PV BASICS PART 2(SOLAR CELLS - HIGH EFFICIENCY CONCEPTS IN SPV

PV BASICS PART 2 SOLAR CELLS - HIGH EFFICIENCY CONCEPTS IN SPV Dr.. LECTURE FORMAT• Basics of Semiconductors • Solar Cell Device P-N Junction • Fabrication Technologies of Silicon Sol

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PV BASICS PART 2 (SOLAR CELLS - HIGH EFFICIENCY CONCEPTS IN

SPV)

Dr O.S SASTRY DIRECTOR, PV TESTING SOLAR ENERGY CENTRE

PH 0124-2579213; Fax: 0124-2579207e.mail:

sankar_sec@yahoo.co.uk

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LECTURE FORMAT

• Basics of Semiconductors

• Solar Cell Device (P-N Junction)

• Fabrication Technologies of Silicon Solar Cell

• Concepts of High Efficiency Silicon Solar Cells

• Third Generation Concepts

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BASICS OF SEMICONDUCTORS

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MAJOR REQUIREMENTS FOR CELL MATERIAL

Technical:

· HIGH ABSORPTION COEFFICIENT

LONG DIFFUSION LENGTH;

· CONVENIENCE OF SHAPES AND SIZES

· SIMPLE AND INEXPENSIVE INTEGRATED PROCESSING

FABRICATION & ECONOMICS RELATED:

· MINIMUM MATERIAL / WATT

· MINIMUM ENERGY INPUT/ OUTPUT WATT

· ENERGY PAY BACK PERIOD < 2 YEARS

COST PAY BACK PERIOD< 5 YEARS

· LONG LIFE (> 20 Years)

· COST (< $1/Watt)

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TYPICAL PARAMETERS

TRANSPORT EQUATIONS

· Generation Rate G(x)= [ E

g (f) [1-R(E)] (E) exp I-d(E)x]dE

· I(V)=IS exp(eV - 1) – IL V  V-IRS & I  I - V-IRS

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POSSIBLE TFSC MATERIALS Single Elements:

Si ( epi, mc, nc, mixed)

Carbon (nanotubes, DLC)

Binary alloys / Compounds:

Cu2S, Cu2O Cu-C, CdTe, CdSe,

GaP, GaAs, InP,ZnP , a-Si : H, Dye coated TiO2

Ternary Alloys / Compounds:

Cu-In-S, Cu-In-Se CdZnSe , CdMnTe, Bi-Sb-S, Cu-Bi-s, Cu-Al-Te, Cu-Ga-Se, Ag-In-S, Pb-Ca-S, Ag-Ga-S, Ga-In-P, Ga-In-Sb ,and so on.

Organic Materials:

Semiconducting Organics / Polymers and Dyes

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SOLAR CELL DEVICE (p-n JUNCTION)

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• A solar cell is a p-n junction semiconductor device

• When p and n type semiconductors are brought together

the Fermi levels try to come to the same level This results in band bending to maintain the charge equilibrium

• When SUN radiation incidents on such junction, current is

generated due to creation of electron – hole pairs, the electric field/ voltage (emf) developed due to band bending drives the light generated charge carriers down the electric field gradient, pushes in to the load In this way both voltage and current, hence power are generated.

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Understanding Operation of Solar Cell

Absorption

Generation Separation Collection Reflection

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Section View of a Solar Cell

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SOLAR CELL

• Solar Cell operations depend on :

o Absorption of light to create electron-hole pairs

(carriers)

o Diffusion of carriers

o Separation of electrons and holes

o Collection of carriers

• A Solar cell is a light driven battery with an open

current voltage (Voc), short circuit current (Isc),

maximum power point current and voltage (In, Vm), and a series and a parallel resistance (Rs, Rsh).

• Solar Cell Efficiency

η – output = Im Vm = I siVIL FT

input Σ nhv Σ nhv depends on quantum efficiency of creation of carriers, effectiveness of separation of carriers before

recombination and collection of the separated carriers.

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Equivalent Circuit of Solar cell

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p-silicon base

n + emitter

Ag contacts SiN x :H ARC

Industrial Silicon Solar Cells

(for 1 sun) (Cell size 125 mm x 125 mm square/pseudo square)

ρ = 0.5-3 Ώ cm, CZ-Si, Area= 78-140cm2: η = 12-15%(1sun)

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FABRICATION TECHNOLOGIES

OF SILICON SOLAR CELL

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PV Value Chain – c-Si and Thin Films Modules

