derek j. hollman undergraduate physics symposium

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Interfacial Charge Transfer in Solar Cells: A Single Molecule Perspective. Derek J. Hollman Undergraduate Physics Symposium. 8 May 08. Dye-Sensitized Solar Cells (DSSC). Interfacial Dynamics Essential to Device Performance!. Understanding the DSSC. - PowerPoint PPT Presentation

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  • Derek J. HollmanUndergraduate Physics Symposium

    Interfacial Charge Transfer in Solar Cells: A Single Molecule Perspective8 May 08

  • Dye-Sensitized Solar Cells (DSSC)Interfacial Dynamics Essential to Device Performance!

  • Understanding the DSSC

    Understanding interfacial charge transfer in DSSC complicated by heterogeneityNecessitates well-defined model system with controlled interfaceBulk properties do not reveal complete dynamics in heterogeneous systems such as DSSCMust observe single molecules to address rates and mechanisms of charge transfer

  • Experimental RealizationWe may observe:Electron transfer ratesDistance dependenceInfluence of interband statesInfluence of surface statesOrientation dependenceSystem:Perylene bisimide dyeGallium Nitride (GaN)Scandium Oxide (Sc2O3)Ultra-high vacuumConfocal Microscopy

  • Thickness from 5 -1000 to slow charge transferNear-perfect, abrupt interfaceSc2O3 (111) grown heteroepitaxially on GaN (0001)The Choice of Sc2O3/GaNChang Liu et al., APL 88 (2006), 222113

  • Single Molecule CT ReporterR = -C4H9 or -C13H27Strong absorber (e = 75000 M-1cm-1) with unity quantum yieldLow intersystem crossing rates and short triplet lifetimePerylene/TiO2 used in DSSCElectronic properties tunable by bay-substitution

  • Towards Single Molecule Spectroscopy in UHV27.570kcpsDistinct on and off states only seen at single molecule level

  • ObjectiveHistograms/distributions: P()

    Autocorrelation function: g(2)() From these analyses, information about CT kinetics can be elucidated

    Simulate 2-state system, develop statistical analyses to recover rate informationMechanism!

  • With kf >> kex >> kfct, 3-state system effectively becomes a 2-state systemSimulation: Signal Generationton, toff exp. deviaterepeaton/offcounts

  • On/off Time DistributionsOn/off transitions may be Poissonian processes; on/off times are exponentially distributedCT kinetics may also be power-law distributedObserving fluorescence intermittency provides information on CT kineticsDistribution contains information on mechanism

  • Dependence on Bin SizeAmbiguity of on/off state

  • Drawing the Linekfct = 100Hzkrecovered = 97 5 Hzoff-time histogramon-time histogramkbct = 100HzAnalysis:Start clock; measure time molecule was on or offWhen a transition occurs, record time, bin it, reset clockRepeat

  • AutocorrelationDetermine correlation between pairs of photons at arbitrarily long times

  • ConclusionsCT kinetics of a DSSC can be understood by analyzing single molecule fluorescence intermittency trajectoriesExperimental design allows for a good model and control of many parametersSimulation provides a framework for developing analysesAnalyses can recover rates for a 2-state system

  • Future Simulation WorkFit autocorrelation functionsPower-law kineticsMultiple dark statesPhoton arrival times for additional informationUse analyses on real data!

  • University of ArizonaDr. Oliver L. A. MontiDr. Brandon S. TackettMichael L. BlumenfeldLaura K. SchirraMary P. SteeleJason M. TylerStefan Kreitmeier (TU Mnchen)University of FloridaDr. Brent P. GilaDr. Stephen J. Pearton

  • DSSC A Complex StructureSEM micrograph of titanium oxide films. M. Grtzel et al., J. Am. Ceram. Soc. 80, 3157.L. Kavan, M. Grtzel, S. E. Gilbert, C. Klemenz, H. J. Scheel, JACS 118, 6716

    Charge transfer in heterogeneous environmentCrystal face- and structure-dependent device performance

  • Kinetics in DSSCT. Hannappel, B. Burfeindt, W. Storck, F. Willig, JPCB 101, 6799S.A. Haque, Y. Tachibana, D.L. Klug, J.R. Durrant, JPCB 102, 1745Result: Non-exponential charge transfer kinetics

  • Ideal Model SystemDonor: Single molecule to model excited state in solar cellAcceptor: Single-crystalline wide bandgap semiconductorSpacer Layer: Heteroepitaxial single crystalline surfaceControllably vary donor-acceptor distanceSlow down charge transfer kineticsConditions: Growth and measurement in ultra-high vacuum

  • Experimental Realization We may observe:Forward and backward electron transfer ratesDistance dependenceInfluence of interband statesInfluence of surface statesOrientation dependenceSystem: Perylene bisimide on Sc2O3 / GaN one molecule at a time!

