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

Ec

EV

Ei

EFp

EFn

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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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V

dp dn

+qND

-qNA

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

10

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