Effect of active layer thickness on opto-electronic characteristics of porphyrin-fullerene photovoltaic cell

The opto-electronic characteristics of porphyrin-fullerene bulk heterojunction photovoltaic cells of different active layer thicknesses were studied. In order to achieve different active layer thicknesses, the photovoltaic cells were prepared by spin coating the active layer of each cell at a different spin speed. To determine the active layer thickness, average of absorption coefficients of the materials constituting the active layer was used along with the optical density. Active layer thicknesses were also measured by using surface profilometer. Atomic force microscope surface scans revealed that there was no considerable change in active layer surface roughness from 1000 to 1500 rpm. However, a decrease in average grain size with increasing spin speed was observed. Current density as a function of voltage curves at different active layer thicknesses were recorded in dark and under a simulated solar spectrum AM 1.5G (100 mW.cm(-2)). Incident photon-to-current conversion efficiency spectra at different active layer thicknesses were also determined. The solar cell having active layer thickness of 68 nm (spin coated at 1200 rpm) showed optimum results. The power conversion efficiency of the photovoltaic cell at this thickness was 0.24%.