Influence of the Iodide to Bromide Ratio on Crystallographic and Optoelectronic Properties of Rubidium Antimony Halide Perovskites

Rubidium antimony halides are a promising low toxic alternative to organo-lead halide perovskites as photovoltaic material. In this contribution, we systematically investigate the influence of varying the bromide to iodide ratio on the structural, optical, and photovoltaic properties of Rb<sub>3</sub>Sb<sub>2</sub>­Br<sub>9–<i>x</i></sub>I<sub><i>x</i></sub> (<i>x</i> = 0–9). Single crystal data reveal that all compounds crystallize in a 2D-layered monoclinic crystal structure. Sequential substitution of iodide with the smaller bromide does not change the crystal system; however, increasing the bromide content results in a shrinkage of the unit cell as well as in a blue shift of the absorption onset, increasing the band gap from 2.02 to 2.46 eV. Whereas the photovoltaic properties of bromide rich compounds are limited due to a preferential orientation of the layered structure parallel to the substrate, which is detrimental to charge transport, solar cells with Rb<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> as absorber material display power conversion efficiencies of 1.37%. Moreover, the devices exhibit low hysteresis properties and are stable for more than 150 days stored under inert atmosphere.