Comment by icodestuff | Hacker News Reader

Comment by icodestuff | Hacker News Reader

If I'm reading this right, this actually has a larger usable band gap range than traditional photovoltaics -- the article talks about capturing the high-energy photons -- in the first pass. This means that the cell can actually capture the electrons knocked out by those high-energy photons, which is something we haven't been able to do.

Silicon's band gap (1.11 eV) corresponds to 1110 nm (NIR), and any photons with more than 3 eV energy (413 nm) are lost (and all the excess energy in photons in between is lost as heat). Newer cells are around 0.6-0.7 eV, but I don't know their maximum capture energy. That's all the violet light and UV. There's a startup that makes a polymer film that can create two lower energy photons in the band gap from a high-energy photon to capture some of that wasted light. This would seem to be a cell that could capture it directly. Very very cool; that's a lot more energy captured per photon.

What I'm unclear on is why you need to heat it up so much to get to those efficiency levels, and it wouldn't just work as an ordinary solar cell.

The article is confusing, but I read it the same way as you: these guys have figured out a way to capture a larger part of the blackbody spectrum than a normal photovoltaic cell. I don't think they are saying that the cell needs to be heated up. I think they are saying that they capture 40% of the power of a blackbody at 2000K. I am not sure why they don't mention what happens for a blackbody at 5600K such as the Sun.

Replying to myself and GP, I found the answers to our questions in the Nature paper linked from the article.

The TPV is distinct from a solar cell because the TPV is meant to reflect energy back to the source. A solar cell also reflects some energy back to the sun, but that energy is effectively lost. In the TPV case, the reflected power helps keeping the source hot, thus effectively recycling photons in a feedback loop involving the source and the TPV.

Some of the energy incident to the TPV is transformed into electricity, some is reflected, and some is dissipated as heat in the TPV. The 40% efficiency is measured as (electricity+reflected)/incident. Thus, it is not true that the TPV converts 40% of its input into electricity. Instead, the TPV converts some fraction, reflects (40%-fraction) to be recycled later, and dissipates 60% into heat. Using the TPV as a solar cell would reduce the efficiency because the reflected energy is wasted. Part of the magic of the TPV is to design a really good mirror that reflects energy back to the source, which is not a concern for solar cells.

The current design operates best for a blackbody source around 2000K and there is probably no point in using it at 5600K.

Cool stuff.

I understood it that the efficiency of the TPV depends on the photons wavelength which is determined by temperature on emission. So the band gap probably has an efficiency curve.

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