Performance Enhancement of Planar Heterojunction Perovskite Solar Cells by n-Doping to Electron Transporting Layer

Abstract

Recently, organic-inorganic hybrid perovskite has attracted great attention as a next generation material for solar cell because of its superior intrinsic properties such as extremely long exciton diffusion length, high absorption coefficient, and excellent carrier transport. Most of state-of-the-art perovskite solar cells utilize TiO2 and 2,2,7,7-tetrakis(N,N-bis(p-methoxy-phenyl)amino)-9,9-spirobifluorene as electron and hole transporting materials, respectively. However, since the formation of mesoporous TiO2 layer requires high temperature sintering process, all solution-processible bilayer structure has been investigated as an alternative device structure by several groups. This planar heterojunction structure utilizes commonly PCBM and PEDOT:PSS as electron and hole transporting materials, respectively. In this architecture, sufficiently thick PCBM layer is required to prevent direct contact between perovskite film and metal electrode. However, relatively low electron mobility and low electric conductivity of PCBM may provide a limit to achieve high power conversion efficiency (PCE) of the device with thick PCBM layer. In this study, an n-type dopant, 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (DMBI), was added into PCBM layer to enhance the electric conductivity of PCBM. Addition of a small amount of DMBI raises the Fermi level of PCBM toward the LUMO energy level, indicative of n-doping and an increase of free electrons. As a result, the solar cell device with n-doped PCBM as electron transporting layer shows a remarkable enhancement of short-circuit current density (JSC) and the PCE. While the device without the dopant exhibits S-shaped curve with a fill factor (FF) of 0.55, the device with 1% doped PCBM shows higher FF of 0.72. Consequently, the doped deice have shown a high PCE of 13.8% with 10% enhanced JSC of 22.0 mA/cm2. Particularly, the effect of doping was more prominent when the thickness of PCBM layer was increased.