Concentration gradient P3OT/PCBM photovoltaic devices fabricated by thermal interdiffusion of separately spin-cast organic layers

A series of organic photovoltaic devices consisting of concentration gradients of poly (3-octylthiophene) (P3OT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) were fabricated by thermally-induced interdiffusion of consecutively spin-cast layers of P3OT and PCBM from solvents of chloroform and pyridine, respectively. The device performance was evaluated as a function of the layer thicknesses, interdiffusion temperature, and interdiffusion time. A maximum power conversion efficiency of 1.0% under AM1.5G simulated solar spectrum was obtained for 70 nm P3OT thickness, 45 nm PCBM thickness, and interdiffusion at 150 °C for 20 min. Auger spectroscopy depth profiling measurements indicated that the optimal devices consist of concentration gradients of P3OT and PCBM extending across the entire film in opposite directions.