Modulating Crystallization Dynamics and Disentangling Degradation Pathways in Pure-Iodide Wide-Bandgap Perovskite Solar Cells

Wide-bandgap perovskite solar cells (WBG-PSCs) function as top cells in tandem architectures to overcome the Shockley-Queisser (S-Q) limit of single-junction devices. However, traditional mixed-halide WBG-PSCs suffer from photoinduced halide segregation. Developing pure-iodide WBG-PSCs offers a promising alternative strategy. Experimental results reveal that using lead acetate (Pb(Ac)₂) induces rapid crystallization, yielding poor-quality crystals with high defect density. Replacing 15% of Pb(Ac)₂ with lead chloride (PbCl₂) slows the crystallization kinetics, significantly improving crystal quality, reducing defects, and enhancing device efficiency to 20.40%. The pure-iodide films maintain phase stability under illumination or humidity. Although thermal aging modifies crystal quality due to secondary crystallization, device efficiency declines. This loss is attributed to degradation at the nickel oxide (NiO_X) buried interface and the phenethylammonium iodide (PEAI) passivation layer under heat. These findings establish that interfacial degradation, not perovskite bulk decomposition, limits the thermal stability of pure-iodide WBG-PSCs.