lecture3 ah
TRANSCRIPT
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Lecture 3Types of Solar Cells (experiment )-
3 generationsGeneration 1:
Single- and poly-Crystalline SiliconGrowth, impurity diffusion, contacts
Modules, interconnection
Generation 2:Polycrystalline thin films, crystal structure, deposition techniques
CdS/CdTe (II-VI) cells
CdS/Cu(In,Ga)Se2 cellsAmorphous Si:H cells
Generation 3:High-efficiency Multi-junction Concentrator Solar Cells based on III-Vs and III-V ternary analogs
Dye-sensitized cells
Organic (excitonic) cells
Polymeric Cells
Nanostructured cells including Multi-carrier per photon cells, quantum dot and quantum
confined cells
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Figure 3. The three generations of solar cells. First-generation cells are based on expensive silicon wafers
and make up 85% of the current commercial market. Second-generation cells are based on thin films of
materials such as amorphous silicon, nanocrystalline silicon, cadmium telluride, or copper indium
selenide. The materials are less expensive, but research is needed to raise the cells' efficiency to the
levels shown if the cost of delivered power is to be reduced. Third-generation cells are the research goal:
a dramatic increase in efficiency that maintains the cost advantage of second-generation materials. Their
design may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlight
concentration, or new materials. The horizontal axis represents the cost of the solar module only; it mustbe approximately doubled to include the costs of packaging and mounting. Dotted lines indicate the cost
per watt of peak power (Wp). (Adapted from ref. 2,) Green.)
http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_60/iss_3/37_1.shtmlhttp://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_60/iss_3/37_1.shtml -
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Generation I.
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Single Crystal Ingot-based PVs
Single crystal wafers made byCzochralski process, as in siliconelectronics
Comprise 31% of market
Efficiency as high as 24.7%
Expensivebatch process involvinghigh temperatures, long times, andmechanical slicing Wafers are notthe ideal geometry
Benefits from improvements
developed for electronics industry
http://hydre.auteuil.cnrs-dir.fr/dae/competences/cnrs/images/icmcb03.jpg
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6.6.06 - 8.6.06 Clemson Summer SchoolDr. Karl Molter / FH Trier / molter@fh-
trier.de
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Production-Processmono- or multi-
crystalline Silicon
crystal growth process
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6.6.06 - 8.6.06 Clemson Summer SchoolDr. Karl Molter / FH Trier / molter@fh-
trier.de
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Production process1. Silicon Wafer-technology (mono- or multi-crystalline)
Tile-production
Plate-production
cleaning
Quality-control
Wafer
Most purely silicon99.999999999%
Occurence:
Siliconoxide (SiO2)
= sand
melting /
crystallization
SiO2 + 2C = Si + 2CO
Mechanical cutting:
Thickness about 300m
Minimum Thickness:
about 100m
typical Wafer-size:
10 x 10 cm2
Link to
Producers of Silicon Wafers
http://mmoll.home.cern.ch/mmoll/links/silicon.htmhttp://mmoll.home.cern.ch/mmoll/links/silicon.htm -
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Energa Fotovoltaica
Celdas Solares
De Silicio monocristalino
Material: Silicio monocristalino
Temperatura de Celda: 25C Intensidad luminosa: 100%
rea de la celda: 100 cm2
Voltaje a circuito abierto: Vca = 0.59 volts
Corriente a corto circuito: Icc = 3.2 A
Voltaje para mxima potencia: Vm = 0.49 volts
Corriente para mxima potencia: Im = 2.94 A
Potencia mxima: Pm = 1.44 Watts
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Polycrystalline Ingot-based PVs
Fastest-growing technology involves casting Si
in disposable crucibles
Grains mm or cm scale, forming columns in
solidification direction
Efficiencies as high as 20% in research
Production efficiencies 13-15% Faster, better geometry, but still requires
mechanical slicing
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Polycrystalline Si Ribbon PVs
String method Two strings drawn through melt stabilize ribbon edge
Ribbon width: 8 cm
Carbon foil method (edge-defined film-fed growth,
EFG) Si grows on surface of a carbon foil die Die is currently an octagonal prism, with side length 12.5
cm
Pros and Cons Method can be continuous Requires no mechanical slicing
Efficiencies similar to other polycrystalline PVs
Balancing growth rate, ribbon thickness and width
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Generation II.
