In dye-sensitized solar cells (DSSCs), dye sensitizers function to capture solar photons. Whether the dye sensitizer can absorb sunlight to the maximum extent directly determines the maximum energy conversion efficiency that the photovoltaic cell can obtain. An ideal dye sensitizer should have high light-harvesting ability, broad absorption spectrum, good photostability and long excited state lifetime. In order to obtain a high-efficiency dye sensitizer, the molecular structure design of the dye should follow the following principles: (1) the absorption spectrum of the dye should cover the ultraviolet-visible region and the near-infrared region; (2) the lowest unoccupied orbital energy level (LUMO) of the dye cannot be lower than the conduction band energy level of TiO2 to facilitate electron injection; (3) the oxidation state and excited state stability of the dye molecule must be high; (4) the dye molecule should have polar groups such as carboxyl and hydroxyl groups, which can be tightly attached to TiO2. Dye sensitizers can be divided into two categories: metal complexes and metal-free organic sensitizers. For metal complexes, the common ones are ruthenium (Ru) complexes and porphyrin zinc (Zn) complexes. For metal-free organic compounds, triphenylamines, indoles and coumarins are common. Among them, ruthenium (Ru) complex dye sensitizers have good thermal stability, chemical stability and photoelectric conversion efficiency, and are currently one of the most widely used dye sensitizers.
Figure 1. Molecular structures of some dye sensitizers