I have one very simple idea. You can place 4 photo sensors in the midle of eatch side of the panel. If the panel is directly oriented in to the sun, you will get equal output from eatch sensor. Otherwise the signals will not be equal. From the difference it is very easy to decide witch direction to move your panel.
_______y1____
___OOOOOOOOOO
___OOOOOOOOOO
X1-OOOOOOOOOO-X2
___OOOOOOOOOO
___OOOOOOOOOO
___OOOOOOOOOO
_______y2____

O - your panel.

So, if Uy1 - Uy2 = 0 and Ux1-Ux2 = 0, then you have max power.
if Uy1 - Uy2 = 0 and Ux1 - Ux2 < 0 you have to rotate clocwise until Ux1 - Ux2 = 0.

I think microcontroller is not necessarily :)