Semiconductor solar cell material with three energy bands traps low-energy photons others miss.

[nodular]

http://www.technologyreview.com/read_article.aspx?id=17577&ch=energy

Researchers at Lawrence Berkeley National Laboratory (LBNL)
have created a new type of semiconductor material designed to
improve the efficiency of solar cells by capturing low-energy
photons.

Traditional solar cells respond only to a narrow spectrum of
sunlight, making them highly inefficient. In the language of
physicists, solar cells convert light with wavelengths
corresponding to the energy it takes for electrons to jump
from the valence band to the conduction band. Photons with
lower energy pass right through the material...

...The new semiconductors have three energy bands instead of
the usual two (valence and conduction). The third band lies
below the conduction band, effectively splitting the gap
between the valence and conduction bands into two smaller
parts. "This helps low-energy photons to participate in
the process because they can excite [electrons] to the
[intermediate] band and then up. It's like a stepping
stone," says Wladek Walukiewicz of LBNL's Materials
Sciences Division, who developed the semiconductor with
colleague Kin Man Yu.

The researchers found that introducing a few atoms of oxygen
into a zinc-manganese-tellurium (ZnMnTe) alloy splits the
compound semiconductor's conduction band into two parts.
Similarly, adding nitrogen to a semiconductor such as gallium
arsenide phosphide will also give a multi-band semiconductor

[Stardust]

1. Cool!

[krispos42]

2. This will help if we can get these things on houses

We have to work on that, especially in the South. The Katrina-hit area would be a great solar-energy project, if we make the power companies pay fairly for every kilowatt-hour the array generates net. As it is, if a private wind turbine or solar array feeds power into the grid, the owner gets almost nothing for his surplus.

And Nodular, please don't use the 'plain text' option for this. It kills hypertext links and any pictures you might post.

Thank you

[nodular]

5. Will comply.

Thanks for the feedback (I do not have a clue.)

[krispos42]

12. LOL

No problem, no problem. You can do a lot of cool stuff with the regular text.

Like italics

Or boldface

Or this excerpt thingy that is great for posting what somebody previously posted so you can tear into them

You can do strike-outs as well.

And even _subscripts and ^superscripts if you have a mind so.

And you can throw in a picture just by pasting in the Internet address of it. Like this one:

[phantom power]

3. Multi-band cells are cool, but cost is unresolved.

I haven't yet seen anybody make claims about the cost/watt of a multiband cell. They strike me as something that would be expensive to make, although I'm not the person to ask.

[nodular]

4. Good point.

I don't know either.

[NNadir]

6. Personally I can't wait to have the tellurides and arsenides spread widely.

The mining and distribution of these materials will have no consequence whatsoever, since we are talking about solar power.

[Porcupine]

7. We can pull them from the smokestacks of coal plants y'know

since coal ash contains a distribution of all of the elements available in the crust. If they weren't in the atmosphere in the first place they wouldn't be present in the coal.

[NNadir]

10. Oh good. Let's burn more coal.

[eppur_se_muova]

9. GaAsP is already widely used in LED's & diode lasers. Fortunately, it's too expensive for larger

devices, or you would see a lot of arsenic dispersed (especially by incinerators).

AFAIK (which may not be much), tellurium poses very little toxicity hazard. According to Merck Index, symptoms of overexposure include dry mouth, anorexia, dermatitis -- and those are the worst ones! I seem to recall that almost all tellurium ingested is quickly converted to elemental tellurium and expelled. This low toxicity is a little surprising when you consider that Te is a congener of selenium, which forms some really insidiously toxic compounds. Apparently, the Se compounds are toxic because their similarity to analogous sulfur compounds allows them to be entrained in the organism's normal metabolism. Te compounds, on the other hand, are sufficiently different in their properties that they don't have the same "Trojan Horse" effect -- they get rejected at the gate. A parallel could be drawn to inorganic arsenate, which subsitutes for phosphate in the cell's energy cycle, leading to fast depletion of stored di- and triphosphates by hydrolysis. Antimony, just below As, does not have the same effect, presumably because antimonate is sufficiently different from phosphate that it does not get caught up in the same pathways.

