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. Metal A has low photoelectric "work function" and B has higher work function. Everything is in vacuum. Sunlight shining on it from top ejects photoelectrons from metal A, and some of them fly to come to rest on metal B, result is solar-powered current source. Example work-function numbers in electron volts: A: Na=2.36, K=2.29, Sr=2.59. B: Ti=4.33, Ni=5.2, Fe=4.7, Mn=4.1. I think lower work functions are available than listed in the "A" row by use of special surface treatments and alloys. I would expect fairly good efficiency for incoming photons in the right frequency range, e.g. if 50% of photons eject an electron and 50% of electrons fly to the right destination, then about 25% efficiency, say 20%. Comparison versus semiconductor solar cells: (a) Unfortunately it appears the vacuum solar cell would be powered by UV light, not visible or IR. (b) Vacuum cell presumably more radiation-tolerant and more immune to temperature change, than semiconductor cells. That plus availability of vacuum makes it seem well suited for space uses. (c) no high purity low-defect crystals of semiconductor needed. (d) vacuum cell could be degraded by surface poisoning due to, e.g, oxidation, so important it really be in vacuum.