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Solar Grade Silicon
One of the most important factors in creating ISRU solar panels is whether a source of photovoltaic effect exhibiting materials can be found. Silicon is the most common of these materials, however to be used for solar panels it must be in one of three forms; see wikipedia: 
Silicon Dioxide is a very common compound, it is quartz, and it can be found in basalt and sand. Luckily, a variety of ways to get pure silicon from silicon dioxide exist, in fact you can do it at home as this article shows: 
Any plus two ions could prove to be useful in stealing the oxygens in this manner. Magnesium, as used in the above experiment, can be found on Mars, but only in Magnesium Oxide. To create a useful ion the oxygen has to be removed, creating the same problem as with silicon dioxide.
The alternative to using positive ions is a process that involves very large amounts of heat. Finding the most efficient pathway from silicon dioxide to solar grade silicon will probably involve researching the heat of decomposition of oxides containing +2 cations.
The most used industrial process is descibed on wikipedia: 
Once pure silicon is obtained, it can be made into a useable crystal for a solar panel by the Czochralski process: .
However, all of that was written before coming across what would appear to be a rediculously convenient holy grail of removing elements from oxides, the FFC Cambridge process: , which directly produces solar grade silicon through electrolysis of soluble molten calcium, as well as breathable oxygen. It still requires a large amount of heat, but that would seem to be a recurring requirement which can hopefully be overcome. How to create a reactor that efficiently retains heat for these processes would seem to be a project of extreme importance for determining habitability.