Electronic properties and open-circuit voltage enhancement in mixed copper phthalocyanine:fullerene bulk heterojunction photovoltaic devices

While bulk heterojunctions (HJ) have been used in organic photovoltaic (OPV) devices, there are few studies on their interface electronics and mechanisms on device performance. Here, we studied the electronic structure of a mixed $CuPc:C60$ and a discrete $CuPc/C60$ junctions using photoemission spectroscopy. The $HOMOCuPc-LUMOC60$ energy offset, which controls the theoretical maximum open-circuit voltage $(Voc)$⁠, was increased from 0.64 to 1.13 eV by mixing CuPc with $C60$⁠. This $Voc$ increase is attributed to the underlying substrate work function and charge transfer between two molecules. The results provide an understanding of the $Voc$ enhancement in OPV devices with bulk HJ.