High crystallization of a multiple cation perovskite absorber for low-temperature stable ZnO solar cells with high-efficiency of over 20%

ZnO as a promising electron transport layer (ETL) to TiO2 for perovskite solar cells (PSCs) has achieved a power conversion efficiency (PCE) of 18.9%; however, this is still lower than that obtained for TiO2-based PSCs (higher than 20%). Herein, we report the fabrication of high-efficiency methylammonium (MA) and Cs co-alloyed formamidinium (FA) triple cation perovskite based ZnO PSCs via delicate control of the cation compositions and annealing temperatures. By virtue of structural, morphological, spectral and electrochemical characterizations and analysis, we found that the incorporation of MA and Cs into FA perovskite enables the formation of a highly crystalline black phase perovskite with reduced surface roughness, which inhibits charge recombination and promotes electron transfer at the ZnO/perovskite/spiro-OMeTAD interfaces and hence improves Jsc and FF values of the cell. As a result, the ZnO PSC based on MA0.1FA0.75Cs0.15PbI3 annealed at 110 °C achieved a PCE as high as 20.09%, exceeding the previous highest efficiency recorded for ZnO ETL based PSCs. The optimized MA0.1FA0.75Cs0.15PbI3 material demonstrated excellent reproducibility and long-term cell durability under ambient conditions within 1000 h. Particularly, the incorporation of a small amount of Br into the triple cation perovskite, i.e., MA0.1FA0.75Cs0.15PbI2.9Br0.1 led to a further enhancement in PCE of up to 20.44%, which is comparable with the best-performing MA and Cs-containing FA-based lead halide TiO2 PSCs.