Formation of intermediate band in environmentally stable and wide band gap semiconductor, CuAlS2, have been studied towards potential, low cost and efficient thin film solar cell material. We investigate the effect of sustainable low-cost dopant (Sn and Fe) in CuAlS2 film by means of density functional theory within the modified Becke-Johnson approximation for the calculation of electronic and optical properties. Polar film of CuAlS2 in (001) direction shows lower band gap than the bulk CuAlS2 due to the excess Cu and Al atoms. Doping of Sn and Fe in CuAlS2 form partially filled, isolated intermediate band. Absorption coefficients for Sn and Fe doped films show higher absorption under visible energy region than the pristine bulk and film. Fe and Sn doped films show efficiencies of 34.67 and 35.64%, respectively within spectroscopic limited efficiency. By means of present study, Sn and Fe doping formed intermediate band, improved absorption in visible light and power conversion efficiency. We suggest Sn doped CuAlS2 as the promising material due to higher effective mass ratio of charge carriers, hence lower rate of recombination, indicates improved performance of CuAlS2 based thin film solar cell.