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yale university
Photovoltaic Properties of a
Revolutionary New Solar Cell
Drew Mazurek Advisor:
Jerry Woodall
April 30, 2002
solar cells in real life
• Cost-effective way to provide
power to remote areas
solar cells in real life
• Cost-effective way to provide
power to remote areas
• Environmentally-friendly
renewable energy source
solar cells in real life
• Cost-effective way to provide
power to remote areas
• Environmentally-friendly
renewable energy source
• Power source for outer space
applications
how solar cells work
The simple idea: photons in,
current and voltage out
how solar cells work
A closer look: photons above the semiconductor’s
band gap energy generate hole/electron pairs…
hn > Eg
- holes
p
pn-junction
- electrons
n
how solar cells work
which then diffuse across the cell’s
concentration gradients
- holes
p
pn-junction
- electrons
n
how solar cells work
Some holes and electrons recombine before they can
reach the other side of the junction. In good cells,
however, there is very little recombination.
- holes
p
pn-junction
- electrons
n
how solar cells work
Most holes and electrons make it to the other side, resulting in a
net charge increase on each side. This net charge increase is
realized outside the cell as current and voltage, or power.
- holes
p
pn-junction
- electrons
n
solar cells in space
To go into space, solar cells must be
• efficient – want to produce as much
power as possible
• lightweight – launching satellites into
space costs $5,000 per pound
Additionally, we’d like them to be
• inexpensive to manufacture – $$$
• radiation-hard – Van Allen Belt ideal place for
satellites, but high radiation environment
solar cells at yale
Indium Phosphide drift-based design
surface
n++
n
Strong electric field
(~1,000-10,000 V/cm)
p
solar cells at yale
Indium Phosphide drift-based design
hn > Eg
surface
n++
n
Strong electric field
(~1,000-10,000 V/cm)
p
solar cells at yale
Indium Phosphide drift-based design
surface
n++
n
Strong electric field
(~1,000-10,000 V/cm)
p
solar cells at yale
Diffusion
(theirs)
vs.
Drift
(ours)
• motion of carriers due
to concentration gradient
• motion of carriers due to
electric field
• material defects shorten
carrier lifetime, causing
more recombination
• not as susceptible to
material defects
solar cells at yale
Strong electric field… so what?
• holes are immediately
swept into the junction,
producing power
• fewer hole/electron
pairs are lost due to
recombination – no time
to recombine!
n++
n
p
solar cells at yale
Strong electric field… so what?
• radiation damage
decreases carrier lifetimes.
Carriers swept by drift
(electric) fields, however,
aren’t affected as much.
n++
n
p
solar cells at yale
Why Indium Phosphide?
• very high ideal efficiency:
~37% at concentration of
1,000 suns
solar cells at yale
Why Indium Phosphide?
• absorbs most light at small
thicknesses – lightweight!
GaAs
InP
100
Absorbed Power (%)
• very high ideal efficiency:
~37% at concentration of
1,000 suns
Si
80
60
40
20
0
0.01
0.1
1
10
Thickness (μm)
100
1000
solar cells at yale
• very high quantum
efficiency across all
wavelengths of visible
light and UV – highly
efficient and it makes
good use of almost the
entire spectrum
Internal Quantum Efficiency
Why Indium Phosphide?
1 µm InP (10 Å dead region)
1 µm GaAs (100 Å dead region)
1 µm Si (10 Å dead region)
10 µm Si (5 Å dead region)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
100
200
300
400
500
600
700
Wavelength (nm)
800
900
1000
solar cells at yale
Yale’s InP solar cells are ideal for outer space applications:
• lightweight
• radiation-hard
• highly efficient
• low cost (~$1/cm2 vs. $10/cm2 for current highefficiency solar cells)
summary
• Solar cells are simply pn-junctions in which hole/electron
pairs are created from photons.
• The holes/electrons diffuse into the junction, and are
immediately swept to the other side.
• The net charge gain is seen outside the cell as current and
voltage, or power.
summary
• At Yale, we have designed and perfected the first ever
drift-dominated solar cell.
• By collecting carriers with an electric field, we are able to
create solar cells that are robust in the strong radiation of
outer space.
• Additionally, our cells are lightweight and inexpensive.
acknowledgements
Many thanks to:
Professor Jerry Woodall
Professor Janet Pan
Yanning Sun