Dr. Lioz Etgar-Abraham Kogan Seminar

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    Excitonic Solar Cells

    Etgar Lioz

    Laboratoire de Photonique et Interfaces, Institut des Sciences et Ingnierie Chimiques,

    Ecole Polytechnique Fdrale de Lausanne (EPFL),Station 6, CH-1015, Lausanne, Switzerland.

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    Outline

    IntroductionQuantum dots based solar cellEnergy transfer in dye sensitized solar cell (DSSC)ZnO NWs as photoanode for DSSCSummaryFuture perspective

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    Introduction

    Why Solar?

    More solar energy is absorbed by the earth every minute than issued in fossilfuels every year.

    Pollution free and wastes/emissions are easily manageable.

    Effective in providing electricity to remote locations where other forms of

    energy are difficult or expensive to get.

    Pollution Fukushima

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    What is a solar cell?

    Solar cells convert sunlight directly into electricity.

    First used in spacecraft and satellites. Traditional types are based on two types of silicon sandwiched

    together (n-type and p-type).

    Based on using photons to separate charges: electron-hole pairs

    Many new types are in research/production stage.

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    First Generation

    Consist of large-area, high quality and single junction devices.

    Involve high energy and labour inputs making these very costly.

    Example: Crystalline Silicon solar cells

    Second Generation

    Consist of Thin film cells.

    Techniques such as vapor deposition and electroplating are advantageous. Expensive production costs.

    Examples: CdTe, Thin film silicon, CIGS.

    Third Generation

    Aim to enhance poor electrical performance of second generation while

    maintaining very low production costs.o Multi junction solar cells.oNanostructures solar cells.o ETA cells. (Extremely thin absorber cells)o Dye-sensitized cells.o Polymer-fullerene cells.

    Introduction

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    Charge separation by electric field

    within a p- and n-doped

    semiconductor material.

    Charge separation by kinetic

    competition like in the photosynthesis

    Dye sensitized solar cellSilicon photovoltaic cell

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    Dye sensitized solar cellsElectrochemical cell

    Voc =

    (EFnE

    Fp) / q

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    Introduction

    Characterization Standard

    Power density of 1000 W/m2Spectral power distribution corresponding toAM1.5

    The Air Mass is the path length which light takes through the atmosphere normalized to the shortest possible

    path length

    The Air Mass quantifies the reduction in the power of light as it passes through the atmosphere and isabsorbed by air and dust.

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    Solar spectrum

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    Metal Contact

    TiO2 NPs film

    FTO glass

    QDs

    Excitonic Solar cells (XSCs)

    e-

    Gel

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    Quantum dots solar cells

    Chemical bath deposition (CBD)Successive ionic layer adsorption and reaction (SILAR)Colloidal quantum dots attached through molecular linker.

    FTO Glass FTO Glass

    CBD,SILAR QDs through molecular linker

    TiO2

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    Metal Contact

    TiO2 NPs film

    FTO glass

    QDs

    TiO2

    NIR-

    QDs

    -4.1

    -7.3

    e-

    h+

    Energy(eV)

    Au

    -3.7

    -5.1

    -5.1

    Cell structure

    Advantages

    1.Solid state device.2.No hole transport material.3.Illumination close to the junction.4.Easy to produce, cost effective.5.Employ many monolayers of the light absorber due to the

    charge transporting functionality of the CQD film.

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    Ligands exchange to MPA-

    PbS QDs deposition by spincoating and ligands exchange

    to 3-mercaptopropionic acid(MPA).

    Ligands can change:

    Energy levels.Solubility.Optical properties.Shape of the QDs.

    PbS QDs

    Gold

    TiO2 NCs

    Compact layerFTO

    Cross section of the device

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    Which factors can affect the QDs solar cell

    performance?

    The thickness of the QDs film.

    QDs Eg (size)Small QDs - more driving force for electron injection and higher Voc, on the other

    hand more particle boundaries to cross until the electrons arrive at the TiO2(higher

    chances for recombination).

    Big QDs- less driving force for electron injection and lower Voc, but should have

    less particle boundaries to cross until the electrons arrive at the TiO2

    If the QD layer is too thick, collection of photogenerated charge carriers will be

    incomplete, while too thin QD layers show poor light harvesting.

    TiO2 thicknessThick TiO2 film the injected electrons wont reach the conductive glass.

    e- e-

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    Voc=0.543 V

    Jsc=16.3mA cm-2

    Fill factor = 41%

    (PCE)= 4.04%(under 90mW cm-2)

    Photovoltaic Performance

    Etgar et al.,ACS nano, 2012, DOI: 10.1021/nn2048153.

