Trifluoromethyl‐Group Bearing, Hydrophobic Bulky Cations as Defect Passivators for Highly Efficient, Stable Perovskite Solar Cells

Abstract

Solution-processed perovskite films are rich in surface defects and grain boundaries, which limits their performance and stability in photovoltaic application. Surface passivation using bulky organic cations can effectively reduce the surface defects of a perovskite film without affecting its fundamental properties. Herein, the use of hydrophobic bulky aromatic molecules, namely 4-trifluoromethyl-benzylammonium iodide/bromide (CF 3 BZA-I/Br), as defectpassivators to heal the surface defects and grain boundaries of perovskite films is introduced. Owing to the presence of the trifluoromethyl (─CF 3 ) moieties, CF 3 BZA-I/Br-passivated perovskite films exhibit a hydrophobic surface with significantly fewer grain boundaries. By suppressing the surface and interfacial imperfections, CF 3 BZA-Br-treated perovskite solar cells achieve an outstanding power conversion efficiency (PCE) of 20.75%. The PCE improvement originates mainly from the reduction of trap states and nonradiative carrier recombination. The ultrathin hydrophobic barrier layer formed after passivation also shields the perovskite film surface from moisture ingress and environmental degradation, leading to improved stability of the devices. By optimizing the passivation conditions, the bulky CF 3 BZA-I/Br molecules could be the ideal defect passivators, with versatile applications in a wide variety of perovskite optoelectronics.

Intercalation of Ammonium Cationic Ligands Enabled Grain Surface Passivation in Sequential‐Deposited Perovskite Solar Cells

“…This is mainly because the surface dispersion force of the target film is retarded by the large steric repulsion in the phenyl ring of the CF 3 BZAI molecules, arising from the high electron cloud density surrounding the -CF 3 moiety. [15,35] A hydrophobic surface can help to repel moisture and act as a moisture barrier that shields the perovskite film from environmental degradation.…”

“…Additionally, the intercalation of CF 3 BZAI also boosted the hydrophobicity of the perovskite film surface, effectively blocking the permeation of oxygen and moisture from the surroundings into the perovskite film. [ 15 ] When placed in the glovebox, the target devices also showed much slower degradation than the control devices, which is understandably due to the mitigated internal ion migration and halide segregation phenomena within the perovskite film. [ 46,47 ] As a result, substantial improvements were observed in both the photovoltaic performance and stability of the CF 3 BZAI‐intercalated devices.…”

“…δ-phase (nonphotoactive) transformation in defect-rich FAPbI 3 films at room temperature, further threatening the performance and stability of PSCs. [6] In this regard, several research strategies including solvent/ antisolvent engineering, [7,8] additive engineering, [9,10] interfacial modifications, [11,12] alloying and doping, [13,14] and defect passivation [15,16] have been attempted to reduce the defects and improve the structural stability of FAPbI 3 films. Among these approaches, grain surface (film surface and grain Solution-processed formamidinium lead iodide (FAPbI 3 ) perovskite typically contains a high number of ionic defects that are intrinsically formed during film formation.…”

“…CF 3 BZAI has been used as a surface passivator to improve the efficiency and stability of one-step antisolvent processed perovskite films. [15,35,36] Nonetheless, the intercalation of CF 3 BZAI in two-step sequential-deposited perovskite films remains unexplored. Our results reveal that the intercalation of CF 3 BZAI passivators has greatly enhanced the morphology, grain size, and crystallinity of FAPbI 3 films.…”

“…In this regard, several research strategies including solvent/antisolvent engineering, [ 7,8 ] additive engineering, [ 9,10 ] interfacial modifications, [ 11,12 ] alloying and doping, [ 13,14 ] and defect passivation [ 15,16 ] have been attempted to reduce the defects and improve the structural stability of FAPbI 3 films. Among these approaches, grain surface (film surface and grain boundaries) passivation is the most promising strategy to increase the carrier diffusion length and reduce the defect trap states of perovskite films, given that the grain surfaces typically exhibit a trap density several orders of magnitude higher than that within the grains.…”