[Artificial Atoms] to harness solar power!

What are quantum dots & how can they be used with solar cells.

8 min readJul 28, 2019

For the past month, I’ve been looking very deeply into Nanotechnology (science that looks at how we can manipulate matter at the molecular and atomic level). Reading TONS of research papers — listed at the bottom, and talking to many different professors and co-founders in the space.

Today, I’ll be talking about a sub-field of that called quantum dots (QD) or artificial atoms and how we can use these for solar.

Think about how cool it would be if we could control individual atoms. Like being able to “turn” them on and off to store bits of information, make them light up with different colors, or control them in all kinds of other ways.

Something very similar to this are called quantum dots (QD) and they look like this [they are pretty fun]:

Semi-conductive tiny atoms — what are quantum dots?

A quantum dot is basically a tiny speck of matter that is so small that it’s concentrated into a single point (or zero-dimensional). Since it is zero-dimensional, the electrons and holes, which are places that are missing electrons are constrained. According to the laws of quantum theory, these have well-defined energy levels like stairs — each step defining a different energy level.

Quantum dots are crystals which are a few nanometers wide. They’re made from a semiconductor like silicon (a material which is neither really a conductor or an insulator, but can be chemically treated so it behaves like either).

A semiconductor is like a raisin cake, where the raisins give the cake the special flavor that makes it sell. By itself, a semiconductor can’t really conduct electricity that well, but if you introduce impurities into it then those impurities turn it into a conductor. The impurities are called dopants, and electric current travels through doped silicon under the right conditions. Think of the dopant atoms as the raisins in the raisin cake, and the raw semiconductor material that’s in between the dopant atoms as the cake in which the raisins sit … this is making me hungry.

Raisins are the dopant atoms and the cake mix in between is the semiconductor material.

They also behave more like individual atoms which is why they are referred to as “artificial atoms”.

Quantum Physics — how do quantum dots work?

If we give an atom energy, you can “excite” it: you can boost an electron inside it to a higher energy level. Just like when you’re playing a video game, and your excitement level goes up & you start moving up levels.

This is followed by the process of relaxation, when the photons can relax and fall back into a lower-lying state. When the electron returns to a lower level, the atom emits a photon of light with the same energy that the atom originally absorbed. The wavelength and frequency of light an atom emits depends on what the atom is. For example, iron looks green when you excite its atoms by holding them in a hot flame, sodium looks yellow, and that’s because of the way their energy levels are arranged. Basically, different atoms give out different colors of light. All this is possible because the energy levels in atoms have set values.

1.& 2. After absorbing energy, an electron inside an atom is promoted to a higher energy level further from the nucleus (core).

3. When it returns, the energy is given out as a photon of light. The color of the light depends on the energy levels and varies from one atom to another.

Wavelength & Frequencies — color

Quantum dots produce light in a similar way because the electrons and holes constrained inside them give them similarly discrete, quantized energy levels. BUT the energy levels are controlled by the size of the dot not the material. For example, dots made from the same material (like silicon) will give out different colors of light depending on how big they are.

The biggest quantum dots produce the longest wavelengths (and lowest frequencies), while the smallest dots make shorter wavelengths (and higher frequencies). Which means that big dots = red light and small dots = blue light. Like when you get really angry, your face turns big and red. When you’re cold, you start to shiver and feel blue (well kind of but you get it).

A small dot has a bigger band gap (the minimum energy it takes to free electrons so they’ll carry electricity through a material), so it takes more energy to excite it; because the frequency of emitted light is proportional to the energy, since smaller dots with higher energy produce higher frequencies (and shorter wavelengths).

Larger dots have more (and more closely) spaced energy levels, so they give out lower frequencies (and longer wavelengths).

Bigger dots produce longer wavelengths, lower frequencies, and redder light; smaller dots produce shorter wavelengths, higher frequencies, and bluer light.

What makes quantum dots unique?

In regular semiconductors like silicon (or bulk matter), the bands are formed by the merger of adjacent energy levels of a very large number of atoms and molecules. But as the particle size reaches the nano-scale and the quantity of atoms and molecules decreases substantially, the number of overlapping energy levels decreases, causing the width of the band to increase. As QDs are so tiny, they have a higher energy gap between the valence and conduction bands, compared to silicon.

The unique properties of quantum dots can be explained by two phenomena’s which we already touched on a bit: quantum confinement effect and the discrete nature (quantized) of the electronic states of these particles.

Quantum confinement effect

Quantum confinement effect is the change in the atomic structure of the particle observed when the energy band is affected by the shifts in the electronic wave range. Confinement energy is the property of a quantum dot that explains the relationship between QDs size and the frequency of light they emit. Because the wave range is comparable to the particle’s size, electrons are constrained by the wavelength boundaries.

Quantum dots properties are size-dependent. This diagram pretty much summarizes all the physics we’ve talked about:

Quantized (or discrete) electronic states of QD

Because of the small size of QD particles, the quantum confinement effect causes a large band gap with observable discrete energy levels. These quantized energy levels in quantum dots lead to electronic structures that are in between single molecules, which have a single gap, and silicon semiconductors, which have continuous energy levels within bands.

How do you make a quantum dot?

Quantum dots are precise crystals, so typical methods include molecular beam epitaxy (MBE, in which beams of atoms are fired at a “base” or substrate so a single crystal slowly builds up):

ion implantation (where ions are accelerated electrically and fired at a substrate):

and X-ray lithography (a kind of atomic-scale engraving process using X rays):

AND we can also make quantum dots using biological processes, like feeding metals to enzymes. I won’t go too deep into how these processes work but these are pretty standard for most fragile crystals.

Now — Quantum Dots for Solar Cells

The solar photovoltaic market is one of the fastest developing energy markets in the world.

By 2030, the solar power industry will see a growth of a factor of 10.

In order for solar energy to succeed, we need a new technology like quantum dots solar cells that can provide higher efficiencies and lower the costs below standard silicon PV panels.

Why are they desirable?

Because the bandgap of the quantum dots can be adjusted, quantum dots are desirable for solar cells. Frequencies in the far infrared that are difficult to achieve with traditional solar cells can be obtained using lead sulfide colloidal quantum dots. Half of the solar energy reaching the Earth is in the infrared region. A quantum dot solar cell makes infrared energy as accessible as any other.

Existing solar cells go as high as 33% conversion efficiency, but when they are installed it’s even lower. Solar cells based on quantum dots could convert more than 65 percent of the sun’s energy into electricity.

Multiple Exciton Generation — how does it work?

Quantum dots acquire surplus photon energy, which is usually lost to heat generation through a process called multiple exciton generation.

The light rays enter through the transparent electrode of a quantum dot solar cell onto a light absorbing layer of dots in order to generate electron hole pairs. The charged particles then separate and eventually travel to their respective electrodes, producing electric current.

In this above image, we can see the structure of the metal-semiconductor junction. It is basically fabricated from quantum dots layers sandwiched between metallic electrode and Indium tin oxide (ITO) ecounter electrode deposited on transparent glass substrate to act as photo-electrode.

Exciting new future!!

There is still a lot of work to be done before quantum dot solar cells can be commercialized, but the potential is great. Researchers, scientists and industry leaders I’ve talked too are all confident that quantum dot solar cells will provide an efficient and stable method of tapping into solar power.

Here are some cool resources / articles I like: