Why does it work?Where arethe losses?ThermodynamicsEnergycounterelectrodesemiconductordyeelectrolyte2(s/s*)4.-0.7±:=0.5AV0.3(-/15)1.1(S+/S)2.7VvsNHEloadPhotovoltageAV:differencebetweenlevelofmediatorredoxcouple,andbottomofTiO,conductionbandnormalhydrogenelectrode(NHE)Diagram:Chem.Rev.2010,110,6595normalhydrogenelectrode(NHE)
Why does it work? Where are the losses? Thermodynamics Photovoltage ΔV: difference between level of mediator redox couple, and bottom of TiO2 conduction band normal hydrogen electrode (NHE) Diagram: Chem. Rev. 2010, 110, 6595 normal hydrogen electrode (NHE) 55
KineticsTimeconstants,inseconds10-13s10-12s10-1110-12sForwardprocess10-8s10-3s102sReverseprocesshv3-/1-104s(recombination)10-6sFTOTiO2SensitizerRedoxmediatorFTO:SnO2:FElectroninjectionca.108timesfasterthanrecombination(10-12/10-4)Regeneration of dyeca.100 timesfasterthanrecombination(10-/10-4)Chargetransportonly10timesfasterthanTiO2-to-1-recombination(10-3/10-2)Diagram:Chem.Rev.2010,110,6595
Kinetics Forward process Reverse process (recombination) Time constants, in seconds Electron injection ca. 108 times faster than recombination (10-12/10-4) Regeneration of dye ca. 100 times faster than recombination (10-6/10-4) Diagram: Chem. Rev. 2010, 110, 6595 Charge transport only 10 times faster than TiO2-to-I- recombination (10-3/10-2) 56 FTO: SnO2:F
ElectrostaticinteractionMesoporousTiO,isnotanBulk defectideal material forchargetransportTransportbydiffusionSurface stateThere are manyinefficienciesRecombinationtoelectrolyteGrain boundaryDead end(a)EcEF.nMicroporous:<2nm,ErodorMesoporous:2-50nmMacroporous:>50nmEvDiagram:Chem.Rev.2010,110,6595FTOmesoporousTiO2/ElectrolyteElectrolyte(b)
Diagram: Chem. Rev. 2010, 110, 6595 Mesoporous TiO2 is not an ideal material for charge transport There are many inefficiencies 57 Microporous: < 2 nm; Mesoporous: 2 – 50 nm; Macroporous: > 50 nm
ResearchEfforts:1991-2017The dyes-several thousand investigatedRequirements:1.Coverage offull visible,and ideallynear-infrared spectrum with highextinctioncoefficient (e)2.Strong binding to semiconductor(SC)(via-CO2H or-PO,H2)3.Excited state(bothS,and T)higher inenergythan SC conduction band edge4.Oxidizedstatelowerinenergythanelectrolyteredoxcouple5.DoesnotaggregateonSCsurface6.Stable-photochemically,electrochemicallyandthermally
Research Efforts: 1991 - 2017 The dyes – several thousand investigated Requirements: 1. Coverage of full visible, and ideally near-infrared spectrum with high extinction coefficient (ε) 2. Strong binding to semiconductor (SC) (via –CO2H or –PO3H2) 3. Excited state (both S1 and T1) higher in energy than SC conduction band edge 4. Oxidized state lower in energy than electrolyte redox couple 5. Does not aggregate on SC surface 6. Stable – photochemically, electrochemically and thermally 58
RutheniumPolypyridylComplexesBOCO,HRuL'(NCS)3HO2CMN-C60-RuL2(NCS)2RIC40CHOC-0-TiO220NCS-ligandsextendabsorptionintotheredand NIR0800400600100051050Wavelength[nm]Accumulated photocurrent (hAcm")L=4.4-COOH-2.2-bipyridine40410*L'm4.4'4*.COOH-2.2:6.2"-lerpyridine(,w,s) xng uoroud310302102021101810SolarSpectrum11003004005006007009008001000Wavelength (nm)
NCS- ligands extend absorption into the red and NIR Ruthenium Polypyridyl Complexes Solar Spectrum 59