Modulating the Exciton Dissociation Rate by up to More than Two Orders of Magnitude by Controlling the Alignment of LUMO+1 in Organic Photovoltaics

Efficient organic solar cells require a high yield of exciton dissociation. Herein we investigate the possibility of having more than one charge-transfer (CT) state below the first optically bright Frenkel exciton state (FE) for common molecular donor (D)/acceptor (A) pairs and the role of the second-lowest CT state (CT2) in the exciton dissociation process. This situation, previously explored only for fullerene acceptors, is shown to be rather common for other D/A pairs. By considering a phenomenological model of a large aggregate, we reveal that the position of CT2 can remarkably modulate the exciton dissociation rate by up to more than two orders of magnitude. Thus, controlling the alignment of CT2 is suggested as a promising rule for designing new D/A heterojunctions.