Photocatalytic activity of TiO2: Dye Sensitised Solar Cells

Dye Sensitised Solar CellsSaurav Chandra Sarma

OutlineSolar cells and their importance.

Dye Sensitized Solar Cell(DSSC) and its composition.

Working of DSSC

Conclusion

What is a Solar Cell?The supply of energy from the Sun to the earth is gigantic.It is about 3x1024 J/yr

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!Importance of Solar Cells

Different types of Solar Cells

Flexible DSSC Module Glass-based DSSC ModuleBorn11 May 1944(age69)Dorfchemnitz,SachsenResidenceSwitzerlandNationalitySwissFieldsphotochemistryInstitutionscole Polytechnique Fdrale de LausanneKnownforDye-sensitized solar cells

Michael Gratzel: Father of DSSCThe 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.

What are the constituents of DSSC?The main processes that occur in a DSSC

The incident photon is absorbed by Ru complex photosensitizers adsorbed on the TiO2surface.

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 TiO2electrode. This results in the oxidation of the photosensitizer (S+).S + h SS S++ e(TiO2)

3. The injected electrons in the conduction band of TiO2are transported between TiO2nanoparticles 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 Iion redox mediator leading to regeneration of the ground state (S), and the Iis 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 Iions.I3+ 2 e 3 I

Mechanism of DSSC

Incident photon is absorbed by Ru complexElectrons are excited from ground sate to the excited stateExcited electrons are injected into the conduction band of TiO2 Oxidized photosensitizer accepts electrons from the IThe oxidized redox mediator, I3, diffuses toward the counter electrodeThe main processes that occur in a DSSCStep 1:The following primary steps convert photons to current:1. The incident photon is absorbed by Ru complex photosensitizers adsorbed on the TiO2surface.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 TiO2electrode. This results in the oxidation of the photosensitizer (S+).S + h S(1)S S++ e(TiO2) (2)3. The injected electrons in the conduction band of TiO2are transported between TiO2nanoparticles 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 Iion redox mediator leading to regeneration of the ground state (S), and the Iis 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 Iions.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 TiO2electrode and the redox potential of the mediator (I/I3) in the electrolyte11Dynamics of Electron Injection

Absorption spectrum of N719 dye(sensitizer) shows the transfer of electron from Ru to Ligands before donation to the conduction band of TiO2Proof of MLCT transition

Some of the Ruthenium SensitizersRuL3(yellow)

cis-RuL2(NCS)2(red)

RuL(NCS)3(green)

DSSC PerformanceConversion 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.

Lets look at an animation to visualise the process better

ReferencesMichael Gratzel, Inorganic Chemistry, Vol. 44, No. 20, 2005 6849

Gratzel, M. Nature 2001, 414, 338.

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