Effects of Molecular Orientation of a Fullerene Derivative at the Donor/Acceptor Interface on the Device Performance of Organic Photovoltaics

Designing donor/acceptor (D/A) interfaces that can efficiently generate free carriers is an attractive research target for organic photovoltaics (OPVs). While many reports suggest that the molecular orientation of the donor at the D/A interface influences the free-charge generation and recombination, the effects of the acceptor orientation on these processes remain elusive. In this work, we demonstrate that [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) changes its molecular orientation at the film surface on crystallization, resulting in the preferential surface exposure of the side chains. Photoelectron spectra of amorphous- and crystalline-PC61BM/sexithiophene (6T) interfaces and analysis of the external quantum efficiency and electroluminescence of bilayer OPVs in the charge-transfer absorption range reveal that the orientational change of PC61BM raises the energy of the charge-transfer state at the D/A interface. In addition, the PC61BM side chain at the crystalline-PC61BM/6T interface reduces the electronic coupling between the charge-transfer and ground states, suppressing carrier recombination without sacrificing photocurrent. These two factors lead to the higher open-circuit voltage of crystalline-PC61BM/6T OPV compared with its amorphous counterpart. This work directly links the interface and photovoltaic properties, highlighting the role of the acceptor's orientation in determining the efficiency of OPVs.