Yeah, the Suntactics ones work great: https://www.amazon.com/Suntactics-Ultralight-Charger-Devices-Assembled/dp/B003ZFQUWO

I had one on my hike. If I had full sunlight I could charge my phone as fast as a wall charger. However, this was 2013 and I know some of the newer Android phones can charge even faster (pushing 9v in), so this might not be the case anymore. I made it up the whole trail with it...but getting into the latter part of August (and during some June storms) it would have been nicer just to have a big battery and not have to think about it. I'm not sure if I would go with a solar panel again to be honest. If my phone was close to dead, I'd sometimes have to stop and charge it knowing I might not get a chance again.