Influence of TFSI post-treatment on surface doping and passivation of lead halide perovskites†

Bis(trifluoromethane)sulfonimide (TFSI) treatment results in near-unity photoluminescence quantum yields in monolayer transition-metal dichalcogenides, such as MoS₂, due to passivation of native defects. Surprisingly, this simple post-treatment process has never been tested in the case of metal halide perovskites which suffer from limited radiative recombination due to charge carrier trapping. Here, we adopt this strategy and treat methylammonium lead iodide perovskite films with TFSI solutions. By employing photoluminescence spectroscopy, the appearance of brighter films proves a net passivation effect, while chemical analysis explains that this is due to strong interactions between SO groups of TFSI and under-coordinated Pb²⁺. A simultaneous passivation of iodide vacancies also leads to a reduction of n-doping at the perovskite surface and thus better hole extraction through spiro-MeOTAD which is deposited on top. These two effects combined (chemical passivation and de-doping) result in enhanced stabilized efficiencies for the as-fabricated n–i–p solar cells. The findings pave the way for the use of TFSI-based solutions to improve the performance of perovskite optoelectronic devices.