Research progress of D-A-D-type hole transport materials in perovskite solar cells

　　Perovskite solar cells (PSCs) have attracted extensive attention due to their advantages of rapidly improving photoelectric conversion efficiency, low preparation cost, solution processing, etc.

　　Perovskite solar cells (PSCs) have attracted extensive attention due to their advantages of rapidly improving photoelectric conversion efficiency, low preparation cost, solution processing, etc. Hole Transport material (HTM) is responsible for hole extraction and preventing charge recombination, which can improve the efficiency and stability of PSCs and is an important component of PSCs. Organic small molecule hole transport materials with linear donor-receptor-donor (D-A-D) structure have simple structure and low synthesis difficulty. In addition, the introduction of an electron absorption unit can reduce the highest occupied molecular orbital (HOMO) energy level and improve the stability of the material, and the linear D-A-D configuration is conducive to enhancing the charge transfer within the molecule, improve the hole transport capability of materials. The application of linear D-A-D hole transport materials in PSCs since 2009 is reviewed. The influence of the molecular structure of each hole transport material on the photoelectric conversion efficiency and device stability of PSCs is introduced in detail. Finally, the future development of linear D-A-D hole transport materials is predicted.

　　As a way to effectively utilize solar energy, solar cells have developed very rapidly. Reducing costs and improving photoelectric Conversion efficiency (Power Conversion efficiency, PCE) to adapt to large-scale commercial manufacturing and applications are currently the focus of solar cell research. Solar cell technology can usually be divided into three categories: the first category is silicon-based solar cells, which have high PCE, but their synthesis conditions are relatively harsh and their manufacturing costs are relatively high; the second type is compound solar cells made of gallium arsenide, cadmium sulfide, cadmium telluride and other compounds. Because their synthetic materials are very scarce and toxic, it is difficult to obtain large-scale commercial applications; the third category is new solar cells that have developed rapidly in recent years, such as organic solar cells (OSCs), dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs), which have low cost, high PCE and mild preparation conditions. Compared with organic and dye-sensitized solar cells, PSCs appear the latest, mostly the plane structure of electron transport layer/perovskite light absorption layer/hole transport layer, and its PCE(3.8%~ 25.2%) has surpassed crystalline silicon cells and become the most promising solar cell.