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

Inorganic CsPbI 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 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 TPID binds more strongly to the CsPbI surface (adsorption energy: -1.51 eV vs -1.18 eV for PPD), 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 CsPbI stability under thermal stress.