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Solar Cells Background• 1839 - French physicist A. E. Becquerel first recognized the photovoltaic

effect.

• Photo+voltaic = convert light to electricity

• 1883 - first solar cell built, by Charles Fritts, coated semiconductor selenium with an extremely thin layer of gold to form the junctions.

• 1954 - Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. Daryl Chapin, Calvin Fuller and Gerald Pearson, invented the first practical device for converting sunlight into useful electrical power. Resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6%.

• 1958 - First spacecraft to use solar panels was US satellite Vanguard 1

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Solar Cell and Photoelectric Effect

1. Light absorptionh

-

+ 2. Generation of „free“ charges

3. effective separation of the charges

Result: wearless generation of electrical Power by light absorption

4.6.07 - 6.6.07 Clemson Summer School 2007Dr. Karl Molter / FH Trier / [email protected]

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energy-states in solids:Band-Pattern

Atom Molecule/Solid

ener

gy-s

tate

s

• • • • • • • •


4

energy-states in solids:Insulator

electron-energyconduction-band

valence-band

Fermi-level EF

bandgap EG

(> 5 eV)


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Terms:Fermilevel EF: limit between occupied and non

occupied energy-states at T = 0 K (absolute zero)

valence-band: completely occupied energy-band just be-

low the Ferminiveau at T = 0 K, theelectrons are „fixed“ (tightly bound)

inside the atomic structure

conduction-band:energy-band just above the valence-band, the electrons can move „freely“

bandgap EG: distance between valance-band andconduction band


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energy-states in solids :metal / conductor

electron-energy

conduction-band

Fermi-level EF


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energy-states in solids:semiconductor

electron-energy

conduction-band

valence-band

Fermi-level EF

bandgap EG

( 0,5 – 2 eV)


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Electron-EnergyAt T=0 (absolute zero of temperature) the electrons occupy the

lowest possible energy-states. They can now gain energy in two ways:

• Thermal Energy: kT (k = Boltzmanns Constant, 1.381x10-23 J/K, T = absolute temperature in Kelvin)

• Light quantum absorption: h (h = Plancks Constant, h = 6.626x10-34 Js, = frequency of the light quantum in s-1).

If the energy absorbed by the electron exceeds that of the bandgap, they can leave the valence-band and enter the conduction-band:


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energy-states in semiconductorsphysical properties:

thermal viewpoint: The larger the bandgap the lower is the conductivity. Increasing temperature reduces the electrical resistance (NTC, negative temperature coefficient resistor)

optical viewpoint: the larger the bandgap the lower is the absorption of light quantums. Increasing light irradiation decreases the electrical resistance (Photoresistor)


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doping of semiconductorsIn order to avoid recombination of photo-induced charges and to „extract“ their energy to an electric-device we need a kind of internal barrier. This can be achieved by doping of semiconductors:

IIIB IVB VB

Si14

B 5

P15

„Doping“ means in this case the replacement of original atoms of the semiconductor-material (e.g. Si) by different ones (with slightly different electron configuration). Semiconductors like Silicon have four covalent electrons, doping is done e.g. with Boron or Phosphorus:


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N - Doping

Si Si

Si

Si

Si

Si

Si

Si

Si

P+

-

n-conducting Silicon

-

crystal view

conduction-band

valence-band

EF

- - - - -P+ P+ P+ P+ P+

majority carriers

donator level

energy-band view


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P - Doping

Si Si

Si

Si

Si

Si

Si

Si

Si

p-conducting Silicon

B- +

+

crystal

conduction band

valence-band

EF B- B- B- B- B-

majority carriersacceptor level

+ + + + +

energy-band view


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p – type region

EFB- B- B- B- B-

+ + + +

n – type region

- - - -P+ P+ P+ P+ P+

p/n-junction without lightBand pattern view

+

--Diffusion

+

Diffusion

internal electrical field

+ -Ed

Ud

depletion-zone


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p–type region

EFB- B- B- B- B-

+ + + +

n–type region

- - - -P+ P+ P+ P+ P+

irradiated p/n-junctionband pattern view (absorption p-zone)

+

-

+

photocurrent

Internal electrical field

+ -Ed

Ud

depletion-zoneE = h

-


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p/n–junction without irradiation(semiconductor diode)

crystal view

n-silicon

- - - - - - - - - - - -

- - - - - - - - - - - -

- - - - - - - - - - - -

- - - - - - - - - - - -

p-silicon

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+

-diffusion -

+electrical fieldE

- - - - - - - - - - - -+ + + + + + + + + + + ++ + + + + + + + + + + +

+ + + + + + + + + + + +

- - - - - - - - - - - -

- - - - - - - - - - - --

+

depletion zone

+

-


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p/n–junction with irradiationcrystal view

n-silicon

- - - - - - - - - - - -

- - - - - - - - - - - -

- - - - - - - - - - - -

- - - - - - - - - - - -

p-silicon

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+

-diffusion -

+electrical fieldE

- - - - - - - - - - - -+ + + + + + + + + + + +

+-

h

-

-

-

+

depletion zone-

+

drift


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Charge carrier separation within p/n–junction

diffusion:from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential)

drift:driven by an electrostatic field established across the device


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Antireflection-coating

The real Silicon Solar-cell

~0,2µm

~300µm

Front-contact

Backside contact

n-region

p-region

-

+

h

depletion zone

- - - - - - - - - -+ + + + + + + + + +


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Equivalent circuit of a solar cell

