Most of the panels sold on Amazon are battery-based chargers that have a small solar panel attached. These are essentially gimmicks and they do not produce enough power to charge a phone in a reasonable time period. A lot of the specifications lie about the actual power-output and they get away with this by including a battery which does the actual phone charging.
That said, you can get solar panels which are capable of charging a phone but they are big and usually expensive. Or you you can get a battery backup to charge a phone.
If you really want to go solar, then look for the ones that do not have an internal battery, that mention the efficiency of the panel (should > 15%) and the material used in the panel (you want "monocrystalline"), and one that specifically mentions the output current and that has reviews that independently back-up the claimed output. And then get something that is at least 20W. You might read the reviews and think "I can get away with 8W" but really in this, you want to overengineer the thing. I would state that the bare minimum should be a 15W, and you really want something >20W.
All Powers have a nice set up that works well. About the only negatives seem to be that it can't easily stand up to the rigors of camping and the wires can break if you put it through a backpacking trip. But the output of the panels seems to be close to what they claim and you can charge a phone with one in a reasonable couple of hours. Something like this:
http://www.amazon.com/dp/B015IGXXSQ