Air-stable molecular semiconducting iodosalts for solar cell applications: Cs₂SnI₆ as a hole conductor

We introduce a new class of molecular iodosalt compounds for application in next-generation solar cells. Unlike tin-based perovskite compounds CsSnI₃ and CH₃NH₃SnI₃, which have Sn in the 2+ oxidation state and must be handled in an inert atmosphere when fabricating solar cells, the Sn in the molecular iodosalt compounds is in the 4+ oxidation state, making them stable in air and moisture. As an example, we demonstrate that, using Cs₂SnI₆ as a hole transporter, we can successfully fabricate in air a solid-state dye-sensitized solar cell (DSSC) with a mesoporous TiO₂ film. Doping Cs₂SnI₆ with additives helps to reduce the internal device resistance, improving cell efficiency. In this way, a Z907 DSSC delivers 4.7% of energy conversion efficiency. By using a more efficient mixture of porphyrin dyes, an efficiency near 8% with photon confinement has been achieved. This represents a significant step toward the realization of low-cost, stable, lead-free, and environmentally benign next-generation solid-state solar cells.