3Solar Energy But it’s very diffuse…… In UK, need ~ 40 m2 of solar cells to supply one persons average electrical demandSo solar conversion systems need tobe cheap / m2!
4Towards the Artificial Leaf Renewable fuel synthesisStorage of solar energy
5Solar Energy Conversion Technologies Dye sensitized solar cellsOrganic semiconductor solar cells2H+H2150 nmChallenges:Efficiency > 10%Cost < $100/m2RobustWaterPhotolysis
6Printing molecular solar cells Konarka’s printed organic solar cell stripsG24i’s production line in Cardiff,producing rolls of dye sensitised solar cells =>
7Dye sensitised solar cells Michael GrätzelSensitizerdyeLab cells upto ~ 11%‘Commercial’ modules 3-4%Nanocrystalline TiO2 filmI- / I2 based electrolytePlatinised TCO coated glassTCO coated glassLightI-I3-External circuitRuNCSOTBAHElectronsdifferentdyes(1000’s tested)
8Polymer/C60 solar cells Lab cells up to 8 % PCBM ~100 tested P3HT Alan HeegerPCBM~100 testedP3HT~1000 tested alreadyLab cells up to 8 %
9Piggy backing on the Organic Electronics Buzz Organic Electronics: electronic devices based on molecular or polymer semiconductorsMotivations: 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 marketSony flexible OLEDSony OLED tv
10Characteristics: Disordered Nanostructures Dye sensitized solar cellsOrganic semiconductor solar cells150 nm2H+H2WaterPhotolysisP3HT/PCBM film150 nmNanocrystallineTiO2SEM image of nc Fe2O3 electrode (EPFL)Interface keyLow cost material and(solution) processing
11Towards Ordered Interfaces? 100 nm dia TiO2 nanowiresDonor / acceptor blockco-polymersSo far disappointing device performance:Still disordered on molecular scale (soft materials)Loose degeneracy of pathways?Hard work…..Similar story for attempts to include antenna structuresTEM of cross-section of block copolymer sampleNanometre phase segregation observed (~ nm domains)
12Photochemical Design Considerations EnergyEnergy cost of achieving a high yield / rateof charge separationSolar Cellsps1P*JSCVOCFFP+/Ph-ms / msP+/QA-Energy versus lifetime for separated chargesP+/QB-nsRecombinationsHigh yield: CurrentEnergetic: Voltage/Optical bandgapLong lived: allows charges to transport to external circuit.High yield of long lived, energeticcharge separated states
13Dye sensitised solar cells Michael GrätzelSensitizerdyeNanocrystalline TiO2 filmI- / I2 based electrolytePlatinised TCO coated glassTCO coated glassLab cells upto ~ 11%‘Commercial’ modules 3-4%LightI-I3-External circuitElectronsdifferentdyes
14Energy losses in DSSCsO’Regan and Durrant Acc. Chem. Res. 2009
15Kinetics versus energetics G/eVHigh yield of long lived, energetic charge separated statesRuNCSOTBAHDye*2Dye+ / e-TiO2ElectronInjection~ 100 psDecay toground~ 10 nshuI3- / eth-TiO2Hole transfer to electrolyte~ 1 msRecombination todye+,ms-ms1Recombination toelectrolyte, ms-sTransportmsDye
16Kinetics versus energetics: short circuit G/eVDye*2Dye+ / e-TiO2ElectronInjection~ 100 psDecay toground~ 10 nshu1I3- / eth-TiO2Hole transfer to electrolyte~ 1 msRecombination todye+,msRecombination toelectrolyte, sTransportmsDye
17Charge separation in DSSCs TiO2 acceptor statesInjection rate increases exponentially with energetics:- Increasing kinj x10 costs ~ 300 meVPhysical origin: exponential increase of CB density of states with energyImpact on injection yield and device current depends upon competition with excited state lifetimeDye*kinjDecay toGroundk0doscb exp(E/100meV)energyN719/TiO2Haque et al.J. Am. Chem. Soc. 2005Koops et al.J. Am. Chem. Soc. 2009SolarCellDye
18The role of excited state lifetime: Singlet versus Triplet Injection 1-10 psS1Injection from both S1 and T1 states of N719 possibleT1 energy ~ 300 meV lower than S1 => injection rate ~ 1 orders of magnitude slower.BUT T1 lifetime 5 orders of magnitude longer than S1Much 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).1.9ISC~ 100 fs100 psT11.6G/eVhνRuNCSOTBAHDecay toground~ 10 nsS0Koops et al. JACS 2009
19Catalysis 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 voltage2I- / I2maximumvoltageS / S+S*/ S+diffusionelectrolytesensitiserdyeTiO2conductingglassE / Vvs. NHE1.00.5–0.5e–regenerationinjectionEFCBcathodehνO’Regan et al. J. Am. Chem. Soc
20Schmidt-Mendes et al. Nanolet. 2005 Interface engineering to minimise recombinationAl2O3coatedUncoatedHaque et al.Angew. Chem.2005Palomares et al. JACS 2003Li+- DFHTMHaque et al.Adv Func Mat2004Schmidt-Mendes et al. Nanolet. 2005
23Charge photogeneration Excitonseparatione-LUMOCharge Dissociatione-e-huGeminate recombination of boundpolaron pairs (charge transfer states)HOMOh+h+Coulomb Attraction:er = 3-4 for organicsPolymerC60Key consideration:How do initially generated polaron pairs overcome their coulomb attraction and dissociate into free charges?
