  Metal halide perovskite-based technologies have been rapidly developed during
the last decade. However, to date, the fundamental question, why are halide
perovskites superior to conventional semiconductors? has remained elusive.
Here, we propose a new theory of lattice energy reservoir (LER) in halide
perovskites and elucidate that LER can comprehensively impact charge carrier
dynamics and thus enhance device performance, from hot carrier cooling, carrier
recombination, anomalous upconversion fluorescence, illumination induced
fluorescence enhancement (photobrightening), to high efficiency solar cells and
light-emitting diodes. An LER is a dynamic nanodomain in halide perovskites
with suppressed thermal transport that can accumulate energy from phonon
coupling and then feedback to subgap carriers and result in subgap carrier
upconversion. The LER directly results in slowed cooling of hot carriers and
significantly prolonged carrier recombination, anomalous upconversion
fluorescence, as usually termed as defect tolerance, as well as the anomalous
ultraslow phenomena including persistent polarization, memory effect, and
photobrightening. The LER theory rationalizes the superior optoelectronic
properties and device performance and provides a novel physical understanding
for anomalous phenomena observed uniquely in halide perovskites.
