Which photo sensor should I use in dust prone areas?
The sensor housing should be smooth so that it cannot accumulate dust. The sensor housing should be mounted so that rain and wind wash away any accumulation. This can be established by testing different sensor housing materials and shapes over a season.

There are special nano-materials which prevent dust from sticking to a surface, but I doubt you will need these. 

If you simply must allow for dust to accumulate on the sensor housing, you can put two sensors in the housing. One sensor watches the sky. The other sensor looks at an LED with fixed brightness--a light reference. When the LED sensor detects loss of light due to dust, the gain of the sensor which watches the sky can be increased.