Facile synthesis of single crystal of two-dimensional mixed-halide copper-based perovskites with tunable band gaps and their capability of exfoliation and reversible thermochromism.

Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic–inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1–10 THz region and liquid-like rotational relaxation in the 1–100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr3 and organic–inorganic hybrid (CH3NH3)PbBr3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1–100 GHz range. We confirmed the absence of the dielectric response in the range of 106–1010 Hz in CsPbBr3, which was clearly present in the hybrid (CH3NH3)PbBr3. This suggests that the response is almost purely due to the rotational motions of the organic dipoles in the hybrid perovskites. We evaluated the lifetimes of the polarons using surface-free transient photoluminescence. The lifetime in CsPbBr3 was up to 1.6 µs, while the lifetime in (CH3NH3)PbBr3 was 18 µs. The lifetime in the hybrid (CH3NH3)PbBr3 was significantly longer than in CsPbBr3, also confirmed by transient infrared spectroscopy. We concluded that the liquid-like dielectric response inhibits polaron recombination due to the efficient separation of opposite charges by the additional dynamic disorder.

We performed in situ x-ray diffraction (XRD) experiments on an inorganic-organic hybrid perovskite, CH 3 NH 3 PbI 3 (MAPbI 3 ), during its interaction with moisture to understand the degradation mechanism. Although the degradation of inorganic-organic hybrid perovskite is an important factor hampering their development as solar cell materials, understanding of the degradation process is currently limited. The moisture-induced degradation mechanism was revealed by the temperature dependence of the in situ XRD pattern sequences and firstprinciples calculations based on the nudged elastic band method. The combination of experimental and computational data suggests that the MAPbI 3 crystal spontaneously changes into the MAPbI 3 mono-hydrate crystal once water molecules activated with an energy of more than $0.6 eV penetrate the (100) outer surface of the MAPbI 3 lattice. These findings have important implications for the development of more robust inorganic-organic hybrid perovskites as light absorbing layers in solar cells and other applications.