Momentarily trapped exciton polaron in two-dimensional lead halide perovskites

Abstract

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 0.25 ps ultrafast interconversion between free and self-trapped exciton polaron with a barrier of ~ 34 meV 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.

Fig. 1. Characterization of 2L CsPbBr₃ NPs.

a TEM image of 2L CsPbBr₃ NPs. Scale bar is 20 nm. b Absorption and PL spectra of 2L CsPbBr3 NPs in toluene solution. The red shaded area is vogit fitting of main peak, which is denoted as FE and the rest as LE.

Fig. 2. Equilibrium of FE and LE emission at room temperature.

a PLE spectra of 2L CsPbBr₃ NPs monitored at FE emission (2.92 eV) and LE emission (2.58 eV). b LE ratio and asymmetric factor as a function of excitation power. c PL decay kinetics and d PL decay rate at different energies (2.38–2.86 eV).

Fig. 3. Dynamics between FE and LE.

a PL spectra at 298 K, 230 K, and 77 K, respectively, and the vogit fits. b LE ratio and asymmetric factor as a function of temperature. c Logarithm plot of LE ratio as a function of temperature and the linear fit according to Eq. 3. Inset: scheme showing the energy difference and conversion between FE state and LE state. d 2D color plot of TA spectra under tail excitation. e The shift of bleach peak maximum E_max with pump-probe decay time.

Fig. 4. Urbach rule in lead halide perovskites.

a Scheme of the energy diagram of free and STE state in different regimes and dominant relaxed species. b Urbach tail and fitting according to Eq. 4 of different thickness perovskites.