I did tour with a small fold-up solar charger (sort of like this—I don't think the exact model is in production anymore) that mostly served to "delay the inevitable" as Trevtassie put it, although that was good enough for my purposes. I had this lashed across my panniers in back.

These days, good high-capacity USB batteries are common and inexpensive, so there's not as much need, as long as you can camp near an outlet every so often.