Efficient Exciton Relaxation and Charge Generation in Nearly Monochiral (7,5) Carbon Nanotube/C₆₀ Thin-Film Photovoltaics

Abstract

We report on photovoltaic diodes based on bilayer heterojunctions between nearly monochiral, polymer wrapped (7,5) semiconducting carbon nanotube photoabsorbing films and C 60. The internal quantum efficiencies (IQEs) for exciton dissociation and subsequent charge collection at the nanotubes’ visible E 22 and near-infrared E 11 and E 11 + X resonances are 84% ± 7%, 85% ± 5%, and 84% ± 14%, respectively. The high IQE at each transition shows that recombination losses during relaxation and/or direct dissociation of “hot” E 11 + X and E 22 excitons are negligible. A peak external quantum efficiency (EQE) of 34% is achieved at the E 11 transition. Zero-bias photoresponsivity is invariant up to short-circuit current densities of at least 23 mA cm –2, indicating negligible losses via trion, charge-exciton, and charge–charge recombination relaxation pathways. An open circuit voltage of 0.49 V and power conversion efficiency of 7.1% are achieved in response to monochromatic excitation of the diodes at the E 11 transition. The high IQE across multiple spectral windows, invariant photoresponsivity, and attractive open circuit voltage relative to the 1.18 eV optical bandgap demonstrate the future promise of using monochiral and multichiral semiconducting carbon nanotube films for broadband solar photovoltaic applications.