The dissociation of excitons at indium tin oxide-copper phthalocyanine interface in organic solar cells

Exciton dissociation process at indium tin oxide (ITO)/copper phthalocyanine (CuPc) interface of ITO/CuPc(370 nm)/Al is studied by transient photovoltage method. A negative-to-positive change in the polarity of photovoltage upon pulsed laser irradiation is observed in CuPc thin film. The polarity change is regarded as a summation of the effect of exciton dissociation at ITO/CuPc interface (fast process) and that of free carrier separation by built-in field (slow process). Further experiments confirm the existence of exciton dissociation at ITO/CuPc interface, and the direction of which is electron injected into ITO, with holes left in CuPc film. This is opposite to that of the interfacial dissociation at donor/acceptor (D/A) interface in single heterojunction cells (ITO/D/A/buffer/Al). 3-nm-thick LiF insulating layer is inserted between ITO and CuPc to inhibit the exciton dissociation at ITO/CuPc interface. Thereby, the open-circuit voltage and power conversion efficiency of the single layer cell have been increased by several times.