Partially-Bright Triplet Excitons in Perovskite Nanocrystals

TL;DR

This paper investigates the excitonic fine-structure of cesium lead-halide perovskite nanocrystals using multi-dimensional coherent spectroscopy, revealing a dark state within the bright triplet manifold that influences their light-emission properties.

Contribution

It provides the first detailed experimental evidence of a dark state within the triplet exciton manifold in perovskite nanocrystals using advanced spectroscopy techniques.

Findings

Identification of a dark exciton state within the bright triplet manifold

Measurement of picosecond dephasing times for triplet coherences

Inference of a unique exciton level-ordering in CsPbI3 nanocrystals

Abstract

Advances in opto-electronics require the development of materials with novel and engineered characteristics. A class of materials that has garnered tremendous interest is metal-halide perovskites, stimulated by meteoric increases in photovoltaic efficiencies of perovskite solar cells. In addition, recent advances have applied perovskite nanocrystals (NCs) in light-emitting devices. It was discovered recently that, for cesium lead-halide perovskite NCs, their unusually efficient light-emission may be due to a unique excitonic fine-structure composed of three bright triplet states that minimally interact with a proximal dark singlet state. To study this fine-structure without isolating single NCs, we use multi-dimensional coherent spectroscopy at cryogenic temperatures to reveal coherences involving triplet states of a CsPbI$_3$ NC ensemble. Picosecond timescale dephasing times are measured for both triplet and inter-triplet coherences, from which we infer a unique exciton fine-structure level-ordering comprised of a dark state energetically positioned within the bright triplet manifold.