Recent advances in metal-free photosensitizers for dye-sensitized photoelectrochemical cells

Dye-sensitized photoelectrochemical cells (DSPECs), which consist an effective approach to achieve total water splitting, have attracted a lot of attention in recent years. Nowadays, most of the photosensitizers used in DSPECs are noble metal complexes, especially polypyridine‑ruthenium complexes. To avoid the use of noble metal ions, metal-free dyes are emerging as promising candidates for the construction of low-cost and environmentally friendly DSPECs, which have been greatly improved in recent years. The metal-free dyes, including perylene dyes, porphyrin/subporphyrin dyes, triphenylamine dyes, and other dyes, have been utilized in photoanode-based, photocathode-based and tandem DSPECs. This review aims at describing the current situation of metal-free dyes used in DSPECs and the relationships between dye structure and device efficiency, and then highlighting the essential role of the molecular design in dyes for the enhancement of durability and efficiency. Finally, the main challenges and their countermeasures are presented and some future opportunities are suggested.