Applications of Photovoltaic
Technologies
Junction under illumination
•Generation of voltage in P-N junction
radiation
P-type
P-type
+
Ln
-
N-type
N-type
W Lp
Direction of current flow under
illumination
•P-N behaves like a forward bias P-N
junction under illumination
2
Solar Cell Structure
• The basic steps in the
operation of a solar
cell are:
• the generation of lightgenerated carriers;
• the collection of the
light-generated carries
to generate a current;
• the generation of a
large voltage across
the solar cell; and
• the dissipation of
power in the load and
in parasitic resistances.
3
No Resistive Losses
Solar Cell model
• The I-V relation is given as:
I
IL
V
I  I L  I0 
q(V )
exp
 1
nkT
ID
Io －dark saturation current ,
IL －light generated current. ,
n －ideality factor .
4
Solar Cell I-V Curve
• A P-N junction in the dark
consumes power, as it can be
operated in 1st or 3rd quadrant
I
I (diffu.)
V
I0
• Effect of solar radiation on the I-V
curve
• Under illumination solar cell can
be operated in the fourth quadrant
corresponding to delivering
power to the external circuit
• Current in the illuminated solar cell is negative, flows against the
conventional direction of a forward diode
5
Solar Cell I-V Curve
Solar cell parameters
I
Isc
Im
Pm
• Voc - open circuit voltage,
• Isc - short circuit current,
• Pm - maximum power point
Vm Voc
V • Im, Vm – current and voltage
at maximum power point
Usual I-V plot of solar cell –
Current is shown on positive y - • FF – fill factor
axis
• η – efficiency
• Rs – series resistance
• Rsh – shunt resistance
6
Short-Circuit Current, Isc
I
• The short-circuit current is the
current through the solar cell when
the voltage across the solar cell is
zero (i.e., when the solar cell is short
circuited).
Pm
X
Im
Vm Voc
I  I L  I 0 (e
qV / kT
At V=0  I = -IL= Isc
 1)
•The short-circuit current is due to
the generation and collection of
light-generated carriers.
• The short-circuit current is the
largest current which may be
drawn from the solar cell.
7
Open Circuit Voltage: Voc
I
Isc
Im
Pm
X
Vm
Voc
I  I L  I 0 (e
qV / kT
At I=0  V = Voc
• The open-circuit voltage, Voc, is the
maximum voltage available from a
solar cell, and this occurs at zero
current.
• The open-circuit voltage corresponds
to the amount of forward bias on the
solar cell junction due to illumination.
 1)
kT
IL
Voc 
ln(  1)
q
I0
8
Maximum power: Pm
I
Im
Isc
Pm
X
• Power out of a solar cell
increases with voltage, reaches
a maximum (Pm) and then
decreases again.
Vm Voc
Pm = Im x Vm
• Remember we get DC power from a
solar cell
9
Fill Factor: FF
I
Ideal diode curve
Isc
Pm
Im
• The FF is defined as the ratio
of the maximum power from
the actual solar cell to the
maximum power from a ideal
solar cell
Vm Voc
• Graphically, the FF is a measure of the "squareness" of the solar
cell
Max power from real cell Vm I m
FF 

Max power from ideal cell Voc I sc
10
Efficiency: η
I
Im
Isc
Pm
X
• Efficiency is defined as the ratio of
energy output from the solar cell to
input energy from the sun.
Vm I m
Max. Cell Power


Incident light Intensity
Pin
Vm Voc
Voc I sc FF

Pin
• The efficiency is the most commonly used parameter to
compare the performance of one solar cell to another.
• Efficiency of a cell also depends on the solar spectrum, intensity
of sunlight and the temperature of the solar cell.
11
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
12
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
13
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
14
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
15
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
16
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
17
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
18
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
19
Light trapping
• Rear side reflector or rear side texturing is used to increase the
(1 for Si is 36 degree)
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
20
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
21
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
22
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
23
Resistive Losses
Solar Cell model
I
Rs
IL
If
Rsh
• Resistive effects (series and shunt
resistance) in solar cells reduce the
efficiency of the solar cell by
dissipating power in the resistances.
• Both the magnitude and impact of
V 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
24
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
25
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“.
26
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/□
27
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
28
Effect of Rs and FF
Isc
Characteristic resistance, Rch
Medium Rs
RCH
Large Rs
I
Vmax

I max
Normalized series resistance,
rs
Voc
V
• Slope of the I-V curve near Voc gives
indication about Rs
Rs
rs 
RCH
• Effect of series resistance on the FF and maximum power
Pm, s  P0 (1  rs )
FFs  FF0 (1  rs )
29
Effect of Rsh on FF
Normalized shunt resistance, rsh
Isc
Rsh
rsh 
RCH
Medium Rsh
I
• Effect of series resistance on
the FF and maximum power
Voc
V
• Slope of the I-V curve near Isc gives
indication about Rsh
Pm, sh
1
 P0 (1  )
rsh
1
FFsh  FF0 (1  )
rsh
30