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.
We thank F. Krieg for providing large CsPbBr3 nanocrystals, S. Yakunin and J. Jagielski for assistance with absolute quantum-yield measurements, and E. Ivchenko, M. Glazov and E. Rashba for discussions. M.A.B., G.R., T.S., M.V.K. and R.F.M. acknowledge the European Union’s Horizon-2020 programme through the Marie-Skłodowska Curie ITN network PHONSI (H2020-MSCA-ITN-642656) and the Swiss State Secretariat for Education Research and Innovation (SERI). J.G.M., S.G.L., N.B., J.L.L. and Al.L.E. acknowledge support from the US Office of Naval Research (ONR) through the core funding of the Naval Research Laboratory. R.V. was funded by ONR grant N0001416WX01849. A.S. acknowledges support from the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center (EFRC) funded by BES, OS, US DOE. D.J.N. and M.V.K. acknowledge partial financial support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC grant agreement number 339905 (QuaDoPS Advanced Grant) and number 306733 (NANOSOLID Starting Grant), respectively.
Extended data figures
This file contains supplementary text S1-S5.