Lecture3 AH

8/13/2019 Lecture3 AH

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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


Amorphous



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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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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&section=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&section=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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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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