You need a solar panel with a working voltage of 5-5.5V.
The best working current of the solar panel is 100mA or more. Of course, it doesn't matter if it is smaller, but the charging time will be longer.
In this way, the cell phone battery can be charged directly. The cell phone battery has already been installed with a protection circuit, so there is no need to worry. about overcharging the cell phone battery and endangering the life of the mobile phone.