Advancements in organic small molecule hole-transporting materials for perovskite solar cells: past and future

This review article discusses the current designs and synthetic procedures for organic small molecules as hole-transporting materials (HTMs) with a focus on their structure-property correlation, conductivity, and photovoltaic performance, as well as their high hole mobility and stability. In comparison to non-planar spiro-like compounds, various pi-conjugated aromatic and planar molecules have been studied as being important for the generation of new HTMs. Since heteroatoms, such as oxygen, sulfur, nitrogen and silica, have been shown to have an impact on the search for more stable and cost-effective HTMs and perovskite solar cells (PSCs), developing a new molecular architecture with efficient pi-pi stacking to increase charge mobility or integrating dopant molecular structure into HTM would be a viable approach for generating dopant-free HTMs. A deeper understanding of perovskite/HTM can also provide insight into the design of novel molecular architectures capable of achieving effective and stable systems.