If you get one that's about the size of a phone, the solar panel is too small to quickly charge it, it will take several days of full, direct sun to charge it. (And that might not be great for the battery either.) If you get one where the panel is a bit larger and the battery is separate, that might work better, but would not be portable at all to carry with the phone (would be more for camping).

tl/dr: the small ones are rather useless if you use it frequently.