1. Charge photogeneration for Solar Energy Conversion
James Durrant
Department of Chemistry & Energy Futures Lab, Imperial College London . 1

2. Solar Energy
There’s lots of it…….

3. Solar Energy But it’s very diffuse……
In UK, need ~ 40 m2 of solar cells to supply one persons average electrical demand
So solar conversion systems need to
be cheap / m2!

4. Towards the Artificial Leaf
Renewable fuel synthesis
Storage of solar energy

5. Solar Energy Conversion Technologies
Dye sensitized solar cells
Organic semiconductor solar cells
2H+ H2
150 nm
Challenges:
Efficiency > 10%
Cost < $100/m2
Robust
Water
Photolysis

6. Printing molecular solar cells
Konarka’s printed organic solar cell strips
G24i’s production line in Cardiff,
producing rolls of dye sensitised solar cells =>

7. Dye sensitised solar cells
Michael Grätzel
Sensitizer
dye
Lab cells upto ~ 11%
‘Commercial’ modules 3-4%
Nanocrystalline TiO2 film
I- / I2 based electrolyte
Platinised TCO coated glass
TCO coated glass
Light I-
I3-
External circuit R u N C S O
TBA H
Electrons
different
dyes
(1000’s tested)

8. Polymer/C60 solar cells Lab cells up to 8 % PCBM ~100 tested P3HT
Alan Heeger
PCBM
~100 tested
P3HT
~1000 tested already
Lab cells up to 8 %

9. Piggy backing on the Organic Electronics Buzz
Organic Electronics: electronic devices based on molecular or polymer semiconductors
Motivations: Harness organic chemistry to synthesis materials which are easily processable, spectrally tunable, cheap ……..
Field first developed to target LED and FET applications. These are now entering market
Sony flexible OLED
Sony OLED tv

10. Characteristics: Disordered Nanostructures
Dye sensitized solar cells
Organic semiconductor solar cells
150 nm
2H+ H2
Water
Photolysis
P3HT/PCBM film
150 nm
Nanocrystalline
TiO2
SEM image of nc Fe2O3 electrode (EPFL)
Interface key
Low cost material and
(solution) processing

11. Towards Ordered Interfaces?
100 nm dia TiO2 nanowires
Donor / acceptor block
co-polymers
So far disappointing device performance:
Still disordered on molecular scale (soft materials)
Loose degeneracy of pathways?
Hard work…..
Similar story for attempts to include antenna structures
TEM of cross-section of block copolymer sample
Nanometre phase segregation observed (~ nm domains)

12. Photochemical Design Considerations
Energy
Energy cost of achieving a high yield / rate
of charge separation
Solar Cells ps
1P*
JSC
VOC FF
P+/Ph-
ms / ms
P+/QA-
Energy versus lifetime for separated charges
P+/QB- ns
Recombination s
High yield: Current
Energetic: Voltage/Optical bandgap
Long lived: allows charges to transport to external circuit.
High yield of long lived, energetic
charge separated states

13. Dye sensitised solar cells
Michael Grätzel
Sensitizer
dye
Nanocrystalline TiO2 film
I- / I2 based electrolyte
Platinised TCO coated glass
TCO coated glass
Lab cells upto ~ 11%
‘Commercial’ modules 3-4%
Light I-
I3-
External circuit
Electrons
different
dyes

14. Energy losses in DSSCs
O’Regan and Durrant Acc. Chem. Res. 2009

15. Kinetics versus energetics
G/eV
High yield of long lived, energetic charge separated states R u N C S O
TBA H
Dye* 2
Dye+ / e-TiO2
Electron
Injection
~ 100 ps
Decay to
ground
~ 10 ns hu
I3- / eth-TiO2
Hole transfer to electrolyte
~ 1 ms
Recombination to
dye+,ms-ms 1
Recombination to
electrolyte, ms-s
Transport ms
Dye

16. Kinetics versus energetics: short circuit
G/eV
Dye* 2
Dye+ / e-TiO2
Electron
Injection
~ 100 ps
Decay to
ground
~ 10 ns hu 1
I3- / eth-TiO2
Hole transfer to electrolyte
~ 1 ms
Recombination to
dye+,ms
Recombination to
electrolyte, s
Transport ms
Dye

