an autodidact meets a dilettante…

Jacinta: So in the last post we were joking about the horrors of physicists and engineers manipulating innocent electrons and forcing them to work for us, gratis. It comes to mind that there are people who are intelligently dubious about the manipulations of scientists – Bernard Beckett, in his 2007 book Falling for science, comes to mind, as does Yuval Noah Harari in Homo deus. ‘Scientism’ was used for a while as a pejorative, especially during the debates on the values of religion ‘versus’ science…

Canto: Yeah, but – I don’t want to dwell on this issue now, except to say that the critics of science are usually not very literate on the subject. So we were talking about dopants, which are impurities that can be added to the silicon crystal lattice to mess up its fine balance, so to speak. Boron is an example – it has three electrons ready for bonding, leaving a ‘hole’, a p-type space, and presumably a loose electron to carry the charge. And then there’s phosphorus, which has five such electrons – so one to spare after bonding, which they call an n-type situation. Positive charge carriers (p-type) and negative charge carriers (n-type) is how they describe it.

Jacinta: Right, so they layer these two types together: ‘The positive holes and negative electrons migrate towards each other’. The electrons will jump into the p-type and the holes jump into the n-type [they don’t explain how holes can jump]. This causes an imbalance of charge, because now the p-type side has more negative charges, and the n-type side has more positive charges’. This apparently creates an ‘electromagnetic valve’, which allows, or perhaps forces, electrons to pass through in one direction only.

Canto: This isn’t very clear to me, but let’s continue. Maybe you have to do it, and so see it working, to get a full grasp. So, a sufficiently energetic photon enters the p-type side (the boron-doped side) of the solar cell, knocking an electron loose to float within the material. It will either recombine with a hole, and fail to create a current, or it can enter the electromagnetic field – that valve thing between the p-types and n-types, also called a depletion layer for some reason. The effect, apparently, is that it accelerates the electron into the n-type side, which of course tends to lack p-type ‘holes’, but the electromagnetic field most cruelly prevents the electron from passing back to the p-type side.

Jacinta: Yes, it’s still a bit fuzzy, but on the n-type side some ‘holes’ are somehow transported across this electromagnetic field junction, where they recombine with electrons. so one side of this junction or valve becomes negatively charged, the other positive. This creates a ‘potential difference’, aka a voltage!

Canto: Explained neatly for us as ‘The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points’. Just saying.

Jacinta: So, as our video-maker tells us, we can then add ‘some mental contacts and an external load circuit’ and we have created a current, presumably, as the electrons will ‘pass along the circuit to recombine with the holes on the other side’. And that’s your solar cell, apparently. But I barely understand a word.

Canto: Well, doing and seeing, as I’ve said. But there’s problem with adding this metal to the upper surface as it blocks some of the light needed for the cell to function effectively. So, problems with solutions that create problems. So engineers keep working on new shapes and materials for optimisation. They’re trying to minimise the metal coverage and electron resistance in getting into the circuit. Topology optimisation is one subject of research, using computerised algorithms.

Jacinta: And it’s fascinating but hardly surprising that this sort of research is producing shapes for solar cells that resemble leaves – which after all are like little solar cells resulting from millions of years of evolution.

Canto: Hmmm, not like ours, plants don’t use the sun to make electricity. But this quote from the video is thought-provoking:

Vascular tissue on a leaf does not perform photosynthesis. It instead brings the water that is essential for photosynthesis to the leaf and extracts the useful products, serving a similar purpose as our electric contacts – so of course plants have developed the perfect shape to optimise the energy they can absorb from the sun… However, most solar cells use a simple grid shape, as it is cheap to manufacture.

Inevitably this means an efficiency loss, measured at around 8%. So, in conclusion, a current silicon solar cell has an efficiency, under lab testing, of around 20%. The drop to 18% shortly after operating has resulted in hundreds of scientific papers, and it seems to have to do with the use of boron, as the drop didn’t occur when boron was replaced with gallium. Something to do with a ‘boron oxygen defect’, so there’s been a lot of work done on trying to reduce the ‘concentration of oxygen impurities in the silicon wafers’, caused by the Czochralski process, the standard process for silicon wafer manufacturing. Almost all silicon solar cells are made this way. Recent research using a special imaging technique found that boron oxygen molecules converted to ‘shallow acceptors’ when exposed to light:

In essence they observed the defects transforming into little electron traps that acted as recombination sites, and thus reduced the time and probability of electrons entering the circuit to do work.

It’s something I can almost grasp. And with this knowledge, engineers, whose grasp is way firmer than mine, can find some kind of fix for the problem and get that efficiency up well beyond the 20% mark.

Jacinta: Well, this has indeed been a knowledge-expanding journey. Pour qu’une chose soit interessante, il suffit de la regarder longtemps. You mentioned the depletion layer, which caught my attention. It’s a central feature of semiconductor physics, also called depletion zone, depletion region, junction region and more. The depletion zone is so called because of the depletion of carriers in the region. Charge carriers presumably. Any rate, this region, and understanding it, is key to understanding the physics of semiconductors. The Wikipedia article on what they call the depletion region is a useful supplementary to our discussion. We might explore all this further, or not, depending on our own depletion levels…

References

The mystery flaw in solar panels  (video)

https://en.wikipedia.org/wiki/Depletion_region