Excitons and their Fine Structure in Lead Halide Perovskite Nanocrystals from Atomistic GW/BSE Calculations

TL;DR

This study combines atomistic GW/BSE calculations with a tight-binding model to analyze exciton properties in lead halide perovskite nanocrystals, revealing size-dependent exciton energies and fine structure details.

Contribution

It introduces a novel computational approach integrating real-space tight binding with GW/BSE for detailed exciton analysis in large nanocrystals.

Findings

Exciton energies vary by nearly 1 eV with nanocrystal size.

Calculated excitation energies are about 0.2 eV higher than experimental photoluminescence.

The lowest exciton is dark and slightly lower in energy than bright triplet states.

Abstract

Atomistically detailed computational studies of nanocrystals, such as those derived from the promising lead-halide perovskites, are challenging due to the large number of atoms and lack of symmetries to exploit. Here, focusing on methylammonium lead iodide nanocrystals, we combine a real-space tight binding model with the GW approximation to the self-energy and obtain exciton wavefunctions and absorption spectra via solutions of the associated Bethe-Salpeter equation. We find that the size dependence of carrier confinement, dielectric contrast, electron-hole exchange, and exciton binding energies has a strong impact on the lowest excitation energy, which can be tuned by almost 1 eV over the diameter range of 2-6 nm. Our calculated excitation energies are about 0.2 eV higher than experimentally measured photoluminescence, and they display the same qualitative size dependence. Focusing on the fine structure of the band-edge excitons, we find that the lowest-lying exciton is spectroscopically dark and about 20-30 meV lower in energy than the higher-lying triplet of bright states, whose degeneracy is slightly broken by crystal field effects.