RP

USG

RSISG

RL

UL

ILID

UD

currentsource

IPH

IPH: photocurrent of the solar-cell

ID /UD: current and voltage of the internal p-n diode

RP: shunt resistor due to inhomogeneity of the surface and loss-current at the solar-cell edges

RS: serial resistor due to resistance of the silicon-bulk and contact materialISG/USG: Solar-cell current and voltage

RL/IL/UL: Load-Resistance, current and voltage

ISG = IL, USG = UL


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Solar-Cell characteristics

ID ISG

RLUD=USG

ID

ISG / PSG

USG

solar-cellcharacteristics

ISG = I0 = IK

RL=0 RL=

Power

UD

diode-characteristic

ID

U0

Load resistance

UMPP

MPP

IMPP

MPP = Maximum Power Point

simplified circuit


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Solar-cell characteristics

• Short-current ISC, I0 or IK:• mostly proportional to irradiation• Increases by 0,07% per Kelvin

• Open-voltage U0, UOC or VOC:• This is the voltage along the internal diode• Increases rapidly with initial irradiation• Typical for Silicon: 0,5...0,9V• decreases by 0,4% per Kelvin


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Solar cell characteristics

• Power (MPP, Maximum Power Point)• UMPP (0,75 ... 0,9) UOC

• IMPP (0,85 ... 0,95) ISC

• Power decreases by 0,4% per Kelvin

• The nominal power of a cell is measured at international defined test conditions(G0 = 1000 W/m2, Tcell = 25°C, AM 1,5) in WP (Watt peak).


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Solar cell characteristics

• The fillfactor (FF) of a solar-cell is the relation of electrical power generated (PMPP) and the product of short current IK and open-circuit voltage U0

FF = PMPP / U0 IK

• The solar-cell efficiency is the relation of the electrical power generated (PMPP) and the light irradiance (AGG,g) impinging on the solar-cell :

= PMPP / AGG,g


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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 300µm

Minimum Thickness:

about 100µm

typical Wafer-size:

10 x 10 cm2

Link to

Producers of Silicon Wafers

http://mmoll.home.cern.ch/mmoll/links/silicon.htm


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Production- Processmono- or multi-

crystalline Siliconcrystal growth process


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Production - Process

EFG: Edge-defined Film-fed Growth

Less energy-consumptively than crystal-growth process

Thickness: about 100µm

Only few Silicon waste, since no cutting necessary

Silicon Band-Growth Process


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Production Process

semiconductor materials are evaporated on large areas

Thickness: about 1µm

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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Solar-Cell Manufacturer


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PV-Systems

The basic photovoltaic or solar cell typically produces only a small amount of power. To produce more power, cells can be interconnected to form modules, which can in turn be connected into arrays to produce yet more power. Because of this modularity, PV systems can be designed to meet any electrical requirement, no matter how large or how small.


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PV ModuleA PV-Module usually is assembled by a certain amount of series-connected solar-cells

typical open-.circuit Voltage using 36 cells: 36 * 0,7V = 25V

Problem: due to series connection, the failure of one cell (defective or shadow) reduces the current through all cells!


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PV Modulein order to avoid this kind of failure, cells or cell strings are bypassed by diodes which shortcut the defective orshaded cell(s) :


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Grid-connected PV-System

Solar-Generator

inverter(virtualload)

DC

AC

protection-Diode

load utility-grid

Grid

The grid is involved as a temporary energy storage


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PV – Solar Home System (SHS)with AC-Load

Solar-Generator

charge-regulator

DC

DC

Protection-Diode

Fuse

inverter

DC

AC

loadAccumulator

(storage)

Main difference to a grid connected System:- a local DC energy storage and DC/DC regulator is necessary- an additional DC/AC converter is necessary


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4. Building Integrated PV

• PV as a multifunctional part of buildings

• Examples

• further informationen


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4.1 Weather Protection

• Rain and wind tightness

• storm resistant

• climate-change resistant

• durable


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Example: Utility Tower in Duisburg


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Example: roof


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4.2 Thermal insulation

• In combination with usual heat-insulating materials

• In combination with heat insulating glass


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4.3 Heating / Air conditioning

• Combination of PV and thermal Energy-conversion (Air / Water)

• Optimization of PV Efficiency


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Information sources in the Internet (selected)

• U.S. Department of Energy (http://www1.eere.energy.gov/solar/technologies.html)and links within these pages

• Wikipedia(http://en.wikipedia.org/wiki/Solar_cells)and links within this page

• Clemson Summer School 2007 Dr. Karl Molter / FH Trier / [email protected]

http://www1.eere.energy.gov/solar/technologies.html

http://en.wikipedia.org/wiki/Solar_cells

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