Functionalization of Graphene Oxide Films with Au and MoO_x Nanoparticles as Efficient p-Contact Electrodes for Inverted Planar Perovskite Solar Cells

A graphene oxide (GO) film is functionalized with metal (Au) and metal-oxide (MoO_x) nanoparticles (NPs) as a hole-extraction layer for high-performance inverted planar-heterojunction perovskite solar cells (PSCs). These NPs can increase the work function of GO, which is confirmed with X-ray photoelectron spectra, Kelvin probe force microscopy, and ultraviolet photoelectron spectra measurements. The down-shifts of work functions lead to a decreased level of potential energy and hence increased V_oc of the PSC devices. Although the GO-AuNP film shows rapid hole extraction and increased V_oc, a J_sc improvement is not observed because of localization of the extracted holes inside the AuNP that leads to rapid charge recombination, which is confirmed with transient photoelectric measurements. The power conversion efficiency (PCE) of the GO-AuNP device attains 14.6%, which is comparable with that of the GO-based device (14.4%). In contrast, the rapid hole extraction from perovskite to the GO-MoO_x layer does not cause trapping of holes and delocalization of holes in the GO film accelerates rapid charge transfer to the indium tin oxide substrate; charge recombination in the perovskite/GO-MoO_x interface is hence significantly retarded. The GO-MoO_x device consequently shows significantly enhanced V_oc and J_sc, for which its device performance attains PCE of 16.7% with great reproducibility and enduring stability.