Solution processing of air-stable molecular semiconducting iodosalts, Cs₂SnI_{6-: X}Br_x, for potential solar cell applications

Organic-inorganic hybrid lead-based perovskite solar cells have already achieved high performance efficiency (∼20%). A major challenge with these cells, however, is their stability and environmental safety concerns due to the presence of lead. This article reports an effort to develop a new class of tin-based and air-stable molecular semiconducting iodosalts, for potential solar cell applications. The compounds are Cs₂SnI_6-xBr_x for a range of x that provides the desired bandgaps from ∼1.3 eV to ∼2.9 eV with x < 3 being suitable for solar cell design. The Sn in this compound is in the 4+ oxidation state, and thus it is much more stable with respect to oxidation and more resistant to hydrolysis during processing and device operation. The focus of this article is on the detailed synthesis challenges in the solution processing of these compounds. A two-step solution synthesis method has been developed whereby during Step-1 a well-defined CsI crystalline film was formed for the appropriate chemical reaction with a solution of SnI during Step-2. By careful tuning of each step of the process and with the use of detailed structural, electrical and optical characterization as feedback, a series of Cs₂SnI_6-xBr_x films were produced. The importance of having a stoichiometric compound for optimal performance of the material was revealed during solar cell fabrication. For the purpose of demonstrating a solar cell device assembly, a sandwich-type photochemical cell with an all solid state ionic conductor blended with inorganic materials and succinonitrile was employed for easy processing to obtain stable results. The cells show a conversion efficiency of 2.1% for the case of the x = 2 compound.