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7/31/2019 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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