Transcript E ph

Applications of Photovoltaic
Technologies
Referenced website:
http://www.udel.edu/igert/pvcdrom/
http://solarpv.itri.org.tw/memb/main.aspx
Why Solar Cells?
•
•
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•
•
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Finite fossil fuel supply
Less environmental damage
No radiation risk (meltdown)
Nearly infinite supply of FREE energy
Sun gives us 32 x1024 joules a year,
Cover 0.1% of the Earth’s surface with 10%
efficient solar cells with an efficiency of would
satisfy our present needs.
2
Greenhouse Effect
• Human activities have now reached a scale where they are
impacting on the planet's environment and its attractiveness to
humans.
3
Spectrum of light
E  h  h
c

h: Planck’s constant 6.626×10-34 (J-s)
ν: frequency (s-1)
λ: wavelength (m)
c : light speed 3.0× 108 (m/s)
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Atmospheric Effects
Hu, C. and White, R.M., "Solar Cells: From Basic to
Advanced Systems", McGraw-Hill, New York, 1983.
5
Solar Radiation
 Power emitted from Sun =3.8×1023 (kw)
 Power direct to Earth=1.8×1014 (kW)
 Solar constant=1353 W/m2
T=5762 K
6
Air Mass (AM)
• AM0 : The standard
spectrum outside the
Earth's atmosphere.
• AM 1: Light incident with
the angle of 0 degree.
• AM 1.5: Light incident
with the angle of 48
degree.
1
AM 
cos 
Intensity
I D  1353  0.7
AM 0.687
I G  1.1I D
Meinel A.B. and Meinel M.P., "Applied Solar
Energy", Addison Wesley Publishing Co., 1976
•ID : Direct beam intensity (W/m2)
•I : Global irradiance (W/m2)
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Standard Solar Spectra
8
Standard Solar Spectra-cont.
• The AM1.5G
Global spectrum is designed for flat plate modules
and has an integrated power of 1000 W/m2 (100
mW/cm2).
• The AM1.5 D
The direct plus circumsolar spectrum has an
integrated power density of 900 W/m2.
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Part of periodic table
II
III
IV
V
VI
B
C(6)
Al
Si(14)
P
S
Zn
Ga
Ge(32)
As
Se
Cd
In
Sb
Te
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Compound semiconductors
•
•
•
•
Elemental semiconductors: Si, Ge
Compound semiconductors: GaAs, InP
Ternary semiconductors: AlGaAs, HgCdTe
Quaternary semiconductors: InGaAsP, InGaAlP
Elemental
IV
Compounds
Binary III-V
Binary II-VI
Si
Ge
As
SiGe
SiC
AlP
GaAs
InP
GaP
CdTe
CdS
ZnS
CdSe
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Crystal Structures
In a crystalline
solid atoms
making up the
crystal are
arranged in a
periodic fashion
Crystalline
Amorphous
In some solids there is no
periodic structure of atoms
at all and called amorphous
solids
Some solids are
composed of
small regions of
single crystal
material, known
as polycrystalline.
Polycrystalline
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Commercial Si solar cells
SINGLECRYSTAL
POLYCRYSTAL
AMORPHOUS
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Photoelectric effect
Photon
Electron
•Semiconductor
Metal
Photon is a particle with
energy E = hv
Photon
Eg
Eph( hv)>Eg
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Direct and indirect semiconductor
E
E
Ec
Ec
Ev
photon
phonon
photon
P
Direct Semiconductor
P
Ev
Indirect Semiconductor
High absorption probability
Low absorption probability
GaAs; InP etc.
c-Si
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Metal-insulator-conductor
Empty States (CB)
Eg
Filled States (VB)
metal semiconductor
• Metal →CB and VB overlap,
insulator
• Insulator and semiconductor CB and VB are separated by an Eg
(energy band Eg).
• Eg for Si is 1.1242eV (semiconductor) ;5eV for diamond (Insulator)
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Absorption of Light
•
Eph < EG Photons with energy Eph less than the band
gap energy EG interact only weakly with the
semiconductor, passing through it as if it were
transparent.
•
Eph = EG have just enough energy to create an
electron hole pair and are efficiently absorbed.
•
Eph > EG Photons with energy much greater than the
band gap are strongly absorbed
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N- and P-type
•Addition of impurities with
five valence electrons results
an extra electron available
current conduction
• P, As, Sb (donor impurities
• Addition of impurities with
three valence electrons
results in available empty
energy state, a hole
• B, Al, In, Ga (Acceptor
impurities)
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Physics of Photovoltaic Generation
※Ehp > EG
※Electron-hole pair (EHP) .
※Electrons go to
electrode; hole to
electrode.
negative
positive
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Physics of Photovoltaic Generation
n-type
semiconductor
+ + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - -
Depletion Zone
p-type
semiconductor
Solar Cell-structure
• A solar cell is a P-N junction device
• Light shining on the solar cell produces both a current and a
voltage to generate electric power.
Busbar
Antireflection
coating
Fingers
Emitter
Antireflection
texturing
Base
(grid pattern)
Rear contact
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Solar cell structure
• How a solar cell should look like ?
• It depends on the function it should perform, it should convert light into
electricity, with high efficiency
• It should be a P-N junction
• It should absorb all light falling on it
It should reflect less light
 Most of the light should go in
•N-type
•P-type
• There should be ohmic contact at both
side
• It should convert all absorb light into
electricity
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Minimizing optical losses
•There are a number of ways to reduce the optical losses: .
• Anti-reflection coatings can be used on the top surface of
the cell.
• Reflection can be reduced by surface texturing
• The solar cell can be made thicker to increase absorption
• The optical path length in the solar cell may be increased by a
combination of surface texturing and light trapping.
•Top contact coverage of the cell surface can be minimized
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Optical properties of surface
• Photons in the spectrum can generate EHP, ideally all the sun light
• falling on the cell should be absorbed
•Short circuit current (ISC) is usually reduced due to optical losses
•What are optical losses:
• Reflection
• Shadowing due to metal contact
• Partial absorption
• Design criteria for small optical losses :
• Mminimize optical loss
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Choice of ARC
• The thickness of a ARC is
•Air, n0
•ARC, n1
•Semiconductor, n2
chosen such that the
reflected wave have
destructive interference 
this results in zero reflected
energy
•n2 > n1 > n0
• The thickness of the ARC is chosen so that the wavelength in the dielectric material is
one quarter the wavelength of the incoming wave (destructive interference).
0  1n1
d1 
0
4n1
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Reflection from various combination
• Index of refraction is
also a function of
wavelength, minimum
reflection is obtained
for one wavelength
• Multilayer structure
reduces the reflection
losses
• More than one ARC
can be used, but
expensive
•Source: PV CDROM - UNSW
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Surface texturing
• Any rough surface decreases the reflection by increasing the
chances of the reflected rays bouncing back on the surface
• Surface texturing can be obtained by selective etching  a process
by which material is removed by chemical reaction
• Selective etching is based on the concept of different material
property in different direction in crystals,
• Etching rate are different in <100> dirn than in <111> dirn
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Surface texturing
• Chemical etching in KOH results in pyramid formation on the
Si surface  etching is faster in <100> direction than in <111>
direction
• Using photolithography, inverted pyramids can be obtained, which
are more effective
•<111>
surface
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Light trapping
• Rear side reflector or rear side texturing is used to increase the
optical path length in solar cell
• Increased optical path is required for thin solar cell (thin solar
cell have higher Voc. It saves expensive Si)
• Total internal reflection (TIR)
condition are used to increase the
optical path length
•Snell’s law
n1 sin 1  n2 sin  2
n2
1  sin ( )
n1
1
•For TIR
• (1 for Si is 36 degree)
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Lambertian Rear Reflectors
• Lambertian reflector is one which reflects
the lights in a random direction  this
together with the front texturing increases
the optical path length
•TIR
• Increases the
path length by
4n2, very good in
light trapping,
path ;length
increases by
about 50
•Random reflector from the rear side
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Current loss due to recombination
• Recombination of carriers reduces both short circuit current as
well as open circuit voltage
Front surface
• Recombination areas
 Surface recombination
•P-N junction
 Bulk recombination
 Depletion region
Bulk semiconductor
recombination
rear surface
•Design criteria: The carrier must be generated within a diffusion
length of the junction, so that it will be able to diffuse to the
junction before recombining
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•Top contact
Design criteria: minimize
losses (resistive, shadow)
d
h
w
w
h
Emitter
 finger and busbar spacing,
•
 the metal height-to-width, aspect ratio,
 the minimum metal line width and
 the resistivity of the metal
One example of top
metal contact design
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Resistive Losses
• Resistive effects (series and shunt resistance)
in solar cells reduce the efficiency of the solar
cell by dissipating power in the resistances.
•Solar Cell model
•I
•Rs
•IL
•If
•Rsh
•V
• Both the magnitude and impact of series and
shunt resistance depend on the geometry of the
solar cell and solar cell area
• Resistance are given in Ω-cm2
• The key impact of parasitic resistance is to reduce fill factor.
I  I L  I0 
q(V  IRs )
exp
nkT
V  IRs

