PHOTOSYNTHESIS PHOTOSYNTHESIS IN A IN A

TEST TUBETEST TUBEDye-Sensitized Solar CellsDye-Sensitized Solar Cells

Atul K. RaturiAtul K. Raturi

Division of PhysicsDivision of Physics

The university of the south pacificThe university of the south pacific

About 1.8 Billion people without electricity

Earth at Night

Pacific Scene Pacific Scene

PNG:~ 85%PNG:~ 85%

Solomon Islands: ~98%Solomon Islands: ~98%

Vanuatu:~ 78%Vanuatu:~ 78%

Fiji: ~ 51% ( mainly rural)Fiji: ~ 51% ( mainly rural)

Population without electricity

Global warming & Climate Global warming & Climate change change

Recent research shows that the global warming in the Recent research shows that the global warming in the New Guinea Island was occurring at the rate 20 times New Guinea Island was occurring at the rate 20 times higher than previously thoughthigher than previously thoughtTemperatures have increased on average 0.3 degrees Temperatures have increased on average 0.3 degrees every decade (among the fastest in the world)every decade (among the fastest in the world)Glaciers on Mount Jaya (highest peak on the Island-Glaciers on Mount Jaya (highest peak on the Island-Indonesian side) have retreated by 300 m in 30 yearsIndonesian side) have retreated by 300 m in 30 yearsThe priceless flora and fauna (some yet to be The priceless flora and fauna (some yet to be discovered) will be lost foreverdiscovered) will be lost forever

Photo:www.rbgkew.org.uk

Source: New scientist, March 2006)

Carteret Atoll, Papua New Guinea

The entire population of 2000 is to be moved to higher grounds.

“First Climate Change Refugees”

By 2015, all the six islands will be submerged. Saltwater has already destroyed the gardens and crops

Two uninhabited islands of Kiribati disappeared in 1999.

Tuvalu, Marshall Islands are under threat.

Global warming effects in the Pacific

FEA Electricity generationFEA Electricity generation

1990: 94.5 % from hydropower

2003: 67.3 % from hydropower

Currently almost 50% is produced using diesel (Fuel bill ~ 98 M$)

FEA generation forecastFEA generation forecast

Wind farm at Butoni, Sigatoka- Commissioned 2007; Capacity 10 MW

Solar Insolation (Fiji)Solar Insolation (Fiji)

All our traditional energy sources All our traditional energy sources ( except wind energy & hydropower) are ( except wind energy & hydropower) are

products of Photo synthesisproducts of Photo synthesis Photosynthesis produces 8 times the Photosynthesis produces 8 times the current energy needs of the worldcurrent energy needs of the world

Coal

Oil

Gas

Solar EnergyPhotosynthesis

Biomass

PhotosynthesisPhotosynthesis An electrochemical Process: A redox reaction of An electrochemical Process: A redox reaction of

excited Chlorophyll moleculesexcited Chlorophyll molecules Chlorophyll, an organic dye, absorbs the light photons to produce Chlorophyll, an organic dye, absorbs the light photons to produce excited electrons. Carbon dioxide acts as electron acceptor while excited electrons. Carbon dioxide acts as electron acceptor while

Oxygen is produced as oxidation productOxygen is produced as oxidation product

Light   nCO2 + n H2 O  ------(CH2O)n + n O2

(STI)

-

+

-

Thylacoid Membrane

0.1 µm

Oxidation Products

Reduction Products

Sunlight

Chlorophyll

Electron-hole pairs are generated when light is absorbed by the chlorophyll molecule . The membrane separates the two charges

A Thin Section Of A Leaf

Electron

Duplication of Photosynthesis has not been very successful

Reasons:

A very thin layer is required to minimize A very thin layer is required to minimize electron losseselectron losses

A thin layer absorbs very little lightA thin layer absorbs very little light

Nature has solved this problem byNature has solved this problem by

Providing successive layersProviding successive layers

Antenna moleculesAntenna molecules

Can afford to recycle the whole system on Can afford to recycle the whole system on annual basisannual basis

Artificial Photosynthesis Artificial Photosynthesis

What is needed? :What is needed? :

1.1. Something to absorb solar energySomething to absorb solar energy

2.2. A mechanism to separate the two types of A mechanism to separate the two types of chargescharges

3.3. A mechanism to pickup one type of A mechanism to pickup one type of chargescharges

4.4. A medium to transport the other type of A medium to transport the other type of chargescharges

Materials RequirementsMaterials Requirements

Semiconductor with appropriate band-gap (1-1.2 eV) and Semiconductor with appropriate band-gap (1-1.2 eV) and High absorption coefficientHigh absorption coefficientA non-reactive electrolyte .A non-reactive electrolyte .A special relationship between the band energies of the A special relationship between the band energies of the semiconductor and the redox potential of the electrolyte semiconductor and the redox potential of the electrolyte is required to drive the reactions is required to drive the reactions For n type: A donor level above or equal to the valence For n type: A donor level above or equal to the valence band edge of the semiconductor must exist in the band edge of the semiconductor must exist in the electrolyte electrolyte For p type: An acceptor level below or equal to the For p type: An acceptor level below or equal to the conduction band edge of the semiconductor must exist in conduction band edge of the semiconductor must exist in the electrolyte.the electrolyte.

