**2.2 Dye/metal oxide interface**

A monolayer dye molecules attached to the surace of the nanocrystalline film. The high surface area of nanocrystalline film ensured s a large concentration of the light absorber leading to good light haversting are critical to be efficient DSSCs. In addition, how and where the photosensitizer bound to the surface of TiO2 is another important issues since unexpected bonding between photosensitizer and the TiO2 surface lead to degrade electron injection performance. In the case of Ru complex, one carboxylic ligand provides good anchoring to metal oxide surface (Long-term stability against moisture condition can be improved by using phosphonic acid) as well as good electronic coupling of the excited states of photosensitizer with TiO2 conduction band states [11].

To be the effective electron transfer, the lowest unoccupied molecular orbital (LUMO) level of excited dye molecules is appropriately lower than the highest unoccupied molecular orbital (HUMO) level of the metal oxide. Under illumination, the photoxcited sensitizers are injected into the metal oxide and diffused from the dye/metal oxide interface to substrate. In an efficient DSSC, a time scale of femtoseconds to picoseconds nanoseconds of injection process is witnessed on the strong electronic coupling between photoexicted sensitizer and metal oxide. The injected electrons experience fast recombination process and thermalisation down to at the electron Fermi level of the electrode [12].