Rsh
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Resistive Losses: Series resistance, Rs
•Finger
s
•Contributing
factors to Rs :
•Bus
•1. the movement bar
of current through•M-S
contact
the emitter and
base of the solar
cell
•Nlayer
•emitte
r
•Base
•player
•2. the contact resistance between the metal contact and the silicon
•3. resistance of the top and rear metal contacts
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Contact resistance
•Metal to semiconductor
• Contact resistance losses occur at the
contact
•Heavy doping under contact
to minimize contact
•N
resistance
interface between the silicon solar cell and
the metal contact. To keep top contact losses
low, the top N+ layer must be as heavily
doped as possible.
• Ohmic contact,
• High doping, tunneling contact
• A high doping creates a "dead layer“.
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Sheet resistance
•In diffused semiconductor layers, resistivity is a strong function of depth. It is
convenient to a parameter called the "sheet resistance" (Rs).
•L
•W•t
L
R
A
 L
L

 Rs
t W
W
• Rs is called sheet resistance with unit of ohms/square
or Ω/□ (actual unit is Ohms)
•The L/W ratio can be thought of as the number of unit squares (of any size)
• Sheet resistance of a solar cell emitter is in the range of 30 to 100 Ω/□
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Emitter resistance: Power loss
•d/2
•N
•P
• t
•d
•L
• Zero current flow exactly at
midpoint of fingers
•dx
•x
• Maximum current density at the
finger edge
d
I max  JL
2
 dx
• Resistance dR in infinitesimally thin dR  
layer of dx
tL
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