Brent Meeker <meekerdb@verizon.net> wrote:
Using a different semi-conductor to capture photons of different energy wouldn't depend on directionality.
Unless each semiconductor is transparent to all wavelengths except the ones at which it's most efficient, yes it does. To direct different wavelengths to different semiconductors, whether with prisms or with diffraction gratings, the light has to be coming from more or less the same direction. If white light is equally bright in all directions, nothing can sort it into different colors. (You could, of course, color it by selectively absorbing and discarding different wavelengths, as with gels on stage lights, but that inefficient process doesn't gain you anything.)
... capturing a photon with just enough energy to boost the electron into the conduction band with little or no excess.
Right. For those without a background in solid state physics, a better mental picture might be a vacuum tube in which light shines on the cathode causing it to emit electrons due to the photoelectric effect. This generates electricity most efficiently if the voltage on the tube is such that the electrons can just barely make it to the anode. Too low a photon energy for the voltage, and the electrons fall back to the cathode, generating no current. Too high a photon energy for the voltage and the electrons hit the anode hard, wasting energy heating it. If the tube is exposed to white light, most photons will have either too little or too much energy, and the efficiency will be low. Both the spread of wavelengths in white light and the spread of directions in diffuse light are forms of entropy. And there's no getting around that. Based on that, I conclude that no semiconductor is transparent to all wavelengths except the ones at which it's most efficient, as that would allow construction of a perpetual motion machine of the second class. The good news is that the entropy of sunlight in space is constant, i.e. you can do just as much useful work with it by capturing it at 100 AU as by capturing it at 1 AU (except for the tiny fraction which is absorbed by planets and other debris between 1 and 100 AU). Of course you need a larger collector if you're further away; that's why nobody powers anything by the light from Sirius. On the other hand, what if you're not trying to maximize energy, but computation? The amount of computation you can do per unit energy is proportional to the absolute temperature of the computer. And how cool you can keep something in space depends on its surface area. So to get as much computation as possible from the sun, build a sphere around it as far away as you can. Of course then you'll suffer from latency getting signals from one part of your immense sphere to another.