Size Control of Reduced Graphene Oxide Nanosheets for Enhancing Hole Transporting Properties in Inverted Organic Solar Cells

Abstract

Reduced graphene oxide (rGO) reduced by functionalized hydrazine from graphene oxide (GO) is a promising alterative material to PEDOT:PSS as hole transport layer (HTL) because rGO based organic solar cells (OSCs) show much better device stability than PEDOT:PSS based OSCs with similar device performance. However, to date, it has not been systematically investigated about effect of sheet size of rGO on the hole transporting properties. In addition, sheet size of rGO can affect the thin film uniformity as well as the elctrical properties of the HTL. The thin film uniformity of HTL has a considerable influence on the device performance in inveted OSCs, because HTLs directily contact with the top metals. Therefore, in this study, we evaluate the size effect of rGO sheets on their thin film uniformity and hole tranporting properties. In atomic force micoscopes analysis, the root mean square roughness of rGO films was decreased from 5.7 to 3.2 nm as the size of the sheets decreased meaning that the film uniformity of rGOs was improved. The hole transporting properties of rGOs having different sheets size were investigated by anlysis of J-V characteristics in the light and dark, Jph-Veff curves, the plots of Jsc and Voc with respect to the light intensity, etc. The small size rGO based HTL, which have more uniform film than large size rGO based HTL, show the more efficient hole extraction and electron blocking abblity due to the improved interface contact with active layer and top metal (Ag), resulting in suppressed carrier recombination at the inferfaces. Thus, a higher power conversion efficiency of 9.07%, corresponding to an approximately 20% efficiency improvement compared with that of a large size rGO HTL (7.65%), was achieved by small size rGO HTL.