The Sun. Big Bang nucleosynthesis 5 billion years old, 5 billion more years before death Radiated...
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Transcript of The Sun. Big Bang nucleosynthesis 5 billion years old, 5 billion more years before death Radiated...
The Sun
• Big Bang nucleosynthesis
• 5 billion years old, 5 billion more years before deathRadiated energy determines age
• Diameter 110 x earth, 99.86% mass of solar system
• ~90 million miles from earth = 1 AU, ~8 minutes for light
• ~75% H, 23% He, rest heavy elements, 2% heavy elements
• Surface temperature ~6,000°C
• 1.4x103 kg/m3, 24 x g ,(water 1.0x103 kg/m3)
• Nuclear Fusion – 500,000 tons H/sec100 earths so far
• Brighter than 85% of stars in Milky Way – Red Dwarfs 2
Blackbody Radiation
Thermal Radiation Is Electromagnetic Radiation Emitted From A Material Which Is Due To Its Temperature
The Sun
• Magnetically active, 11 year cycle:Sunspots, Solar flaresSolar wind (disruption of communications and electric power)
• Made of gas and plasmaH fusion now, He in 5 billion years
• Stellar nucleosynthesis
5
A Brief History Photovoltaic Technology
• 1839 – Photovoltaic effect discovered by Becquerel• 1870s – Hertz developed solid selenium PV (2%)• 1905 – Photoelectric effect explained by A. Einstein • 1930s – Light meters for photography commonly
employed cells of copper oxide or selenium• 1954 – Bell Laboratories developed the first
crystalline silicon cell (4%)• 1958 – PV cells on the space satellite U.S. Vanguard
(better than expected)
Things Start To Get Interesting...
• mid 1970s – World energy crisis = millions spent in research and development of cheaper more efficient solar cells
• 1976 – First amorphous silicon cell developed by Wronski and Carlson
• 1980’s - Steady progress towards higher efficiency and many new types introduced
• 1990’s - Large scale production of solar cells more than 10% efficient with the following materials: Ga-As and other III-V’s CuInSe2 and CdTe
TiO2 Dye-sensitized
Crystalline, Polycrystalline, and Amorphous Silicon
• Today, prices continue to drop and new “3rd generation” solar cells are researched
Energy Bands in a Semiconductor
• Conduction Band – Ec – empty
• Valence Band – Ev – full of electrons
3 Types of Semiconductors
1. Intrinsic2. n-type3. p-type
• Types 2 and 3 are semiconductors that conduct electricity - How?
By alloying semiconductor with an impurity, also known as doping
Carriers placed in conduction band or carriers removed from valence band
Schottky Barriers and Heterojunctions
• A Schottky barrier is a potential barrier formed at a metal–semiconductor junction which has rectifying characteristics, suitable for use as a diode
• The largest differences between a Schottky barrier and a p–n junction are its typically lower junction voltage, and decreased (almost nonexistent) depletion width in the metal
Light Absorption by a Semiconductor• Photovoltaic energy relies on light• Light → stream of photons → carries energy• Example: On a clear day 4.4x1017 photons hit 1 m2
of Earth’s surface every second.• Eph()=hc/
h = plank’s constant = 6.625 x 10-34 J-s = wavelength c = speed of light =3 x 108 m/s f = frequency
• However, only photons with energy in excess of bandgap can be converted into electricity by solar cells
Generation of Electron Hole Pairs with Light
• Photon enters, is absorbed, and lets electron from VB get sent up to CB
• Therefore a hole is left behind in VB, creating absorption process: electron-hole pairs
• Because of this, only part of solar spectrum can be converted
• The photon flux converted by a solar cell is about 2/3 of total flux
Generation Current• Generation Current = light induced electrons across bandgap as
electron current
• Electron current:= Ip=qNA
N = # of photons in highlighted area of spectrum A = surface area of semiconductor that’s exposed to light
• Because there is current from light, voltage can also occur
• Electric power can occur by separating the electrons and holes to the terminals of device
• Electrostatic energy of charges occurs after separation only if its energy is less than the energy of the electron-hole pair in semiconductor
• Therefore, Vmax=Eg/q
• Vmax= bandgap of semiconductor is in EV’s, therefore this equation shows that wide bandgap semiconductors produce higher voltage
Diffusion
• n-type and p-type are aligned by the Fermi-level
• When a photon comes in n-type, it takes the place of a hole, the hole acts like an air bubble and “floats” up to the p-type
• When the photon comes to the p-type, it takes place of an electron, the electron acts like a steel ball and “rolls” down to the n-type
Recombination
• Opposite of carrier generation, where electron-hole pair is annihilated
• Most common at:impuritiesdefects of crystal structuresurface of semiconductor
• Reducing both voltage and current
Tandem Cells• Tandem cell- several
cells, Top cell has large
bandgap Middle cell mid
eV bandgapBottom cell small
bandgap
Indium Tin Oxidep-a-Si:H
Blue Cell i-a-Si:Hn-a-Si:H
pGreen Cell
i-a-SiGe:H (~15%)
np
Red Celli-a-SiGe:H (~50%)
nTextured Zinc Oxide
SilverStainless Steel Substrate
Silver Grid
Schematic diagram of state-of-the-art a-Si:H based substrate n-i-p triple junction cell structure