Tailoring Lead Iodide Crystals Through Organic Templating and Inorganic Heterostructuring for Optoelectronics

Metal-halide perovskites are leading candidates for next-generation solar cells and light-emitting diodes. Because the absorber or emitter forms by reaction of an inorganic scaffold with a cation salt, the crystallographic quality of the lead iodide (PbI2) precursor strongly influences device performance: small, disordered crystallites propagate grain boundaries, trap states and incomplete conversion. This article examines the structural chemistry linking PbI2 crystal engineering to optoelectronic outcomes. We contrast the electronic structure of halide and oxide perovskites and discuss how the A-site cation, rationalised through the Goldschmidt tolerance factor, determines whether three-dimensional or moisture-resistant two-dimensional phases form. Attention is given to template-assisted strategies, notably lead-fumarate and related metal–organic frameworks (MOFs) used as sacrificial scaffolds or interfacial buffers that promote large, oriented PbI2 crystallites and suppress halide-ion migration. We distinguish which degradation pathways such engineering can and cannot mitigate and assess its transferability to lead-free halides. We then present an original case study in which pressed PbI2/SnO2 composite plates (5 and 15 wt% SnO2), prepared by aqueous crystallisation, serve as a macroscopic model of heterojunction-mediated charge transport. Powder X-ray diffraction confirms that the PbI2 host retains its hexagonal structure in both composites. Under dark, daylight and 3–100 W UV (365 nm) illumination, the 15 wt% composite delivers roughly two orders of magnitude higher photocurrent than pristine PbI2, consistent with Type-II band-offset-driven electron extraction into a connected SnO2 network, while the diminishing gain per added wt% points to an optimum filler loading. These results provide device-independent support for the interfacial charge-separation principles discussed throughout.