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 Sn-119 NMR measurements we show that hydrochloride containing additives undergo halide exchange with SnI4 to form SnIxCly, 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 FASnI(3) is not capable of reducing SnI4 or forming SnIxCly, 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.