electron injection kinetics in dye-sensitized solar cells
DESCRIPTION
Overview of kinetic impacts in design of dye-sensitized solar cells. Considerations include cell structure, competing processes, kinetic rates, dye adhesion geometry, dye electronic structure, quantum yield, electron injection, density of states, triplet states lifetime, & sources of quenching.TRANSCRIPT
ELECTRON INJECTION KINETICS IN DYE-SENSITIZED SOLAR CELLS
CHELSEY CROSSE
LEVINGER GROUP | COLORADO STATE UNIVERSITY
CHEMICAL KINETICS | DECEMBER 10, 2013
2
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
3
• 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)
4
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.
5
ENERGETIC ANALYSIS OF DSSC
Listorti, A., O’Regan, B., Durrant, J. Chem. Mater. 2011, 23, 3381-3399.
6
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.
7
• 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
8
• 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.
9
• 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.
10
• 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
11
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.
12
• 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
13
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
14
• 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
16
SUMMARY
Listorti, A., O’Regan, B., Durrant, J. Chem. Mater. 2011, 23, 3381-3399.
17
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)
18
ACKNOWLEDGEMENTS• Dr. Elliot Bernstein
• Dr. Mike Elliot
• All of you
• Winter Break!