Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene)

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a promising candidate for polymer photovoltaic research due to its stability and absorption in the red region. In this manuscript, we report polymer photovoltaic devices based on RR-P3HT:methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) 1:1 weight-ratio blend. We studied the effects of annealing temperature and time on the device performance for devices annealed before and after cathode deposition. Thermal annealing shows significant improvement in the performance for both types of annealing conditions, with postproduction annealing being slightly better. For devices with a 43-nm-thick active layer, maximum power conversion efficiency (PCE) of 3.2% and fill factor up to 67% is achieved under Air Mass 1.5, 100-mW∕cm2 illumination. We performed atomic force microscopy and ultraviolet-visible absorption spectroscopy on the P3HT:PCBM films to explain the effect of thermal annealing. By keeping the optimized thermal annealing condition and by varying the active layer thickness, we fabricated devices with PCE up to 4.0%, which is the highest efficiency reported so far for this system.