Unfortunately, this cannot be done with a wall charger. The wall charger input is 125v AC (Alternating current) in US and Canada and is higher is some other countries. Small solar cells have an output of only 2-6v DC (Direct current). Like Jarren said, you could wire only the plug to a solar cell, as long as the voltage isn't too high for the phone. You seem like you want to still be able to plug the charger into a wall outlet, so I would suggest cutting the charger cable and connecting each half to a plug/socket and having another plug or socket (opposite of whichever is connected to the charging end of the cable (not the wall outlet end. That way you can plug the charging end into the wall or into the solar cell. Remember to use a blocking diode and also make sure the solar cells aren't overpowering the phone.