Download What is a Solar Cell? - Charles W. Davidson College of Engineering

Solar Cell Technology
Engineering 10
October 11, 2007
Professor Richard Chung
Department of Chemical and Materials Engineering
San Jose State University
San Jose, California 95192-0082
(408) 924-3927, [email protected]
What is a Solar Cell?
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It is also known as Photovoltaic cell (PV cell)
A device that converts light energy (solar energy) directly to
electricity.
The term solar cell is designated to capture energy from
sunlight, whereas PV cell is referred to an unspecified light
source.
It is like a battery because it supplies DC power.
It is not like a battery because the voltage supplied by the cell
changes with changes in the resistance of the load.
Made from a single
crystalline silicon wafer
History
The photovoltaic effect was first recognized
in 1839 by French physicist AlexandreEdmond Becquerel. However, it was not until
1883 that the first solar cell was built, by
Charles Fritts, who coated the semiconductor
selenium with an extremely thin layer of gold
to form the junctions. The device was only
around 1% efficient.
 Russell Ohl patented the modern solar cell in
1946 (U.S. Patent 2,402,662)
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Applications of Solar Cells
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Renewable energy
Can be powered for remote locations
It’s free, limitless, and
environmentally friendly…
Physics of Solar Cells
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Semiconductor material can be p-type (hole
carriers) or n-type (electron carriers)
P+
n-type
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B-
p-type
N-type has impurities with an extra electron
(phosphorus)
P-type has impurities with one fewer electron (boron)
Put them together: p-n junction
A solar cell is a very large p-n junction (or diode)
Basic Physics of Solar Cells
P+
n-type
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e
B-
h
p-type
The holes from the p-type side diffuse to the n-type
side.
The electrons diffuse to the p-type side.
This leaves behind charged ions (missing electrons
or holes).
Built-In Electric Field
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The charged atoms (ions) create an electric field.
This electric field makes it easy for current to
flow in one direction, but hard to flow in the
opposite direction.
n-type
P+
B-
P+
B-
P+
B-
E-field
p-type
Generating Charges From The Sun
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Light breaks silicon bonds and creates “free”
electrons and holes “missing electrons”
Holes are positive charges
Built-in field separates electrons and holes
n-type
P+
B-
eP+
B-
P+
B-
E-field
h
p-type
Generating Charges From The Sun
Connect diode to a circuit
 Photocurrent goes through resistor
 Causes a voltage drop

P+
eP+
n-type
P+
BBB-
h
p-type
E-field
V=IR
IPC
Generating Charges From The Sun
Forward biases the diode
 Causes a current in opposite direction
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-V
n-type
P+
B-
P+
B-
Pe+
B-
+V
h
p-type
IFB
V=IR
IPC
Generating Charges From The Sun
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If R is very large, V is very large
If V is very large, IFB = IPC
I=0
Open Circuit condition
-V
n-type
P+
B-
P+
B-
e
P+
+V
h
B-
p-type
IFB=IPC
V=IR
IPC
Generating Charges From The Sun
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If R is very small, V is very small
If V = 0, IFB = 0
I= IPC
Short Circuit condition
-V
n-type
P+
B-
P+
B-
eP+
B-
+V
h
p-type
IFB=0
V=IR=0
IPC
Construction of Solar cells
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They are constructed by layering special materials
called semiconductors into thin, flat sandwiches.
These are linked by electrical wires and arranged on
a panel of a stiff, non-conducting material such as
glass. The panel itself is called a module.
Modules are then interconnected, in series or
parallel, or both, to create an array with the desired
peak DC voltage and current.
http://www.specmat.com/Overview%20of%20Solar%20Cells.htm
a. Encapsulate
b. Contact Grid
c. Antireflective Coating
d. N-type Silicon
e. P-type Silicon
f. Back Contact
How Solar cells work
Function 1: Photogeneration of charge
carriers (electrons and holes) in a lightabsorbing material
 Function 2: Separation of the charge
carriers to a conductive medium such as a
metal contact or a wire in order to transmit
the electricity
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 It supplies a voltage and a current to a resistive load
(light, battery, motor).
 Power = Current x Voltage
1. A solar cell is a sandwich
of n-type silicon (blue)
and p-type silicon (red).
2. When sunlight shines on
the cell, photons (light
particles) bombard the
upper surface.
3. The photons (yellow
dot) carry their energy
down through the cell.
4. The photons give up
their energy to electrons
(green dot) in the lower,
p-type layer.
5. The electrons use this
energy to jump across
the barrier into the
upper, n-type layer and
escape out into the
circuit.
6. Flowing around the
circuit, the electrons
make the lamp light up.
http://www.explainthatstuff.com/solarcells.html
Solar Cell Properties
 Open circuit voltage (VOC)
 Short circuit current (ISC)
 Maximum power
 Efficiency
Factors affecting Solar Cell Performance
Light intensity (type of light)
Light wavelength (color of light)
Angle of incident light
Surface condition of solar cells (cleanness)
Temperature on solar cells
Peak Power Point (Maximum Power)
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A solar cell may operate over a wide range of
voltages (V) and currents (I). By increasing the
resistive load on an irradiated cell continuously from
zero (a short circuit) to a very high value (an open
circuit) one can determine the maximum-power
point, the point that maximizes V×I, that is, the load
for which the cell can deliver maximum electrical
power at that level of irradiation.
Dynamically adjust the load so the maximum power
is always transferred, regardless of the variation in
lighting.
Efficiency
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A solar cell's energy conversion efficiency (η,
"eta"), is the percentage of power converted
(from absorbed light to electrical energy) and
collected, when a solar cell is connected to an
electrical circuit. This term is calculated using
the ratio of Pm, divided by the input light
irradiance under "standard" test conditions
(E, in W/m²) and the surface area of the solar
cell (Ac in m²).
Pm
 
