Abstract
Small-molecule organic photovoltaic cells (OPVCs) with a hetero- structure of indium tin oxide (ITO)/copper phthalocyanine (CuPc)/C60/ metal (Ag), ITO/CuPc/C60/Al, ITO/CuPc/C60/CuPc/Al and ITO/Au/CuPc /C60/CuPc/Al were fabricated by thermal evaporation (under vacuum level of about 210-5 torr). Meanwhile the short circuit current Isc and open circuit voltage Voc for these OPVCs under different illumination conditions were performed. The following changes for these devices were made to find their optimum performance: (1) For electrodes, different way of deposition condition (ion sputtering or thermal evaporation) was chosen. The device with optimum power conversion efficiency was obtained by the thermal evaporation. (2) Among these frameworks, the best one is ITO/Au/CuPc/C60/CuPc/Al, because that the inserted CuPc buffer layer can effectively prevent C60 active layer from oxygen exposure and damage during the deposition of upper electrode. (3) The thickness of the film was controlled by the deposition time, the best active layers of thickness were CuPc: 30 nm, C60: 60 nm and CuPc buffer layer 10 nm. These results are consistent with the exciton diffusion length in each active layer. (4) A optimum power conversion efficiency and a small series resistance were obtained when a thin Au film was deposited on top of ITO. Finally, based on the abovementioned results, a mechanism related to the transport and separation of the photo-generated excitons and to the relation of power conversion efficiency versus film thickness was discussed.
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