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Charge photogeneration for Solar Energy Conversion James Durrant Department of Chemistry & Energy Futures Lab, Imperial College London www.imperial.ac.uk/people/j.durrant.

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Presentation on theme: "Charge photogeneration for Solar Energy Conversion James Durrant Department of Chemistry & Energy Futures Lab, Imperial College London www.imperial.ac.uk/people/j.durrant."— Presentation transcript:

1 Charge photogeneration for Solar Energy Conversion James Durrant Department of Chemistry & Energy Futures Lab, Imperial College London www.imperial.ac.uk/people/j.durrant

2 Solar Energy Theres lots of it…….

3 Solar Energy But its very diffuse…… In UK, need ~ 40 m 2 of solar cells to supply one persons average electrical demand So solar conversion systems need to be cheap / m 2 !

4 Towards the Artificial Leaf Renewable fuel synthesis Storage of solar energy http://www3.imperial.ac.uk/energyfutureslab/research/grandchallenges/artificialleaf

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

6 Printing molecular solar cells G24is production line in Cardiff, producing rolls of dye sensitised solar cells => Konarkas printed organic solar cell strips

7 Dye sensitised solar cells Light External circuit Electrons Platinised TCO coated glass I - / I 2 based electrolyte TCO coated glass Nanocrystalline TiO 2 film I3-I3- I-I- Sensitizer dye www.iq.usp.br/geral/dyecell different dyes (1000s tested) Michael Grätzel Lab cells upto ~ 11% Commercial modules 3-4%

8 Polymer/C60 solar cells P3HT ~1000 tested already PCBM ~100 tested Alan Heeger 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 OLED tv Sony flexible OLED http://www3.imperial.ac.uk/plasticelectronics

10 2H + H 2 Characteristics: Disordered Nanostructures SEM image of nc Fe 2 O 3 electrode (EPFL) 150 nm Nanocrystalline TiO 2 P3HT/PCBM film Dye sensitized solar cells Organic semiconductor solar cells Water Photolysis 150 nm Interface key Low cost material and (solution) processing

11 Towards Ordered Interfaces? TEM of cross-section of block copolymer sample Nanometre phase segregation observed (~ 50-60 nm domains) Donor / acceptor block co-polymers 100 nm dia TiO 2 nanowires 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

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

13 Dye sensitised solar cells Light External circuit Electrons Platinised TCO coated glass I - / I 2 based electrolyte TCO coated glass Nanocrystalline TiO 2 film I3-I3- I-I- Sensitizer dye www.iq.usp.br/geral/dyecell different dyes Michael Grätzel Lab cells upto ~ 11% Commercial modules 3-4%

14 Energy losses in DSSCs ORegan and Durrant Acc. Chem. Res. 2009

15 Kinetics versus energetics G/eV 0 1 2 Dye * Dye Dye + / e - TiO2 Electron Injection ~ 100 ps Decay to ground ~ 10 ns I 3 - / e th - TiO2 Hole transfer to electrolyte ~ 1 s Recombination to dye +, s-ms h Recombination to electrolyte, ms-s Transport ms High yield of long lived, energetic charge separated states

16 Kinetics versus energetics: short circuit G/eV 0 1 2 Dye * Dye Dye + / e - TiO2 Electron Injection ~ 100 ps Decay to ground ~ 10 ns I 3 - / e th - TiO2 Hole transfer to electrolyte ~ 1 s Recombination to dye +,ms h Recombination to electrolyte, s Transport ms

17 Charge separation in DSSCs energy Dye Dye* TiO 2 acceptor states Decay to Ground k 0 k inj dos cb exp(E/100meV) Haque et al. J. Am. Chem. Soc. 2005 Koops et al. J. Am. Chem. Soc. 2009 N719/ TiO 2 Solar Cell Injection rate increases exponentially with energetics: - Increasing k inj 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

