How your calculator works is surprisingly controversial

Solar calculator casio fx115ES cropThe calculator to the left is solar powered, via the little solar panel at the top right. Small cheap solar panels like those in calculators are made from amorphous silicon, because its a lot cheaper than its more efficient but pricey, cousin crystalline silicon. In crystalline silicon the silicon atoms are arranged in a regular crystal lattice — as it happens the arrangement is similar to that of water molecules in ice.

So, what is the structure of amorphous silicon, the stuff in your calculator’s solar panel? We don’t know for certain, but there are ideas and some scientists have strong opinions. Three years, the prestigious journal Science published a paper claiming that amorphous silicon was ‘paracrystalline’, meaning that locally, the amorphous silicon actually looks crystalline, but that instead of the crystalline periodic arrangement repeating itself for billions of silicon atoms, as it does in crystalline silicon, it only repeats for a handful of silicon atoms, a handful of periods, and then decays away.

This idea did not impress everyone, a few months later Science published a comment on the paper, i.e., a short paper disagreeing with the original paper’s conclusions. The authors of the original paper replied. I cannot judge who is right, the original paper used quite a rare experimental technique, one I am not familiar with at all. But it seems controversial. This is not too much of a surprise for Science, journals are often publishing controversial stuff, sometimes controversial because it is pushing the envelope, and sometimes controversial because its just wrong.

As far I can tell, we don’t know for certain what the structure is of amorphous silicon like that in solar-powered calculators. This may matter, as it will be hard to improve the efficiency of power generation in amorphous silicon solar panels if we are clueless about the arrangement of atoms in them. For a calculator efficiency is not so important, they only take a tiny amount of power, but if we are going to generate large amounts of power from large solar arrays then efficiency really matters.

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