Mehdi,
A lot will "hinge" on your mechanical arrangement. Are you proposing a "heliostat" - a device which tracks the sun, so is pointed in roughly the right direction on an equatorial mount, or is this going to be on an alt-az mount hunting for the sun ?

If its a simple equatorial heliostat, then a couple of LDRs, a shadow mask and an op amp might be all you need.

An Alt-az mount, because the movement isn't constrained is likely to be harder, but again, this is a electro-optical-mechanical problem, which might stand or fall on the way you shield the diodes from stray light.

Steve