Photovoltaic Charge Generation in Organic Semiconductors Based on Long-Range Energy Transfer

For efficient charge generation in organic solar cells, photogenerated excitons must migrate to a donor/acceptor interface where they can be dissociated. This migration is traditionally presumed to be based on diffusion through the absorber material. Herein we study an alternative migration route-two-step exciton dissociation-whereby the exciton jumps from the donor to acceptor before charge creation takes place. We study this process in a series of multilayer donor/barrier/acceptor samples, where either poly(3-hexylthiophene) (P3HT) or copper phthalocyanine (CuPc) is the donor, fullerene (C-60) is the acceptor, and N,N-diphenyl-N,N-bis(3-methylphenyl)[1,1-bisphenyl]-4,4-diamine (TPD) acts as a barrier to energy transfer. By varying the thickness of the barrier layer, we find that energy transfer from P3HT to C-60 proceeds over large distances (similar to 50% probability of transfer across a 11 nm barrier), and that this process is consistent with long-range Forster resonance energy transfer (FRET). Finally, we demonstrate a fundamentally different architecture concept that utilizes the two-step mechanism to enhance performance in a series of P3HT/CuPc/C-60 devices.