Rationally designed hole transporting layer system for efficient and stable perovskite solar cells

In this review, p-type doping technologies for organic/polymeric semiconductors in hole transporting layer (HTL) for perovskite solar cells (PSCs) are examined. Initially, we investigate the conventional dopant systems used for HTL in terms of dopants and additives, as well as their doping principles and limitations. Second, we recapitulate the current research strategies for overcoming the limitations of conventional dopant systems: (i) dopants/additives with large cations, (ii) hydrophobic dopants/additives, (iii) locking-capable dopants/additives, (iv) rinsing or ion exchange methods, and (v) other methods. Afterwards, we provide a comprehensive analysis of alternative dopants based on ionic liquids, Lewis acids, strong acceptors, and others. In addition, the review emphasizes current achievements based on multiple research approaches to diverse hole transporting materials, surface/interfacial treatment, and architectural modification of HTL, and provides a perspective on developments of desirable HTL system for efficient and stable PSCs.