Pb-free layered all-inorganic metal halides RbSn2Br5: Mechanochemical synthesis, band gap tuning, optical and dielectric properties

Abstract

Recently, two-dimensional (2D) layered metal halide perovskites stimulate substantial attention in photovoltaics and optoelectronics. Nevertheless, one of the main hurdles to their commercialization is the Pb toxicity. Although challenging, the emergence of Sn(II)-based halides considered as a most favourable environmentally benign substitute to Pb based material. Herein, we investigate the optical and dielectric properties of 2D Pb-free all-inorganic metal halide, RbSn2Br5, which is synthesized by all-solid state mechanochemistry. The material exhibited a band gap of ∼3.20 eV with room-temperature photoluminescence (PL) centered at ∼2.51 eV. The Raman spectroscopic investigation demonstrated distinct vibrational modes related to the Sn-Br bond from 2D polymeric [Sn2Br5]nn− layer units. The thermal stability indicated that the material is stable up to ∼205 °C. An excitonic absorption with high exciton binding energy is perceived for ultrasonic treated toluene-dispersed solution of RbSn2Br5. Enhanced PL intensity with red-shifted and broad emission line were demonstrated at 77 K than the room-temperature emission. The charge-carrier recombination took place from the band-edge states and intrinsic defect sites (shallow states) at both temperatures. Further, the halide mixing strategy is applied to tune the band gap (2.68–3.36 eV) and PL emission (2.31–2.58 eV) by synthesizing RbSn2Br3Cl2 and RbSn2Br4I. Finally, the dielectric measurement of RbSn2Br5 was carried out as a function of temperature (4−300 K), which exhibited a strong frequency dependence.