EE580 – Solar CellsTodd J. Kaiser
• Lecture 05
• P-N Junction
1Montana State University: Solar Cells Lecture 5: P-N Junction
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)
2Montana State University: Solar Cells Lecture 5: P-N Junction
p-n Junctionp n
PPositivepie
NNegativeMinus Sign
An electric “check valve”
CurrentFlow
No CurrentFlow
3Montana State University: Solar Cells Lecture 5: P-N Junction
Creation of PN Junction
• 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
4Montana State University: Solar Cells Lecture 5: P-N Junction
Si Si Si Si Si
Si
Si
Si Si
P
Si Si Si Si
Si Si Si Si Si P Si
Si
P
Si Si Si Si Si
Si Si Si Si Si
Si
Si
P P
P P
B
B
B
B
B
B
B
--
--
- --
++
+++
++Charge
Position5Montana State University: Solar Cells
Lecture 5: P-N Junction
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)
6Montana State University: Solar Cells Lecture 5: P-N Junction
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
7Montana State University: Solar Cells Lecture 5: P-N Junction
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
8Montana State University: Solar Cells Lecture 5: P-N Junction
pn Junction in Thermal Equilibriump:NA n:ND
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9Montana State University: Solar Cells Lecture 5: P-N Junction
pn Junction in Thermal Equilibrium
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10Montana State University: Solar Cells Lecture 5: P-N Junction
pn Junction in Thermal Equilibrium
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p:NA n:ND
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Built-in voltage11Montana State University: Solar Cells
Lecture 5: P-N Junction
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
12Montana State University: Solar Cells Lecture 5: P-N Junction
Photovoltaic Effect
Absorption of Light
Creation of extra electron
hole pairs (EHP)
Voltage(V)
Current(I)
Power = V x I
Excitation of electrons
Separation of holes and electrons by Electric Field
Movement of charge by Electric Field
13Montana State University: Solar Cells Lecture 5: P-N Junction
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
14Montana State University: Solar Cells Lecture 5: P-N Junction
Diode Equilibrium Behavior
N-sideMany ElectronsFew Holes
P-sideMany HolesFew Electrons
Depletion Region
Conduction Band
Valence Band
DRIFT = DIFFUSION
Potential Barrier Stops Majority of Carriers from Leaving Area
15Montana State University: Solar Cells Lecture 5: P-N Junction
Forward Bias Behavior
N-sideMany ElectronsFew Holes
P-sideMany HolesFew Electrons
Depletion Region
Conduction Band
Valence Band
Reduces Potential BarrierAllows Large Diffusion Current
16Montana State University: Solar Cells Lecture 5: P-N Junction
Reverse Bias Behavior
N-sideMany ElectronsFew Holes
P-sideMany HolesFew Electrons
Depletion Region
Conduction Band
Valence Band
Increases Potential BarrierVery Little Diffusion Current
17Montana State University: Solar Cells Lecture 5: P-N Junction
Diode I-V Characteristics
Voltage
Current
ForwardBias
ReverseBias
Exponential Growth
10
kT
qV
eII
18Montana State University: Solar Cells Lecture 5: P-N Junction
Diode Nonequilibrium BehaviorLight Generated EHP
N-sideMany ElectronsFew Holes
P-sideMany HolesFew Electrons
Depletion Region
Conduction Band
Valence Band
EHP are generated throughout the device breaking the equilibrium causing current flow
19Montana State University: Solar Cells Lecture 5: P-N Junction
Solar Cell I-V Characteristics
Voltage
Current
Dark
Light
Twice the Light = Twice the Current
Current from Absorption of Photons
LkT
qV
IeII
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20Montana State University: Solar Cells Lecture 5: P-N Junction
Active Region
P-type Base
N-type emitter
Depletion region
E-field
Neutral p-region
Neutral n-region
Long Wavelength
DiffusionDrift
Le
Medium WavelengthShort Wavelength
Diffusion Drift
Lh
Active region = Lh + W + Le
heL LWLqAGI 21Montana State University: Solar Cells
Lecture 5: P-N Junction
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)
heL LWLqAGI
22Montana State University: Solar Cells Lecture 5: P-N Junction