Fullerenes and carbon nanotubes as acceptor materials in organic photovoltaics

The development of new acceptor materials has played an important role in advancing the efficiency of organic photovoltaics (OPV). In this review we survey two types of acceptors, fullerene derivatives and single-walled carbon nanotubes (SWCNTs). The former has been the most successful type of acceptor in OPV, while the latter is a new type of acceptor that has shown encouraging spectroscopic results but its implementation in efficient devices is work in progress. We present some of the essential properties of an OPV acceptor, and the main experimental methodology used to characterize the photoconductivity of an OPV active layer comprising a donor-acceptor blend. We then discuss different aspects of donor-acceptor blends, or bulk heterojunctions, with a polymer donor and a fullerene or SWCNT acceptor. We show the basic charge-generation and recombination properties of a prototypical polymer-fullerene system, and discuss the significance of the molecular structure of the fullerene in terms of the mixing of the two components and its effect on photocarrier dynamics. In the case of the fullerene acceptors we also compare to OPV device efficiency. We discuss the role of charge and energy transfer from the polymer in exciton quenching in polymer-SWCNT blends, and show that the former results in the generation of long-lived charge carriers, similar to observations in polymer-fullerene blends. We show that SWCNT enrichment, to yield samples with only a few percent of metallic SWCNTs results in an enhancement of the photoconductance and carrier lifetime in P3HT-SCWNT blends. We also discuss the role that SWCNT diameter plays on the driving force for carrier generation, and suggest that smaller SWCNTs may be critical for efficient OPV device performance. Finally, we discuss the implications of the results of the photoconductivity studies on OPV device performance, and outline future challenges and open questions pertaining to the optimization of acceptors in OPV.11 page(s