Optimization of thermally reduced graphene oxide for an efficient hole transport layer in polymer solar cells

► Thermally reduced graphene oxide (GO) was used as hole transport layer (HTL) in polymer solar cells. ► Annealing temperature plays an important role in recovering GO conductivity. ► High-temperature reduced GO exhibited a low resistance due to removal of oxygen functional groups. ► Large fill factor of 65.7% was obtained for devices with GO HTL annealed at 230 °C. ► Optimized GO device had a PCE that is 26% higher than bare ITO device.