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Metal-halide perovskites are optoelectronic materials applied to solar cells as a light absorber due to their excellent optoelectronic properties. The power conversion efficiency of perovskite solar cells (PSCs) reaches 25.7% certified, which stands in comparison with Si solar cells. Importantly, compositional engineering of perovskites has been one of the keys to the breakthrough. However, the presence of defects within perovskites is a matter of importance as it can cause nonradiative recombination of charge carriers. In addition, defect migration can degrade the photovoltaic performance and stability of PSCs. Previous studies have commonly addressed that iodide-related defects such as interstitial iodide and iodide vacancy are problematic due to their low formation energy. Thus, halide engineering is imperative to mitigate the defect-related dynamics and improve the materials quality of perovskites. In this sense, superhalogen is a promising candidate for defect passivation and stabilization of perovskites based on its higher electronegativity and electron affinity than halides, which are beneficial to the formation of a more robust interaction with adjacent elements in perovskites. This perspective gives an overview of studies regarding the use of superhalogen to develop efficient and stable PSCs and concludes with an outlook of further research directions.

Three benzodipyrrole (BDP)-based organic small molecules with substituted 4-methoxyphenyl (CB-1), 3-fluorophenyl (CB-2), and 3-trifluoromethylphenyl (CB-3) are designed, synthesized, and used as a hole-transporting material (HTM) for perovskite solar cells (PSCs). The electrochemical, optical, thermal, electronic, and optoelectronic properties of the HTMs are characterized to verify their suitability for PSCs. The terminal functional groups of the HTMs having different heteroatoms mainly target effective defect passivation of perovskites. Photoluminescence studies and molecular dynamic simulations reveal that fluorine atoms within CB-2 and CB-3 can contribute to the defect passivation via interaction with Pb of the perovskite. In particular, a highly planar conformation of CB-2 on the perovskite surface can facilitate more efficient hole transfer at the interface. Thus, the PSCs employing CB-2 achieve the highest power conversion efficiency (PCE) of 18.23% while the devices using CB-1 and CB-3 exhibit a lower PCE of 16.78% and 16.74%, respectively. PSCs with the BDP-based HTMs demonstrate excellent long-term storage stability without degradation in their PCEs over 6 months. The highly planar geometry, defect passivation effect, and hydrophobicity of CB-2 show its great potential as an HTM for efficient and stable PSCs.