Dark current and photovoltage models on the formation of depletion region in C60/NPB organic heterojunctions

► We study the C60/NPB organic heterojunction photodiodes (OPD) by adjusting the thicknesses of semiconductor layers. ► Theoretical models are proposed to numerically fit the dark current and photovoltage of different OPD structures. ► The critical thicknesses of C60/NPB heterojunction are deduced to be 5 nm/70 nm in the dark and 8 nm/60 nm in light. ► Comparison of the performance parameters of C60/NPB OPDs verifies the correctness of the fitting results. ► The finally optimized structure of C60/NPB heterojunction is 8 nm/70 nm.