17. Charge separation in DSSCs
TiO2 acceptor states
Injection rate increases exponentially with energetics:
- Increasing kinj x10 costs ~ 300 meV
Physical origin: exponential increase of CB density of states with energy
Impact on injection yield and device current depends upon competition with excited state lifetime
Dye*
kinj
Decay to
Ground k0
doscb  exp(E/100meV)
energy
N719/
TiO2
Haque et al.
J. Am. Chem. Soc. 2005
Koops et al.
J. Am. Chem. Soc. 2009
Solar
Cell
Dye

18. The role of excited state lifetime: Singlet versus Triplet Injection
1-10 ps S1
Injection from both S1 and T1 states of N719 possible
T1 energy ~ 300 meV lower than S1 => injection rate ~ 1 orders of magnitude slower.
BUT T1 lifetime 5 orders of magnitude longer than S1
Much easier to achieve efficient electron injection from T1 than S1 state.
Results in injection efficiency being key limitation for singlet injector and low bandgaps dyes (e.g.: porphyrins).
1.9
ISC
~ 100 fs
100 ps T1
1.6
G/eV hν R u N C S O
TBA H
Decay to
ground
~ 10 ns S0
Koops et al. JACS 2009

19. Catalysis of the iodide/iodine redox couple
I2 + 2TiO2(e-) → 2 I-
Regeneration: Dye+ + 2 I- → dye + I2-
Recombination: I2 + 2TiO2(e-) → 2 I-
Counter Elec. I2 + 2FTOPt(e-) → 2 I-
-Recombination rate constant krecom strongly dependent upon sensitizer dye
-Key in determining cell voltage
2I- / I2
maximum
voltage
S / S+
S*/ S+
diffusion
electrolyte
sensitiser
dye
TiO2
conducting
glass
E / V
vs. NHE
1.0
0.5
–0.5 e–
regeneration
injection EF CB
cathode hν
O’Regan et al. J. Am. Chem. Soc

20. Schmidt-Mendes et al. Nanolet. 2005
Interface engineering to minimise recombination
Al2O3
coated
Uncoated
Haque et al.
Angew. Chem.
2005
Palomares et al. JACS 2003
Li+- DFHTM
Haque et al.
Adv Func Mat
2004
Schmidt-Mendes et al. Nanolet. 2005

21. Interfacial redox relays
Log (krecom / s-1)
~ 1 s
krecom  e-br , b ~ 1 Ǻ-1
Haque et al. Angew Chem. 2005

22. Polymer/C60 solar cells
P3HT
PCBM

23. Charge photogeneration
Exciton
separation e -
LUMO
Charge Dissociation e - e - h u
Geminate recombination of bound
polaron pairs (charge transfer states)
HOMO h + h +
Coulomb Attraction:
er = 3-4 for organics
Polymer
C60
Key consideration:
How do initially generated polaron pairs overcome their coulomb attraction and dissociate into free charges?

24. The energy cost of charge photogeneration
Yield of dissociated charges
‘Gen 1’ Polymers (polythiophenes) blended with C60:
Efficient charge photogeneration only achieved at high energy cost (half the photon energy!)
Energy lost in charge separation
Ohkita et al. JACS 2008, Clarke et al. Adv. Func. Mat. 2009

25. Onsager Theory
Key issue: Electron thermalisation length (a) versus coulomb capture radius
Clarke and Durrant Chem Rev 2010 ACS ASAP

26. Reducing the energy cost of charge separation
HOMO
Charge separation yield (a.u.) CT
Energy Loss during charge separation
LUMO
Adding intramolecular charge separation in the polymer appears to enable charge separation at lower energy costs
Clarke et al. Chem Comm 2009, Chem Rev 2010

27. Charge Photogeneration as the key determinant of photocurrent
Outlier: polyfluorene based polymer
-electric field dependent photogeneration?
Data plotted for all polymer / PCBM blend films where both DOD and JSC measured. Remarkably good correlation between charge photogeneration yield and device JSC. Suggests charge photogeneration is key (primary?) determinant of photocurrent rather than collection/transport (at least for polymer / PCBM blend films).
Device
photocurrent
All optical assay of
charge photogeneration in films

28. Solar to fuels
2H+ H2
UV + nc TiO2+Pt
H2O
H2 + ½ O2

29. Protein immobilisation
Proteins
Protein loading of
Nanomoles / cm2
ZnO +
Applications:
Biosensing
Spectroelectrochemistry
Artificial Photosynthesis
SnO2
Topoglidis et al.
Anal Chem. 1998

