Photocatalytic Activity of TiO2

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Dye Sensitised Solar Cells

Saurav Chandra Sarma

Int. Ph.D NCU 2ndSemester

17 April 2014


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Outline

Solar cells and their importance.

Dye Sensitized Solar Cell(DSSC) and its comp

Working of DSSC

Conclusion


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What is a Solar

Cell?

A solar cell (also calleda photovoltaic cell) is anelectrical device that convertsthe energy of light directlyinto electricity

Generates an electriccurrent without beingattached to any voltagesource


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It exploits a renewable sources ofenergy

It is environmental friendly

Solar cells can be used in remote areaswhere it is too expensive to extend theelectricity power grid.

Solar cells last a longer time and havelow running costs

Importance of Solar Cells


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Buried contactsolar cell

Cadmiumtelluride solar

cell

Copper indiumgallium selenide

solar cells

Dye-sensitizedsolar cell

Galliumarsenide

germaniumsolar cell

Hybrid solarcell

Different types of Solar Cells


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Flexible DSSC Module

Glass-based DSSC Module


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Born 11 May 1944 (age 69)Dorfchemnitz, Sachsen

Residence Switzerland

Nationality Swiss

Fields photochemistry

Institutions cole PolytechniqueFdrale de Lausanne

Known for Dye-sensitized solar cells

Achievements:

Author of over 900 publications, two books and inventor or co-inventor ofover 50 patents

On 9 June 2010, Grtzel received Millennium Technology Prize, fordevelopment of dye-sensitized solar cells.

Michael Gratzel: Father of DSSC
http://en.wikipedia.org/wiki/Dorfchemnitzhttp://en.wikipedia.org/wiki/Sachsenhttp://en.wikipedia.org/wiki/%C3%89cole_Polytechnique_F%C3%A9d%C3%A9rale_de_Lausannehttp://en.wikipedia.org/wiki/Dye-sensitized_solar_cellshttp://en.wikipedia.org/wiki/Dye-sensitized_solar_cellshttp://en.wikipedia.org/wiki/Dye-sensitized_solar_cellshttp://en.wikipedia.org/wiki/%C3%89cole_Polytechnique_F%C3%A9d%C3%A9rale_de_Lausannehttp://en.wikipedia.org/wiki/%C3%89cole_Polytechnique_F%C3%A9d%C3%A9rale_de_Lausannehttp://en.wikipedia.org/wiki/Photochemistryhttp://en.wikipedia.org/wiki/Switzerlandhttp://en.wikipedia.org/wiki/Sachsenhttp://en.wikipedia.org/wiki/Dorfchemnitz


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The material of choice has been TiO2(anatase), although alternativewide-band-gap oxides such as ZnO and Nb

2

O5

have also beeninvestigated.

Nanoparticles of the oxide are deposited, for example, by screenprinting onto a glass or flexible plastic support.

The surface is then coated with layers of sensitizer.

What are the constituents of DSSC?


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The main processes that occur in a DSSC

1. The incident photon is absorbed by Ru complex photosensitizers adsorbed on theTiO2surface.

2. The photosensitizers are excited from the ground state (S) to the excited state (S). Theexcited electrons are injected into the conduction band of the TiO2electrode. This results inthe oxidation of the photosensitizer (S+).

S + hS

S

S++ e(TiO2)

3. The injected electrons in the conduction band of TiO2 are transported betweenTiO2 nanoparticles with diffusion toward the back contact (TCO). And the electrons finallyreach the counter electrode through the circuit.

4. The oxidized photosensitizer (S+) accepts electrons from the Iion redox mediator leadingto regeneration of the ground state (S), and the Iis oxidized to the oxidized state, I3.

S++ eS

5. The oxidized redox mediator, I3, diffuses toward the counter electrode and then it isreduced to Iions.

I3+ 2 e3 I

Mechanism of DSSC


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Incident photon is absorbed

by Ru complex

Electrons are excitedfrom ground sate to the

excited state

Excited electrons are injected

into the conduction band of

TiO2

Oxidized photosensitizer

accepts electrons from the

I

The oxidized redoxmediator, I3

, diffuses

toward the counter

electrode


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Dynamics of Electron Injection

The dyes should incorporatefunctional group such as , fore.g, carboxylate, hydroxymate,or phosphate moieties thatanchor the sensitizer to theoxide surface.

Metal to Ligand ChargeTransfer(MLCT) occurs whichfacilitates the rapid electron

injection from the ligand to thesemiconductor.


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Absorption spectrum of N719 dye(sensitizer) shows

the transfer of electron from Ru to Ligands before

donation to the conduction band of TiO2

Proof of MLCT transition


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


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Some of the Ruthenium Sensitizers

RuL3(yellow) cis-RuL2(NCS)2(red) RuL(NCS)3(green)


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DSSC Performance

Conversion of light to

elecric current bymesoscopic 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 theabsorption maximum of the sensitizer,which is located around 530 nm


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Lets look at an animation to

visualise the process better


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The transport of the electroactive ions is expected to playa significant role in determining DSSC efficiency

The search for suitable solid materials that can replacethe liquid electrolyte is an additional interesting andactive area of research.

Research on dye sensitizers are mainly focused on

transition metal complexes, but a considerable of work isnow directed towards the optimization of organicsensitizers and of natural sensitizers extracted from

fruits.

Conclusion


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References

Michael Gratzel, Ino rganic Chemis try, Vol. 44, No. 20,

2005 6849

Gratzel, M. Natu re 2001, 414, 338.


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Photocatalytic Activity of TiO2

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