Exciton–phonon coupling in quasi-two-dimensional Ruddlesden–Popper perovskites: impact of a mixed-phase structure

Two-dimensional (2D) metal halide perovskites (MHPs) hold great potential for optoelectronic and spintronic device applications due to their outstanding optical and electronic properties. In this study, we conducted a systematic temperature-dependent photoluminescence (PL) investigation to elucidate the influence of exciton-phonon scattering on the optoelectronic properties of 2D Ruddlesden-Popper (RP) perovskites, (C₆H₇SNH₃)₂ (CH₃NH₃)_<i>n</i>-1Pb_<i>n</i>I_3<i>n</i>+1 (<i>n</i> = 1-4). The results from our micro-PL study suggest that the optical band gap of each 2D phase (a perovskite with a specific number of inorganic layers, <i>n</i>) varies with temperature due to thermal lattice expansion and exciton-phonon interactions. The strength of exciton-phonon interactions differs in each 2D phase within a mixed-phase sample, with a notable increase as the layer number (<i>n</i>) rises. This enhancement is attributed to greater lattice mismatch, increased interface complexity, and a higher degree of disorder within the system. Additionally, we found that parameters such as exciton-phonon coupling strength and exciton binding energy exhibit significantly different behaviours across various mixed-phase perovskite samples, likely due to changes in the local environment around excitons in each specific phase. These findings offer valuable insights into the mechanisms underlying nonradiative processes and scattering phenomena and provide guidance for optimizing the efficiency of 2D-RP mixed-phase perovskite-based optoelectronic and spintronic devices.