There are some threads on this in the Electronics forum, and I think there's one buried here. Some solar chargers are available off the shelf. A solar panel's peak output only happens on a clear day with the sun at 12:00 relative to the solar cells, but you get very significant power in the shade, and throughout mid-day. And it's better than nothing whenever there's enough light to see by. But don't expect a recharge in 20 minutes - unless there's a battery that's a lot bigger than the cell phone battery, a solar panel that will do that will be pretty big and $$. And then how will that battery be maintained, if it can't get you through the whole trip?
Anyway, some sort of solar is possible, but will take some planning and trials, in the context of the tour plan.