Momentarily trapped exciton polaron in two-dimensional lead halide perovskites

Two-dimensional (2D) lead halide perovskites with distinct excitonic feature have shown exciting potential for optoelectronic applications. Compared to their three-dimensional counterparts with large polaron character, how the interplay between long- and short- range exciton-phonon interaction due to polar and soft lattice define the excitons in 2D perovskites is yet to be revealed. Here, we seek to understand the nature of excitons in 2D CsPbBr3 perovskites by static and time-resolved spectroscopy which is further rationalized with Urbach-Martienssen rule. We show quantitatively an intermediate exciton-phonon coupling in 2D CsPbBr3 where exciton polarons are momentarily self-trapped by lattice vibrations. The 0.25ps ultrafast interconversion between free and self-trapped exciton polaron with a barrier of similar to 34meV gives rise to intrinsic asymmetric photoluminescence with a low energy tail at room temperature. This study reveals a complex and dynamic picture of exciton polarons in 2D perovskites and emphasizes the importance to regulate exciton-phonon coupling.

Automated summary

The nature of excitons in 2D CsPbBr3 perovskites was studied through static and time-resolved spectroscopy and rationalized with the Urbach-Martienssen rule. Quantitatively, an intermediate exciton-phonon coupling in 2D CsPbBr3 was demonstrated, showing that exciton polarons are momentarily self-trapped by lattice vibrations.