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 calleda photovoltaic cell) is anelectrical device that convertsthe energy of light directlyinto electricity

Generates an electriccurrent without beingattached to any voltagesource

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

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

Different types of Solar Cells

Flexible DSSC Module

Glass-based DSSC Module

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.

Michael Gratzel: Father 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.

What are the constituents of DSSC?

The main processes that occur in a DSSC

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∗

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−

Mechanism of DSSC

Incident photon is absorbed

by Ru complex

Electrons are excited

from 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 redox

mediator, I3−, diffuses

toward the counter

electrode

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.

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

The most widely used sensitizer for the DSC has been cisRu(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

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.

Conclusion

References

•Michael Gratzel, Inorganic Chemistry, Vol. 44, No. 20,

2005 6849

•Gratzel, M. Nature 2001, 414, 338.