  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.
