Stage #1

The incident light (photon) is absorbed via a photosensitizer, and therefore, due to the photon absorption, electrons get promoted from the ground state (S+/S) to the excited state (S+/S*) of the dye, where the absorption for most of the dye is in the domain of 700 nm which corresponds to the photon energy almost about 1.72 eV.

Stage #2

The excited electrons with a lifetime of nanosecond span are injected into the conduction band of nanoporous TiO2 electrode which lies below the excited state of the dye, where the TiO2 absorbs a small fraction of the solar photons from the UV region [43] . Consequently, the dye gets oxidized.

S + / S + h ν S + / S (1)

S + / S S + / S + e ( TiO 2 ) (2)

Stage #3

Such injected electrons are transported among TiO2 nanoparticles and diffuse towards the back contact (transparent conducting oxide [TCO]). Via the external circuit, electrons attain the CE.

Stage #4

The electrons at the CE reduce I 3 to I; therefore, dye regeneration or the regeneration of the ground state of the dye occurs thanks to the acceptance of electrons from I ion redox mediator, and I gets oxidized to I 3 (oxidized state).

S + / S + e S + / S (3)

Stage #5

Plus, the oxidized mediator ( I 3 ) diffuses towards the CE and reduces to I ion.

I 3 + 2e 3I (4)