A review on triphenylamine (TPA) based organic hole transport materials (HTMs) for dye sensitized solar cells (DSSCs) and perovskite solar cells (PSCs): evolution and molecular engineering

In this review article, the important features (optical, thermal and electrochemical) of triphenylamine (TPA) based organic hole transport materials (HTMs) in accordance with their structural diversities are discussed from their evolution to recent advancements. The literature here covers past and ongoing work mostly relevant to HTMs used in DSSCs and PSCs. Besides the good optical properties and high hole mobility, the stability of the amorphous state of the HTM layer is a crucial factor in the commercialization of PSCs. The stability of the amorphous glassy state of HTMs is defined by an important physical parameter, i.e., the glass transition temperature (T-g) which is discussed on the basis of the molecular structure of HTMs.