Reduction

Oxidation

Electrolyte

Metal: CathodeN-type Semiconductor: Photo anode

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External ConnectionHole

Electron

Light

Electron

A PHOTOELECTROCHEMICAL CELL- The Next Best Thing

Fermi Level

VB

I

PEC OPERATIONPEC OPERATION

Electron and hole pairs are created by photoexcitation of Electron and hole pairs are created by photoexcitation of the semiconductor electrode .the semiconductor electrode .Electron and holes drive chemical reactions: For n-type Electron and holes drive chemical reactions: For n-type semiconductorsemiconductorPhotoanode (SC) : Hole injection- Photoanode (SC) : Hole injection- Oxidation reactionOxidation reactionMetal Cathode: Electron injection – Metal Cathode: Electron injection – Reduction reactionReduction reaction

Red + hRed + h++--------------Ox --------------Ox + + ( At the anode) ( At the anode)OxOx++ + e + e-----------------Red ( At the cathode)---------------Red ( At the cathode)No Change in the electrolyte composition. Light energy is No Change in the electrolyte composition. Light energy is converted into electrical energy converted into electrical energy

Energy Levels Energy Levels EEredred ---- Ionization energy of the reduced species ---- Ionization energy of the reduced species

EEoxox====Electron affinity of the oxidized specie====Electron affinity of the oxidized specie

EEFredoxFredox– the average of E– the average of Eredred and E and Eoxox

Semiconductor Redox System

EFredox

Eox

Ered

EF

The energy levels in the electrolyte belong to the localized states bound to the component of the redox system

Valence band

Conduction band

The Band bending at the junction is given by

VB = UF – UFB

Where UF is the Fermi Level and the UFB is the Flat Band Potential

We need the light to be absorbed in the depletion layer so that the charges can be separated efficiently.

The width of the depletion layer is 2

1

2

eN

VW B

2

1

2

eN

VW B

For larger W , N (no. of donors/acceptors/Vol) should be low but that results in higher resisitivity.

Find materials with large minority carrier lifetime and hence low recombination.

PHOTOELECTROCHEMICAL PHOTOELECTROCHEMICAL CELLSCELLS

AdvantagesAdvantagesThe junctions can be The junctions can be made very easily unlike in made very easily unlike in the case of solid state the case of solid state cellscells

Polycrystalline materials Polycrystalline materials can be used (lower cost)can be used (lower cost)

Diffusion and doping Diffusion and doping processes are eliminatedprocesses are eliminated

No front metallization No front metallization requiredrequired

ProblemsProblemsStability of the Stability of the semiconductor electrodes semiconductor electrodes in the electrolyte is the in the electrolyte is the biggest problem: holes biggest problem: holes oxidize the oxidize the semiconductorsemiconductor

Examples of PEC systems: nCdSe/S-2/S, nCdTe/S-2/S, nCdS/{Fe(CN)6}4+ / {Fe(CN)6}3+

NEW GENERATION PECsNEW GENERATION PECsDYE SENSITIZED NANOCRYSTALLINE DYE SENSITIZED NANOCRYSTALLINE

SOLAR CELLSOLAR CELL

Third generation Solar CellsThird generation Solar Cells

Mimics the natural photosynthesis processMimics the natural photosynthesis process

Light absorption process is separated from Light absorption process is separated from charge transport process (unlike in older cells)charge transport process (unlike in older cells)

An organic dye absorbs the sunlight and a An organic dye absorbs the sunlight and a nanocrystalline wide band-gap semiconductor is nanocrystalline wide band-gap semiconductor is used to transport the chargesused to transport the charges

Also called “Graetzel Cells”Also called “Graetzel Cells”

ElectrolyteI/I3

-

TiO2

Conducting glass

Sunlight

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Carbon electrode

Organic Dye: Light harvester (sensitizer)

Nanocrystalline TiO2: Transports electrons

Electrons reach the carbon electrode via external connection and a current flows

Reactions:

I3- + e- -- I- ( at carbon electrode)

I- +h+ ---- I3- + e- ( at the

photoelectrode)

Max. Photovoltage= Difference between the Redox Potential of the electrolyte and the Fermi level of the semiconductor

Electron

DSSC

Organic Dye: e.g. Cyanin

I

e-

Efficiency achieved ~ 10.5 %

Commercial Solar Cell efficiency ~ 15-17%

Tropical Forest Ecosystem efficiency ~ 1%

Theoretical limit for natural Photosynthesis ~ 13%

Commercial Production : Sustainable Technologies International (Australia)- Under license from EPFL

DSSC Façade Panel

Various colours Transparent

Hitachi’s 9% efficient Building integrated

Source: M.Gratzel:Nano Roadmap conference

DSSCs

DSSC Photosensitizers (specially prepared Dyes)DSSC Photosensitizers (specially prepared Dyes)

(i) Ru Dye (ii)N3 Dye (iii) Black Dye

Coumarin Dye

Properties:

1: Must contain a chemical group that can attach to the TiO2 surface

2. Must have energy levels at the proper positions

The dye is obtained from

the berries (Rubiaceae family)

Experimenting with natural dyes

Step1:

A nanocrystalline (porous) film of TiO2 was coated on a conducting glass slide.

Step2:

Sensitizer dye was derived from the berries.

Step3:

TiO2 film was coated with the dye

Step4:

A cell was constructed by putting a second conducting glass slide on top of the coated one with few drops of electrolyte between them

Step4:

Cell characterization: Voc , ISC, ,FF, Efficiency

DSSC commercialization

DSSC Market

2007- 3.5 billion Yen

2010- 58.1 billion Yen

Manufacturing costs ; ½ to 1/10 of crystalline cells

CONCLUSIONCONCLUSIONDSSCs have great potential for providing low-cost Photovoltaic power to billions of people around the world , living in remote areas without any access to electricity. These cells can be produced easily without the expensive set-ups needed for conventional solid-state solar cells.

This is an example of BIOMIMICRY proving that Nature is our greatest teacher.