Advances in anchoring-based self-assembly hole-transporting materials for perovskite solar cells

Inverted perovskite solar cells (PSCs), owing to their simple preparation process, excellent performance, and the advantage of being able to prepare tandem devices with other photovoltaic cells, have gained significant attention as a next-generation photovoltaic technology. In recent years, the development and application of anchored self-assembled hole-transporting materials, also known as self-assembled monolayers (SAMs), have emerged as a focal point of research in inverted PSCs. Depositing such materials on transparent conducting oxide electrodes (TCO) can form a uniform thin layer, which minimizes charge transport loss and optical loss, thereby simultaneously improving the open-circuit voltage, fill factor, and short-circuit current of perovskite solar cells, significantly enhancing the power conversion efficiency. Beyond their electrical advantages, anchored self-assembled hole-transporting materials possess unique molecular self-assembly capabilities that allow for precise regulation of interfacial energetics and morphology. By tuning the alignment of energy levels between the hole-transporting layers (HTLs) and the perovskite layer, suppressing charge recombination, and improving the uniformity and stability of the interface, these materials contribute to both efficiency and durability improvements. Starting from the terminal groups, spacer groups, and anchoring groups of the anchored assembled hole-transporting molecules, this paper focuses on the structure-performance relationship and reviews the research progress and challenges of anchored assembled hole-transporting materials in inverted perovskite solar cells. The terminal group, typically consisting of carbazole, triphenylamine, or phenothiazine derivatives, plays a critical role in energy level alignment, hole extraction efficiency, and interfacial passivation. π-conjugated and functionalized terminal units have been shown to enhance the wettability, dipole moment, and film-forming properties of anchored assembled hole-transporting materials, contributing to improved device stability and reduced non-radiative recombination. Spacer units, including alkyl chains and aromatic bridges, modulate the molecular conformation, interfacial coverage, and charge tunneling dynamics. Notably, the transition from flexible alkyl chains to rigid conjugated linkers enhances molecular packing, dipole orientation, and operational stability. Anchoring groups such as phosphonic acid, carboxylic acid, sulfonic acid, and boric acid govern the chemical binding to metal oxide substrates like ITO and NiOx, impacting film robustness and interfacial stability. This review highlights the critical interplay between molecular design and device-level performance, providing comparative analyses of key anchored assembled hole-transporting molecules, and aims to offer valuable insights into the key design principles. Future research should address scalability, long-term stability, and the elucidation of dynamic self-assembly mechanisms through in situ and non-destructive characterization techniques. Such advancements are expected to accelerate the rational design of next-generation interfacial materials for high-efficiency, stable, and commercially viable perovskite solar technologies.