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Flat-Plate Thin-Films
Potential for cost advantages over crystalline silicon
Lower material use
Fewer processing steps
Simpler manufacturing technology
Three Major Systems
Amorphous Silicon
Cadmium Telluride
Copper Indium Diselenide (CIS)
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6.6.06 - 8.6.06
Clemson Summer School
Dr. Karl Molter / FH Trier / molter@fh-
trier.de
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Production Process
semiconductor materials are evaporated on
large areas
Thickness: about 1m
Flexible devices possible
less energy-consumptive than c-Silicon-process
only few raw material needed
Typical production sizes:
1 x 1 m2
Thin-Film-Process (CIS, CdTe, a:Si, ... )
CIS Module
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Photon Energy
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Material Level of
efficiencyin % Lab
Level of efficiency in %
Production
Monocrystalline
Silicon Approx. 24 14 to 17
Polycrystalline
Silicon Approx. 18 13 to 15
Amorphous
Silicon Approx. 13 5 to 7
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Basic Cell Structure
p-i-n structure
Intrinsic a-Si:Hbetween very thin p-n
junction Lower cells can be a-
Si:H, a-SiGe:H, ormicrocrystalline Si
Produces electricfield throughout thecell
http://www.sandia.gov/pv/images/PVFSC36.jpg
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CdTe
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Cadmium Telluride
One of the most
promising approaches
Made by a variety of
processes
CSS HPVD
http://www.nrel.gov/cdte/images/cdte_cell.gif
http://www.sandia.gov/pv/images/PVFSC29.jpg
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John A. Woollam, PV talk UNL 2007 31
CdTe and CIGS Review: 2006 World PV ConferenceNoufi and Zweibel, NREL/CP -520-39894, 2006
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John A. Woollam, PV talk UNL 2007
Cadmium Telluride Solar CellsD.E.Carlson, BP Solar
CdS/CdTe heterojunction: typically
chemical bath CdS deposition, and
CdTe sublimation.
Cd Toxicity is an issue.
Best lab efficiency = 16.5%
First Solar plans 570 MWp
production capacity by end of2009.
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Nano-Structured CdS/CdTe Solar Cells
Nanocrystalline CdS
CdTe
ITO
Glass
Graphite
Band gap of CdS can be tuned in the range 2.4 - 4.0 eV.
Nano-structured CdS can be a better window material and may
result in high performance, especially in short circuit currents.
Nano CdS/ CdTe device Structure.
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Pros and Cons
Pros A material of choice for thin-flim PV modules
Nearly perfect band-gap for solar energy conversion
Made by a variety of low-cost methods
Future efficiencies of 19% "CdTe PV has the proper mix of excellent efficiency and manufacturing cost to make
it a potential leader in economical solar electricity." Ken Zweibel, NationalRenewable Energy Laboratory
Pros Health Risks
Environmental Risks Safety Risks
Disposal Fees
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Modulos Solares de CdTe
Costo 60% de Si
20 aos garantia
Modulos de peliculasdelgadas
Potencia 50 60 W
Eficiencia 9%
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Modulos Solares de CdTe
Costo 60% de Si
20 aos garantia
Modulos de peliculasdelgadas
Potencia 50 60 W
Eficiencia 9%
100 kW
1 MW
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Tandem Cells
Current output matched for individual cells Ideal efficiency for infinite stack is 86.8%
GaInP/GaAs/Ge tandem cells (efficiency 40%)
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6.6.06 - 8.6.06
Clemson Summer School
Dr. Karl Molter / FH Trier / [email protected]
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Tandem-
cell
Pattern of a multi-
spectral cell on the
basis of the
Chalkopyrite
Cu(In,Ga)(S,Se)2
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Generation III.
