Discussion about quantum dots photovoltaics

What is solar photovoltaics ? What is the solar spectrum ? What is energy in physics ? If you know the answers to these questions, we can carry on. If not, you will not understand.

I Attracts
II Nodes and disappointment
III Towards a simple design


I Attracts

First, let us say that quantum dots solar cells are in the state of research. They are not yet interesting to use.

Quantum dots are crystals of the size of tens to hundreds of atoms1 that exhibit two interesting properties for solar photovoltaics : multiple exciton generation and a gap energy at will.

Multiple exciton generation is the fact that 1 photon can free more than one electron in the active material. It means than instead of being lost as heat, the excessive energy of the photon is harvested into other free electrons2.

The gap of quantum dots is tunable with the size of the crystal3. I guess that the gap increases when the size of the crystal diminishes. Why ? Because I’ve learnt in the university that the energy involved in small systems is more important than in big systems, and this is why nuclear chemistry has bigger energies than classical chemistry.
The freedom of choice for gaps makes quantum dot solar cells interesting for multi-junction solar cells, that are the best solar cells in terms of efficiency with 44 % concentrated efficiency in space aeronautics4.

II Nodes and disappointment

Let’s read between the lines of the Wikipedia Mutiple exciton generation article :

“high-energy photons excite on average more than one electron across the band gap, and so in principle the solar cell can produce more useful work.”.

High energy photons. What does that mean ?

Let us consider that a high-energy photon is a photon with a bigger energy than a normal photon.
Let us consider that a normal photon is a photon with an energy at the middle of the visible spectrum.

The wonderful solar spectrum figure tells us that high-energy photons are the ones in pink.

high_energy_photons

(Pink shits, right ?)

First, it means that most of the solar spectrum frees 1 electron for 1 photon. But hey, it’s gonna be amazing, maybe we shall be able to free 2, 3, 4, or even more electrons with 1 high-energy photon !

When reading Wikipedia, we understand that this is not natural numbers, but non-integer numbers like 1.3, that looks like more an average. We are in quantum mechanics, it’s all about collective phenomenoms.

So, how many electrons does a high-energy photon free ?

It begins to be more than 1 at 550 nm, then it gradually increases and stops at 300 nm.

550/300 = 1.833 etc

so it means that the maximum electrons a photon of some energy can free on average is below 2.

In the calculus, that means you put 1 instead of 1 for most of the spectrum, and you put 1.9 or even less for a small part of the spectrum. Such a little gain !

That means that multiple exciton generation is just little energy conversion gain for photovoltaics.

The tunable gap remains interesting.

I’ve written about nodes. They are social ones.

In the video above, you see the reality of quantum dot photovoltaics. There are a lot of different materials, including cadmium, a toxical one, the results do not look like photovoltaics modules.

Sincerely, the problem of the quantum dot social middle is that :

  • they are extremely complicated ;
  • they have understood nothing to the ecological mind of solar photovoltaics ;
  • they are tens of years of industry away to know if it is properly recyclable ;
  • they are disconnected from the reality of energy economics ;
  • and (as many others unfortunately) they do not have a clear understanding of solar photovoltaics.

Wow, and we need to work with those lazy guys ?

Ecology doesn’t need good for nothing people. It needs people with a strong motivation, and an environmental conscience.

I’ve spitted enough venom by now. Let us see something more positive 🙂 .

III Towards a simple design

When I was wondering about what a good solar cell should look like, I’ve run into a comment on the Internet that said there was a need for the simplicity of the design.

Simplicity of the design.

I plan to make an article to explain what design is, because this notion is often misunderstood. In short, the simplicity of the design means that the figure of the solar cell is simple.

cdte_thin_layers
This solar cell has a superposition of layers design. I’ve seen the technical part, and I’ve noticed that CdTe and CdS is a good choice in terms of active materials. Not a good choice in terms of ecology as Cd is highly poisonous.

Simple things are the most powerful. With a simple and positive state of mind, things are gonna be OK 🙂 .

Notes :

1 : Source : Quantum dots, pv magazine, Ångström, Wikipedia, Atom sizes, WebElements. The size of an atom is about 1 ångström.
2 : Source : Multiple exciton generation, Wikipedia
3 : Source : Quantum dot solar cell, Wikipedia
4 : Source : NREL world record, energy informative. Take a look at the NREL figure.

Advertisements