Chapter 9 - Solar energy harvesting with 2D materials

Question 1

What are optoelectronic devices?

Answer 1

Electronic devices that either detect, generate, control or interact with light.

Question 2

Give some examples of optoelectronic devices using the photoelectric effect.

Answer 2

Photodiode, solar cells, phototransistors, photoamplifier.

Question 3

Give some examples of optoelectronic devices using photoconductivity.

Answer 3

Photoresistors, switches, charge-coupled imaging devices (CCDs).

Question 4

Give some examples of optoelectronic devices using stimulated emission

Answer 4

LEDs, laser diodes.

Question 5

How can radiative transition (absorption and emission) rates be calculated?

Answer 5

Using Fermi’s Golden Rule (with the transition matrix element).

Question 6

What determines how strongly a material interacts with light`

Answer 6

The magnitude of the momentum matrix element.

Question 7

What can be said about optical transitions in an E-k diagram?

Answer 7

They are vertical, due to the negligible momentum of the photon.

Question 8

How does the photon absorption rate depend on the joint density of states?

Answer 8

It is proportional to it.

Question 9

What is the joint density of states?

Answer 9

All the pairs of CB and VB states that can facilitate an excitation by a photon with a given energy.

Question 10

What are the requirements of solar energy conversion?

Answer 10

We need absorption of sunlight, the ability to separate the electrons and holes and transport them to the electrodes. Then we need to transport the generated electricity to where we want it, or store it (e.g. in batteries).

Question 11

How can we store energy generated by sunlight?

Answer 11

Electricity generated by PVs can be stored in for example batteries or supercapacitors. We could also use the sunlight to produce solar fuels, such as hydrogen or methanol through artificial photosynthesis.

Question 12

What are the requirements for an efficient photocatalyst?

Answer 12

1. High photoabsorption efficiency in visible regime.
2. Separation of e-h pairs.
3. Appropriate charge carrier mobility.
4. High stability in water, also under illumination.
5. Bandgap above 1.23 eV (due to potential needed to split water).
6. Band edge suitable for water redox potential.
7. High earth abundance.

Question 13

What are the absorbance of TMD monolayers? How does this compare to GaAs and Si?

Answer 13

5-10%. For the same amount of absorbance, we need 15 nm of GaAs or 50 nm of Si.

Question 14

What is required for separation of e-h pairs?

Answer 14

A built-in in-plane electric field.

Question 15

Give three examples of how one can introduce built-in electric field?

Answer 15

1. Schottky barrier using MoS2 / Graphene. The electrons will go from MoS2 to graphene, and holes can’t go to graphene.
   2) 2D Material Heterojunction - using a type 2 heterojunction from WS2 / MoS2 interface.
   3) MoS2 multilayer Schottky junction using different electrodes.

Question 16

What is the fill factor when talking about solar cell efficiency?

Answer 16

The fraction of the area of maximum power point compared to the maximum power generated if I-V curve was square.

Question 17

How does the power density of graphene/MoS2 or WS2/MoS2 compare to Si or GaAs?

Answer 17

They are much, much higher.

Question 18

To what extent are MoS2 stable in water under illumination?

Answer 18

Pristine MoS2 is highly stable. Edges and vacancies allow for water assisted laser cutting with energy of photon is above band gap.

Question 19

How does the different materials do when it comes to absorption of sunlight?

Answer 19

GaAs has a low band-gap, and absorb most sunlight. The same does WSe2. MoS2 absorb less, but still on the right side of the maximum peak. TiO2 absorbs only a tiny fraction of the sunlight.

Question 20

Sketch the principle of solar water splitting. Include the reaction equations.

Answer 20

For sketch, see slides. p28

Equations:
in semiconductor: 2photons -> 2e + 2h
oxidation: H2O + 2h+ -> 2H+ + 1/2 O2
reduction: 2H+ + 2e -> H2
overall: H2O + 2photons -> H2 + 1/2 O2

Question 21

How can one visualize the fit of a semiconductor for solar water splitting?

Answer 21

See slides. p29

Question 22

What is the overpotential?

Answer 22

The overpotential is the extra potential difference needed to get the reaction going. Even though water splitting theoretically happens at 1.23 V, it doesn’t always go, and will need a bit of extra potential to get going.

Question 23

Draw schematically how a an n-type photo anode water splitting device could work.

Answer 23

See slides. p30

Question 24

Which of the seven requirements for a good photocatalyst does MoS2 fulfill?

Answer 24

All of them.

Question 25

Are MoS2 most efficient as a photocatalyst as a monolayer, bilayer or trilayer?

Answer 25

As monolayer. The overpotential increases with number of layers.

Question 26

Why could it be a good idea to grow vertically aligned MoS2 or MoSe2 films for water splitting purposes?

Answer 26

Because the edge site plays an important role for hydrogen evolution. This increases the number of edge sites exposed to water.