Multiple Emission Peaks Challenge Polariton Condensation in Phenethylammonium-Based 2D Perovskite Microcavities

Abstract

Two-dimensional metal halide phases, commonly known as 2D perovskites, have emerged as promising materials for exciton-polaritons, particularly for polariton condensation. This process entails the spontaneous accumulation of population in the polariton ground state and relies on efficient energy relaxation. In this class of materials, this relaxation is mediated by exciton reservoir emission, which pumps polariton states through radiative pumping. To achieve strong light-matter coupling and sustain a high polariton density, the material must possess excitations with large oscillator strength and high exciton binding energy. While 2D perovskites exhibit these desirable characteristics, there are no reports of room-temperature polariton condensation and only one successful demonstration at cryogenic temperatures. In this work, we systematically explore the role of energy alignment between the exciton reservoir emission and the lower polariton branch in populating the polariton ground state via radiative pumping. Through cavity detuning, we shift the lower polariton energy minimum to overlap with the emission of the exciton reservoir at different energies. We identify that the multiple radiative pathways of 2D perovskites lead to inefficient radiative pumping of the lower polariton branch at the lowest-energy state, ultimately posing challenges for polariton condensation in this class of materials.

Keywords: exciton-polariton condensation; excitons; halide perovskites; polaritons.

1

(a) Schematic of the (PEA)₂PbI₄ 2D perovskite microcavity, produced with a TiO₂/SiO₂ distributed Bragg reflector, a (PEA)₂PbI₄ layer, a poly­(methyl methacrylate) (PMMA) spacer film, and a Ag layer serving as a semitransparent top mirror. (b) Absorption and photoluminescence of the neat (PEA)₂PbI₄ film (left). The energy dispersion measured at 5 K with Fourier microscopy including the expected cavity mode and distinct exciton energies (red dashed line) as well as the simulated polariton modes (right). (c) 200 K k⃗-space photoluminescence dispersion showing accumulations of PL intensity at higher $|\overrightarrow{k_{\parallel }}|$ . (d) 5 K photoluminescence dispersion showing accumulation around $|\overrightarrow{k_{\parallel }}|=0$ .

2

Photoluminescence energy dispersion, measured through Fourier microscopy, of a (PEA)₂PbI₄ microcavity with a detuning that maximizes the energy overlap between (a, b) the lower polariton and PL1 at a pumping fluence of 0.2 and 8 mJ/cm², respectively. (c, d) The lower polariton and PL2 of the material at a pumping fluence of 0.2 and 8 mJ/cm², respectively.

3

Fluence-dependent photoluminescence energy dispersion from thin-top mirror microcavities showing leakage from the exciton reservoir that increases with fluence, from 0.2 mJ/cm² (a) to 0.9 mJ/cm² (b) and 3 mJ/cm² (c). The dashed horizontal cuts indicate the energies of the LP at $|\overrightarrow{k_{\parallel }}|=0$ (2.34 eV) and LP2 leakage (2.32 eV), respectively. (d) Maximum PL intensity at the lower polariton and exciton leakage energy. (e) PL from the 2D perovskite bare films as a function of fluence.

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(a) Excitation correlation photoluminescence (ECPL) dynamics as the fractional change in the PL due to nonlinear interactions, plotted as a function of time delay and measured at different pump fluences. (b) Spectrally resolved map of ECPL dynamics taken with total pump fluence of 10 mJ/cm². (c) Normalized ECPL dynamics integrated over the spectral region marked with the dotted line in (b). These correspond to the ECPL dynamics of the lower polariton emission and the emission from the PL2 peak.

5

Exciton reservoir radiative recombination pathways. The two emission peaks lead to inefficient population of the lower polariton branch.