Mechanistic Insights into the Role of the Bis(trifluoromethanesulfonyl) imide Ion in Coevaporated p-i-n Perovskite Solar Cells

Hybrid lead halide perovskites have reached comparable efficiencies to state-of-the-art silicon solar cell technologies. However, a remaining key challenge toward commercialization is the resolution of the perovskite device instability. In this work, we identify for the first time the mobile nature of bis-(trifluoromethanesulfonyl)imide (TFSI-), a typical anion extensively employed in p-type dopants for 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'spirofluorene (spiro-OMeTAD). We demonstrate that TFSI- can migrate through the perovskite layer via the grain boundaries and accumulate at the perovskite/electrontransporting layer (ETL) interface. Our findings reveal that the migration of TFSI- enhances the device performance and stability, resulting in highly stable p-i-n cells that retain 90% of their initial performance after 1600 h of continuous testing. Our systematic study, which targeted the effect of the nature of the dopant and its concentration, also shows that TFSI- acts as a dynamic defect-healing agent, which self-passivates the perovskite crystal defects during the migration process and thereby decreases nonradiative recombination pathways.