Halide

Exciton

Chemistry

Materials science

Optoelectronics

Composite material

Physics

Nanocrystal

Crystallography

Nanotechnology

Perovskite (structure)

Absorption (acoustics)

Atomic physics

Biexciton

Semiconductor

Inorganic chemistry

Condensed matter physics

Nature

The lowest-energy exciton state in caesium lead halide perovskite nanocrystals is shown to be a bright triplet state, contrary to expectations that lowest-energy excitons should always be dark. Lead halide perovskite semiconductor nanocrystals are attracting considerable interest as materials for solar cells and light-emitting diodes because of their excellent photophysical properties. But what makes them so special? Excitons are the electronic excitations that are ultimately responsible for the emissive properties of nanostructured semiconductors, and prevailing wisdom is that the lowest-energy excitonic state will be long-lived and hence poorly emitting (or 'dark'). Michael Becker et al. now show that caesium lead halide perovskites disobey this rule: the lowest-energy excitons are instead unusually 'bright', emitting much faster than any other semiconductor nanocrystal. Furthermore, they identify the structural and electronic factors responsible for this anomalous behaviour, providing vital clues for the identification of other semiconducting materials that might behave similarly. Nanostructured semiconductors emit light from electronic states known as excitons1. For organic materials, Hund’s rules2 state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules3 predict an analogue of this triplet state known as the ‘dark exciton’4. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin–orbit coupling in the conduction band of a perovskite is combined with the Rashba effect5,6,7,8,9,10. We then apply our model to CsPbX3 nanocrystals11, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster12 than any other semiconductor nanocrystal at room13,14,15,16 and cryogenic4 temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting17, lasers18 and displays19, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

Bright triplet excitons in caesium lead halide perovskites