The problem with solar is going to be that either it's going to be reasonably sized, cheapish, and charge super slowly or it's going to be big, bulky and pretty expensive. The typical consumer-oriented cell phone chargers that I see are going to charge at something like 25-50mA - which is going to take a long long time to charge a phone (meaning "days"). Look for at least a 5W panel, ideally monosilicon for efficiency, which should be somewhere in the vicinity 8"x8" (20cm x 20cm) sizewise. If it's one of these amorphous silicon 4"x2" panels that I see on Amazon, you are going to be disappointed, I think because they will take ~40 hours of direct sunlight to fully charge the phone (based on my back-of-the-envelope guessing).
Something like this would work great:
http://www.amazon.com/Durable-Reliab.../dp/B003ZFQUWO
And yeah, you'll say "wait! $140, are you crazy?" but I'm saying that if you really want a portable solar panel recharger you want the more expensive monocrystalline cells (because they are more efficient ) and you want a large panel (which is expensive).