EE580 – Solar Cells Todd 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 Junction pn P Positive pie N Negative Minus Sign An electric “check valve” Current Flow No Current Flow 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 P P P PP PP B B B B B B B Charge Position 5Montana 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 Equilibrium p:N A n:N D EcEc EVEV EiEi E Fp E Fn Montana State University: Solar Cells Lecture 5: P-N Junction

pn Junction in Thermal Equilibrium p:N A n:N D EcEc EVEV EiEi E Fp E Fn qV bi 10Montana State University: Solar Cells Lecture 5: P-N Junction

pn Junction in Thermal Equilibrium p:N A n:N D  E V dpdp dndn +qN D -qN A Built-in voltage 11Montana 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 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-side Many Electrons Few Holes P-side Many Holes Few 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-side Many Electrons Few Holes P-side Many Holes Few Electrons Depletion Region Conduction Band Valence Band Reduces Potential Barrier Allows Large Diffusion Current 16Montana State University: Solar Cells Lecture 5: P-N Junction

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

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

Diode Nonequilibrium Behavior Light Generated EHP N-side Many Electrons Few Holes P-side Many Holes Few 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 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 Diffusion Drift LeLe Medium Wavelength Short Wavelength DiffusionDrift LhLh Active region = L h + W + L e 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 (L e + W + L h ) Make diffusion length long (very pure materials) 22Montana State University: Solar Cells Lecture 5: P-N Junction