High-performance inverted planar perovskite solar cells using a pristine fullerene mixture as an electron-transport layer

Abstract

Fullerene derivatives are widely used as efficient electron-transport materials for inverted perovskite solar cells (PSCs). Among them, phenyl-C-x-butyric acid methyl ester (PCxBM; x = 61 or 71) is the most frequently used because of its high solubility and solution processability. However, chemical modification of the original fullerenes (C-60 or C-70) will decrease the charge mobility, which is detrimental to the performance of PSCs. Moreover, the high cost of fullerene derivatives (such as PCBMs) is an obstacle hindering the commercialization of PSCs. In this study, we developed a pristine fullerene mixture (FM) composed of C-60 and C-70 as an electron-transport layer (ETL). Because of the increased configurational entropy, the solubility of the C-60 and C-70 mixture was significantly improved to 52.9 g L-1 (in dichlorobenzene), which may reduce the speed of crystallization of the FM during the spin-coating process, in which the organic solvent is evaporated. With this FM-based ETL, the PSCs exhibited an average power-conversion efficiency (PCE) of 16.9%, which was higher than that (15.2%) of PC61BM-based ones. Additionally, the FM-based ETL exhibited greater hydrophobicity than PC61BM, which led to better moisture tolerance. As a result, the long-term stability of the FM-based PSCs was improved significantly, with reduced PCE degradation (from 26% to 15%) after 150 h under ambient conditions.