1. Case Study: Solar cells
Uses the principle of the photoelectric effect (Einstein: Nobel prize, 1919): light hitting on a material creates current
Solar cell
Sun light
current
current

2. Silicon based Solar cells
Band gap of Si small enough (1.1 eV) for visible light ( 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
Exposure to light
Electron-hole pair
Full band
~ 1.1 eV
3.1 eV (violet)
1.7 eV (red)
2.4 eV (yellow)
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

3. 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 As
3 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

4. Impurities in Si: physical picture
Phosphorus atom
Aluminum atom
“Hole” 4 + 4 +
Free electron 5+ 3 +
valence
electron
no applied
Si atom
no applied
electric field
electric field
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 !

5. Impurities in Si: band picture
Phosphorous impurity
Aluminum impurity
Empty band
Empty band
Donor level
1.1 eV
Acceptor level
Full band
Full band
Hole
n-type semiconductor
(charge carriers are negatively charged)
p-type semiconductor
(charge carriers are positively charged)

6. 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
Phosphorus atom
Aluminum atom
“Hole” 4 + 4 + 5+ 3 +
valence
electron
Free electron
Si atom
Free electrons
Free holes
Bound electrons

7. 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)

8. p-n junction solar cell
n-type Si
neutral
p-type Si
Full band
Positively charged
Negatively charged
Some holes neutralized by electrons
Full band
Exposure to light creates electron-hole pairs
Electric current generated !!

9. 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

10. 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 …

11. Sources of Energy (US) Oil 38.8 % Natural gas 23.2 % Coal 22.9 %
Nuclear %
Hydroelectric 3.8 %
Biomass %
Geothermal 0.3 %
Solar %
Wind %
FUEL CELLS ???

12. 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

13. 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.

14. IR as art

15. Surveillance/targeting

16. Thermal non-destructive-testing (thermal-NDT)

17. Aerial imaging
IR can be used to detect features that can be hidden from visual observation (camouflaged)

18. 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