That's not nearly as easy as your comment implies. You can't just melt the soil into glass -- you have to create glass, dope it with the appropriate elements to make the individual solar cell, then mount it on a substrate and wire all those individual cells together. Then you have to install framing and cable the cell panels into arrays. With low efficiency amorphous cells of the type you'll be making, you might get arrays of a few percent efficiency, so you'll need tens of acres of these panels to be deployed. You will need to have some way to clean the solar arrays, as windblown dust will accumulate on them rapidly and reduce your power output to near zero. Finally, solar cells degrade with time, so you would have to make these arrays continuously.
By the way, you'll need considerable power to do all these things above, so you still have a mass problem.
Oh, and you have to do this all with teleoperated robots, so that you have the power system set up and running when people arrive. And you have to operate those machines with a round-trip time delay that lasts up to 40 minutes. Hope that nothing breaks down beyond repair -- it will take you another 22 months for a launch window before you can ship a replacement up to Mars.
Good luck and Godspeed.
Using 2015 technology, you are correct. Using 2050 technology or 2100 technology, who knows?
Then again, by the time we have the technology to create autonomous robot solar cell factories, maybe we'll have finally gotten compact fusion power.