BlitzWolf® 15W 2A Foldable Portable Dual USB SunPower Solar Cell Panel Charger with Power3S
is now in promotion for $24.99 (lowest yet).
This panel can easily charge any smartphone and will also charge in overcast conditions as well (with decreased rate of course).
Pair with LiitoKala Lii-100 0.5A/1A Li-ion Ni-MH USB Battery Charger for example and you can have independent power source for your flashlight needs:)