On earth, the photovoltaic cell junction for mass production tends to be just silicon, although there is a variety of material types - single crystal vs multicrystal or polycrystal, or even amorphous.
I just read that the Voyagers used indium phosphide (InP), Wikipedia says that Galium Arsenide (GaAs) are common, and I have read some satellites use triple-junction solar cells multi-layered devices used when the absolute highest efficiency is achieved.
Is there any reason - besides purchase price - that would keep all new satellites from using the highest efficiency cells? And actually, do different cells have best efficiency under different conditions encountered in spaceflight?
Pics of Juno's photovoltaics from NASA: