Excitonic-trion population in two-dimensional halide perovskites

TL;DR

This paper models the thermodynamic populations of excitons and trions in 2D halide perovskites, revealing that trions can dominate charge carriers at room temperature despite their lower binding energy.

Contribution

It derives and solves Saha equations for excitonic complexes in 2D perovskites, providing a detailed understanding of their population dynamics and potential charge transport mechanisms.

Findings

Excitonic trions can dominate charge carriers at room temperature.

The population balance is governed by large binding energies and temperature.

Trionic hopping may involve tunneling, influencing charge transport.

Abstract

There are many reports of a surprisingly high charge-carrier density with sizable mobility in photo-excited two-dimensional (2D) halide perovskites despite their unusually high exciton binding-energy. In this work we study the thermodynamic quasi-equilibrium of the relative population of photoexcited free quasi-electron/quasi-hole pairs, neutral-excitons and excitonic trions, in 2D materials that support such excitonic complexes with large binding energy. We derive and solve the general Saha equations which describe the detailed balance of such a system of photo-excited electronic degrees of freedom forming a multi-component fluid of excitations in thermodynamic quasi-equilibrium.The solution to these equations, for the special case of 2D perovskites where the reported exciton and excitonic trion binding-energies are of the order of 0.3-0.4 eV for the former and 30-40 meV for the latter, reveals that while the charge-neutral excitonic population dominates all other excitations, at room temperature and below, the excitonic trion component can be the dominant population among charge carriers. We also argue that trionic hopping can take place via a tunneling mechanism which is speculated to play a role in a novel charge-transport mechanism.