Design of small molecular hole-transporting materials for stable and high-performance perovskite solar cells

Perovskite solar cells represent one of the most exciting developments in photovoltaics in the past decade, with the power conversion efficiencies of over 25% being achieved to date. In high-performance perovskite solar cells, hole-transporting materials are generally employed to extract and transport holes from perovskite. Among them, small molecular hole transporting materials have attracted intense interest due to their tunable energy levels, structural variety, and simple synthesis. The commonly used hole-transporting material is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl amino)-9,9′-spirobifluorene (spiro-OMeTAD). Considering the high synthetic cost of spiro-OMeTAD and the device stability issue associated with the use of dopants, much research has been focused on the development of alternative high-performance hole-transporting materials. Herein, this review summarizes the recent developments in highly efficient small molecular hole-transporting materials with a power conversion efficiency close to or over 20%. On the basis of their structural features, three categories of small molecules are identified and discussed as highly efficient hole-transporting materials: spiro molecules with new terminal groups or a new spiro skeleton, star-shaped small molecular hole-transporting materials with three or four branches, and linear hole-transporting materials with a D-A, D-π-D, D-A-D, or D-A-π-A-D structure. The relationships of the optoelectronic properties of these hole-transporting materials and the device performance are discussed, with a comparison to those of model compounds in some cases. Finally, an outlook is addressed on the future development of hole-transporting materials for high-performance perovskite solar cells. We hope that this review can provide important guidance for the design and synthesis of new hole-transporting materials and finally help to promote the commercialization of perovskite solar cells.