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![The p-n Junction Diode
Voltage-Current (VI) characteristics for a diode
/
0 ( -1)
d
qV kT
d
I I e
38.9
0 ( -1) (at 25 C)
d
V
d
I I e
k = Boltzmann’s constant
1.381x10-23 [J/K]
T = junction temperature [K]
Vd = diode voltage
Id = diode current
q = electron charge 1.602x10-19 C
I0 = reverse saturation current
18](https://image.slidesharecdn.com/reslecture11solarpv-220102063128/85/Res-lecture-11-solar-pv-18-320.jpg)
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Res lecture 11 solar pv
- 1. EE421 Renewable EnergySystems Solar PV System Mr. Ijaz Husnain
- 2. Photovoltaics (PV) Photovoltaic definition-a material or device that is capable of converting the energy contained in photons of light into an electrical voltage and current Rooftop PV modules on a village health center in West Bengal, India http://www1.eere.energy.gov/solar/pv_use.html "Sojourner" exploring Mars, 1997 University of Illinois Solar Decathalon House – 2nd place overall in 2009 http://www.solardecathlon.uiuc.edu/gallery.html# 2
- 3. PV History • EdmundBecquerel (1839) observed PV effect • Adams and Richard Evans Day (1876) observed that illuminating a junction causes photo voltaic effect • Albert Einstein (1904) • Czochralski (1940s) pronounced as chokh-RAHL-skee • Vanguard I satellite (1958) • Costs have recently dropped quite substantially 3
- 4. PV System Overview Shadows •Solar cell is a diode • Photopower coverted to DC • Shadows & defects convert generating areas to loads • DC is converted to AC by an inverter • Loads are unpredictable • Storage helps match generation to load 4
- 5. Some General Issuesin PV • The device • Efficiency, cost, manufacturability automation, testing • Encapsulation • Cost, weight, strength, yellowing, etc. Yellowing” of PV modules is defined as the optical degradation of the ethyl vinyl acetate (EVA) where the clear encapsulant becomes visibly yellow or even brown. • Accelerated lifetime testing • 30 year outdoor test is difficult • Damp heat test, light soaking test, etc. • Inverter & system design • Micro-inverters, blocking diodes, reliability 5
- 6. 6 Damp Heat Testmachine Problems that can be identified with the pv damp heat test are: • Corrosion • De-lamination • Junction box and module connection failures
- 7. What are SolarCells? Current Voltage Open-circuit voltage Short-circuit current Maximum Power Point n-type p-type - + Load • Solar cells are diodes • Light (photons) generate free carriers (electrons and holes) which are collected by the electric field of the diode junction • The output current is a fraction of this photocurrent • The output voltage is a fraction of the diode built-in voltage 7
- 8. Standard Equivalent CircuitModel Photocurrent source Diode Shunt resistance Load Series resistance Where does the power go? (minimize) (maximize) 8
- 9. Photons • Photons arecharacterized by their wavelength (frequency) and their energy • In this context energy is often expressed in electronvolts (eV), which is defined as 1.6 10-19 J – This is the amount of energy gained by a single electron moving across a voltage difference of one volt c v hc E hv 9 Planck's constant (h) is 6.626 10-34 J-s Velocity of light (c) is 3108 m/s 1.242 , EV m hc E E Above equation for E can be rewritten with E in eV and in m
- 10. Band-Gap Energy • Electricalconduction is caused by free electrons (electrons in conduction band) • At absolute zero temperature metals have free electrons available and hence are good conductors – Metal conductivity decreases with increasing temperature • Semi-conductors, such as silicon, have no free electrons at absolute zero and hence are good insulators 10
- 11. Band-Gap Energy, cont. •Silicon conductivity increases with increasing temperature; at room temperature they only have about 1 in 1010 electrons in the conduction band • Band gap energy is the energy an electron must acquire to jump into the conduction band 11 Quantity Si GaAs CdTe InP Band gap (eV) 1.12 1.42 1.5 1.35 Cut-off wavelength (μm) 1.11 0.87 0.83 0.92 Band Gap and Cut-off Wavelength Above Which Electron Excitation Doesn’t Occur 1.242 EV m E
- 12. Silicon Solar CellMax Efficiency • Wavelengths above cutoff have no impact • This gives an upper bound on the efficiency of a silicon solar cell: – Band gap: 1.12 eV, Wavelength: 1.11 μm • This means that photons with wavelengths longer than 1.11 μm cannot send an electron into the conduction band. • Photons with a shorter wavelength but more energy than 1.12 eV dissipate the extra energy as heat 12
- 13. Silicon Solar CellMax Efficiency • For an Air Mass Ratio of 1.5, 49.6% is the maximum possible fraction of the sun’s energy that can be collected with a silicon solar cell • Efficiencies of real cells are in the ~20-25% range 13
