Organic Photovoltaic Cells Based on a Medium-Bandgap Phosphorescent Material and C60

Organic photovoltaic (OPV) cells using a medium-bandgap phosphorescent material of bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate) [(pbi)(2)Ir(acac)] as an electron-donating layer were fabricated. An S-shaped kink was observed in the current-voltage curves with the increase in (pbi)(2)Ir(acac) thickness. Two ways of using different hole transport layers to reduce interfacial energy step and doping to improve charge carrier mobility were used to investigate the origin of the kink. The results showed that this anomalous feature is due to the presence of large interfacial energy step between the anode/donor-interface and the low hole mobility of (pbi)(2)Ir(acac). An improved power conversion efficiency of 2.23% under AM 1.5 solar illumination at an intensity of 100 mW/cm(2) was obtained using N,N'-bis-(1-naphthyl)-N,N'-dipheny1-1,1'-biphenyl-4,4'-diamine to minimize the energy barrier for charge injection.