Amidinium versus Ammonium Ligands for Thermal Stability and Efficiency Boost in CsPbI₃ Perovskite Solar Cells

Inorganic CsPbI₃ perovskite solar cells (PSCs) hold notable promise but suffer from instability in maintaining the photoactive black phase under ambient and thermal conditions; this work systematically investigates the impacts of p-phenylenediamine dihydrochloride (PPD·2HCl, containing ammonium groups) and terephthalimidamide dihydrochloride (TPID·2HCl, featuring amidinium groups) ligands on device performance and stability, with a focus on dissecting TPID's thermal stabilization mechanisms. While both ligands enhance room-temperature phase stability via surface defect passivation, TPID·2HCl-treated devices exhibit superior thermal resilience: its amidinium group features delocalized electron density across the N-C-N skeleton, strengthening coordination with undercoordinated sites and mitigating lattice distortion under heat; thermogravimetric analysis (TGA) shows TPID·2HCl decomposes at ∼350 °C with gradual mass loss, surpassing PPD·2HCl's rapid degradation at ∼300 °C; density functional theory (DFT) calculations reveal TPID²⁺ binds more strongly to the CsPbI₃ surface (adsorption energy: -1.51 eV vs -1.18 eV for PPD²⁺), ensuring stable surface interaction; X-ray photoelectron spectroscopy (XPS) confirms TPID·2HCl retains its chemical environment even after 100 °C aging for 150 min, whereas PPD·2HCl dissociates. These mechanisms collectively boost device performance, with TPID·2HCl-treated cells achieving a power conversion efficiency (PCE) of 18.59% (vs 17.23% for PPD·2HCl and 16.22% for untreated) and delaying efficiency decay at 85 °C, highlighting amidinium ligands as a viable approach to enhance CsPbI₃ stability under thermal stress.