An overview of metal-free diazine-based dyes for dye-sensitized solar cells: Synthesis, optical, and photovoltaic properties

The constant rise in global energy usage has depleted fossil fuel reserves. Therefore, researchers explored solar energy as an alternative energy reserve due to the need for limitless power. In particular, dye-sensitized solar cells (DSSCs) hold great promise in meeting the growing demand for renewable energy supplies due to a low-cost and environmentally friendly photovoltaic technology for harnessing solar energy. The sensitizers' molecular engineering is essential for accomplishing high conversion efficiencies. Numerous research activities have been concentrated on diazine scaffolds (substituted diazines, benzodiazines, and fused heterocyclic diazines) among the many different types of sensitizers because of their strong absorption bands in the visible light region, electron-withdrawing ability, and the ease of modifications that can be made to their skeleton. This review classified the diazines according to their scaffolds (pyridazine, pyrimidine, pyrazine). The improvements to the performance of metal-free DSSCs according to the molecular engineering of the sensitizer are discussed. The effect of the donors, auxiliary donors, auxiliary acceptors, mono or di-anchoring groups, and incorporation of π-spacers or alkyl chains in the sensitizer molecule on the photovoltaic performance are discussed. The synthetic approaches, optical properties in solution, and their photovoltaic performances in DSSCs were also summarized. The present work aims to give readers a distinct picture of the subject and enlighten the researchers by developing future ideas about designing sensitizers employing other heterocyclic scaffolds with electron-deficient properties to achieve an excellent solar energy conversion.