Intrinsic and Extrinsic Determinants of Stability in Spiro-OMeTAD-Based Hole-Transporting Layers in Perovskite Solar Cells: Mechanistic Insights and Strategic Perspectives

Spiro-OMeTAD has remained the benchmark hole-transporting material (HTM) in state-of-the-art perovskite solar cells, owing to its favorable energy level alignment and excellent interfacial compatibility. However, its practical implementation is critically hindered by the intrinsic instabilities introduced by conventional dopants such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP). While these dopants enhance electrical conductivity, they concurrently initiate multiple degradation pathways—including ionic migration, radical deactivation, and moisture/thermal-induced morphological failure—thereby compromising device longevity and reproducibility. This review presents a comprehensive and mechanistic perspective on dopant-induced instabilities in spiro-OMeTAD-based hole-transporting layers, systematically unraveling the physicochemical origins of performance loss under operational stress. Recent advances in dopant design, additive engineering, and post-oxidation-independence doping strategies that aim to circumvent the trade-offs inherent to traditional systems are further highlighted. Emphasis is placed on the interdependence among dopant formulation, charge transport kinetics, and environmental resilience. By integrating insights from advanced characterization and molecular-level design, rational guidelines toward the development of next-generation dopant systems and HTM architectures that reconcile high efficiency with long-term operational stability are proposed. This review offers a forward-looking framework to steer the evolution of robust and commercially viable perovskite photovoltaics.