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Design, tools & equipments

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SOLAR CELL (SILICON) FABRICATION TECHNOLOGY

MINIMUM MATERIAL LOSS DURING PROCESSING :About 20 TO 30%ENERGY CONSUMPTION : ~ 3 kWh/Wp

COST PAY BACK PERIOD : < 5 YEARS

QUARTZ (SiO2)SAND METALLAZRY GRADE -Si SOLAR GRADE -Si SINGLE CRYSTAL INGOT

WAFERING DOPING (p-n JUNCTION FORMATION) TEXTURIZATION AND AR COATING BACK SURFACE METALLIZATION FRONT SURFACE GRIDDING

TESTING STRINGING MODULE LAMINATION

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Pull rod Rotation

& Lift

Gas convection

Liquid convection

Si melt

Crystal rod

Graphite susceptor

Crucible rotation

Radation radiation shield

Quartz crucible

& Lift

Gas convection

Si melt Crystal

& Lift

Gas convection

Si melt

Crystal rod

Graphite susceptor

Crucible rotation

Radation radiation shield

Quartz crucible

Float- zone pulling

Feed rod holder Feed rod (poly silicon)

Melting interface

RF heating coil Moltem zone Freezing interface Single crystal silicon

shoulder neck seed hold

Float- zone pulling

Feed rod holder Feed rod (poly silicon)

Melting interface

RF heating coil Moltem zone Freezing interface Single crystal silicon

shoulder neck seed hold

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Cast mc-Si solar cells

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END PART-II

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CONCEPTS OF HIGH EFFICIENCY SILICON

SOLAR CELLS

(LOSSES IN SOLAR CELLS)

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LOSSES IN SOLAR CELLS

• THERMODYNAMICAL

• OPTICAL

• ELECTRICAL

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Solar Radiation Spectrum

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Spectral Distribution of Extraterrestrial Radiation

• In addition to the total energy in the solar spectrum (i.e the

solar constant), it is useful to know the spectral distribution of the extraterrestrial solar radiation, that is, the solar radiation that would be received in the absence of the atmosphere.

• A standard spectral irradiance curve based on high altitude and space measurements is shown here which is found to be similar

to the 5777K blackbody spectrum

• From this figure following observations are made:

– The peak solar intensity is 2028.8 w/m2 at a wavelength of 0.48 m.

– The solar spectrum varies from 0.2 – 3.0 m,

– The energy in various spectral ranges is as follows:

Ultravoilet Visible Infrared

(0.38 – 0.78 m)

656 48

(0.78 – 3.0 m)

623 46

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LOSSES AT DIFFERENT COMPONENTS

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Burried Contact Silicon Solar cells

Green et al (1988)

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High Efficiency Silicon Solar Cells

(for normal sunlight: 1sun)

These cells have effective BSF ( d/L <<1), inverted pyramids on front surface, surface passivation, reduced metal to silicon contact area, Selective emitter and reduced metal shadowing.

Passivated Emitter Rear-Locally diffused solar cell (PERL cell)

FZ-Si, ρ = 1Ω cm, R =150 Ω/, Area = 4 cm2, η= 24.7% (1 sun)

Green et al

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Highest Lab Efficiency – UNSW

25%

PERL – Designer Cell

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PERL OF UNSW & Suntech’s PLUTO

PERL CELL

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25 12.5 8.3 6.25

Estimated S (cm/sec)

M Tanaka, et al., 3 rd WCPVEC, Osaka, May 11-18, 2003

Sanyo’s HIT cells on n-type Si ( H eterojunctions with I ntrinsic T hin Layers

~10nm

~20nm

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THIRD GENERATION CONCEPTS

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First generation cells

Courtesy : UNSW

VERY HIGH COST??

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Chalcogenides: CdTe and CIGS

TCO window absorber

ZnO

glass

Cu (Ga,In) (Se,S)2 CdS

substrate

Courtesy : UNSW

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Second Generation: thin-film

(COST REDUCTION MAIN TARGET!)

Advantages

low materials cost

large manufacturing unit

fully integrated modules

aesthetics, ruggedness?

.

Thin-film Technologies Silicon

amorphous microcrystalline

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Efficiency- cost The 3 generations

?

II

100 80 60

abundant non-toxic durable

Thin-film

Courtesy : UNSW

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Third generation options

impurity PV & band, up-converters tandem (n = 3)

thermal, thermoPV, thermionics

impact ionisation tandem (n = 2) down-converters single cell

tandem (n = 6)

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tandem (n = 6)

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100nm 100nm

Fabrication of Si quantum dots

SiOx, SiyNx, SiCx SiO2, Si3N4, SiC

Zacharias et al., APL 80, 661, 2002

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Si quantum dot photoluminescence

Norm PL Spectra (2-5nm dots; 300K)

0

Photon energy, eV

5nm (270s) 4nm (240s) 3nm (180s) 2nm (120s)

100nm

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tandem (n = 6)

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Hot-carrier cell concept

tunneling contact

absorber

100nm 100nm Bandgap (eV)

hole contact electron contact

Ta

Tc

Efficiency > 4 cell tandem

Courtesy : UNSW

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tandem (n = 6)

Luque & Marti, PRL 78,

5014 (1997)

Courtesy : UNSW

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END PART-III

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FOR YOUR ATTENTION

THANK YOU !!

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QUESTIONS ??

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