OP again.
Thanks for your input, and interesting to learn a lot of this.
However, I meant a solar battery charger just for emergency use, not as the sole source of energy (although that would be cool!).
I am thinking of hikers, or people who just need enough power to call 911 or maybe make a quick call home - the solar charge would work just well enough for someone to make, say, a 5 minute call. It would be just an “emergency” option if your cell died and you really, really needed to get it up and running to make a call for help.