Poly(N-vinylpyrrolidone)-decorated reduced graphene oxide with ZnO grown in situ as a cathode buffer layer for polymer solar cells.

A ZnO@reduced graphene oxide-poly(N-vinylpyrrolidone) (ZnO@RGO-PVP) nanocomposite, prepared by in situ growth of ZnO nanoparticles on PVP-decorated RGO (RGO-PVP) was developed as a cathode buffer layer for improving the performance of polymer solar cells (PSCs). PVP not only favors homogeneous distribution of the RGO through the strong π-π interactions between graphene and PVP molecules, but also acts as a stabilizer and bridge to control the in situ growth of sol-gel-derived ZnO nanoparticles on the surface of the graphene. At the same time, RGO provides a conductive connection for independent dispersion of ZnO nanoparticles to form uniform nanoclusters with fewer domain boundaries and surface traps. Moreover, the LUMO level of ZnO is effectively improved by modification with RGO-PVP. Compared to bare ZnO, a ZnO@RGO-PVP cathode buffer layer substantially reduces the recombination of carriers, increases the electrical conductivity, and enhances electron extraction. Consequently, the power conversion efficiency of an inverted device based on thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71 -butyric acid methyl ester (PC71 BM) with ZnO@RGO-PVP as cathode buffer layer was greatly improved to 7.5 % with improved long-term stability. The results reveal that ZnO@RGO-PVP is universally applicable as a cathode buffer layer for improving the performance of PSCs.