Nanostructured Solar Cells - University of California,...
Transcript of Nanostructured Solar Cells - University of California,...
Outline
• Introduction Solar energy
Photoelectric effect
Photoexcitation in semiconductors
Physics of solar cells
• Solar cells Criteria for cost-effective solar cells
Conventional solar cells
Nanostructured solar cells Polymer solar cells
Dye Sensitized solar cells
Quantum dot solar cells
Nanowire solar cells
Plasmonic solar cells
Perovskite solar cells
Power to Earth = 1.2 x 1017 W
Composition:
74% H2 + 24% He
Nuclear fusion:
P = 4 × 1026 W
Lifetime: 5 billion years
1 hour (of solar energy to earth) = 1 year (of energy consumption) !
Solar Farm in Nevada Desert
Copper Mountain Solar 3; completed in 2015
Located in the desert town of Boulder City about 20 miles from
Las Vegas; 1,400 acres of land; 250 MW; supply power to
about 80,000 homes.
Light: wave and particle
E= hf = hc/l
E: photon energy
h: plank constant
c: light speed
l: wavelength
Test your understanding
Question:
1. Which of the following has longer wavelength:
A. red light
B. blue light
Test your understanding
Question:
1. Which of the following has longer wavelength:
A. red light
B. blue light
A is correct.
Which one has higher energy photons?
Test your understanding
Question:
2. Sort the following electromagnetic radiation from
high to low photon energy:
ultraviolet, infrared, red light, microwave, x-ray,
blue light, radio wave
Test your understanding
Question:
3. You are trying to knock electrons out of
hydrogen atoms with light. You find
that a light bulb does not work. Will two
light bulbs work any better?
A. Yes. B. No +
-
Test your understanding
Question:
4. What is a better strategy?
A. Use ten light bulbs
B. Put the light bulb closer to the
hydrogen
C. Put hydrogen in a microwave oven.
D. Put the hydrogen in your mouth when
you get your teeth x-rayed.
Test your understanding
Question:
4. What is a better strategy?
A. Use ten light bulbs
B. Put the light bulb closer to the
hydrogen
C. Put hydrogen in a microwave oven.
D. Put the hydrogen in your mouth when
you get your teeth x-rayed.
D is correct. Merely increasing the number of photons do not help!
Hydrogen energy levels
+
-
Solid matter made of many atoms has so many
energy levels that they form energy bands.
1 eV = 1.6 x 10-19 Joule
From hydrogen atom to solid matter
Solid matter made of many atoms has so many
energy levels that they form energy bands.
Energy Bands
In a semiconductor, electrons fill up the low energy valence band
and leave the high energy conduction band empty.
Unfilled
Filled by electrons
Conduction band
Valence band
Energy
Photoexcitation
A photon can be absorbed by a semiconductor, knocking an
electron from low energy valence band to high energy conduction
band, leaving a vacancy (called hole) behind.
Conduction band
Valence band
Photon
Energy
Recombination
After a while, the high energy electron in the conduction band will
relax back into valence band and recombines with the hole. The
excess energy can be converted into a photon or heat.
Conduction band
Valence band
Photon
Energy
Test your understanding
Question:
5. If an electron is negatively charged, the
hole left behind in the valence band must
be
A. also negatively charged
B. positively charged
C. it depends on the type of the atoms
Test your understanding
Question:
6. Silicon is a semiconductor with a bandgap of 1.1 eV.
Germanium has a smaller bandgap of 0.67 eV.
Which one can absorb more sun light?
A. Silicon
B. Germanium
Test your understanding Question:
6. Silicon is a semiconductor with a bandgap of 1.1
eV. Germanium has a smaller bandgap of 0.67
eV. Which one can absorb more sun light?
A. Silicon
B. Germanium
B is right. Photons with
energy above bandgap
can be absorbed by the
semiconductor.
Silicon Germanium
Test your understanding
Question:
7. Then germanium should make a better solar cell,
since it absorbs more sun light.
A. Yes
B. No
Test your understanding Question:
7. Then germanium should make a better solar cell,
since it absorbs more sun light.
A. Yes
B. No
B is right. The small
bandgap means each
electron has less energy.
Though sun light can
generate more electrons in
Germanium, the total
energy is less than Silicon. Silicon Germanium
Bandgap vs. Efficiency
Bandgap too large not much absorption
Bandgap too small electron energy is too low
Optimal bandgap for solar cells is about 1-2 eV.
Physics of Solar Cells
Recombination is bad for light to electricity conversion, as the
energy absorbed by the semiconductor is “wasted” back into
photons or heat. How do we separate electrons and holes?
Conduction band
Valence band
Energy
Photon
Physics of Solar Cells
Electrons are negatively charged and holes are positively charged.
So an electric field would separate them.
Conduction band
Valence band
Energy
+
-
Electric field
Physics of Solar Cells
Built in
Practically, the electric field is achieved by putting two different semiconductors
(or same semiconductor with different types) together, forming a p-n junction.
Basically, electrons want to go into n-type semiconductor, holes into p-type.
Outline
• Introduction Solar energy
Photoelectric effect
Photoexcitation in semiconductors
Physics of solar cells
• Solar cells Criteria for cost-effective solar cells
Conventional solar cells
Nanostructured solar cells Polymer solar cells
Dye Sensitized solar cells
Quantum dot solar cells
Nanowire solar cells
Plasmonic solar cells
Perovskite solar cells
Criteria for cost-effective solar materials
Low cost: Earth abundant, non-toxic, stable
High efficiency: bandgap 1-2 eV, low defects
(High defects faster recombination energy loss)
Conventional Planar Solar Cells
Photons
Conventional Si Solar Cells
p-type
n-type - +
- +
- +
- +
Contact Grid
Back Contact
Cells need to be thick to
absorb sunlight and hence
charge needs to travel a
long distance. Electrons
and holes can recombine
during traveling unless the
recombination lifetime is
long.
Need very pure (high cost)
materials.
10
0 m
m
Nanostructured Solar Cells
Polymer solar cells
Dye Sensitized solar cells
Quantum dot solar cells
Nanowire solar cells
Plasmonic solar cells
Perovskite solar cells
Polymer solar cells
Pro: solution processing, solar cells can be made like printing newspapers.
Con: polymers are often sensitive to moisture, unstable in air.
efficiency ~10%
Dye sensitized solar cells
efficiency ~12%
Light is absorbed by dye molecules; electrons are collected by TiO2;
holes collected by electrolyte.
Quantum dot solar cells
Quantum dots are very small crystals.
The color of quantum dots can be controlled by their sizes to
match solar spectrum.
Nanowire Solar Cells
Photons
Conventional Solar Cells
p-type
n-type - +
- +
- +
- +
Contact Grid
Back Contact
Photons
- +
- +
- +
- +
Nanowire Solar Cells
Charges do not need to travel long any more in nanowire solar cells.
Plasmonic solar cells
Metal nanoparticles are placed on top of semiconductors to
enhance light intensity so that a thinner silicon is sufficient to
absorb most sunlight.
Perovskite solar cells
Methyl ammonium lead iodide
There are millions of compounds. The best has to be discovered!
Conclusion
Sun provides abundant, clean energy to earth.
Photons knock electrons to higher energy. Taking out
the high energy electrons before they relax results in
electric power.
Conventional silicon solar cells are still too expensive to
compete with cheap (but dirty) coal electricity
New solar cells taking advantage of nanostructures may
revolutionize energy generation in the near future.