Hi All

SM... there are new solar cell materials in the offing with efficiencies of ~ 50%. Zinc-manganese-tellurium materials seem ideal, but there's also Gallium-indium-nitride. Here are the links...

http://www.trnmag.com/Stories/2004/042104/Material_grabs_more_sun_042104.html [ZnMnTe]

http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Full_Solar_Spectrum.html [InGaN]

...the first is actually combined in a single material, whereas as the other is multi-junction but covers the whole spectrum.

From my point-of-view I reckon solar concentrator cells are the best option. From what I have read cells improve in efficiency with light intensity - that's the 31% you quoted - and also less semi-conductor is needed since the rest is mirror. Ultra-thin mirrors, perhaps aluminium vapour deposited on carbon-nanotube sheeting, would be ideal to cut back on the mass requirement which I reckon is the real killer for both SPS and your ARMXER project. Ultra-light carbon sheets like these...

http://www.space.com/businesstechnology/technology/carbonsail_000302.html

...which should be easier to make in quantity, especially with the new nanotube making techniques.

Adam