Charge transport and extraction of PTB7:PC71BM organic solar cells: effect of film thickness and thermal-annealing

Charge carrier transport in the active layer and charge extraction at the electrode have significant impact on the performance of solar cells. In this study, the effect of active layer thickness and thermal-annealing treatment on the charge transport and extraction performance of PTB7:PC71BM organic solar cells was studied comprehensively. Thin films of active layer couldn't utilize enough sunlight, while thick films could bring about large bulk resistance and deteriorate carrier transport. There is a trade-off between active layer thickness and carrier transport. The optimized active layer thickness is about 100 nm for the PTB7:PC71BM bulk heterojunction organic solar cells. Thermal-annealing could improve the morphology of the active layer, and facilitate charge transport in the active layer and charge collection at the electrode. The improved carrier transport and extraction were verified by the transient photocurrent/transient photovoltage and photo-induced charge carrier extraction by linearly increasing voltage measurements. The optimal power conversion efficiency was obtained as 8.28% for the device with an active layer thickness of 100 nm and treated with 90 degrees C thermal-annealing.