1. Dye Sensitised Solar Cells
Saurav Chandra Sarma

2. Outline Solar cells and their importance.
Dye Sensitized Solar Cell(DSSC) and its composition.
Working of DSSC
Conclusion

3. What is a Solar Cell?
A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity 
Generates an electric current without being attached to any voltage source
The supply of energy from the Sun to the earth is gigantic.It is about 3x1024 J/yr

4. Importance of Solar Cells
It exploits a renewable sources of energy
It is environmental friendly
Solar cells can be used in remote areas where it is too expensive to extend the electricity power grid.
Solar cells last a longer time and have low running costs
One research study suggests that covering just 1% of the world's deserts with
solar panel arrays could provide one fifth of the world's electricity needs!

6. Different types of Solar Cells
Buried contact solar cell
Cadmium telluride solar cell
Copper indium gallium selenide solar cells
Dye-sensitized solar cell
Gallium arsenide germanium solar cell
Hybrid solar cell

7.  Glass-based DSSC Module
Flexible DSSC Module 

8. Michael Gratzel: Father of DSSC
Born
11 May 1944 (age 69) Dorfchemnitz, Sachsen
Residence
Switzerland
Nationality
Swiss
Fields
photochemistry
Institutions
École Polytechnique Fédérale de Lausanne
Known for
Dye-sensitized solar cells
Achievements:
Author of over 900 publications, two books and inventor or co-inventor of over 50 patents
On 9 June 2010, Grätzel received Millennium Technology Prize, for development of dye-sensitized solar cells.

9. What are the constituents of DSSC?
The material of choice has been TiO2 (anatase), although alternative wide-band-gap oxides such as ZnO and Nb2O5 have also been investigated.
Nanoparticles of the oxide are deposited, for example, by screen printing onto a glass or flexible plastic support.
The surface is then coated with layers of sensitizer.

10. Mechanism of DSSC The main processes that occur in a DSSC
The incident photon is absorbed by Ru complex photosensitizers adsorbed on the TiO2 surface.
2. The photosensitizers are excited from the ground state (S) to the excited state (S∗). The excited electrons are injected into the conduction band of the TiO2 electrode. This results in the oxidation of the photosensitizer (S+).
S + hν → S∗ 
S∗ → S+ + e− (TiO2)
3. The injected electrons in the conduction band of TiO2 are transported between TiO2 nanoparticles with diffusion toward the back contact (TCO). And the electrons finally reach the counter electrode through the circuit.
4. The oxidized photosensitizer (S+) accepts electrons from the I− ion redox mediator leading to regeneration of the ground state (S), and the I− is oxidized to the oxidized state, I3−.
S+ + e− → S
5. The oxidized redox mediator, I3−, diffuses toward the counter electrode and then it is reduced to I− ions.
I3− + 2 e− → 3 I− 

11. Oxidized photosensitizer accepts electrons from the I−
Excited electrons are injected into the conduction band of TiO2
The oxidized redox mediator, I3−, diffuses toward the counter electrode
Electrons are excited from ground sate to the excited state
The main processes that occur in a DSSC
Step 1:The following primary steps convert photons to current:
1. The incident photon is absorbed by Ru complex photosensitizers adsorbed on the TiO2 surface.
2. The photosensitizers are excited from the ground state (S) to the excited state (S∗). The excited electrons are injected into the conduction band of the TiO2 electrode. This results in the oxidation of the photosensitizer (S+).
S + hν → S∗ (1)S∗ → S+ + e− (TiO2) (2)3. The injected electrons in the conduction band of TiO2 are transported between TiO2 nanoparticles with diffusion toward the back contact (TCO). And the electrons finally reach the counter electrode through the circuit.
4. The oxidized photosensitizer (S+) accepts electrons from the I− ion redox mediator leading to regeneration of the ground state (S), and the I− is oxidized to the oxidized state, I3−.
S+ + e− → S (3)5. The oxidized redox mediator, I3−, diffuses toward the counter electrode and then it is reduced to I− ions.
I3− + 2 e− → 3 I− (4)The efficiency of a DSSC is depends on four energy levels of the component: the excited state (approximately LUMO) and the ground state (HOMO) of the photosensitizer, the Fermi level of the TiO2 electrode and the redox potential of the mediator (I−/I3−) in the electrolyte
Incident photon is absorbed by Ru complex

12. Dynamics of Electron Injection
The dyes should incorporate functional group such as , for e.g, carboxylate, hydroxymate, or phosphate moieties that anchor the sensitizer to the oxide surface.
Metal to Ligand Charge Transfer(MLCT) occurs which facilitates the rapid electron injection from the ligand to the semiconductor.

13. Proof of MLCT transition
Absorption spectrum of N719 dye(sensitizer) shows the transfer of electron from Ru to Ligands before donation to the conduction band of TiO2

14. The most widely used sensitizer for the DSC has been cis Ru(SCN)2L2(L)2,2′-bipyridyl-4,4′-dicarboxylate), abbreviated as N3

15. Some of the Ruthenium Sensitizers
RuL3(yellow)
cis-RuL2(NCS)2(red)
RuL′(NCS)3(green)

16. DSSC Performance
Conversion of light to elecric current by mesoscopic solar cells sensitized with the ruthenium dye N-719. The IPCE is plotted as a function of the excitation wavelength.
IPCE: Incident Photon to Current conversion Efficiency
The IPCE values exceed 80% in the wavelength range near the absorption maximum of the sensitizer,which is located around 530 nm

17. Lets look at an animation to visualise the process better

19. Conclusion
The transport of the electroactive ions is expected to play a significant role in determining DSSC efficiency
The search for suitable solid materials that can replace the liquid electrolyte is an additional interesting and active area of research.
Research on dye sensitizers are mainly focused on transition metal complexes, but a considerable of work is now directed towards the optimization of organic sensitizers and of natural sensitizers extracted from fruits.

20. References
Michael Gratzel, Inorganic Chemistry, Vol. 44, No. 20,
Gratzel, M. Nature 2001, 414, 338.

21. Thank You