Liquid-like dielectric response is an origin of long polaron lifetime exceeding 10 μs in lead bromide perovskites.

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 CsPbBr and organic-inorganic hybrid (CHNH)PbBr 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 10-10 Hz in CsPbBr, which was clearly present in the hybrid (CHNH)PbBr. 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 CsPbBr was up to 1.6 µs, while the lifetime in (CHNH)PbBr was 18 µs. The lifetime in the hybrid (CHNH)PbBr was significantly longer than in CsPbBr, 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.