Interfacial modification, which serves multiple roles, is vital for the fabrication of efficient and stable perovskite solar cells. Here, a multifunctional interfacial material, biguanide hydrochloride (BGCl), is introduced between tin oxide (SnO2) and perovskite to enhance electron extraction, as well as the crystal growth of the perovskite. The BGCl can chemically link to the SnO2 through Lewis coordination/electrostatic coupling and help to anchor the PbI2. Better energetic alignment, reduced interfacial defects, and homogeneous perovskite crystallites are achieved, yielding an impressive certified power conversion efficiency (PCE) of 24.4%, with an open‐circuit voltage of 1.19 V and a drastically improved fill factor of 82.4%. More importantly, the unencapsulated device maintains 95% of its initial PCE after aging for over 500 h at 20 °C and 30% relative humidity in ambient conditions. These results suggest that the incorporation of BGCl is a promising strategy to modify the interface and control the crystallization of the perovskite, toward the attainment of highly efficient and stable perovskite solar cells as well as other perovskite‐based electronics.

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Morphology optimization of active layer plays a critical role in improving the performance of organic solar cells (OSCs). In this work, a volatile solid additive‐assisted sequential deposition (SD) strategy is reported to regulate the molecular order and phase separation in solid state. The OSC adopts polymer donor D18‐Cl and acceptor N3 as active layer, as well as 1,4‐diiodobenzene (DIB) as volatile additive. Compared to the D18‐Cl:N3 (one‐time deposition of mixture) and D18‐Cl/N3 (SD) platforms, the D18‐Cl/N3(DIB) device based on DIB‐assisted SD method exhibits a finer phase separation with greatly enhanced molecular crystallinity. The optimal morphology delivers superior charge transport and extraction, offering a champion power conversion efficiency of 18.42% with significantly enhanced short‐circuit current density (Jsc) of 27.18 mA cm−2 and fill factor of 78.8%. This is one of the best performances in binary SD OSCs to date. Angle‐dependent grazing‐incidence wide‐angle X‐ray scattering technique effectively reveals the vertical phase separation and molecular crystallinity of the active layer. This work demonstrates the combination of volatile solid additive and sequential deposition is an effective method to develop high‐performance OSCs.

2,2′,7,7′-Tetrakis­(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), as an organic small molecule material, is the most commonly employed hole transport material (HTM) in perovskite solar cells (PSCs) because of its excellent properties that result in high photovoltaic performances. However, the material still suffers from low conductivity, leading to the necessary use of dopants and oxidative processes to overcome this issue. The spiro-OMeTAD oxidation process is highlighted in this review, and the main parameters involved in the process have been studied. Furthermore, the best alternatives aiming to improve the spiro-OMeTAD electrical properties have been discussed. Lastly, this review concludes with suggestions and outlooks for further research directions.