Solar Cell / Panel Efficiency

A couple of things I failed to mention recently with respect to solar panels and their efficiencies. Polycrystalline solar cells basically “leak” at the crystal boundaries. That is to say, electrons and holes recombine at the edges of the crystal without going through an external circuit, and it is this that causes these panels to be less efficient than monocrystalline cells.

But there is more to this difference in efficiency. The rate of electrons and holes recombining at these crystal boundaries tends to be relatively constant as long as there are electrons and holes available to recombine, while the rate that electrons are ejected by photons varies with light intensity.

While a monocrystalline panel might be 17% efficient at full lighting and a polycrystalline panel 15%, which isn’t a huge difference, in 50% lighting that monocrystalline panel will still be close to 17% efficient but the polycrystalline panel will be much less than 15%, perhaps 13%, and as the lighting falls the efficiency of the polycrystalline panel falls off rapidly, and a point is reached where no power at all is produced because electrons combine with holes at the crystal boundaries as fast as they are ejected by photons. However, the monocrystalline panel will produce electricity corresponding to illumination relatively linearly down to very low light levels.

Consequently, a monocrystalline panel will produce less than full power but still what might be usable power under overcast skies, a polycrystalline panel will produce much less under these circumstances, if any power at all.

So if you live in a place like the Pacific Northwet, you may want to consider shelling out a little extra for monocrystalline panels. You’ll get more usable power out of them relative to polycrystalline than the difference in efficiency would suggest.

This consideration really applies only to unconcentrated solar arrays. If you’re concentrating the light with mirrors, lenses, or reflectors, the concentration won’t be effective in overcast skies anyway and in that case the array is only going to operate in direct sunlight effectively regardless of which type of cell you use. Also, concentration will reduce the differences in efficiency making polycrystalline cells more attractive.

One of the problems with silicon cells is that of gathering the energy they produce. A physicist by the name of Bram Hoex discovered that he could increase the efficiency by more than 1% by adding a thin layer of aluminum oxide to the surface. I’m curious how this works since aluminum oxide is normally an insulator. Still if it works, that is what’s important, if it works and it’s cheap enough that it doesn’t rise the cost of the panel more than the efficiency.

I’ve included a link on the title to a page in Science Daily that provides more information on this.

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