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Multijunction Concentrators
Similar in technique
Exotic Materials
More expensive processing (MBE)
http://www.nrel.gov/highperformancepv/entech.html
S t l b T i l J ti S l C ll
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John A. Woollam, PV talk UNL 2007
Spectrolabs Triple-Junction Solar CellD.E.Carlson, BP Solar
Spectrolab: 40.7% conversion efficiency at ~ 250 suns.
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[edit] Gallium arsenide substrateTwin junction cells with Indium gallium phosphideand gallium arsenide can be made on gallium
arsenide wafers. Alloys of In.5Ga.5P through
In.53Ga.47P may be used as the high band gap
alloy. This alloy range provides for the ability to
have band gaps in the range of 1.92eV to 1.87eV.
The lower GaAs junction has a band gap of
1.42eV.
The considerable quantity of photons in the solar
spectrum with energies below the band gap of
GaAs results in a considerable limitation on theachievable efficiency of GaAs substrate cells.
http://en.wikipedia.org/w/index.php?title=Multijunction_photovoltaic_cell&action=edit§ion=5http://en.wikipedia.org/wiki/Indium_gallium_phosphidehttp://en.wikipedia.org/wiki/Indium_gallium_phosphidehttp://en.wikipedia.org/wiki/Indium_gallium_phosphidehttp://en.wikipedia.org/w/index.php?title=Multijunction_photovoltaic_cell&action=edit§ion=5 -
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Dye-sensitized Solar Cells
ORegan and Grtzel 1991
Organic dye molecules + nanocrystalline
titanium dioxide (TiO2)
11% have been demonstrated
Benefits: low cost and simplicity of
manufacturing
Problems: Stability of the devices
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Operation
Sunlight enters the cell through the transparent SnO2:F top
contact, striking the dye on the surface of the TiO2. Photonsstriking the dye with enough energy to be absorbed will create an
excited state of the dye, from which an electron can be "injected"
directly into the conduction band of the TiO2, and from there it
moves by diffusion (as a result of an electron concentration
gradient) to the clear anode on top.
Meanwhile, the dye molecule has lost an electron and themolecule will decompose if another electron is not provided. The
dye strips one from iodide in electrolyte below the TiO2, oxidizing
it into triiodide. This reaction occurs quite quickly compared to the
time that it takes for the injected electron to recombine with the
oxidized dye molecule, preventing this recombination reaction
that would effectively short-circuit the solar cell.
The triiodide then recovers its missing electron by mechanically
diffusing to the bottom of the cell, where the counter electrode re-
introduces the electrons after flowing through the external circuit.
http://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Gradienthttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Iodidehttp://en.wikipedia.org/wiki/Triiodidehttp://en.wikipedia.org/wiki/Short-circuithttp://en.wikipedia.org/wiki/Counter_electrodehttp://en.wikipedia.org/wiki/Counter_electrodehttp://en.wikipedia.org/wiki/Short-circuithttp://en.wikipedia.org/wiki/Short-circuithttp://en.wikipedia.org/wiki/Short-circuithttp://en.wikipedia.org/wiki/Triiodidehttp://en.wikipedia.org/wiki/Iodidehttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Gradienthttp://en.wikipedia.org/wiki/Diffusion -
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Organic Solar Cells
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Fig. 1. The scheme of plastic solar cells. PET -
Polyethylene terephthalate, ITO - Indium Tin
Oxide, PEDOT:PSS - [[Poly(3,4-
ethylenedioxythiophene)
poly(styrenesulfonate), Active Layer (usually apolymer:fullerene blend), Al - Aluminium.
http://en.wikipedia.org/wiki/Polyethylene_terephthalatehttp://en.wikipedia.org/wiki/Indium_Tin_Oxidehttp://en.wikipedia.org/wiki/Indium_Tin_Oxidehttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Indium_Tin_Oxidehttp://en.wikipedia.org/wiki/Indium_Tin_Oxidehttp://en.wikipedia.org/wiki/Polyethylene_terephthalatehttp://en.wikipedia.org/wiki/Polyethylene_terephthalatehttp://en.wikipedia.org/wiki/File:Solarcells4.gif -
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Nanostructured Solar cells
d l ll
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Nanostructured Solar Cells
Nanomaterials as lightharvesters leading todirect conversion orchemical productionalone or imbedded ina matrix.
