Bright triplet excitons in lead halide perovskites

Abstract

Nanostructured semiconductors emit light from electronic states known as excitons[1]. According to Hund's rules[2], the lowest energy exciton in organic materials should be a poorly emitting triplet state. Analogously, the lowest exciton level in all known inorganic semiconductors is believed to be optically inactive. These 'dark' excitons (into which the system can relax) hinder light-emitting devices based on semiconductor nanostructures. While strategies to diminish their influence have been developed[3-5], no materials have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in quasi-cubic lead halide perovskites is optically active. We first use the effective-mass model and group theory to explore this possibility, which can occur when the strong spin-orbit coupling in the perovskite conduction band is combined with the Rashba effect [6-10]. We then apply our model to CsPbX3 (X=Cl,Br,I) nanocrystals[11], for which we measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright character of the lowest exciton immediately explains the anomalous photon-emission rates of these materials, which emit 20 and 1,000 times faster[12] than any other semiconductor nanocrystal at room[13-16] and cryogenic[17] temperatures, respectively. The bright exciton is further confirmed by detailed analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals[18], which are already used in lighting[19,20], lasers[21,22], and displays[23], these optically active excitons can lead to materials with brighter emission and enhanced absorption. More generally, our results provide criteria for identifying other semiconductors exhibiting bright excitons with potentially broad implications for optoelectronic devices.

14 pages and 3 figures in the main text, Methods and extended data 16 pages which include 11 figures, and supporting information 28 pages

References in corpus (4)

• Restoring the density-gradient expansion for exchange in solids and surfaces
• Generalized gradient approximation for solids and their surfaces
• Local polar fluctuations in lead halide perovskite crystals
• Polarization analysis of excitons in monolayer and bilayer transition-metal dichalcogenides

Cited by in corpus (83)

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• Coherent Spin Precession and Lifetime-Limited Spin Dephasing in CsPbBr3 Perovskite Nanocrystals
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• Fine Structure of Excitons in Vacancy Ordered Halide Double Perovskites
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• Fuzzy band structure of quantum dots by Bloch Orbital Expansion, unconventional insights into geometry-electronic structure relations
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• Electrically controlled emission from triplet charged excitons in atomically thin heterostructures
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