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EE580 – Solar Cells Todd J. Kaiser

EE580 – Solar Cells Todd J. Kaiser. Lecture 05 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

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EE580 – Solar Cells Todd J. Kaiser

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  1. EE580 – Solar CellsTodd J. Kaiser • Lecture 05 • P-N Junction Montana State University: Solar Cells Lecture 5: P-N Junction

  2. 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

  3. p-n Junction p n P Positive pie N Negative Minus Sign An electric “check valve” Current Flow No Current Flow Montana State University: Solar Cells Lecture 5: P-N Junction

  4. 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

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

  6. 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

  7. 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

  8. 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

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

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

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

  12. 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

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

  14. 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

  15. Diode Equilibrium Behavior DRIFT = DIFFUSION P-side Many Holes Few Electrons Conduction Band Depletion Region N-side Many Electrons Few Holes Valence Band Potential Barrier Stops Majority of Carriers from Leaving Area Montana State University: Solar Cells Lecture 5: P-N Junction

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

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

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

  19. Diode Nonequilibrium BehaviorLight Generated EHP P-side Many Holes Few Electrons Conduction Band Depletion Region N-side Many Electrons Few Holes Valence Band EHP are generated throughout the device breaking the equilibrium causing current flow Montana State University: Solar Cells Lecture 5: P-N Junction

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

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

  22. 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

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