EE 580 Solar Cells Todd J Kaiser Lecture
- Slides: 22
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: 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 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 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 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 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 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 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 - - - - - - - + + 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 - - - - - - - + + 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 - - - - - - - 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 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 (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 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 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 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 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: Solar Cells Lecture 5: P-N Junction 18
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 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 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) • 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
- Seorang siswa akan mengukur tinggi pohon yang berjarak
- Nsa 580
- Nia 930
- 580-490
- Isa 580
- Pachimetria 580
- Elite 580
- El matematico griego pitagoras nacio en el año 580
- Nia 530 ejemplos prácticos
- 01:640:244 lecture notes - lecture 15: plat, idah, farad
- Prokaryotic cells vs eukaryotic cells
- Prokaryotic v. eukaryotic cells
- Masses of cells form and steal nutrients from healthy cells
- Pineal gland
- Nondisjunction in meiosis
- Plant animal cell venn diagram
- Younger cells cuboidal older cells flattened
- How are mitosis and meiosis similar
- Cell substance
- Prokaryotic cells vs eukaryotic cells venn diagram
- Paranasal sinus at birth
- What animals have prokaryotic cells
- Red blood cells and white blood cells difference