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 sp² 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.