    Cell conditions-

    1. TiO2 18nm NPs, 500nm thickness film.

    2. PbS QDs- size of 3.2nm corresponding tofrist excitonic peak at wavelength of 920nm.

    3. 12 layers of 50mg/ml.

    (thickness of ca.300nm)

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    At low bias potential.

    Electrochemical Impedance Spectroscopy (EIS)

    Nyquist plots

    Two different features were observed,

    1. A high frequency arc (frequency range ~1MHzto 100kHz), whose size depends strongly onthe applied bias potential.

    2. A low frequency arc (frequency range~kHz to mHz).From each feature we can extract the Resistance, capacitance and lifetime.

    At high forward bias with the indication of thefrequencies.

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    Electrochemical Impedance Spectroscopy (EIS)

    Low frequency

    Recombination resistance and capacitance

    High frequency

    PbS/Au interfaceTiO2/PbS interface

    Lifetime

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    Voc incresing- C

    OH

    O

    SH

    C=O

    CO2-

    TiO2

    QDsVoc

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    001

    101

    Anatase 101 has the most stable surface, its surface energy is the lowest 0.91 Jm-2Anatase 101 surface is unreactive.Anatase 001 has the higher surface energy (1.43 Jm

    -2

    ) and its surface is reactive.

    Changing the TiO2 dominant facets

    Exposed (001) anatase TiO2 Standard-Exposed (101) anatase TiO2

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    Cross section

    TiO2 nanoplatelets

    PbS QDs absorbance

    Ph t lt i f

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    Photovoltaic performance

    Etgar et al,Advanced Materials, 2012, 24(16), 2202-2206.

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    Summary QDs cell architecture without hole conductor was presented. Around 12 layers of QDs is the optimum thickness for the QDs layer, using PbS

    QDs with energy gap of 1.38eV.

    EIS technique helping us to see more deeply into the device electronic properties. Anatase TiO2 nanosheets having 30 and 80nm size, with dominant (001) facets

    were synthesized and employed in PbS QDs/TiO2 heterojunction solar cell.

    The best photovoltaic performance was achieved using 30nm TiO2 nanosheetswith PbS QDs having Eg of 1.38eV. The photovoltaic parameters give a Jsc of

    20.5 mA/cm2, a Voc of 0.545V, a FF of 0.38 corresponding to a solar to electricpower conversion efficiency () of 4.73% under 0.9 light intensity.

    The higher surface area related to the small nanosheets and the high reactivity ofthe (001) facets contribute to the better performance of the PbS QDs/TiO2

    heterojunction solar cell compared to standard TiO2 NPs.

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    Overlap spectra

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    R06= 9000(ln10)

    2

    QDJ128

    5n

    4N

    AV

    The frster radius-the distance at which the frster energy transfer (FRET) efficiency is 50%.

    n is the refractive index of the host : 1.4-1.5 for the electrolyte in DSSC.

    k2

    is the orientation factor -2/3 for random orientation,

    QD is the quantum efficieny of the donor (28%),

    Nav is the Avogadro number.

    J is the overlap integral, J = ID()A()4d

    0

    R0= 3.7nm

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    QDs emission

    (Donor)

    VG1-C10 emission

    (Acceptor)

    PL lifetime of VG1C10 +QDs On Glass

    The arrows indicate theincrease of the QD/VG1-C10

    ratio.

    E =1DA

    D= 69%

    FRET efficiency

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    Cell type Jsc (mA/cm2

    ) Voc (mV) FF PCEVG1-C10 dye 2.73 542 0.536 0.79

    VG1-C10+CdSe

    QDs (FRET cell)

    3.25 653.4 0.69 1.48

    +19% +20% +29% +87%

    Etgar et al,RSC Advances, 2012, 2 (7), 2748 - 2752.

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    Summary

    This work presents the enhancement of the light harvesting in dye sensitized solarcell due to Frster resonance energy transfer.

    The donors are CdSe QDs and the acceptors are a new design squaraine dyes withan additional carboxcilic group and two long hydrocarbon chains as compared to

    the standard squaraine dye.

    The use of the cobalt complex (Co+2/Co+3) as electrolyte in the cells permitsdirect contact between the QDs and the electrolyte without affecting the QDs.

