A review on organic hole transport materials for perovskite solar cells: Structure, composition and reliability

Charge transport materials in heterojunction solar cells (e.g. perovskite solar cells (PSCs)) play critical roles in determining charge dynamics, photovoltaic performance and device stability. Currently, the conventional hole transport materials (HTMs), spiro-OMeTAD and PTAA, exhibit remarkable power conversion efficiencies in PSCs owing to high thin-film quality and matched energy alignment. However, they often show high material cost, low carrier mobility and poor stability, which greatly limit their practical applications. Tremendous efforts have been devoted to design of alternative low-cost HTMs and to engineer the doping composition. This review summarizes recent advances made in structural optimization and doping engineering of organic HTMs for efficient and stable PSCs. It begins with fundamental roles of HTMs in different device architectures, followed by the strategies to tune the charge dynamics through optimizing the molecular structures and properties. The working principles of the dopants and additives are discussed to provide a comprehensive understanding of compositional roles in device efficiency and stability. Different approaches in managing material structures and doping composition to improve the device reliability have been summarized in both regular and inverted PSCs. Moreover, mechanical stability and scalable deposition techniques are briefly discussed. Finally, we give our perspectives on the ways to further develop efficient and stable HTMs for PSCs.