This is about double what current solar cell efficiency is. Definitely a big improvement.
Now for the downside. The thin film technologies rely on some rather exotic, rare, and non eco friendly III-V materials, such as GaAs, InGaP, etc. Most of these are toxic and all of them are "rare"/expensive. And the technology to mfg them is not as widespread and as understood as Silcon.
The good thing with multi-junction cells is that the different layers can each be optimized to capture different parts of the spectrum. Since Si is completely transparent in the infrared, you can add backing to reflect the light back through the wafer and a Si3N4 layer to try to trap the photons.
So for many applications it may be what is more important, cost/Watt or space/watt? In the case of space exploration, space/watt wins. For large scale use??? This technology does seem to offer a 2x improvement in space/watt, which is good.
A friend of mine has spent decades in the solar cell development/mfg with several large solar companies. There has been lots of research on efficiency improvements, but in most cases big improvements came at the cost of mfg efficiency, use of rare/expensive/toxic materials etc. that didn't come to fruition based on available mfg technologies, processes, and material issues. So improvement in large volume applications so far have been incremental, and the high efficiency cells religated to space exploration and a few other cases where cost isn't an issue but weight and size are extremely important.
it will be interesting to watch and see how this technique plays out.
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