  • Single Molecule CT ReporterR = -C4H9 or -C13H27Strong absorber (e = 75000 M-1cm-1) with unity quantum yieldLow intersystem crossing rates and short triplet lifetimePerylene/TiO2 used in DSSCElectronic properties tunable by bay-substitution

  • PTCDI/Sc2O3/GaN so farELUMO(PTCDI) = 0100 meV vs. Sc2O3/GaN CBM

  • Excitation/Emission GaNThere are states within the bandgap!

  • Fluorescence IntermittencySingle molecules exhibit blinkingOn/Bright state: continual excitation, fluorescence cyclingOff/Dark state: non-fluorescing state resulting from ISC or CT eventton, on-time: period of continual excitation/fluorescing until a single molecule ISC or CT eventtoff, off-time: period until a charge recombination or reverse ISC event

  • Time ScalesISC events occur with low transition rate and short lifetime, typically microsecond or shorterCT events occur with much longer lifetimes, millisecond to seconds, also tunable (insulator layer)Data acquisition rate much slower than ISC event rateISC events only lower average cps

  • What it looks likeDistinct visible states, on and off, only seen at single molecule level

  • Model System With kf >> kex >> kfct, 3-state system effectively becomes a 2-state system

    Experimental acquisition rate: 103 - 104 Hz

    kf ~ 109 Hz, kex ~ 106 Hz, kfct ~ 103 Hz

  • Poissonian ProcessesOn/off transitions are Poissonian processesOn or off times may be characterized by Poisson distributionke-ktExponential because Transfer of charge may be a tunneling process Kinetics may follow well-defined rate constant

  • Power-law KineticsCT kinetics may be power-law distributed:

    Fluctuating rate constant; molecule sampling multiple surface sites

    Observing fluorescence intermittency provides information on CT kinetics

  • Motivation for a SimulationShot-noise limited signals with low S/N, need sophisticated methods of analyzing dataSimulation provides framework for developing various analysesControl of input rate parameters, want to recover themDo not know experimental rates a priori, can not verify analyses otherwise

  • Simulated Fluorescence TrajectorySignal generated at rate much faster than real acquisition rate, then re-binned

  • Re-binning Simulated TraceSimulated data generated on 1s time stepReal data acquisition rate closer to 0.1-1ms

  • On/off HistogramsWill investigate dependence on threshold, bin size

    off times histogramon times histogram

  • krecovered = 97 5 HzRecoveryFit histograms to exponential; decay rate should be input rateRecovery!

    kfct = 100Hzm = -0.097 0.005off times histogram

  • AutocorrelationDetermine correlation between pairs of photons at arbitrarily long timesShape of autocorrelation contains kinetics of system

    Algorithm implemented:

    *How its built, how important interfaces areHow is such a cell made: nanocrystalline TiO2, liquid I/I3, electrodes, dye molecules attached to TiO2interface controls charge generation/transport in the solar cell

    8% for ionic liquid DSSC1% for all-solid DSSC with solid HTM6% with polymer electrolytes* *Energy transfer to intra-gap states?Forward: LUMO -> CBBackward: CB -> HOMO*Dots indicate perfect crystallinity

    Well-defined system*Low ISC rate in contrast to Ru dyes such as N3Evaporated onto substrate at low concentration, in isolation*10uW, This shows stuff we cant see in the ensemble. Say: We dont expect any interesting dynamics on glass.**distribution tells about mechanism***use photon arrival times for new information***left figure: only TiO2 filmDifference in efficiency from different crystals Different device performance based on crystal surfacesensitized with cis-Ru[L2(SCN)2], L= 2,2-bipyridyl-4,4-dicarboxylic acid)*Transient absorption, short light pulse to excite the dye. Bring in another short light pulse (white), record spectral change after white light moves through device; what has changed, what has been absorped, will see absorption peaks from dye, dye+, TiO2 e-, can watch evolution of signal from dye+. Its decay says something about recombination kinetics.

    Non-exponential, not a simple decay of an excited state, some other physics going on

    Crystal face sensitivity,

    CT (with liquids) are non-exponential, difficult to extract mechanistic informationWillig: N3 dye on anatase TiO2, recombinationDurrant: RuII-(2,2-bipyridyl-4,4dicarboxylate)2(NCS)2 sensitized nanocrystalline TiO2, recombination

    *Forward AND backward CTPrecise distribution of on and off periods determined by mechanism!

    Heteroepitaxial (lattice matching between two different materials)

    Charge xfer from dye to GaN very fast, fs to ps. Recombination in liquid much slower, could be competitive in solids

    If want to do this at single molecule, must space it. Rate scales exponentially with distance (tunneling process). Can slow it to us length and longer quite comfortably*Energy transfer to intra-gap states?Forward: LUMO -> CBBackward: CB -> HOMO*Low ISC rate in contrast to Ru dyes such as N3Evaporated onto substrate at low concentration, in isolationIn process of identifying/verifying these *Single slide with a figure (with time scales, what it looks like, model system)********New Jason data, 3D fluorescence data*New Jason data, 3D fluorescence data*