A positional isomerism strategy for fluorophenyl substituents in self-assembled monolayers for fabricating perovskite solar cells

Abstract

The application of self-assembling molecules that form ordered thin films on substrates offers a novel strategy for the development of high-performance perovskite solar cells (PSCs). However, the enhancement of hole transport and interface-defect passivation in self-assembled monolayer (SAM)-based materials remains challenging. In this work, we designed three fluorinated isomeric SAM molecules (ZY4, ZY5, and ZY6) based on the 4PACz core unit. A combination of density functional theory (DFT), time-dependent DFT (TD-DFT), and molecular dynamics (MD) simulations was employed to investigate various properties, including photoelectronic characteristics, excited-state properties, stability, solubility, and hole transport. Furthermore, theoretical calculations revealed that the three SAM molecules exhibit higher dipole moments (3.50 D, 2.81 D, and 2.95 D, respectively) than the reference molecule (4-(9H-carbazol-9-yl)butyl)phosphonic acid (1.93 D). Their HOMO energy levels are close to the valence band maximum (VBM) of perovskite, while their LUMO levels lie above the conduction band minimum (CBM). Additionally, they demonstrate more negative solvation free energies than 4PACz (−0.46 eV). Among them, ZY4 exhibited the highest hole mobility of 6.71 × 10⁻¹ cm² V⁻¹ s⁻¹. Moreover, the fluorine atoms could form coordination bonds with the Pb²⁺ ions on the perovskite surface. This study demonstrates that the isomerization of fluorinated substituents in 4PACz-core-based SAMs is an effective strategy for enhancing hole transport mobility and optimizing interfacial properties.