Photodarkening relaxation under light exposure of a-As₂Se ₃ amorphous films doped with 0.5-5.0 at.% Sn was studied for its dependence on the concentration of impurities and thermal treatment. Both factors reduce photodarkening with the degree of reduction dependent on the concentration of impurity. The relaxation process may be described by a stretched exponential with the dispersion parameter 0.1 ≤ α < 0.6 and the time constant increasing with tin concentration or thermal annealing. The results are discussed in the frame of the "slip-motion" model of photodarkening in chalcogenide glasses taking into account the peculiarities of the glass structure.