EE 580 Solar Cells Todd J Kaiser Lecture

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EE 580 – Solar Cells Todd J. Kaiser • Lecture 05 • P-N Junction

EE 580 – Solar Cells Todd J. Kaiser • Lecture 05 • P-N Junction Montana State University: Solar Cells Lecture 5: P-N Junction 1

P-N Junction • Solar Cell is a large area P-N junction or a diode:

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 2

p-n Junction p P Positive pie n N Negative Minus Sign An electric “check

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 3

Creation of PN Junction • • High concentration of electrons in n-side High concentration

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 Montana State University: Solar Cells Lecture 5: P-N Junction 4

Si B Si Si Si B Si P Si Si Si B Si Si

Si B Si Si Si B Si P Si Si Si B Si Si P Si Si B Si Si Si P Si + + + + Charge - - Montana State University: Solar Cells Lecture 5: P-N Junction Position 5

Creation of Electric Field • Electric fields are produced by charge distributions • Fields

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 6

Creation of Depletion Region • The local dopant ions left behind near the 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 Montana State University: Solar Cells Lecture 5: P-N Junction 7

Creation of a Potential • Changes in the electric field create a potential barrier

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 8

pn Junction in Thermal Equilibrium p: NA n: ND - - - - -

pn Junction in Thermal Equilibrium p: NA n: ND - - - - - - - + + Ec + + + + + + + + + + + EFn Ei EFp EV Montana State University: Solar Cells Lecture 5: P-N Junction 9

pn Junction in Thermal Equilibrium p: NA n: ND - - - - -

pn Junction in Thermal Equilibrium p: NA n: ND - - - - - - - + + Ec + + + + + + + + + + + EFn Ei EFp q. Vbi EV Montana State University: Solar Cells Lecture 5: P-N Junction 10

pn Junction in Thermal Equilibrium p: NA n: ND - - - - -

pn Junction in Thermal Equilibrium p: NA n: ND - - - - - - - dp r + + + + + + + + + + + + dn +q. ND -q. NA E V Built-in voltage Montana State University: Solar Cells Lecture 5: P-N Junction 11

Operation of PN Junction • When sunlight is absorbed by the cell it unbalances

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 12

Photovoltaic Effect Separation of holes and electrons by Electric Field Absorption of Light Voltage

Photovoltaic Effect Separation of holes and electrons by Electric Field Absorption of Light Voltage (V) Creation of extra electron hole pairs Excitation (EHP) of electrons Power = V x I Current (I) Movement of charge by Electric Field Montana State University: Solar Cells Lecture 5: P-N Junction 13

Solar Cell Voltage • In silicon, the electrons will need to overcome the potential

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 14

Diode Equilibrium Behavior DRIFT = DIFFUSION P-side Many Holes Few Electrons Valence Band Depletion

Diode Equilibrium Behavior DRIFT = DIFFUSION P-side Many Holes Few Electrons Valence Band Depletion Region Conduction Band 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 15

Forward Bias Behavior P-side Many Holes Few Electrons Valence Band Depletion Region Conduction Band

Forward Bias Behavior P-side Many Holes Few Electrons Valence Band Depletion Region Conduction Band N-side Many Electrons Few Holes Reduces Potential Barrier Allows Large Diffusion Current Montana State University: Solar Cells Lecture 5: P-N Junction 16

Reverse Bias Behavior P-side Many Holes Few Electrons Valence Band Depletion Region Conduction Band

Reverse Bias Behavior P-side Many Holes Few Electrons Valence Band Depletion Region Conduction Band N-side Many Electrons Few Holes Increases Potential Barrier Very Little Diffusion Current Montana State University: Solar Cells Lecture 5: P-N Junction 17

Diode I-V Characteristics Current Exponential Growth Voltage Reverse Bias Forward Bias Montana State University:

Diode I-V Characteristics Current Exponential Growth Voltage Reverse Bias Forward Bias Montana State University: Solar Cells Lecture 5: P-N Junction 18

Diode Nonequilibrium Behavior Light Generated EHP P-side Many Holes Few Electrons Valence Band Depletion

Diode Nonequilibrium Behavior Light Generated EHP P-side Many Holes Few Electrons Valence Band Depletion Region Conduction Band 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 19

Solar Cell I-V Characteristics Current Dark Current from Absorption of Photons Light Voltage Twice

Solar Cell I-V Characteristics Current Dark Current from Absorption of Photons Light Voltage Twice the Light = Twice the Current Montana State University: Solar Cells Lecture 5: P-N Junction 20

Active Region Neutral n-region Depletion region Long Wavelength P-type Base Medium Wavelength Short Wavelength

Active Region Neutral n-region Depletion region Long Wavelength P-type Base Medium Wavelength Short Wavelength Neutral p-region Lh Drift Diffusion Le E-field Diffusion Drift N-type Active region = Lh + W + Le emitter Montana State University: Solar Cells Lecture 5: P-N Junction 21

Light Current • Proportional to: – The Area of the solar cell (A) •

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) Montana State University: Solar Cells Lecture 5: P-N Junction 22