The photosensitizers absorb photons and have a wide spectral absorption range, which is an important part of the dye-sensitized solar cells (DSSCs) and directly affects the energy conversion efficiency of the cell. DSSCs combine the advantages of photosensitizers and inorganic semiconductors, have a wide spectral response range, simple manufacturing process, low cost, environmental friendliness, and broad application prospects, so they have attracted people's attention.

Requirements for Photosensitizer

A photosensitizer is considered efficient for DSSCs when it fulfills these requirements:

a) The absorption spectrum of the photosensitizers should cover the whole visible region and even the part of the near-infrared.

b) The photosensitizers should have anchoring groups (-COOH, -H₂PO₃, -SO₃H, etc.) to strongly bind the dye onto the semiconductor surface.

c) For n-type DSSCs, the excited state level of the photosensitizer should be higher in energy than the conduction band edge of n-type semiconductor, so that an efficient electron transfer process between the excited dye and conduction band of the semiconductor can take place. In contrast, for p-type DSSCs, the HOMO level of the photosensitizers should be at more positive potential than the valene band level of p-type semiconductor.

d) For dye regeneration, the oxidized state level of the photosensitizer must be more positive than the redox potential of electrolyte.

e) The photosensitizers should be photostable, electrochemical and thermal stability are also required.

Mechanism

In generally, an exciton (e-h pair) is produced after photon absorption by the dye molecule, which moves toward the conduction band (CB) of TiO₂ and the conductive substrate. Then, an electron is transferred through the external load to the counter electrode, reduces the electrolyte (I⁻/I³⁻), and regenerates the dye.