18 The role of excited state lifetime: Singlet versus Triplet Injection Injection from both S 1 and T 1 states of N719 possible T 1 energy ~ 300 meV lower than S 1 => injection rate ~ 1 orders of magnitude slower. BUT T 1 lifetime 5 orders of magnitude longer than S 1 Much easier to achieve efficient electron injection from T 1 than S 1 state. Results in injection efficiency being key limitation for singlet injector and low bandgaps dyes (e.g.: porphyrins). G/eV 0 h Decay to ground ~ 10 ns 1.9 1.6 S 0 T 1 S 1 ISC ~ 100 fs 100 ps 1-10 ps Koops et al. JACS 2009

19 Catalysis of the iodide/iodine redox couple -Recombination rate constant k recom strongly dependent upon sensitizer dye -Key in determining cell voltage ORegan et al. J. Am. Chem. Soc. 2008 I 2 + 2TiO 2 (e - ) 2 I - Regeneration: Dye + + 2 I - dye + I 2 - Recombination: I 2 + 2TiO 2 (e - ) 2 I - Counter Elec. I 2 + 2FTO Pt (e - ) 2 I - 2I - / I 2 maximum voltage S / S + S*/ S + diffusion electrolyte sensitiser dye TiO 2 conducting glass E / V vs. NHE 1.0 0.5 0 –0.5 e–e– regeneration injection EFEF CB cathode e–e– hν

20 Interface engineering to minimise recombination Al 2 O 3 coated Uncoated Haque et al. Angew. Chem. 2005 Palomares et al. JACS 2003 Haque et al. Adv Func Mat 2004 Li + - DFHTM Schmidt-Mendes et al. Nanolet. 2005

21 Interfacial redox relays ~ 1 s k recom e - r, ~ 1 Ǻ -1 Haque et al. Angew Chem. 2005 Log (k recom / s -1 )

22 Polymer/C60 solar cells P3HT PCBM

23 Charge photogeneration e - Polymer C 60 h h Exciton separation h + h + LUMO HOMO e - e - h + h + Geminate recombination of bound polaron pairs (charge transfer states) Key consideration: How do initially generated polaron pairs overcome their coulomb attraction and dissociate into free charges? Coulomb Attraction: r = 3-4 for organics e - e - Charge Dissociation

24 The energy cost of charge photogeneration Ohkita et al. JACS 2008, Clarke et al. Adv. Func. Mat. 2009 Yield of dissociated charges Gen 1 Polymers (polythiophenes) blended with C 60 : Efficient charge photogeneration only achieved at high energy cost (half the photon energy!) Energy lost in charge separation

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

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

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

28 Solar to fuels H2OH2O UV + nc TiO 2 +Pt H 2 + ½ O 2 2H + H 2

29 Protein immobilisation + ZnO TiO 2 SnO 2 Proteins Protein loading of Nanomoles / cm 2 Topoglidis et al. Anal Chem. 1998 Applications: Biosensing Spectroelectrochemistry Artificial Photosynthesis

30 Bio/inorganic electrodes for hydrogen evolution Long lived (200 ms) charge separation Hydrogen evolution observed with ~ 20% internal QY using visible light Probably never a technology, but hopefully inspiring science. ZnCyt-c/TiO 2 - Pt TiO 2 -Pt Astuti et al. JACS 2005 H2H2 H+H+ Pt D D+D+ e-e- e-e- e-e- hv TiO 2 nanoparticle

31 Hole dynamics in TiO 2 Electron scavenging by Ag + ions results in long lived TiO 2 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 O 2 O 2 QY data as function of excitation intensity TiO 2 hole decay dynamics Tang, Durrant & Klug JACS 2008 Timescale: ms -seconds

32 light dark Hole dynamics in Fe 2 O 3 Visible e-e- h+h+ CB VB TCO 2H 2 O 4H + +O 2 Bulk charge recombination EFeEFe - 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 s/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 ORegan, 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 ORegan, 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: 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)


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