Sulfated Graphene Oxide as a Hole-Extraction Layer in High-Performance Polymer Solar Cells

In this study, we have rationally designed and successfully developed sulfated graphene oxide (GO-OSO3H) with -OSO3H groups attached to the carbon basal plane of reduced GO surrounded with edge-functionalized -COOH groups. The resultant GO-OSO3H is demonstrated to be an excellent hole extraction layer (HEL) for polymer solar cells (PSCs) because of its proper work function for Ohmic contact with the donor polymer, its reduced basal plane for improving conductivity, and its -OSO3H/-COOH groups for enhancing solubility for solution processing. Compared with that of GO, the much improved conductivity of GO-OSO3H (1.3 S m(-1) vs 0.004 S m(-1)) leads to greatly improved fill factor (0.71 vs 0.58) and power conversion efficiency (4.37% vs 3.34%) of the resulting PSC devices. Moreover, the device performance of GO-OSO3H is among the best reported for intensively studied poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) devices. Our results imply that judiciously functionalized graphene materials can be used to replace existing HEL materials for specific device applications with outstanding performance.