Emission characteristics of metal halide perovskites play a key role in the current widespread investigations into their potential uses in optoelectronics and photonics. However, a fundamental understanding of the molecular origin of the unusual blueshift of the bandgap and dual emission in perovskites is still lacking. In this direction, we investigated the extraordinary photoluminescence behavior of three representatives of this important class of photonic materials, that is, CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃, which emerged from our thorough studies of the effects of temperature on their bandgap and emission decay dynamics using time-integrated and time-resolved photoluminescence spectroscopy. The low-temperature (<100 K) photoluminescence of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ reveals two distinct emission peaks, whereas that of CH(NH₂)₂PbBr₃ shows a single emission peak. Furthermore, irrespective of perovskite composition, the bandgap exhibits an unusual blueshift by raising the temperature from 15 to 300 K. Density functional theory and classical molecular dynamics simulations allow for assigning the additional photoluminescence peak to the presence of molecularly disordered orthorhombic domains and also rationalize that the unusual blueshift of the bandgap with increasing temperature is due to the stabilization of the valence band maximum. Our findings provide new insights into the salient emission properties of perovskite materials, which define their performance in solar cells and light-emitting devices.

The methylammonium lead iodide perovskites at the core of recently proposed solar cells with exceptionally large quantum conversion efficiency are studied by first-principles methods. Large absorption coefficients (0.03\char21{}0.04 nm${}^{$-${}1}$ for wavelength $$\sim${}$500 nm) and small effective masses suited for both $n$-type and $p$-type transport are obtained as a consequence of their peculiar structural and electronic characteristics. In particular, the presence of a direct gap between highly dispersed Pb(6$s$)-I($5p$) valence bands and Pb($6p$) conduction bands is the key ingredient at the basis of their excellent performance in photovoltaic applications.

Halide perovskites are emerging as revolutionary materials for optoelectronics. Their ionic nature and the presence of mobile ionic defects within the crystal structure have a dramatic influence on the operation of thin‐film devices such as solar cells, light‐emitting diodes, and transistors. Thin films are often polycrystalline and it is still under debate how grain boundaries affect the migration of ions and corresponding ionic defects. Laser excitation during photoluminescence (PL) microscopy experiments leads to formation and subsequent migration of ionic defects, which affects the dynamics of charge carrier recombination. From the microscopic observation of lateral PL distribution, the change in the distribution of ionic defects over time can be inferred. Resolving the PL dynamics in time and space of single crystals and thin films with different grain sizes thus, provides crucial information about the influence of grain boundaries on the ionic defect movement. In conjunction with experimental observations, atomistic simulations show that defects are trapped at the grain boundaries, thus inhibiting their diffusion. Hence, with this study, a comprehensive picture highlighting a fundamental property of the material is provided while also setting a theoretical framework in which the interaction between grain boundaries and ionic defect migration can be understood.