency exciton dissociation at poly(3-hexylthiophene)/single-walled carbon nanotube interfaces in planar nano-heterojunction photovoltaics

There is significant interest in combining nanowires or 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 (SWNT) underneath a poly(3-hexylthiophene) (P3HT) layer. The resulting junctions display photovoltaic efficiencies per nanotube of 3%, which exceeds those of polymer/nanotube BHJ devices by a factor of 50-100. We attribute the increase to the absence of their aggregates in this planar device geometry. The photovoltaic properties were found to depend on the number of SWNT and the thickness of P3HT layer with a trend described using a Monte Carlo model of exciton diffusion and reaction. It is suggested that this device structure provides a suitable platform for further understanding the potential role of polymer/nanotube interfaces for photovoltaic applications.