24The 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 separationOhkita et al. JACS 2008, Clarke et al. Adv. Func. Mat. 2009
25Onsager TheoryKey issue: Electron thermalisation length (a) versus coulomb capture radiusClarke and Durrant Chem Rev 2010 ACS ASAP
26Reducing the energy cost of charge separation HOMOCharge separation yield (a.u.)CTEnergy Loss during charge separationLUMOAdding intramolecular charge separation in the polymer appears to enable charge separation at lower energy costsClarke et al. Chem Comm 2009, Chem Rev 2010
27Charge 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).DevicephotocurrentAll optical assay ofcharge photogeneration in films
30Bio/inorganic electrodes for hydrogen evolution PtDD+e-hvTiO2 nanoparticleZnCyt-c/TiO2-PtTiO2-PtLong lived (200 ms) charge separationHydrogen evolution observed with ~ 20% internal QY using visible lightProbably never a technology, but hopefully inspiring science.Astuti et al. JACS 2005
31Hole dynamics in TiO2 Timescale: ms -seconds TiO2 hole decay dynamicsTimescale:ms -secondsO2 QY data as function of excitation intensityElectron 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 / nanoparticleConsistent with needing to accumulate 4 oxidising equivalents to generate one O2Tang, Durrant & Klug JACS 2008
32Hole dynamics in Fe2O3 TCO e- light -0.1 V EFe dark h+ 2H2O 4H+ +O2 CB Visiblee-h+CBVBTCO2H2O4H+ +O2Bulk chargerecombinationEFe-0.1 V+ 0.4 VPhotoelectrode for visible light driven water oxidationPhotocurrent only observed under positive biasPositive bias increases hole lifetime from ms/ms to seconds
33Lessons for the artificial leaf For the synthetic chemists: Key features areNanostructures, self-assembly, multi-function componentsFor the photochemists: Kinetics versus thermodynamics key as for photosynthesisFor 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.
34Acknowledgements Dye sensitized solar cells Brian O’Regan, Piers Barnes, Assaf Anderson, Li Xiaoe,Andrea Listorti, Joe Mindagaus.plus Michael Grätzel, Nazeeruddin et al. (EPFL), DavidOfficer et al (Wollangong), Corus, g24iPolymer / fullerene solar cellsTracey 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-workersplus 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 FuelsJunwang 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
35Lessons for the artificial leaf For the synthetic chemists: Key features areNanostructures, self-assembly, multi-function componentsFor the photochemists: Kinetics versus thermodynamics key as for photosynthesisFor 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.
36Lessons from DSSCs and OPV for solar to fuels Yield versus lifetime versus energy determines efficiencyElegant structures are so far not functionally betterMultifunctional components are painful to develop but necessaryReal (disordered) materials properties are critical to determining function (rather than Marcus, Forster, Redfield……..)Catalysis and multi-electron chemistry can be exploited to aid kineticsA lot of the key action happens on unfashionably slow timescalesThe efficiency gap between lab scale champion cells and cheap, stable modules can be critical (and painful)