I once worked in a group that made organotellurium compounds. One of the first things I did was to check what was known about toxicity -- turned out there was little to fear.

When I saw these guys were working with ZnMnTe, it struck me as very promising, since this should be much cheaper than semiconductors made from scarcer elements, such as CuIn(S,Se)2 (toxic selenium!) or Ga(As,P). And certainly better than the Cd and Hg used in some current thin-film PV cells!

[NNadir]

11. Occupational Exposure limits for Tellurium are given here.

http://www.gr.nl/pdf.php?ID=558&p=1

Hydrogen telluride is indeed an extremely toxic compound. The order of toxicity is H₂S < H₂Se < H₂Te. Ralph Nader doesn't know this, but it's probably more toxic than plutonium. However it is unstable to light.

I have made a number of organoselenols (phenol analogues) and the corresponding thiophenols. For instance, I made 2,4,6 triisopropylselenophenol. (The purpose was to model cysteine complexes of certain metal centers in proteins.) One of the interesting things about these compounds was their consistently horrible smell which could not (as with the corresponding thiophenols) be oxidized away. If you got some of the compounds on your hands on your gloves, irrespective of how many gloves you wore, it would permeate the glove and cause a horrible intractable smell on your hands. I suspect that this effect was related to the extreme sensitivity of nasal receptors to these compounds, suggesting that they do have biological activity.

But I have never made organotellurides.

As is the case with thiols, after a time you became desensitized to the odor, but people who didn't work in the lab could smell it nonetheless. This played a role in my life inasmuch as I was pursing my future wife while doing this work, and later she told me that the main reason she resisted dating me was because of my odor. (She was able to deal with my extreme ugliness, my horrible body, my obnoxious personality, but not the smell of the selenols.)

Actually I'm quite certain that tellurium and selenium based solar cells will not have the environmental impact of burning coal. However, since solar energy is not continuously available, this is irrelevant. Solar energy is not a base load power source - it is a peak load power source. If it were broadly affordable for people other than wealthy people, it would fill a niche to displace natural gas, not coal.

I am quite sure that the external cost of solar cells is not as high as natural gas, natural gas being a fuel that is unacceptably dangerous owing to global climate change. Still this is also irrelevant since solar cells are not broadly affordable and represent a trivial form of energy. The world solar production capacity is at a pace where the best it can do is to displace a tiny fraction of the world's natural gas capacity.

If solar power ever became an important source of power - and it's not even close despite all the endless discussion of it - it's external costs would become more obvious. However solar power has the luxury of being such an insignificant industry that its external costs can for now be overlooked and/or disregarded. That said, it is difficult to imagine that if solar cells ever got to the point where they were producing a few exajoules of energy, that even with the point source pollutants discarded solar cells would represent, that the health risks would ever be as high as the health risks of any fossil fuel. I recall that in the 1980's there was very little discussion of what to do with discarded computer monitors and discarded computers. There is more discussion of that subject now and it is regarded as an intractable international problem. The chemistry of solar cells and the chemistry of computers are, of course, related.

I am, of course, being disingenuous by applying the same kind of "what if" speculative "could happen" reasoning that is applied to a much cleaner and safer form of energy, nuclear power. It's a put-on on my part. But the intent of my sarcasm is to remind everybody that solar energy is not risk free. There is no such thing as risk free energy. For now the biggest risk associated with solar energy has nothing to do with the chemistry of solar cell manufacture or the long term hazards of solar cell materials. The biggest risk is in fact that encourages magical thinking and the ridiculous assumption that solar power will be enough in the face of the vast risk of climate change. Such thinking to the extent that it is not discredited - and largely it is discredited - will be fatal to much, if not all, of humanity.

[eppur_se_muova]

8. Discussed here earlier ... ZnMnTe should be oodles cheaper than GaAsP

http://www.democraticunderground.com/discuss/duboard.php?az=show_mesg&forum=115&topic_id=71115&mesg_id=71115