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

Abstract

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.

Synergy Effect of a π‐Conjugated Ionic Compound: Dual Interfacial Energy Level Regulation and Passivation to Promote Voc and Stability of Planar Perovskite Solar Cells

“…[38] Therefore, the diffusion of [BF 4 ] À along the perovskite polycrystalline GBs is possibly due to the integrated chemical environmental factors of the precursor and SnO 2 interface layer. [42] Similar ion distribution of QAPyBF 4 was also found when using titanium oxide or zinc oxide instead of SnO 2 as ETLs (Figure S13). This suggested that the different distribution behaviors of the cations and anions of QAPyBF 4 in perovskite were universal among common metal-oxide-based ETLs for planar PSCs, indicating its potential multifunctionality in PSC applications.…”

“…Thus, the synergistic effect of [QAPy] 2+ and Pb 2+ has been hypothesized to release a large amount of [BF 4 ] − in the perovskite precursor solution, which coordinates with the positively charged surface metal centers (metal cations or oxidized metal atoms) in the perovskite preparation [38] . Therefore, the diffusion of [BF 4 ] − along the perovskite polycrystalline GBs is possibly due to the integrated chemical environmental factors of the precursor and SnO 2 interface layer [42] …”