The limiting factor of producing solar panel at these immense scales is the cost of raw materials. As other posters pointed out this study is already assuming that the cost of solar panels and wind will be barely greater than the cost of materials.

And yes, if the US tries to increase it's solar capacity by a factor of 50 in the next 15 years (and especially while other countries try to do the same) these materials will become more scarce and thus more expensive.