Here's a kit you can put together for around $70.00 to keep your phone charged. It consists of:
20W Aukey solar panel
Koar battery pack
Micro USB cable (to charge the battery)
Cable to charge your phone
Use the solar panel to charge the battery, and then the battery can be used to charge your phone.
You can charge your phone directly from the panel if desired and forego the battery, but I find it easier and less risky to leave the battery unattended with the panel rather than your phone.
Bonus: You can even strap this to a backpack or tent.
The entire package folds up to a very compact size for packing and travel, and can be purchased on Amazon:
Aukey Solar Panel: $52.99
https://www.amazon.com/gp/product/B011UH0ZCY/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Koar Power Vault: $14.99
https://www.amazon.com/Koar-8000mAh-Portable-Battery-Outputs/dp/B019VUYA9Q/ref=sr_1_fkmr0_2?ie=UTF8&qid=1466175807&sr=8-2-fkmr0&keywords=koar+power+vault+18000