Fluorinated Reduced Graphene Oxide as an Efficient Hole-Transport Layer for Efficient and Stable Polymer Solar Cells

In this work, we have rationally designed and successfully synthesized a reduced graphene oxide (GO) functionalized with fluorine atoms (F-rGO) as a hole-transport layer (HTL) for polymer solar cells (PSCs). The resultant F-rGO has an excellent dispersibility in dimethylformamide without any surfactants, leading to a good film-forming property of F-rGO for structuring a stable interface. The recovery of conjugated C=C bonds in GO oxide after reduction increases the conductivity of F-rGO, which enhances the short-circuit current density of photovoltaic devices from 15.65 to 16.89 mA/cm². A higher work function (WF) (5.1 eV) of F-rGO than that of GO (4.9 eV) is attributed to the fluorine group with a high electronegativity. Naturally, the better-matched WF with the highest occupied molecular orbital level of the PTB7-Th (5.22 eV) donor induces an improved energy alignment in devices, resulting in a superior open-circuit voltage of the device (0.776 vs 0.786 V). Consequently, the device with F-rGO as the HTL achieves a higher power conversion efficiency (8.6%) with long-term stability than that of the devices with GO HTLs and even higher than that of the poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) control device. These results clearly verify that the F-rGO is a promising hole-transport material and an ideal replacement for conventional PEDOT/PSS, further promoting the realization of low-cost, solution-processed, high-performance, and high-stability PSCs.