Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising for solution-processed, thin film photovoltaics due to their strong near-infrared absorptivity and excellent transport properties. We report on the generation yield and recombination kinetics of free charge carriers in photoexcited thin films of polymer-wrapped s-SWCNTs with and without an overlying electron-accepting C 60 layer, using time-resolved microwave photoconductivity (TRMC). Free carriers are generated in neat s-SWCNT films, even without an obvious driving force for exciton dissociation. However, most carriers recombine in <10 ns. Adding C 60 increases the yield and extends the lifetime of a significant fraction of free carriers to ≫100 ns via interfacial charge separation. Spectral dependencies indicate that the driving force for interfacial electron transfer vanishes for large-diameter SWCNTs, from which we approximate (9,7) s-SWCNT energetics. We estimate a free carrier generation yield of ∼6% in neat s-SWCNT films and 9 GHz SWCNT hole mobility of >1.3 cm 2 V –1 s –1 . These studies improve understanding of s-SWCNT photoresponses in solar cells and photodetectors.