Small solar panel dc to 5vdc for charging
you can try two options. 


With a battery
Without battery


I suggest the second option with LiPo battery. A solar charge controller like LT3652 will be very usefull in this case. This uses the concept of  maximum power point tracking (MPPT) . There are breakoutboards available for those as well, like Sunnu Buddy from sparkfun.

I tried the same thing with a mobile powerbank and it worked perfectly. There is a chip inside power bank which trasfer 3.6v battery voltage to 5v. Tou can use dc to dc converters as well.