Hydrostatic Pressure Driven Band Gap Tuning and Self-Trapped Exciton Formation in (4FPEA)$_2$SnBr$_{4}$ Halide Perovskite

Abstract

Two-dimensional tin halide perovskites provide a highly tunable platform for exciton phonon coupling and local lattice distortions, enabled by their intrinsically soft lattice. We report a combined temperature and pressure dependent photoluminescence study of the layered perovskite (4FPEA)$_{2}$SnBr$_{4}$. At room temperature, its optical response is dominated by near band edge (NBE) excitons, which redshift linearly under hydrostatic pressure up to $\sim$3 GPa, indicating a rigid band edge behavior without phase transitions. Cooling reveals a broad, strongly Stokes shifted self-trapped exciton (STE) emission, evidencing a crossover from delocalized to self localized excitonic states. Strikingly, while NBE emission redshifts under pressure, STE emission exhibits an anomalous blueshift, reflecting pressure induced modification of the exciton phonon energy landscape. In contrast, the iodide analogue (4FPEA)$_{2}$SnI$_{4}$ shows no STE emission under identical conditions, highlighting the critical role of lattice rigidity and dielectric screening in stabilizing self-trapped excitons.

Figures (5)

• Figure 1: Normalized photoluminescence spectra of (4FPEA)$_{2}$SnBr$_{4}$ under varying hydrostatic pressure at (a) 300 and (b) 200 K. Free exciton is indicated as NBE and position changes are marked by black dashed line. (c) Pressure-induced band gap shift, extracted from the PL spectra at 300 (blue circles) and 200 K (green circles), along with corresponding linear fits (solid lines). A clear temperature dependence of the pressure coefficient is observed.
• Figure 2: Pressure dependent normalized PL spectra of (4FPEA)$_{2}$SnBr$_{4}$ at (a) 120 and (b) 40 K. Self-Trapped Exciton is indicated as component STE. Positions changes are marked by black dashed lines. (c) Pressure-induced band gap shift $\Delta E = E(P) - E(P=0)$, extracted at 120 (red circles) and 40 K (grey circles). Solid circles and lines correspond to the NBE feature; open circles and solid lines represent STE feature. Black line shows DFT calculations (crystal strucutre is presented in Figure S2 in the SI). (d) Pressure dependence of the Stokes shift from the same data.
• Figure 3: Pressure dependent photoluminescence spectra of two layered halide perovskites measured at variable temperature. (a-d) PL spectra of (4FPEA)$_{2}$SnI$_{4}$ collected at 300, 200, 120, and 40 K under hydrostatic pressure. (e-h) Corresponding PL spectra of (4FPEA)$_{2}$SnBr$_{4}$ acquired under indentical temperature but different pressure conditions. Bright yellow traces denote the evolution of NBE emission, whereas the violet trace indicates the spectral position of STE emission.
• Figure 4: Schematic configuration diagram illustrating the pressure-induced redshift of the NBE emission and blueshift of STE emission in (4FPEA)$_{2}$SnBr$_{4}$. P$_{0}$ denotes ambient pressure, while P$_{1}$ larger than P$_{0}$ represents elevated pressure. GS indicates the ground state of the system.
• Figure 5: Pressure-dependent band structures of (4FPEA)$_{2}$SnBr$_{4}$ at 0 GPa and 3 GPa, revealing a persistent direct band gap and a significant pressure induced redshift. Values of pressure coefficients for VBM and CBM are also given.