Enhanced Lattice Coherences and Improved Structural Stability in Quadruple A-Site Substituted Lead Bromide Perovskites

Abstract

Lead halide perovskites (LHPs) are promising materials for efficient photovoltaic devices; however, they often encounter limited structural stability and degradation problems that limit their technological potential. This study investigates a novel perovskite composition consisting of (Cs, MA, FA, GA)PbBr_3, abbreviated as (4cat)PbBr₃, to effectively enhance phase stability and optoelectronic characteristics. The spectroscopic data reveal improved structural order, electronic properties, and dynamic lattice response in a cubic phase, which is uniquely stabilized by the specific cation composition down to 80 K. Superior optoelectronic properties are verified by increased photoluminescence (PL) and 20-fold higher electron mobility, when compared to the single-cation composition, MAPbBr₃. Notably, the ultrafast Terahertz-induced Kerr effect (TKE) reveals a dominating 1.1 THz octahedral twist mode, also observed in MAPbBr₃, however with a doubled phonon coherence time in (4cat)PbBr₃ at 80 K. The observation of higher structural order in the 4-cation composition is thus reflected by the prolonged lattice coherences, indicating enhanced dynamic screening effects that can explain the improved optoelectronic properties of (4cat)PbBr₃. This study therefore sheds light on the influence of the A-site cation composition on the inorganic sublattice and its coherent dynamics, highly relevant to perovskite-based photovoltaic and optoelectronic technologies.

Keywords: A‐site cation engineering; coherent phonons; lattice dynamics; lead halide perovskites; nonlinear THz spectroscopy; optoelectronic properties; structural phase stability.

Figure 1

Structure and surface properties A) ABX₃ perovskite crystal structure and chemical structure of incorporated A‐site cations used in the synthesis of the (4cat)PbBr₃. B) SEM images of MAPbBr₃ and (4cat)PbBr₃ perovskite films. (4cat)PbBr₃ shows full coverage, uniform grain size distribution, and a pinhole‐free surface evaluated at room temperature. C) The non‐contact (NC) height mode AFM image reveals a root mean square (RMS) roughness of 10 nm, indicating a smooth surface at room temperature.

Figure 2

Optical and charge transport properties comparing various LHP thin films: A) Absorption spectra of LHP thin films and Tauc plots (inset). The direct optical band gaps of perovskite thin films are determined from the Tauc plots fit to ≈ 2.36 eV and 2.37 eV for the (4cat)PbBr₃ and benchmark MAPbBr₃ LHP. B) PL at 300 K with excitation wavelength at 450–460 nm for various thin films shows highest emission for the (4cat)PbBr₃ thin film, indicating the radiative efficiency increases with multiple A‐site cations. C) Impedance spectroscopy measurements using a simple RC model (FTO/Perovskite/gold): Comparison of MAPbBr₃ (dot markers) and (4cat)PbBr₃ (star markers) with 350 nm thickness. (4cat)PbBr₃ shows more than twenty times higher diffusion coefficient and electron mobility compared to MAPbBr₃.

Figure 3

Coherent phonon dynamics of LHP thin films via Terahertz‐induced Kerr effect (TKE): A) Comparison of THz‐induced birefringence in various A‐site cation lead bromide perovskites; all ≈350 nm thin films on 500 µm BK7 glass substrate at 80K. Left hand section: First 5 ps responses normalized to the instantaneous electronic responses (at t = 0). Right hand section: Long lived signals (>14 ps), rescaled by factor 10³. B) FTs of the long‐time TKE responses after 14 ps. C) Long‐lived oscillatory TKE features of the (4cat)PbBr₃ thin film as a function of temperature (normalized to t = 0 peak signal and offset by 1 for each curve). D) FTs of the temperature dependent (4cat)PbBr₃ TKE responses in C. Inset: THz field dependence of the 1.1 THz peak at 80 K, including fit to quadratic dependence on the THz electric field peak E _THz.

Figure 4

Temperature dependent static and dynamic lattice properties A) Temperature‐dependent XRD of (4cat)PbBr₃ single crystal confirms the presence of cubic phase perovskite down to 80K. Dashed line: Einstein model for average lattice constant. B) Phonon lifetime and C) Phonon frequency of the 1.1 THz mode of (4cat)PbBr₃ in the low temperature range. Details: B) fit to anharmonic decay model assuming three‐phonon (black line) and four‐phonon scattering (grey line) C) Frequency shift of the 1.1 THz mode Δ₀ due to lattice expansion based on thermal expansion coefficient obtained from the Einstein model fit in A (dashed purple line). Solid lines: total shift due to contributions from lattice expansion and three‐phonon scattering (black) and four‐phonon scattering (grey).