- 14. Solar Cell Efficiency •Factors that add to losses – Recombination of electrons/holes – Internal resistance – Photons might not get absorbed, or they may get reflected – Heating • Smaller band gap - easier to excite electrons, so more photons have extra energy – Results in higher current, lower voltage • High band gap – opposite problem • There must be some middle ground since P=VI (DC), this is usually between 1.2 and 1.8 eV 14
- 15. Maximum Efficiency forCells 15 • The maximum efficiency of single-junction PV cells for different materials was derived in 1961 by Shockley and Queisser
- 16. Review of Diodes •Two regions: “n-type” which donate electrons and “p-type” which accept electrons • p-n junction- diffusion of electrons and holes, current will flow readily in one direction (forward biased) but not in the other (reverse biased), this is the diode http://en.wikipedia.org/wiki/File :Pn-junction-equilibrium.png 16
- 17. The p-n JunctionDiode • Can apply a voltage Vd and get a current Id in one direction, but if you try to reverse the voltage polarity, you’ll get only a very small reverse saturation current, I0 • Diode voltage drop is about 0.6 V when conducting 17
- 18. The p-n JunctionDiode Voltage-Current (VI) characteristics for a diode / 0 ( -1) d qV kT d I I e 38.9 0 ( -1) (at 25 C) d V d I I e k = Boltzmann’s constant 1.381x10-23 [J/K] T = junction temperature [K] Vd = diode voltage Id = diode current q = electron charge 1.602x10-19 C I0 = reverse saturation current 18
- 19. Circuit Models ofPV Cells • The simplest model of PV cell is an ideal current source in parallel with a diode • The current provided by the ideal source ISC is proportional to insolation received • If insolation drops by 50%, ISC drops by 50% ISC Id I VLoad + - I VLoad + - PV cell Vd + - 19
- 20. Circuit Models ofPV Cells • From KCL, ISC = Id + I, and the current going to the load is the short-circuit current minus diode current • Setting I to zero, the open circuit voltage is / 0 ( -1) qV kT SC I I I e 0 ln 1 SC OC I kT V q I I ISC Id VLoad + - Vd + - 20 The subscript SC is for short circuit
- 21. PV Cell I-VCharacteristic • For any value of ISC , we can calculate the relationship between cell terminal voltage and current (the I-V characteristics) • Thus, the I-V characteristic for a PV cell is the diode I-V characteristic turned upside-down and shifted by ISC (because I = ISC – Id) • The curve intersects the x-axis at VOC and the y-axis is ISC / 0 SC ( -1)= f(I ,V) qV kT SC I I I e 21
- 22. PV Cell I-VCurves • I-V curve for an illuminated cell = “dark” curve + ISC dark curve 22
- 23. PV Cell I-VCurves • More light effectively shifts the curve up in I, but VOC does not change much • By varying the insolation, we obtain not a single I- V curve, but a collection of them 23
- 24. Need for aMore Accurate Model • The previous circuit is not realistic for analyzing shading effects • Using this model, absolutely NO current can pass when one cell is shaded (I = 0) 24
- 25. PV Equivalent Circuit •It is true that shading has a big impact on solar cell power output, but it is not as dramatic as this suggests! Otherwise, a single shaded cell would make the entire module’s output zero. • A more accurate model includes a leakage resistance RP in parallel with the current source and the diode • RP is large, ~RP > 100VOC/ISC • A resistance RS in series accounts for the fact that the output voltage V is not exactly the diode voltage • RS is small, ~RS < 0.01VOC/ISC 25
- 26. PV Equivalent withParallel Resistor • From KCL, I = ISC - Id – IRP • For any given voltage, the parallel leakage resistance causes load current for the ideal model to be decreased by V/RP ( ) SC d P V I I I R Photocurrent source R P Load (maximize) Parallel-Only ISC I V + - Vd + - Id Shunt resistance drops some current (reduces output current) 26
- 27. PV Equivalent withSeries Resistor • Add impact of series resistance RS (we want RS to be small) due to contact between cell and wires and some from resistance of the semiconductor • From KVL • The output voltage V drops by IRS d S V V I R Photocurrent source Load Rs (minimize) Series-Only ISC Vd + - V + - I Id Series resistance drops some voltage (reduces output voltage) 27
- 28. General PV CellEquivalent Circuit • The general equivalent circuit model considers both parallel and series resistance Photocurrent source R P Load Rs (minimize) (maximize) Equivalent Circuit Id I ISC Vd + - V + - d S V V I R ( ) d SC d P V I I I R / 0 ( ( -1)) S q kT V R I S SC P V R I I I I e R 28 Load current Load Voltage
- 29. Series and ShuntResistance Effects • Parallel (RP) – current drops by ΔI=V/RP • Series (RS) – voltage drops by ΔV=IRS 29
- 30. Fill Factor andCell Efficiency Cell Efficiency (h) = Isc•Voc•FF Incident Power Imax•Vmax Incident Power = Fill Factor (FF) = VR•IR Isc•Voc area = VRIR area = VOCISC 30 Fill factor is ratio at the maximum power point
- 31.