Pushing up the efficiency of planar perovskite solar cells to 18.2% with organic small molecules as the electron transport layer

Compared to the traditional-architecture perovskite photovoltaic solar cells (n-i-p type), which use metal oxide as electron transport layers (ETLs) and organic semiconducting materials as hole transport layers, the fabrication of metal-oxide-free, solution-processed inverted perovskite solar cells (PSCs) is more desired because of low-temperatures and all-solution-based applications in future commercial PSC modules. In a typical configuration of inverted PSCs, the widely used ETL compound is the fullerene-based phenyl-C61-butyric acid methyl ester (PCBM), which currently is the best organic ETL material. The cost of this compound is very high, and the morphology and electrical properties are very sensitive to experimental conditions. We here propose a new organic ETL material for the replacement of PCBM in inverted PSCs. We demonstrate metal-oxide-free solution-processed inverted PSCs using the n-type sulfur-containing azaacene 10,14-bis(5-(2-ethylhexyl)thiophen-2-yl)-dipyrido[3,2-a:2′,3′-c][1,2,5]thiadiazolo[3,4-i]phenazine (TDTP) as the ETL with a power conversion efficiency of ∼18.2%, which is higher than that of the corresponding non-sulfur-containing azaacene 10,17-bis((triisopropylsilyl)ethynyl)dipyrido[3,2-a:2′,3′-c]quinoxalino[2,3-i]phenazine (PYPH)-based PSCs (up to 9.5%) or PCBM-based PSCs (up to 17.0%). This superior performance is attributed to the stronger interaction between TDTP and the perovskite surface than that between PYPH and the perovskite surface, which is supported by theoretical calculations. Our results show that easily-accessible simple n-type sulfur-containing small molecules are promising ETL candidates to further propel inverted PSCs to practical applications.