Low-Cost, High-Performance Organic Small Molecular Hole-Transporting Materials for Perovskite Solar Cells

Perovskite solar cells (PSCs) have become the focus of interest among next-generation photovoltaic technologies attributed to their outstanding power conversion efficiency (PCE) (the highest certified PCE of 25.2% being achieved to date), low cost and fabrication feasibility. Perovskite itself is hole-conductive, albeit with a low efficiency. The use of hole transporting material (HTM) remains indispensable for the efficient charge extraction in high-performance PSCs. The recent design and development of low-cost organic small molecules as HTMs in high-performance PSCs with a PCE over 19% are summarized. These HTMs are categorized into materials with spiro core structures, thiophene derivatives, and others, on the basis of their structural features. The relationship between molecular structure and device performance is discussed from the perspective of synthetic strategy and chemical modification. Finally, an outlook is given on the future development of small molecular HTMs.