Transcript Periodic Table - Montana State University

EE580 – Solar Cells
Todd J. Kaiser
• Lecture 05
• P-N Junction
Montana State University: Solar Cells
Lecture 5: P-N Junction
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P-N Junction
• Solar Cell is a large area P-N junction or a diode:
electrons can flow in one direction but not the other
(usually)
• Created by a variation in charge carriers as a function of
position
• Carriers (electrons & holes) are created by doping the
material
– N: group V (Phosphorus) added (extra electron
negative)
– P: Group III (Boron)added (short electron (hole)
positive)
Montana State University: Solar Cells
Lecture 5: P-N Junction
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p-n Junction
p
P
Positive
pie
n
N
Negative
Minus Sign
An electric “check valve”
Current
Flow
No Current
Flow
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Creation of PN Junction
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High concentration of electrons in n-side
High concentration of holes in p-side
Electrons diffuse out of n-side to p-side
Electrons recombine with holes (filling valence
band states)
• The neutral dopant atoms (P) in the n-side give
up an electron and become positive ions
• The neutral dopant atoms (B) in the p-side
capture an electron and become negative ions
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Si
B
Si
Si
Si
B
Si
P
Si
Si
Si
B
Si
Si
Si
P
Si
Si
P
Si
Si
Si
B
Si
Si
P
Si
Si
B
Si
Si
Si
B
Si
P
Si
Si
P
Si
Si
B
Si
Si
Si
P
Si
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Charge
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Montana State University: Solar Cells
Lecture 5: P-N Junction
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Creation of Electric Field
• Electric fields are produced by charge
distributions
• Fields flow from positive charges (protons,
positive ions, holes) and flow toward negative
charges (electrons, negative ions)
• Free charges move in electric fields
– Positive in the direction of field (holes)
– Negative opposite to the electric field
(electrons)
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Creation of Depletion Region
• The local dopant ions left behind near the
junction create an electric field area called the
depletion region
• Any free carriers would be swept out of the
depletion region by the forces created by the
electric field (depleted of free carriers)
• The depletion area grows until it reaches
equilibrium where the created electric field stops
the diffusion of electrons
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Creation of a Potential
• Changes in the electric field create a
potential barrier to stop the diffusion of
electrons from the n-side to the p-side
• The p-n junction has a built-in potential
(voltage) that is a function of the doping
concentrations of the two areas
Montana State University: Solar Cells
Lecture 5: P-N Junction
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pn Junction in Thermal Equilibrium
p:NA
n:ND
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EFn
Ei
EFp
EV
Montana State University: Solar Cells
Lecture 5: P-N Junction
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pn Junction in Thermal Equilibrium
p:NA
n:ND
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EFn
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EFp
qVbi
EV
Montana State University: Solar Cells
Lecture 5: P-N Junction
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pn Junction in Thermal Equilibrium
p:NA
n:ND
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dn
+qND
-qNA
E
V
Built-in voltage
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Operation of PN Junction
• When sunlight is absorbed by the cell it
unbalances the equilibrium by creating
excessive electron-hole pairs.
• The internal field separates the electrons from
the holes
• Sunlight produces a voltage opposing and
exceeding the electric field in the internal
depletion region, this results in the flow of
electrons in the external circuit wires
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Photovoltaic Effect
Separation of holes and
electrons by Electric
Field
Absorption of
Light
Excitation
of
electrons
Voltage
(V)
Creation of
extra
electron hole
pairs (EHP)
Power = V x I
Current
(I)
Movement of charge
by Electric Field
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Solar Cell Voltage
• In silicon, the electrons will need to overcome
the potential barrier of 0.5 - 0.6 volts  any
electrons(electricity) produced will be produced
at this voltage
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Diode Equilibrium Behavior
DRIFT = DIFFUSION
P-side
Many Holes
Few Electrons
Conduction Band
Valence Band
Depletion
Region
N-side
Many Electrons
Few Holes
Potential Barrier Stops Majority of Carriers from Leaving Area
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Forward Bias Behavior
P-side
Many Holes
Few Electrons
Valence Band
Conduction Band
Depletion
Region
N-side
Many Electrons
Few Holes
Reduces Potential Barrier
Allows Large Diffusion Current
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Reverse Bias Behavior
P-side
Many Holes
Few Electrons
Valence Band
Conduction Band
Depletion
Region
N-side
Many Electrons
Few Holes
Increases Potential Barrier
Very Little Diffusion Current
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Diode I-V Characteristics
Current
 qV

kT
I  I 0  e  1


Exponential Growth
Voltage
Reverse
Bias
Forward
Bias
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Diode Nonequilibrium Behavior
Light Generated EHP
P-side
Many Holes
Few Electrons
Valence Band
Conduction Band
Depletion
Region
N-side
Many Electrons
Few Holes
EHP are generated throughout the device breaking the equilibrium causing current flow
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Solar Cell I-V Characteristics
Current
 qV

kT
I  I 0  e  1  I L


Dark
Current from Absorption of Photons
Voltage
Light
Twice the Light = Twice the Current
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Active Region
Neutral
n-region
Depletion
region
Neutral p-region
Long
Wavelength
P-type Base
Medium
Wavelength
Short
Wavelength
Drift
Lh
Diffusion
Le
E-field
Diffusion Drift
N-type
Active region = Lh + W + Le
emitter
I L  qAGLe  W  Lh 
Montana State University: Solar Cells
Lecture 5: P-N Junction
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Light Current
• Proportional to:
– The Area of the solar cell (A)
• Make cells large
– The Generation rate of electron hole pairs (G)
• Intensity of Light
– The active area (Le + W + Lh)
• Make diffusion length long (very pure materials)
I L  qAGLe  W  Lh 
Montana State University: Solar Cells
Lecture 5: P-N Junction
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