I have a couple choices: the Bushnell Solar Wrap Mini and the Brunton Ember. The first has a small storage battery but can completely recharge in the field. The second has a much bigger battery but only partly recharges from the sun due to its small panel. Either can keep a small gizmo collection going, but a device with a big battery like a phone (typically >3,000mA) will completely drain it in the process. The Petzl Core battery (discontinued, but still around for the persistent) that powers my 3-AAA Tikkas is less than 1,000 mA, so charges easily. The only situation where I wouldn't be able to keep it going indefinitely would be in winter.

Cheers,