E x Ac
I-V Curv e "scie nce shop" single ce ll, A= 10cm^2, V
=0.5 Voltage at le v e l 10.5 of Haloge n light source
0
100
V (m Volt)
200
300
0.00
-0.20
-0.40
J (mA/cm2)
-0.60
-0.80
Pmax = 471.2 W/cm^2
Jmax = 1.366 mA per cm^2
-1.00
-1.20
-1.40
-1.60
-1.80
I max = 16.27 mA
400
500
600
Solar Cell Process Flow
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Start with n-type silicon wafers
Cleaning the wafers
Sulfuric: peroxide - removes organics
Buffered Hydrofluoric acid - removes residual oxide
HCl: peroxide - removes heavy metals
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Spin on dopant: a liquid source of boron (p-type
impurity)
Anneal: 1000oC furnace step drives B into wafer
(forming diode).
Solar Cell Process Flow
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Metallization: Aluminum deposited on the front and
backside of the wafer.
Patterning: Resist is spun on the front and back sides of
the wafer and exposed using a mask and UV light. The
exposed resist is removed during developing.
Metal Etch: The pattern from the mask is transferred to
the metal using a wet metal etch. The remaining
photoresist is then removed.
Metal Anneal: The wafer is annealed at 400C to improve
the conductivity of the metal.
MASK LEVEL 1
Semiconductor
Light from aligner
Positive photoresist
Semiconductor
N- Type Silicon
P-Type Silicon
Metal
Metallization (Al)
P-type
Boron
Diffusion
(P-Type)
N-type
To make your own solar cells
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MatE 153 – Electronic Materials Properties
MatE/EE 129 - Basic IC Processing
MatE/ChE 166 - Advanced Thin Film Processes
MatE/EE 167 - Microelectronics Manufacturing
Methods
Major/Minor in Materials Engineering
Dr. Emily Allen of Chemical and Materials
Engineering and Dr. David Parent of Electrical
Engineering
Current Obstacles
Efficiency vs. cost
Solar cell efficiencies vary from 6% for amorphous siliconbased solar cells to 42.8% with multiple-junction research
lab cells. Solar cell energy conversion efficiencies for
commercially available multicrystalline Si solar cells are
around 14-16%. The highest efficiency cells have not
always been the most economical — for example a 30%
efficient multijunction cell based on exotic materials such
as gallium arsenide or indium selenide and produced in low
volume might well cost one hundred times as much as an
8% efficient amorphous silicon cell in mass production,
while only delivering about four times the electrical power.
Future Developments
The first generation photovoltaic, consists of
a large-area, single layer p-n junction diode,
which is capable of generating usable
electrical energy from light sources with the
wavelengths of sunlight. These cells are
typically made using a silicon wafer.
 The second generation of photovoltaic
materials is based on the use of thin-film
deposits of semiconductors. These devices
were initially designed to be high-efficiency,
multiple junction photovoltaic cells.
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Third generation photovoltaics are very different from the previous
semiconductor devices as they do not rely on a traditional p-n
junction to separate photogenerated charge carriers. These new
devices include photoelectrochemical cells, polymer solar cells, and
nanocrystal solar cells. Dye-sensitized solar cells are now in
production. Examples include Amorphous silicon, Polycrystalline
silicon, micro-crystalline silicon, Cadmium telluride, copper indium
selenide/sulfide.
Fourth generation Composite photovoltaic technology with the use
of polymers with nano particles can be mixed together to make a
single multispectrum layer. Then the thin multi spectrum layers can
be stacked to make multispectrum solar cells more efficient and
cheaper based on polymer solar cell and multi junction technology
used by NASA on Mars missions.
Review Question 1
A solar cell is designated to
capture energy from:
A.Sunlight
B.White light
C.Incandescent light
D.Halogen light
E.All of the above
Review Question 2
A P-type semiconductor is a
________ carrier?
A.Photon
B.Electron
C.Hole
D.Ion
E.None of the above
Review Question 3
Which of the following is NOT
a property of a solar cell?
A. Open circuit voltage (VOC)
B. Short circuit current (ISC)
C. Resistor in the circuit
D. Maximum power
E. Efficiency
Review Question 4
Which of the following will impair a
solar cell’s performance?
A. Size of the cell
B. A water stain
C. Shape of the cell
D. All of the above
E. None of the above
Review Question 5
What is the challenge in solar
cell development?
A. Cost
B. Maximum power
C. Efficiency
D. New thin film material
E. All of the above
Thank you!