structured zno growth using electrochemical deposition for light management in solar cells
TRANSCRIPT
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Structured ZnO growth usingelectrochemical deposition for lightmanagement in Solar cells
Graduate Seminar
Nanomolecular Science Program
Jacobs University Bremen, Germany
Uddin Md. Jalal
March 02, 2012
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Outline
Basic science of Solar Energy conversion
Thin-films in Solar Cell
Overview of conventional light trappingtechnology and their limitations
Proposed research project
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What is Photovoltaics (PV)?
Photovoltaics (PV) is a technology to generate electrical energy
in the form of current and voltage from semiconductor when
they are illuminated by photons.
Why Photovoltaics?
Direct conversion of sunlight to electricity is possible usingSolar cells
Environment-friendly renewable energy source
Guaranteed energy availability
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Basic Process of Photovoltaics
Energy Generation
The basic processes behind the PV
effect are:
Generation of the charge carriersdue to the absorption of photons
in the materials that form a junction
Subsequent separation of the
photo-generated charge carriers
in the junction
Collection of the photo-generated
charge carriers at the terminals of
the junction
Fig.1 Illuminated semiconductor illustratingincoming, reflected and absorbed light in thesemiconductor and light passing through thesemiconductor.
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Fig.3 Boosting electron at higher energyFig.2 Electron-hole pair generation
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Fig.4 Transfer electron to contact terminals Fig.5 Transfer of energy to the external circuit
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Conversion of Solar Energy
Fig.6 Solar spectrum at different airmasses
The solar spectrum shows that forAM1.0, a maximum intensity is at
the wavelength l = 0.5 mm and itfalls to half at l = 1 mm.
AM = 1/cos, is the angle of theposition of the sun with a vertical
line.
Stephen J. Fonash (2010) Solar Cell Devices Physics, Elsevier, USA, 2nd Ed., 1-7
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Fig.7 Cross-section of a typical Solar Cell
The total input power PIN :
The output power POUT :
POUT = JV
Fig.8 J-V Characteristic of Solar Cell underillumination
The best efficiency of the PV energy
conversion process for the cell of Fig. 4:
The fill factor:
cs AAWhen
in
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P
VJ
cs
c
s
AAWhenA
A
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P
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ocsc
mpmp
VI
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Stephen J. Fonash (2010) Solar Cell Devices Physics, Elsevier, USA, 2nd Ed., 1-7
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Motivation with Thin-film Silicon
Material
Silicon is most commonly used material for photovoltaic device.Because-
Silicon is abundant and easy to obtain from earth crust
Well characterized and use of Si material in thin-film leads to the costminimization
The thickness varies for a few mm to 10 mm with diffusion lengthof 10-20 nm
Best efficiency of single Si is 24.7% with maxm theoretical value of 30%
K. L. Chopra et al. (2004) Prog. Photovolt: Res. Appl. 12, 69
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Thin film Si forms active layer of the thickness of a few mm
Most of the photons dont contribute to the electron-hole pair generation
Requires additional light trapping techniques
Major Challenge with Si Solar cell
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Overview of Light Trapping Technology
Light trapping mechanism is required
To increase the optical path length inside the active layer To improve absorption in the active layer
To prevent light that otherwise would be lost
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Methods to trap light are-
Anti reflection coating Use of back reflector
Surface texturing
Methods to Trap Light Inside Solar Cell
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Materials that has lower refractive index (nARC)
Materials with refractive index of 1.87
Materials thickness of d1= lo/4n1 Prominent ARC materials are ZnO, Si3N4
Fig.9 Simplified schematic of thin film Si solarcell showing anti-reflecting coating
Anti Reflection Coating (ARC):
Albert Polman,Light management in thin- film solar cells, Center for Nanophotonics
FOM-Institute AMOLF, Amsterdam, The Netherlands
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Fig.10 (a) Thin film Si solar cell showing the effect of thicknessof anti-reflecting coating on (b) Comparison of surface reflectionwith and without antireflection coating
(a) (b)
http://pveducation.org/pvcdrom/design/anti-reflection-coatings
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Back Reflector & Surface Texturing:
Fig.11 Cross-section of microcrystalline thin film Si solar device showing (a) Flat backreflector (b) Textured back reflector
Kenji Yamamoto (2003)JSAP International, 7, 12-19
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Fig.12 Efficiency of microcrystalline thin film Si solar device having (a) Flat backreflector (b) Textured back reflector
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Limits of the Conventional Light Trapping
The maximum possible enhancement has an upper limit which is given
by the absorption coefficient of 4n2/ Sin2,
where, n = the refractive index of the active material
= the angle of emission cone in the medium
surrounding the solar cell
But this limit is no applicable in nanophotonic regime using nanostructures
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Different Approaches to Enhance Light
Trapping to Avoid Conventional Limits
Different approaches:
Metal nanostructures
Photonic crystals
Optical interference in multilayer stacks
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Metal Nanostructures to Enhance
Light Trapping
Fig.13 Light trapping by (a) scattering from metallic or dielectric nanostructures at theSurface of the solar cell, (b) scattering of light from a corrugated metal back surfacecouple to surface plasmon polariton
H.A. Atwater and A. Polman (2010),Nature Materials 9, 205
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Proposed ModelStructured ZnO growth using electrochemical
deposition for light management in Solar cell
Why ZnO?
Its a wide band gap (3.3 eV) material with semi conducting properties
The optical and chemical properties as well as the structures of ZnO
rely on its preparation method
Solution chemistry can be suitably used for ZnO growth with low
temperature of less than 100C.
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Methodologyfor Fabrication of Solar Cell
Step 1:Collection of Indium Tin Oxide (ITO) on glass from the commercial source
Step 2:Growth ZnO nanorods on top of ITO using electrochemical deposition
Fig.14 Schematic setup for the ZnO growth experiment
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Fig.15 Proposed design of nanorod-based hybrid solar cell
Proposed Structure
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Working mechanism of Proposed Model
Fig.16 Explanation of working of Proposed model with charge separating diagram
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Working Mechanism of Proposed Model
Top metal electrode should be transparent
Light comes through the top transparent electrode
and falls on the P3HT semiconductor
Electrons from HOMO in P3HT get excited to LUMO
level and then they go to the LUMO level of ZnO
and get collected by the transparent top electrode
Absence of electrons in the HOMO level of P3HT
creates holes, and they are collected at the ITO
Thus charge carriers generate current
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Conclusion
Use of indium-tin oxide (ITO) as the cathode
It has good conducting properties
Use of poly-3-hexylthiophene (P3HT) as active layer
It possesses high charge-carrier mobility
It has good optical absorption properties
Use of P3HT in organic solar cell as donar material provides the
efficiency of about 5%
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Further Research to Continue
To replace highly cost contact materials with lower cost materials such ascopper or carbon-based materials (nanotubes)
Low-cost (i.e. abundant) materials must be incorporated into reliable, highperformance PV modules and systems
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Thanks for your kind attention!!!