  In lead halide perovskites (APbX3), the effect of the A-site cation on
optical and electronic properties has initially been thought to be marginal.
Yet, evidence of beneficial effects on solar cell performance and light
emission is accumulating. Here, we report that the A-cation in soft APbBr3
colloidal quantum dots (QDs) controls the phonon-induced localization of the
exciton wavefunction. Insights from ab initio molecular dynamics and
single-particle fluorescence spectroscopy demonstrate that anharmonic lattice
vibrations and the resulting polymorphism act as an additional confinement
potential. Avoiding the trade-off between single-photon purity and optical
stability faced by downsizing conventional QDs into the strong confinement
regime, dynamical phonon-induced confinement in large organic-inorganic
perovskite QDs enables bright (10^6 photons/s), stable (> 1h), and pure (> 95%)
single-photon emission in a widely tuneable spectral range (495-745 nm). Strong
electron-phonon interaction in soft perovskite QDs provides an unconventional
route toward the development of scalable room-temperature quantum light
sources.
