Control of molecular orientations by sequential deposition to enhance organic photovoltaic performance

Bulk heterojunction (BHJ) structures prepared by coating mixed solutions of a donor and an acceptor have been used for efficient organic photovoltaics (OPVs), but this approach allows little control of the molecular orientations in the films. Herein we show that a sequential deposition strategy, in which the molecular acceptor is deposited sequentially on a pristine polymer donor film, reliably controls the molecular orientations in the resulting BHJ films. Spectroscopic ellipsometry indicates that both the donor and the acceptor adopt higher degrees of orientation with the π-planes parallel to the film plane compared with mixed BHJ films. The sequentially deposited films enhance the power conversion efficiency of OPVs by 30% compared with mixed BHJ films, indicating the usefulness of this strategy for improving OPV performance. •Molecular orientations were controlled in bulk heterojunction films prepared by a sequential deposition process.•The sequential desposition showed 30% higher photovoltaic performance than the conventional mixed bulk heterojunction.•The film morphology, the molecular orientation, and the composition at the film surface were characterized.