I borrowed a solar phone charger. I set it up in bright sunshine and all was going well. After sunset last night it was still connected for about an hour and nearly discharged the phone. The solar panel and the phone battery must act as emfs in parallel. When the sun in shining, the emf of the panel will be higher than that of the battery and so will send charge that way. However when the sun sets, the emf of the phone battery will be higher. At any point in time, charge will move in such a way as to try to equalise the emfs.The higher emf will always send energy to the lower. The difference is that the solar panel is constantly having its energy replaced by an external source.
