It also occurs to me that a vacuum-tube analogue of the semiconductor "solar cell" could be built.
/\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | /\ | ========================================
where the === denotes an insulating substrate, /\ denotes triangular-prism strips of metal A, (going into the page) and | denotes "walls" of metal B.
I don't completely understand the physics of this device, but why do you need lots and lots of tiny pieces of the two metals, rather than just 2 or 3 large ones?
--if the strips are too wide then an ejected electron will be likely to merely re-land on the same one it started from, which would be a huge waste, so they cannot be too wide. Other remarks: if the metal strips were actually extremely narrow, e.g. 50 nm, then the same idea the vacuum-tube paper had should also hold here: we would not actually need vacuum, noble gas atmosphere would work. Indeed, it seems plausible to me that just the right gas pressure might actually yield MORE efficiency than vacuum.
The fact that the 50% numbers for both ejection and flying to the right destination appear to have been derived by rectal extraction, rather than some calculation that includes the size of the metal pieces and distances between them, does not provide much confidence that the engineering to build a device like this is feasible.
--indeed, I just made up the 50% numbers. A larger error I made in my previous post, was claiming that the vacuum solar cell would be best suited to UV light. Actually, here is a table of wavelength=h*c/E in nanometers where h=planck constant, c=light speed, E = photon energy in electron volts: E(eV) wavelength(nm) 1 1240 (infra red) 2 620 (red) 3 413 (violet) 4 310 (ultraviolet) 5 248 UV 6 207 UV indicating that 2-4 eV work functions for the anodes will work starting from the near UV into the red range... lower work function is better since then more of the range will be covered. If you went really low, it'd be counterproductive... but I'd actually guess that about 2eV would be near optimal for our sun. In other words, this is a more promising idea than I'd initially thought.
I were to say "I would expect that only .5% of the electrons would fly to the right destination, giving the cell an overall efficiency of .2%", how would we determine which of our expectations is more reasonable? Andy
--well, it is largely just a guess at present, but my suspicion is, with good design, my 50%*50%=25% numbers should be achievable while yours are very pessimistic. Evidence: Commercial "photomultiplier tubes" used to detect single photons indeed do achieve detection efficiencies in the 25% range, illustrating that this concept probably has some legs. For example here is a plot of photon detection efficiency for one random commercial such tube: http://www.thorlabs.com/images/TabImages/PMTSS_Quantum_Efficiency_Graph_780.... indicating efficiency>30% between about 210 to about 420 nm, and above 20% from about 150 to 570 nm. My solar cell idea should probably be able to obtain slightly greater photon-->electron efficiency than a photomultiplier tube because the latter have geometric constraints caused by their desires for multi-stage "multiplication" while the solar cells do not have to worry about that. And also the phototube guys are counting only photons which generate electrons which successfully start a multiplication "avalanche" whereas in the solar cell application, we would count all electrons which reach the cathode, a wider class. On the other hand: these solar cells if commercialized would want to use low priced materials in contrast to expensive photomultiplier tubes where they feel free to use expensive materials. And the phototubes are artificially charged to high voltages in an effort to guide the electrons where they want, and give them more oomph. So of course, the real way to answer this is experimentally. I'm going to send the authors of the vacuum tube paper, my thoughts, after some further editing, since they actually do do experiments. I just armchair theorize. Important question: is it possible to fabricate simple repetitive patterns like parallel metal strips in some very economical manner like "rolling them" as opposed to photolithography? -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)