The future of solar cells?

As mentioned in one of the last posts we had a project at the university last week which we will finish tomorrow with a presentation. In this project we measured the reflectance of nanowire arrays depending on their diameter. What? And why?

One of the most promising energy sources ever is the sun. Solar cells are used to take advantage of this mega-source. But common solar cells are not that efficient. (20 - 30 percent I think.) So some intelligent physicists had the idea to investigate different approaches than using silicon for the cells. Especially here in Lund nanowires are of big interest. And I have to say the guys here are very good (means respected world-wide ;) ) at  growing nanowires. Now what are nanowires and why are they special? The name implies already what is behind the technique: Nanowires are wires with a very small diameter from a few nanometers up to maybe 100 nm. (And yes, this is true, we saw them under a scanning electron microscope!) The special thing about them is that they can be formed out of semimetals that have very nice electrical properties but cannot be used to form layers or bulk samples. This is because if they are present in solid form the distance between their atoms are very different for different metals. That causes strain in the material and this again causes cracks. Not good, solar cell broken. In nanowires this does not happen as the strain from the different atom spacing can be compensated due to the large surface-to-volume ratio of the wires.

Figure 1: Schema of the arrangement of the wires on the sample.
Indium phosphide was used during this project. On the right there
is a schema of the sample that we measured. Each array on the
sample contains nanowires of different diameter.

But back to nanowires in solar cells. As mentioned these semimetal compounds have nice electrical properties which promise higher efficiencies than common solar cells. If they are placed for example in arrays like shown in figure 1 they can - put simply - absorb sunlight which can be used to generate energy. Just like in common solar cells, only more efficient. What we did in the project was to measure the reflectance of such nanowire arrays. If energy is generated by absorbing sunlight it is of course very desirable that the cells/arrays absorb as much of the light as possible. And thus show a reflectance  as low as possible. That was what our measurements were about. We varied the diameter of the nanowires and measured reflectance spectra. Some are shown below (figure 2). Every colour represents a different nanowire diameter and you can see that the reflectances vary depending on the wire diameter.

The reason why the reflectance depends on the diameter is that the absorption and thus the reflection of light depends on how strongly the electromagnetic light fields couple into the nanowires. This again depends on the diameter of the wires so the reflectance depends on the diameter as well. Now only somebody needs to make this applicable for industrial fabrication and here we go - energy crisis solved. ;)

This is very recent research by the way! An extensive paper on this topic which we used to prepare for the project was released only in May this year!

FIgure 2: Reflectance spectra of nanowire arrays. Wavelength on the x-axis,
reflectance on the y-axis. Diameter of the nanowires was varied from 30 nm to 80 nm.
The length of the wires was 1.1 µm.

Trapped single states

Figure 1: Sample structure: Different
semiconductor materials stacked.

Today I have been working on a lab report all day ... Only after some time I realised how awesome the physics is that is behind the experiment! (At least I think it is awesome. :) ) We trapped single electrons in a quantum well! I mean really single electrons, not the assumed single electrons or only few single electrons. So as I am working on this anyway I thought I should share parts of it in a very high-level (that means not too much crazy science, only a little ;) ) way. I hope it is understandable. :)

The setup consists of a stacked semiconductor structure as shown in figure 1. The gallium arsenide and and the aluminium gallium arsenide have different band gaps which the electrons in the device see as a well in energy. This is like if you jump into a well in the ground - you see the walls around you once you are in the well. (See figure 2 for the wells.)

Figure 2: Two bands resulting from
the different band gaps in the structure
from figure 1 and possible ways
to excite an electron from the bottom
to the top. Laser!

The next prerequisite is to know that physicists assume that there are differen bands in semiconductors that define the energies that electrons can have. (In which they can move if you want.) Now there are not only bands for electrons but also for holes which are the conception of missing electrons. Just like if you take a chocolate marshmallow out of its box leaving a blank space. These holes can move as well just like electrons.

So if there is an electron in one of the bands (the valence band to be precise) it can be moved to another band (the conduction band) by giving energy to it. (For example by shooting at it with a laser - pew, pew!) From before now it is known that this will leave a hole behind.

Figure 3: Spectrum recorded
during the experiment.

After some time the excited electron (the one that was shot to the other band) and the hole recombine because it is more suitable for them. (Particles always want to reach the lowest possible energy state.) If they recombine they emit the energy that the electron got from the laser before. This is visible as light so it can be detected. By a detector ...

Because we are not sure (unless we calculate it before, as I just had to for the lab report ...) which energy this light will have we let the detector detect a lot of possible energies. For some of the detected energies there will be nothing special but for some other energies there will be peaks which means that at these energies there is something going on. For example recombination of electrons and holes. This measurement of different energies is called a spectrum. Figure 3 shows an original spectrum from our experiment. :)

But how do we know that we trapped only one single electron in our well? That we know from the calculations. The movement of particles can be described by the Schrödinger equation. The setup with the quantum well gives boundary conditions to this equation so we get discrete solutions for it. As the semiconductor setup used is a little more complicated than a "normal" quantum well (whatever a >normal >quantum well may be ... <.< ) the solutions for the Schrödinger equation could not be obtained analytically. Instead we used a graphical method. The clue behind this is to plot the graph of the equation. Only at points where the graphs intersect a bound (trapped) state can exist. If you look at figure 4 you see yourself that there is only one intersection point. So one single trapped electron.! BÄM, science! ;)

Figure 4: Graphical solution of the Schrödinger equation. There is only
one intersection point (blue and red) so there is only one(!) single(!) electron
trapped in the quantum well!

So this was a very much broken down overview of what happened. I hope you could understand something. I am still fascinated that something like this is possible and that we did it. O.O

Last thing: Why do we do stuff like this? Well if the behaviour of the electrons in the well is better understood one can build some fancy devices out of this! Maybe new transistors which result in faster computers? Maybe quantum computers? Maybe better solar cells? (Actually I participated in another project where we measured nanowire solar cell arrays! I hope that I will tell something about this as well.)

If I am that fascinated this seems to be the right stuff to study! :)