electron injection kinetics in dye-sensitized solar cells

ELECTRON INJECTION KINETICS IN DYE-SENSITIZED SOLAR CELLS

CHELSEY CROSSE

LEVINGER GROUP | COLORADO STATE UNIVERSITY

CHEMICAL KINETICS | DECEMBER 10, 2013

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WHY RENEWABLE ENERGY?

Climate change 2001, Synthesis report, Contribution of working groups I, II and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 2001.

Climate

Health

DiversificationClimate

“China Smog Photos Show How Bad Its Air Pollution Problem Has Become,” Huffington Post [Online], 21 October 2013 <http://www.huffingtonpost.com/2013/10/21/china-smog-photos-pollution_n_4137675.html> (8 December 2013).

Climate

Health

Tverberg, G. “World Energy Consumption since 1820 in Charts,” Our Finite World, 12 March 2012 <http://ourfiniteworld.com/2012/03/12/world-energy-consumption-since-1820-in-charts/>

Climate

Health

Diversification

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• Monocrystalline Silicon

• Polycrystalline Silicon

• Thin-film

• Dye-Sensitized Solar Cell (DSSC)

TYPES OF SOLAR CELLS

Sinclair, K. “Solar 101: What Are Solar Panels?” RenewableEnergyWorld.com, 8 September 2011 <http://www.renewableenergyworld.com/rea/blog/post/2011/09/what-are-solar-panels> (Accessed 8 December 2013).

Semi-conductor Solar Cell

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

Dye-Sensitized Solar Cell(DSSC)

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1. Absorption

Dye + hν Dye*

2. Electron Injection v. Decay to Ground

Dye* e-TiO2 + Dye+ (100 ps)

Dye* Dye + hν (~ns)

3. Regeneration Reaction v. Recombination to Dye

Dye+ + I- Dye + I3- (~1 μs)

Dye+ + e-TiO2 Dye (μs – ms)

4. Charge Transport v. Recombination to Electrolyte

I3- + e-

Pt I- (ms – s)

I3- + e-

TiO2 I- (ms – s)

DSSC KINETIC PROCESSES

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

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ENERGETIC ANALYSIS OF DSSC

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

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Must be optimized to account for:

• Disadvantages of dye-dye quenching

• Advantages of shorter spacers for electron injection

ANATOMY OF SENSITIZER DYE ADHESION/BONDING

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

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• Injection kinetics depend on:

• Energetic coupling• Density of states in

conduction band

• Decay kinetics depend on:

• Overlap of states• Quantum yield

ELECTRON INJECTION V. DECAY TO GROUND

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

qinj

injinj kkk

k

0

ηinj: quantum yield of electron injectionk0: rate constant for decay of isolated dyekq: rate constant for quenching in cell system

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• E absolute energy relative to NHE of the semiconductor acceptor state

• E*ox redox potential of dye excited state

• ρ(E) density of semiconductor acceptor states @ E

• V average electronic coupling @ E

• f(E,EF) Fermi occupancy factor for each semiconductor acceptor state

• λ total reorganization energy for election injection

ELECTRON INJECTION

dETk

EEEEEfVAk

B

ox

Finj

4

)(exp)()),(1(

2*2

Dye SC NP

E*ox

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

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• Shape is exponential, not parabolic

• Injection kinetics can be measured using TRANSIENT ABSORPTION

ELECTRON INJECTION: DENSITY OF STATES

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

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• Changes the position of the excited state relative to the conduction band

• Affects the density of states available in the semi-conductor nano-particle (SC NP)

ELECTRON INJECTION: DYE EXCITED STATE OXIDATION POTENTIAL

dETk

EEEEEfVAk

B

ox

Finj

4

)(exp)()),(1(

2*2

Dye SC NP

E*ox

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Depends on:

• Distance between dye & semiconductor

• Electronic coupling between LUMO & semiconductor states

• Choice of anchoring group

• Dominated by electronic coupling, distance is too large for tunneling to compete

ELECTRON INJECTION: ELECTRONIC COUPLING

dETk

EEEEEfVAk

B

ox

Finj

4

)(exp)()),(1(

2*2

Anderson, N.A., Lian, T. Coordination Chemistry Reviews (2004) 1231.

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• Injection from triplet states can contribute to total injection

• Intersystem crossing can occur on 100 fs timescale

• Beneficial effects for kinetics

ELECTRON INJECTION: TRIPLET STATES

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

S0

S1

Dye TiO2 Nanoparticle

T1

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Decay kinetics depend on:

• Overlap of states within dye molecule

• Cell quenching

ELECTRON INJECTION V. DECAY TO GROUND

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

qinj

injinj kkk

k

0

k0: rate constant for decay of isolated dyekq: rate constant for quenching in cell system

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• Decay rates for triplet states are much lower

• This leads to less competition between decay & injection

• Increased injection yield

DECAY TO GROUND: TRIPLET STATES

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

qinj

injinj kkk

k

0

15

• Dye aggregation

• Electronic transitions

• Thermal Relaxation

• Electrolyte quenching

• Anionic process may still produce electron injection

DECAY TO GROUND: SOURCES OF QUENCHING

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

qinj

injinj kkk

k

0

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SUMMARY

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

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SUMMARY

Listorti, A., O’Regan, B., Durrant, J. Chem. Mater.  2011, 23, 3381-3399.

1. Absorption

Dye + hν Dye*

2. Electron Injection v. Decay to Ground

Dye* e-TiO2 + Dye+ (100 ps)

Dye* Dye + hν (~ns)

3. Regeneration Reaction v. Recombination to Dye

Dye+ + I- Dye + I3- (~1 μs)

Dye+ + e-TiO2 Dye (μs – ms)

4. Charge Transport v. Recombination to Electrolyte

I3- + e-

Pt I- (ms – s)

I3- + e-

TiO2 I- (ms – s)

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ACKNOWLEDGEMENTS• Dr. Elliot Bernstein

• Dr. Mike Elliot

• All of you

• Winter Break!