Tetraiodide-Dianion-Bridged3D Perovskite with EnhancedConductivity via Interlayer Iodine Integration

The formation of 3D cubic-phase perovskites is typically constrained by the size of the cation, leading to the emergence of 2D quantum well structures with large organic cation spacers. To overcome this limitation, we propose a novel method that uses molecular I₂ to link the interlayers of 2D perovskites, forming the I₄^2– anion. A 3D perovskite, (4ATHP)₂PbI₄·I₂ (4ATHP: 4-aminotetrahydropyran cation), was synthesized by controlling the usage of H₃PO₂. This compound features interlayers of inorganic components connected by molecular iodine, resulting in a 3D structure. With the incorporation of molecular iodine, the bandgap was significantly decreased to 1.76 eV compared to its analogous 2D perovskite (CHA)₂PbI₄ (CHA: cyclohexanaminium) with a bandgap of 2.29 eV. The conductivity increased by 3 orders of magnitude along the out-of-plane direction compared to (CHA)₂PbI₄, due to defect formation driven by the release of molecular iodine, which acted as a charge carrier. Transient photoconductivity increases while steady-state photoconductivity decreases over time. Additionally, the compound exhibits an extrinsic broadband emission. These findings demonstrate the direct impact of iodide ion oxidation into iodine on the photoelectrical properties of perovskite solar cells, specifically increasing the conductivity while decreasing the photoconductivity. Furthermore, this compound serves as a valuable model to explore the role of molecular iodine in perovskite solar cells.