Origin of the open-circuit voltage in small-molecular organic photovoltaic devices: Effects of deposition rate control of donor material

Small-molecular organic photovoltaic devices with different open-circuit voltage (V_OC) were fabricated by simply controlling the deposition rate of donor material. Higher V_OC was obtained when the donor deposited at a higher rate and vice versa. The origin of the improved V_OC was studied by means of morphological change and temperature-dependent current density-voltage characteristics. The presence of pinholes as shown in atomic force microscopic images indicated the strong molecular interaction at the lower donor deposition rate, resulting in the severe leakage current from acceptor to anode as observed in the dark current. Equivalent circuit model and temperature-dependent dark currents were utilized to realize the effect of reverse saturation current on V_OC. The higher barrier height at the donor-acceptor interface was attributed to the improved V_OC for the device with higher donor deposition rate.