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.