Case Study: Solar cells
• Uses the principle of the photoelectric effect
(Einstein: Nobel prize, 1919): light hitting on a
material creates current
Sun light
Solar cell
current
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
Exposure to light
3.1 eV (violet)
2.4 eV (yellow)
1.7 eV (red)
Electron-hole
pair
~ 1.1 eV
Full band
•
•
•
Full band
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
As
3 valence
electrons
4 valence
electrons
5 valence
electrons
Phosphorous impurity
Aluminum impurity
Empty band
Empty band
Donor level
1.1 eV
Acceptor level
Full band
Full band
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
Phosphorus atom
Aluminum atom
4+ 4+ 4+ 4+
4+ 4+ 4+ 4+
4+ 5+ 4+ 4+
4+ 3+ 4+ 4+
4+ 4+ 4+ 4+
no applied
electric field
Free electron
valence
electron
Si atom
“Hole”
4+ 4+ 4+ 4+
no applied
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 !
Impurities in Si: band picture
Phosphorous impurity
Aluminum impurity
Empty band
Empty band
Donor level
1.1 eV
Acceptor level
Full band
n-type semiconductor
(charge carriers are
negatively charged)
Hole
Full band
p-type semiconductor
(charge carriers are
positively charged)
Response to electric field
• Say we have two pieces of Si, one is doped with phosphorous (ntype 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
4+ 4+ 4+ 4+
4+ 4+ 4+ 4+
4+ 5+ 4+ 4+
4+ 3+ 4+ 4+
4+ 4+ 4+ 4+
Free electron
valence
electron
4+ 4+ 4+ 4+
Si atom
Free electrons
Free holes
Bound electrons
“Hole”
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 ntype 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 cell
n-type Si
p-type Si
neutral
neutral
Some holes
neutralized by
electrons
Full band
Positively
charged
Negatively
charged
Full band
Electric current generated !!
Exposure to light
creates electron-hole
pairs
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
Natural gas
Coal
Nuclear
Hydroelectric
Biomass
Geothermal
Solar
Wind
FUEL CELLS
38.8 %
23.2 %
22.9 %
7.6 %
3.8 %
3.2 %
0.3 %
0.07 %
0.04 %
???
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