Fullerene derivative with a branched alkyl chain exhibits enhanced charge extraction and stability in inverted planar perovskite solar cells

Fullerene derivatives are known for their excellent electron transport properties, which can effectively serve as electron transport layers (ETLs) deposited from solution at low temperatures in perovskite solar cells (PSCs). However, it remains unclear how functional groups attached to the fullerene derivatives influence device performances. Herein, the performance of an ETL based on [6,6]-phenyl-C-61 butyric acid 2-ethylhexyl ester (PC61BEH) was investigated. It was demonstrated that the branched alkyl chain of the fullerene derivative can highly influence the solubility, film morphology, passivation ability, ETL-perovskite contact, and electron extraction ability of the fullerene derivative. Overall device performance and stability are dependent on these characteristics. The results suggest that further design and improvement of fullerene-based PSCs performance may be possible through optimization of the addend structure and functionality.