I'm not sure what @raschemmel has in mind ?
I would just use a pair of Light Dependent Resistors shielded so the sun only falls evenly on the two of them when the panel is facing the sun. Then write a few lines of Arduino code to move the servo (and panel) towards the LDR that is in brighter sunlight.
You probably should arrange for the Arduino to shut off power to the servo when movement is not needed or the servo may consume more power than the panel produces.
...R