Transcript Chetan

Applications of Photovoltaic
Technologies
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
2
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
3
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
4
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
5
Choice of ARC
Air, n0
ARC, n1
Semiconductor, n2
• The thickness of a ARC is
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
6
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
7
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
8
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
9
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
(1 for Si is 36 degree)
For TIR
10
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
11
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
 Bulk recombination
 Depletion region
recombination
Bulk semiconductor
P-N
junction
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
12
Top contact
• Design criteria: minimize
losses (resistive, shadow)
d
h
w
h
w
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
13
Resistive Losses: Series resistance, Rs
Fingers
Contributing
factors to Rs :
1. the
movement of
current
through the
emitter and
base of the
solar cell
Bus bar
M-S
contact
N-layer
p-layer
emitter
Base
2. the contact resistance between the metal contact and the silicon
3. resistance of the top and rear metal contacts
14
Contact resistance
•Metal to semiconductor
contact
• Contact resistance losses occur at
•Heavy doping under contact to
minimize contact resistance
•N
the 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“.
15
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
Wt
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/□
16
Emitter resistance: Power loss
•N
•P
d/2
t
d
L
• Zero current flow exactly at
midpoint of fingers
dx
x
d
I max  JL
2
 dx
• Resistance dR in infinitesimally thin dR  
layer of dx
tL
• Maximum current density at the
finger edge
17