title: Photovoltaic charge generation in organic semiconductors based on long-range energy transfer.
abstract: 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 (C60) 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 C60 proceeds over large distances (∼50% probability of transfer across a 11 nm barrier), and that this process is consistent with long-range Förster 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/C60 devices.
authors.affiliation: Chemical and Material Sciences Center, National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, Colorado 80401, USA.
authors.forename: David C
authors.initials: DC
authors.lastname: Coffey
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Ferguson
authors.forename: Nikos
authors.initials: N
authors.lastname: Kopidakis
authors.forename: Garry
authors.initials: G
authors.lastname: Rumbles