How additives for tin halide perovskites influence the Sn⁴⁺ concentration

Tin halide perovskite (Sn-HPs) photovoltaics could potentially equal or exceed the performance of their more toxic Pb-based analogues if defect state densities, particularly originating from the presence of Sn⁴⁺, can be significantly decreased. Numerous additives are incorporated into Sn-HPs to minimize the amount of Sn⁴⁺, including SnF₂, reducing agents such as hydrazine derivatives, and various antioxidants. Despite the frequent use of additives to reduce Sn⁴⁺ content, there is limited understanding of how they function and consequently limited guidance for the development of new additives. Herein, we use cyclic voltammetry to probe the redox behavior of SnI₂, SnI₄, Sn-HP precursor solutions, and 18 different additives. Through ¹¹⁹Sn NMR measurements we show that hydrochloride containing additives undergo halide exchange with SnI₄ to form SnI_xCl_y, which results in decreased Sn⁴⁺ concentrations and less p-type character in the Sn-HP films. We find that the most effective additive at lowering the Sn⁴⁺ content in FASnI₃ is not capable of reducing SnI₄ or forming SnI_xCl_y, but rather it acts as a sacrificial and coordinating antioxidant. In general, when selecting additives for Sn-HPs it is important to account for the redox potential, coordination with Sn species, ability to react with oxygen, and the potential for halide exchange.