Improving the performance of polymer solar cells by efficient optimizing the hole transport layer-graphene oxide

Graphene oxide (GO) materials have emerged as a promising alternative for hole transport layer (HTL) in polymer solar cells (PSCs) due to their unique structures and properties. However, insulating properties and eco-contaminative production of GO still need to be solved. Here, we report on the preparation of GO through an improved Hummers method without using NaNO3, which is an eco-friendly option because it avoids the emissions of NO2 and N2O4 toxic gases. Subsequently, the GO as HTL in PSCs is reduced by simple heat treatment of different temperatures in air, and the performance of devices is obviously improved. The FT-IR and XPS spectra show oxygenated functional groups in GO thin films are gradually removed with the increase of annealing temperature, which restores sp(2) hybridized graphitic structure, and makes the GO thin films more conducive to the charge transfer. The highest power conversion efficiency of PSCs based on the P3HT: PC71BM system with GO as HTL is 3.39%, which approaches that of PSCs with PEDOT: PSS as HTL (3.41%). Moreover, the devices with annealed GO as HTL have better stability compared to devices with PEDOT: PSS.