Case Study: Solar cells• Uses the principle of the photoelectric effect

(Einstein: Nobel prize, 1919): light hitting on a material creates current

current

Solar cell

Sun light

current

Silicon based Solar cells• Band gap of Si small enough (1.1 eV) for visible light (1.7-3.1 eV) to

excite electrons• Thus visible light will make Si a conductor! So Si is not exposed to

light in devices; it is packaged

Full band

~ 1.1 eV

3.1 eV (violet)

1.7 eV (red)2.4 eV (yellow)

Full band

Exposure to light

Electron-hole pair

• In solar cells, Si is exposed to light to create electron hole pairs • However, electron-hole pairs created will annihilate themselves, as electron will fall back into the hole

re-emitting light again• So, a p-n junction is used which will prevent the re-emission process, and will result in a net current

Impurities in Si• Impurities are added to Si in a

controlled manner (by a process called “doping”) to create donor and acceptor levels

B C N

Al Si P

Ga Ge As3 valence electrons

4 valence electrons

5 valence electrons

Empty band

Full band

1.1 eV

Donor level

Phosphorous impurity

Empty band

Full band

Acceptor level

Aluminum impurity

Both impurities result in levels that are about 0.03 eV from the main band; thus room temperature thermal energy is sufficient to excite electrons to and from these levels

Impurities in Si: physical picture

• A “hole” is a missing electron, just like a vacancy is a missing atom in an atomic lattice

• A hole has the properties of an electron but has an effective positive charge !

no applied electric field

5+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

Phosphorus atom

no applied electric field

Aluminum atom

valence electron

Si atom

3+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

Free electron

“Hole”

Impurities in Si: band picture

Empty band

Full band

1.1 eV

Donor level

Empty band

Full band

Acceptor level

Phosphorous impurity Aluminum impurity

Hole

n-type semiconductor(charge carriers are negatively charged)

p-type semiconductor(charge carriers are positively charged)

Response to electric field• Say we have two pieces of Si, one is doped with phosphorous (n-

type Si), and the other doped with aluminum (p-type Si)• At room temperature, the first Si piece has a lot of free electrons,

and the second one has free holes• When an electric field is applied, the two types of charge carriers

move in opposite directions, as they are oppositely charged

5+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

Phosphorus atom Aluminum atom

valence electron

Si atom

3+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

“Hole”

Free electrons

Free electron

Free holes

Bound electrons

The p-n junction rectifier

• When a p-type and a n-type Si are joined together, we have a p-n junction

• A p-n junction has high electron conductivity along one direction, but almost no conductivity along the other! Why?

• Electrons can cross the p-n junction from the n-type Si side easily as it can jump into the holes

• However, along the other direction, electrons have to surmount a ~ 1.1 eV barrier (which is impossible at room temperature in the dark)

p-n junction solar cellp-type Sin-type Si

neutral neutral

Full band

Positively charged

Negatively charged

Full band Exposure to light creates electron-hole pairs

Electric current generated !!

Some holes neutralized by electrons

Basic solar cell

• Anti-reflective coating prevents reflection at top surface to increase efficiency

• Top and bottom contacts help collect the electron and hole currents generating electricity in an external circuit

Prospects of solar cells• Today, only 0.1% of all energy produced come from

solar energy; maximum demonstrated efficiency is 30 %• We want large pieces of crystalline Si to make solar cells

counter to the trend of miniaturization, and difficult to produce large crystalline Si

• Although large, high efficiency amorphous Si solar cells have been demonstrated, production of these is slow

• Lack of sunshine in some parts of the world, and unpredictability in others

• Solar cells produce DC, but AC current required for transmission to large distances

• At present, the most promising applications are in rural and remote areas

• However, this is a very “clean” source of energy, and research is continuing …

Sources of Energy (US)

• Oil 38.8 %• Natural gas 23.2 %• Coal 22.9 %• Nuclear 7.6 %• Hydroelectric 3.8 %• Biomass 3.2 %• Geothermal 0.3 %• Solar 0.07 %• Wind 0.04 %• FUEL CELLS ???

Camera photocells & night vision goggles

• Photocells work due to the fact that Si is an insulator in darkness, but is a conductor when exposed to light

• Night vision goggles are of 2 types: active and passive– Passive: uses the low intensity light in dark situations,

and will not work in total darkness• This uses the reverse of the solar cell principle: light creates

electrons, electrons hit other electrons, and create more electrons, which are all accelerated towards a phosphor screen

– Active: uses infrared radiation

How can we use non-visible radiation?

• All radiation can theoretically be focused just like visible light.– Really only practical for visible, IR, and UV.– Otherwise, wavelengths are too short or long to

be able to build a useful device.

• This provides opportunities as certain wavelengths transmit better through the atmosphere than others, especially as a function of weather (e.g. fog).– IR

• IR is also a strong function of temperature, and thus can be used for thermal measurements.

IR as art

http://www.ir55.com/infrared_IR_camera.html

Surveillance/targeting

Thermal non-destructive-testing (thermal-NDT)

Aerial imaging

• IR can be used to detect features that can be hidden from visual observation (camouflaged)

http://www.photo.net/photo/edscott/ap000010.htm

Summary

• Doping Si produces n-type or p-type semiconductors

• Solar cells created by forming a junction between n-type and p-type semiconductors

• Next class (next Tuesday):– A-J: Prof. Leon Shaw’s guest lecture– K-W: Dr. Dan Goberman’s lab tour (UTEB 269)

• Next regular class (next Thursday): Optical properties of materials (Chapters 28 & 29)

• April 14: Pratt-Whitney tour• April 19: Quiz 3