Dye Sensitised Solar Cells Saurav Chandra Sarma

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

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

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!

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

 Glass-based DSSC Module Flexible DSSC Module 

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.

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.

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− 

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

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.

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

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

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

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

Lets look at an animation to visualise the process better

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.

References Michael Gratzel, Inorganic Chemistry, Vol. 44, No. 20, 2005 6849 Gratzel, M. Nature 2001, 414, 338.

Thank You