3 points to consider...
1. sort out if you want amorphous or polycrystaline panels. The former are less efficient but they work/produce for more time cos they are less sun angle specific.
But I you can constandly move the polycrystalines around, you get the cake and eat it too.
2. if your panel is a 5v -and not 12V out- you will have a small selection of AA&AAA chargers. Panasonic/Eneloop, Soshine T2 and the new klarus one. Do your reasearch on amp output, these chargers need 1amp to charge 2 batteries at a time.
And your charger MUST MUST MUST charger batteries indepedently.
3. Solar does not make any sense without having the means to store the power. Are you really gonna sit around for 2-4 hours waiting for your cell to charge? What if it rains or you just finished charging your phone in the afternoon, and all hell breaks loose and you have to talk and talk to your phone? You will be left dry before nightime.
Invest in a good power bank first, which you will be able to charge from a wall socket and your car. Then buy the solar panel
Really, how much time we spent in our cars daily and how much next to a solar panel? Car always wins.
And if all hell breaks loose there will be a full battery for every car left on the side of the street without gas.