The panel voltage will only go above the main source voltage if the sun is strong enough for the panel to provide *ALL* of the load current. Up to that point, the panel will provide as much current as it can at the voltage set by the AC source, which functions as a low-impedance "sink" at its voltage.
Solar panels behave strangely, and you have to keep in mind that they are basically current sources, not voltage sources.
Below is a modified circuit which adds a schottky diode across the mosfet. In effect, this diode takes the place of the body diode, and should be more able to handle the charge current. When the AC source is not present, the mosfet turns on, which bypasses the extra diode as well as the body diode, and there is no voltage drop. But even when AC is present, some current will flow through the extra diode depending on how bright the sun is.
Both schottky diodes should be 3A or whatever your maximum charge current is. The SB220 shown in the previous reply is a 2A diode, so it wouldn't be enough.