@lrhodes I forgot to add some details: 1000W/m2 of solar incident radiation, for a phone typical surface 140mmx70mm and let's suppose 16% of PV efficiency gives about 15W. It's not a fast charge, quite decent. The problem is that you'd have to keep it oriented perfectly over two axes to maintain this power. If you casually leave it on a horizontal surface you may get 5W by the cosine law, that gives you about 1000mAh at 5V after an hour. That's roughly equal to half a day of normal phone use. All this would be OK if you don't count the battery overheating under direct sun for an hour or more...
See this https://www.nature.com/articles/srep12967 to see how much Li-ion batteries degrade for temperatures from 25 to 55C after 250 cycles.