Bidentate Pyridine Passivators Attaching Trifluoromethyl Substitute Groups in Varied Positions for Efficient Carbon-Based Perovskite Solar Cells

Multifunctional passivation agents have been demonstrated to have an important effect on defect suppression and performance improvement in perovskite solar cells (PSCs). Rational molecular engineering of passivators can significantly boost passivation effects for achieving superior photovoltaic performance in PSCs. Thus, the relationship of passivators' molecular structure-solar cell performance has great merit to be investigated. With this regard, a series of regioisomeric passivators (2-amino-n-(trifluoromethyl)pyridine, n-TFMAP (n = 3, 4, 5, or 6)) were purposely chosen to surface passivate perovskite films. The trifluoromethyl (-CF₃) substitute group with good hydrophobic and electron-withdrawing characteristics is placed in different positions of the pyridine ring, and their passivation effect on carbon-based PSCs (C-PSCs) were systematically compared. It reveals that all n-TFMAP can passivate the uncoordinated Pb²⁺ defects via bidentate coordination (pyridinic N and -NH₂), and the different positions of the -CF₃ group mainly changed the molecular electron density and the steric hindrance effect. The Lewis basicity (pK_a value) was varied, and the binding energy and defect formation energy also correlate with the molecular structure when the passivators interact with the perovskite films. Among these passivators, 5-TFMAP showed the highest defect formation energy and moderate adsorption energy. 4-TFMAP exhibited exceptional humidity stability due to the sterically enabled vertical alignment of the -CF₃. The trade-off between coordination strength (5-TFMAP) and interfacial coverage (4-TFMAP) highlighted the need for balanced molecular design. Consequently, 5-TFMAP with the strongest bidentate passivation and substantial hydrophobicity achieved the champion PCE of 14.14% compared with the control device (11.74%). After 30 days of storage in the dark with 35-45% relative humidity, the 5-TFMAP-passivated device retained 88% of their initial PCE, compared with control devices that retained only 63% of their initial PCE. This work suggests the necessity of precise site engineering to balance electronic properties, molecular geometry, and surface functionality, offering a valuable insight for designing high-performance passivators in PSCs.