From Part to Whole: Advances against Perovskite Phase Segregation in Subcells for Stable Tandem Photovoltaics

Wide-bandgap (WBG) lead halide perovskite materials are indispensable top-cell materials for tandem solar cells (TSCs), whose high power-conversion potential, tunable bandgaps, and solution processability dictate the efficiency ceiling and commercialization prospects. However, mixed-halide WBG perovskites are plagued by light/thermal-induced phase segregation, inducing rapid photovoltaic performance degradation and undermining long-term operational stability. This review summarizes state-of-the-art strategies for WBG perovskite device optimization and phase instability mitigation. Then, detailed clarification of the pivotal roles of WBG absorbers in spectral splitting and bandgap matching is provided, and key materials and interface engineering tactics including solvent engineering, compositional modulation, multifunctional additives, interfacial modification, cutting-edge fabrication approaches, etc., are elaborated The utility of in situ grazing incidence small-angle X-ray scattering (GIWAXS) and photoluminescence techniques to track phase-segregation dynamics and unravel the underlying thermodynamic and ion-migration mechanisms is highlighted. Key remaining challenges involve large-area crystallization nonuniformity, unbalanced cost-process compatibility-stability for phase suppression, and unclear real-world multifield degradation mechanisms, alongside forward-looking research directions encompassing scalable green manufacturing and machine learning-enabled rational material discovery. Prospects for breakthroughs in WBG perovskite stability for further commercial deployment of TSCs are also discussed.