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

Salts based on bis(trifluoromethanesulfonyl)imide (TFSI) have been developed as additives for photoabsorbers in perovskite solar cells (PSCs) to enhance their photovoltaic (PV) performance. However, the effects of their TFSI anions have remained elusive. Herein, a novel methylammonium bis(trifluoromethanesulfonyl)imide (MA-TFSI) additive, comprising MA cations that are removed from the perovskite layer during heating, is verified. This is the first implementation of alkyl-primary-ammonium-based TFSI additives for perovskite layers. MA-TFSI addition exhibited unique chemical capping effects; the TFSI moieties were spontaneously coated on the outer surface of the perovskite and on the crystal grains during deposition, leading to the prevention of defect formation in the perovskite layer. Notably, the TFSI-capped perovskite surface displays high wettability to water droplets yet improved PV performance stability against humidity, contradicting the school of thought in the PSC research field. Parameter-differentiated contact angle (PDCA) measurements suggest that the high wettability of water droplets is attributed to the active hydrogen bonds derived from the TFSI capping. Meanwhile, the improved stability against humidity is attributable to the low dispersion energy of the CF₃ moiety in the TFSI capping on the crystal grains. The presented deviation from the prevailing viewpoint will lead to the advancement of materials science.