Surface passivation of perovskite thin films by phosphonium halides for efficient and stable solar cells

Surface passivation of metal halide perovskite thin films has proved to be critical for efficient and stable perovskite solar cells (PSCs), i.e. suppressing charge recombination at the interfaces between perovskite and charge transport layers for high device efficiency and preventing the penetration of degrading agents into the perovskite layer for high device stability. Here, we report the use of a new class of materials, phosphonium halide salts, i.e. triphenyl(9-ethyl-9H-carbazol-3-yl)-phosphonium bromide (TCPBr) and iodide (TCPI), as surface passivation agents for efficient and stable PSCs. Suppressed nonradiative recombination and enhanced interfacial hole extraction, enabled by this facile phosphonium halide passivation, resulted in PSCs with power conversion efficiencies (PCEs) of >20.1% (∼18.5% for pristine MAPbI3 devices). Phosphonium halide passivation also reduced the hysteresis index from 11.2% (pristine device) to 5.4% (TCPBr passivated device). Moreover, the hydrophobic phosphonium halide passivation layers significantly suppressed moisture penetration and ion migration, greatly enhancing the device stability. It was found that unencapsulated solar cells based on TCPBr treated MAPbI3 thin layers retained more than 90% of the initial PCEs after more than 1000 hours storage under ambient conditions. Our work established the potential of phosphonium-based materials for surface passivation of metal halide perovskites, providing a new route towards efficient and stable perovskite optoelectronic devices.