Phase-stable Cs-doped MAPbI₃ hybrid perovskite investigated through DFT-assisted structural, spectroscopic, EQE, and nonlinear optical analysis

This paper presents an extensive experimental and theoretical investigation of the cesium-doped methylammonium lead triiodide (Cs: MAPbI₃), a hybrid inorganic-organic perovskite that exhibits high photovoltaic and nonlinear optical (NLO) promise. The reaction mechanism at the molecular level suggests coordination of the Pb²⁺ with the iodide ions and Cs⁺-induced zwitterionic nature, which favors exciton dissociation and charge separation. Crystallographic analysis reveals favored crystallization in the (101) plane and reveals a three-dimensional intricate system of molecular slabs, tubular morphologies, helical molecular strips, and sloping planes, all for anisotropic charge transfer. The pseudo-tetragonal lattice exhibits low Microstrain (ε = 3.49 × 10⁻⁴), high crystallite dimension (812.2 nm), and high phase purity, as established by XRD and SEM studies. Electrochemical impedance spectroscopy gives low series resistance (Rs = 15.2 Ω) and high recombination resistance (Rrec = 600 Ω), suggestive of optimal carrier transport. Band gap value optimized to ~ 1.55–1.63 eV is comparable with the Shockley–Queisser limit, and the material has a high external quantum efficiency (EQE ~ 86%). Spectroscopic analyses (FT-IR, FT-Raman, NMR) authenticate polar strong interactions (e.g., Pb-I, Pb-H, Cs-I), which are responsible for vibrational coupling and dielectric modulation. Theoretical calculations, such as APT, Mulliken population, HOMO-LUMO analysis, UV-Vis absorption, and NBO analysis, indicate extensive delocalization of electrons, strong orbital hybridization, and favourable dipole alignment, along with highly developed hyperpolarizability (βtot = 528.19 × 10⁻³³ esu) to facilitate intense second-order NLO activity. MEP map and exciton cavity analysis emphasize dipolar energy landscapes conducive to exciton confinement and dissociation. Collectively, these photophysics, electronics, and structure result in Cs: MAPbI₃ as a structurally stable, optically active, and electronically tuneable material that is best suited for high-performance applications in solar energy and nonlinear photonics.