Spontaneous Chemical Capping with Bis(trifluoromethanesulfonyl)imide-Based Additive for Photoabsorbers in Perovskite Solar Cells

The improvement in the performance of perovskite photoabsorbers plays a crucial role in the development of perovskite solar cells (PSCs). Thus far, salts based on bis(trifluoromethanesulfonyl)imide (TFSI) have been developed as additives for perovskite photoabsorbers and have improved the quality of the perovskite layers. However, the effects of TFSI anions require clarification because most TFSI-based additives are multifunctional; both the counter cation and TFSI anion improve the quality of the perovskite layers. Therefore, we synthesized a novel methylammonium bis(trifluoromethanesulfonyl)imide (MATFSI) and revealed the chemical capping effects of TFSI-based additives on the perovskite layer. MATFSI includes the same cation as the co-additive MA chloride, which is used to create narrow-bandgap perovskite layers. Hence, the use of the MATFSI additive can reveal the effect of the TFSI anion, independent of its counter cation. MATFSI addition resulted in the spontaneous coating of the TFSI moiety on the perovskite layer during its deposition process, and facilitated the crystal growth of the perovskite layer; thus, TFSI chemical capping effectively hampered defect formation in the perovskite layer. Hence, the optimal MATFSI addition improved the photovoltaic (PV) performance of the PSCs. Notably, the resulting perovskite surface that was chemically capped with TFSI displayed high wettability to water droplets, yet exhibited improved PV performance stability against humidity, which contradicts the overarching opinion in this research field. Parameter-differentiated contact angle (PDCA) measurements indicated that the high wettability of water droplets was attributed to the formation of active hydrogen bonds derived from the TFSI capping on the outer perovskite surface; moreover, the improvement in stability against humidity could be attributed to the low dispersion energy of the CF3 moiety in the TFSI capping. The insights obtained in this study could assist in improving perovskite additives and highlight the effectiveness of the simple PDCA method, which offers insights that conventional contact angle measurements cannot provide.