Questions: [email protected]
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Fig.2 (a) Nanostructure of anodically formed Al2O3 template. (b) its cross-section,
(c) catalyst deposited at the bottom of the pores, (e) vertically aligned nanotubes, and (f) TEM
image of a nanotube.
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z
z
n-CdS
Alumina
p-CIS
Mo/Glass
ITO
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PTCBI
Porous Al2O3
CuPc
ITO
ITO
Al or Ag
CuPc
PTCBI
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PV M d l C i Effi i i
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John A. Woollam, PV talk UNL 2007
PV Module Conversion EfficienciesD.E.Carlson, BP Solar
Modules Lab
Dye-sensitized solar cells 3 5% 11%
Amorphous silicon (multijunction) 6 - 8% 13.2%
Cadmium Telluride (CdTe) thin film 8 - 10% 16.5%
Copper-Indium-Gallium-Selenium (CIGS) 9 - 11% 19.5%
Multicrystalline or polycrystalline silicon 12 - 15%20.3%
Monocrystalline silicon 14 - 16%23%
High performance monocrystalline silicon 16 - 19%24.7%
Triple-junction (GaInP/GaAs/Ge) cell (~ 250 suns) - 40.7%
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Generation III Solar Cells not yetrealized experimentally
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Multiband Cells
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Multiband Cells
Intermediate band formed by impurity levels. Process 3 also assisted by phonons
Limiting efficiency is 86.8%
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Quantum Dots
Multiple Quantum Well
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Multiple Quantum Well
Principle of operation similar to multibandcells
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Multiple E-H pairs
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Multiple E H pairs
Many E-H pairs created by incident photonthrough impact ionization of hot carriers
Theoretical efficiency is 85.9%
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Figure 3. Photoexcitation at 3Eg creates a 2Pe-2Ph exciton state.This state is coupled to multiparticle states with matrix element V
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This state is coupled to multiparticle states with matrix element V
and forms a coherent superposition of single and multiparticle
exciton states within 250 fs. The coherent superposition dephases
due to interactions with phonons; asymmetric states (such as a 2Pe-1Sh) couple strongly to LO phonons and dephase at a rate of -1.
To study MEG processes in QDs we detect
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To study MEG processes in QDs, we detect
multiexcitons created via exciton multiplication
(EM) by
monitoring the signature of multiexciton decay in
the
transient absorption (TA) dynamics, while
maintaining a
pump photon fluence lower than that needed to
create
multiexcitions directly. The Auger recombination
rate is
proportional to the number of excitons per QD
with the
decay of a biexciton being faster than that of the
single
exciton. By monitoring the fast-decay componentof the
TA dynamics at low pump intensities we can
measure the
population of excitons created by MEG.
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The work reported here provides a confirmation of the
previous report of efficient MEG in PbSe. We observed a
previously unattained 300% QY exciting at 4Eg in PbSe QDs,indicating that we generate an average of three excitons per
photon absorbed. In addition, we present the first known
report of multiple exciton generation in PbS QDs, at an
efficiency comparable to that in PbSe QDs. We have shown
that a single photon with energy larger than 2Eg can
generate
multiple excitons in PbSe nanocrystals, and we introduce a
new model for MEG based on the coherent superposition of
multiple excitonic states. Multiple exciton generation incolloidal QDs represents a new and important mechanism
that may greatly increase the conversion efficiency of solar
cell devices.
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For the 3.9 nm QD (Eg = 0.91 eV), the QY reaches a
surprising value of 3.0 at Ehn/Eg = 4. This means that on
average every QD in the sample produces three
excitons/photon.
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Fig. 2. Calculated efficiencies for different QYII
models.
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