    PL lifetime measurements showed that FRET is the dominant mechanism fromthe QDs to the dye.

    IPCE measurements exhibit a full coverage of the visible region.All the cell photovoltaic parameters were enhanced proving the efficient energy

    transfer within the QD-dye-sensitized solar cell.

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    Wh Z O NW i t d f TiO NP ?

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    Why ZnO NWs instead of TiO2 NPs?ZnO TiO2

    Crystal structure rocksalt,zinc blende and wurtzite rutile, anatase and brookite

    Energy gap (eV) 3.2-3.3 3-3.2

    Electron mobility

    (cm2VS-1)

    205-300(bulk ZnO),

    1000 (single NW)

    0.1-4

    Growth Low temperature, milder conditions,

    controllable conditions

    High temperature

    NWs-improve charge carrier transportation by

    providing a facile direct electron pathway

    NPs

    Problems??

    1. High efficiencies require high dye-loading. Typically TiO2-based DSSCs usesemiconducting layers of around 20 m thick to ensure a high amount of dye can be

    incorporated into the active layer. Growing ZnO NWs of comparable length and

    internal surface area under hydrothermal conditions is not trivial due to competing of

    homogeneous and heterogeneous nucleation processes.

    2. The surface of ZnO is known to be chemically unstable and contain surface trap states.

    Z O NW l h i l ll

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    N

    O

    O

    S SCN

    COOH

    C55

    H68

    N2O

    4S

    2

    Exact Mass: 884.46

    C218 Dye

    Z960 electrolyte based acetonitrile High molar absorption coefficient ()

    of 62.7103 M-1 cm-1 at 555nm

    ZnO NWs electrochemical cell-

    Etgar et al.Energy & Environment Science, 2011, 4, 2903-2908 .

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    10 m length of ZnO NWs-

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    The overall energy conversion efficiencies of the cell was measured

    under AM 1.5 solar radiation to be 1.25%.Voc= 524.1 mV;

    Jsc= 5.49 mA/cm2;

    ff= 0.43

    10 m length of ZnO NWs-

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    Core Shell ZnO/TiO2 NWs

    0 10 20 30 40

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    1.0

    Efficiency(%)

    Shell thickness (nm)

    Efficiency (%)

    Jsc

    0.0

    0.5

    1.0

    1.5

    2.0

    2.5

    Jsc

    (mAcm

    -2)

    0 10 20 30 40

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    FillfactorandV

    OC

    (V)

    Shell thickness (nm)

    Voltage (V)

    Fill factor

    Shell thickness optimization

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    Open-circuit voltage (Voc) of 819.6 mV

    Short circuit current density (Jsc) of 5.08mA cm-2

    Fill factor of 60.6%

    Power conversion efficiency of 2.53% under AM1.5

    10 m thick of ZnO/TiO2 core shell NWs

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    Full-scale semilogarithmic plots in the dark of the ZnO NWs DSC and the ZnO/TiO2 DSC.

    Dark current of the bare ZnO NWs DSC is higher than the dark current of the ZnO/TiO2NWs DSC.

    A smaller dark current suggest a lower rate of recombination which results an increasingof the Voc and the fill factor.

    Dark Current

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    Summary

    DSSC using new type of photo anode, ZnO NWs was presented.Using 10m thick of ZnO NWs photoanode increase the dyeloading and hence increase the efficiency.

    One of the highest efficiencies using ZnO NWs was received 1.25%.Coating the ZnO NWs with 20nm TiO2 shell increase the fill factor and theVoc dramatically resulting an efficiency of 2.53%.

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    Summary

    PbS Quantum dots/TiO2 heterojunction solar cells were presented achievingpower conversion efficiency of 4.7% with cuurent density of 20.5 mA cm-2.

    Energy transfer between CdSe QDs as donors and Squarine dyes as acceptors indye sensitized solar cell show an enhancement of 87% in power conversion

    efficiency .

    ZnO NWs coated with 20nm TiO2 shell as photoanodes in DSSC presentingPCE of 2.5%.

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    qNanostructures oxide films as photoanode for XSCs.qNanostructured inorganic-organic heterojunction

    solar cells.

    q Stability

    Future Perspective

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    Marie Curie, Intra European Fellowships(IEF), FP7-PEOPLE-2009-IEF.PIEF-GA-2009-252228.

    Innovasol, FP7-energy-NMP 2008- Novel materials for energyapplication.

    Prof. Graetzel Michael. LPI members. (around 50!!)

    Acknowledgment

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