nanostructured solar cells - university of california,...

Nanostructured Solar Cells

Dong Yu

COSMOS, July 17, 2015

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) !

Human power needs can be met by covering the black spots with solar cells!

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


Question:

1. Which of the following has longer wavelength:

A. red light

B. blue light

A is correct.

Which one has higher energy photons?


Question:

2. Sort the following electromagnetic radiation from

high to low photon energy:

ultraviolet, infrared, red light, microwave, x-ray,

blue light, radio wave

Light: wave and particle

Solar Spectrum

Photoelectric Effect

A photon knocks out an electron from an atom


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 +

-


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.


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


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


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


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


Electrons are negatively charged and holes are positively charged.

So an electric field would separate them.

Conduction band

Valence band

Energy

+

-

Electric field


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






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 Solar Cells

The cost of solar electricity is several times coal electricity.

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






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 dots are very small crystals.

The color of quantum dots can be controlled by their sizes to

match solar spectrum.


efficiency ~10%

Nanowire Solar Cells

Nanowires can be grown into an array vertical to substrate.

Efficiency 14%


Photons

Conventional Solar Cells

p-type

n-type - +

- +

- +

- +

Contact Grid

Back Contact

Photons

- +

- +

- +

- +


Charges do not need to travel long any more in nanowire 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.


Very rapid increase of efficiency


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.

nanostructured solar cells - university of california,...

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