I copied this from another site that someone mentioned this happening
"Solar panel output power fluctuations can also cause a phones’ screen to fluctuate on and off. This can happen if you are in motion with the solar panel like walking under trees. The phone sees this as a power drop too. The phone screen will turn off and on in succession which causes power drain. Another words, when the phones' screen toggles off and on as you are walking under shadows, the power drain happens. A workaround is to use a powerbank. These are not a finicky with shadows.
Charging in twilight hours or deep cloudy conditions may also cause a phone screen to turn on. some chargers have Auto-Retry. Auto-Retry tries to reconnect devices, it has advantages and some disadvantages.The advantages out-weight the disadvantages. Just unplug the solar charger if you suspect that the Sun may not be enough this day.
If your phone overheats, which can happen very quickly under the sun, your phone will slow to a crawl. It is imperative that you shade your phone. Don’t worry, overheating will not cause damage. These phones are designed to sit in a windshield. However, when they get too hot, they will slow down in charge.
Placing your device behind the solar panel is not a shade spot. Do not place your phone underneath the solar panel thinking this is good enough shade. This is very important. The heat underneath the solar panel is like an oven. There is no harm if you do this, but your phone will not charge until it is cooled down. Place your phone in a towel, under a chair, out of the sun."