# Luminescence Materials for Dye-Sensitized Solar Cells: Advances and Directions
## 2 Fundamental Principles
### Photovoltaic Effect in DSSCs  
The photovoltaic effect in DSSCs is driven by the absorption of photons by the dye sensitizer and the subsequent injection of electrons into the TiO2. The excited electrons with a lifetime within a nanosecond range are injected into the conduction band of the TiO2 semiconductor, which lies below the excited state of the dye where the TiO2 absorbs a small fraction of the solar photons from the UV region [48]. This is accompanied by the electron transport through the external circuit to the counter electrode. Then, the dye undergoes regeneration, where the excited dye returns to the ground state, and, finally, the electrolyte undergoes regeneration, where the I\({}^{-}_{3}\) is reduced to the I\({}^{-}\). The schematic diagram in Figure 1b shows the representation of the assembled DSSCs, which encompasses the dye molecules adsorbed into the TiO2 and coated on the transparent conductive oxide glass, like the FTO. The dye molecules harvest photons and yield the excited electrons from its highest occupied molecular orbital (HOMO) in the ground state to the lowest unoccupied molecular orbital (LUMO) in the excited state. The electrolyte is for redox mediation and the platinum coated on the FTO to provide counter electrode. The processes involved in the working operation can be broken down into various key steps:  
1. **Light Absorption:** The dye molecules absorb incident photons, leading to their excitation: \[Dye+h\upsilon\ \rightarrow\ Dye^{*}\] The excited \(Dye^{*}\) has an electron promoted to a higher energy state.
2. **Electron Injection:** The excited electron is injected into the conduction band of TiO\({}_{2}\): \[Dye^{*}\to Dye^{+}+e^{-}\left(\textsc{TiO}_{2}\right)\] This leaves the dye in an oxidized state (\(Dye^{+}\)).
3. **Electron Transport:** The injected electrons diffuse through the TiO\({}_{2}\) nanoparticles to the fluorine-doped tin oxide (FTO) layer and flow through an external circuit, generating electrical power: \[e^{-}\left(\textsc{TiO}_{2}\right)\to FTO\ \rightarrow\ External\ Circuit\ \rightarrow\ Counter\ Electrode\] Electrons travel through the TiO\({}_{2}\), move to the FTO, flow through the external circuit, and finally reach the counter electrode.
4. **Dye Regeneration:** The oxidized dye is reduced back to its ground state by the redox mediator in the electrolyte: \[Dye^{+}+3I^{-}\ \rightarrow\ Dye+\ I_{3}^{-}\]
5. Electrolyte Regeneration: The tri-iodide (I\({}_{3}^{-}\)) formed is reduced back to iodide ions (I\({}^{-}\)) through gaining electrons at the counter electrode: \[I_{3}^{-}+2e^{-}_{(Counter\ Electrode)}\ \rightarrow 3I^{-}\]  
This completes the cycle, allowing continuous operation of the cell. These steps collectively result in the generation of electrical power, with the efficiency of the DSSC being influenced by factors such as the light absorption properties of the dye, the electron transport efficiency in the TiO\({}_{2}\) and the regeneration kinetics of the redox mediator [47].  
Figure 1: (a) Working operation of a DSSC and (b) DSSC schematic diagram of the layers.