30. Bio/inorganic electrodes for hydrogen evolutionPt D D+ e- hv
TiO2 nanoparticle
ZnCyt-c/TiO2-Pt
TiO2-Pt
Long lived (200 ms) charge separation
Hydrogen evolution observed with ~ 20% internal QY using visible light
Probably never a technology, but hopefully inspiring science.
Astuti et al. JACS 2005

31. Hole dynamics in TiO2 Timescale: ms -seconds
TiO2 hole decay dynamics
Timescale:
ms -seconds
O2 QY data as function of excitation intensity
Electron scavenging by Ag+ ions results in long lived TiO2 holes (~ 200 ms)
Oxygen QY strongly dependent upon excitation density, peaking at ~ 18% for 4 photons absorbed / nanoparticle
Consistent with needing to accumulate 4 oxidising equivalents to generate one O2
Tang, Durrant & Klug JACS 2008

32. Hole dynamics in Fe2O3 TCO e- light -0.1 V EFe dark h+ 2H2O 4H+ +O2 CB
Visible e- h+ CB VB
TCO
2H2O
4H+ +O2
Bulk charge
recombination
EFe
-0.1 V
+ 0.4 V
Photoelectrode for visible light driven water oxidation
Photocurrent only observed under positive bias
Positive bias increases hole lifetime from ms/ms to seconds

33. Lessons for the artificial leaf
For the synthetic chemists: Key features areNanostructures, self-assembly, multi-function components
For the photochemists: Kinetics versus thermodynamics key as for photosynthesis
For the theoreticians: Marcus (and inorganic device physics) often less useful as a design tools than we might have hoped – appreciating the chemical complexity and impact of disorder often more important.
For the device people: The jump from small lab devices to a scaleable, stable, low cost module can be larger! Key driver for new materials / processing etc.

34. Acknowledgements Dye sensitized solar cells
Brian O’Regan, Piers Barnes, Assaf Anderson, Li Xiaoe,
Andrea Listorti, Joe Mindagaus.
plus Michael Grätzel, Nazeeruddin et al. (EPFL), David
Officer et al (Wollangong), Corus, g24i
Polymer / fullerene solar cells
Tracey Clarke, Safa Shoee, Chris Shuttle, Brian O’Regan, Rick Hamilton, Andrea Maurano, Mattias Eng, Fiona Jamieson, Dan Credgington, Yvonne Soon
. Jenny Nelson, Donal Bradley, Iain McCulloch and co-workers
plus Steve Tierney et al (Merck Chemicals), Christoph Brabec et al (Konarka), Nazario Martin et al. (Madrid), Seth Marder / Jean-Luc Bredas et al. (Georgia Tech)
Solar Fuels
Junwang Tang, Monica Barroso, Stephanie Pendlebury, Wenhua Leng, Alex Cowan, David Klug, Steve Dennison, Geoff Kelsall, Klaus Heldgardt,
plus Kevin Sivula, Michael Grätzel et al. (EPFL)
Alumini: Saif Haque, Hideo Ohkita, Emilio Palomares, Yeni Astuti, Ana Morandeira, Sara Koops etc..
Financial Support: EPSRC, EU, TSB, BP Solar, Solvay, Konarka, Carbon Trust

35. Lessons for the artificial leaf
For the synthetic chemists: Key features areNanostructures, self-assembly, multi-function components
For the photochemists: Kinetics versus thermodynamics key as for photosynthesis
For the theoreticians: Marcus (and inorganic device physics) often less useful as a design tools than we might have hoped – appreciating the chemical complexity and impact of disorder often more important.
For the device people: The jump from small lab devices to a scaleable, stable, low cost module can be larger! Key driver for new materials / processing etc.

36. Lessons from DSSCs and OPV for solar to fuels
Yield versus lifetime versus energy determines efficiency
Elegant structures are so far not functionally better
Multifunctional components are painful to develop but necessary
Real (disordered) materials properties are critical to determining function (rather than Marcus, Forster, Redfield……..)
Catalysis and multi-electron chemistry can be exploited to aid kinetics
A lot of the key action happens on unfashionably slow timescales
The efficiency gap between lab scale champion cells and cheap, stable modules can be critical (and painful)