The Impact of Residual Triphenylphosphine Oxide on the Crystallisation of Vapour-deposited Metal Halide Perovskite Films

Thermal evaporation is an industrially compatible technique for fabricating metal halide perovskite thin films, without the requirement for hazardous solvents. It offers precise control over film thickness and is a good candidate for large-scale production of commercial optoelectronic metal halide perovskite devices, such as solar cells. The use of additives to passivate electronic defects in solution-processed metal halide perovskite has led to dramatic increases in device performance. However, there are few reports of vapor-deposited films with co-evaporated passivating agents. Triphenylphosphine oxide (TPPO) has been used as an effective surface passivating agent in solution-processed metal halide perovskite films. It is a promising candidate passivating agent for co-evaporation, where it is beginning to be used with encouraging results. However, here we report that triphenylphosphine oxide is incompatible with thermal deposition in the same deposition chamber. Such TPPO remnants are found to result in severe suppression of the perovskite phase, long-range crystalline ordering and optical absorption of lead iodide perovskites films subsequently deposited in the same chamber. TPPO contamination persists even through repeat baking cycles, with reduction of the contaminant to acceptable levels requiring vacuum chamber dismantling and manual cleaning. We conclude that TPPO should not be co-evaporated in order to prevent contamination of future batches.