The Sun

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

Sunspots and the Sunspot Cycle

Photovoltaics

How Much Solar Irradiance Do You Get?

6 Boxes at 3.3 TW Each

Solar Cell Land Area Requirements for the World’s Energy with Solar PV

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

Types of Solar Photovoltaic Materials

Photovoltaic Materials

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

p-n and p-i-n Junctions

Ef Ef

Vbi Vbi

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

I-V Curve for Solar Cells

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

The Solar Spectrum

The entire spectrum is not available to single junction solar cell

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

Electron Flow in a PV Cell

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

Different Types of Photovoltaic Solar Cells

DiffusionDrift

Excitonic

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

Power Losses in

Solar Cells

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