CdTe is one of the leading materials for low cost,high efficiency thin-film solar cells with a nearly ideal band gap of 1.48 eV.However,its solar to electricity power conversion efficiency (PCE) is hindered by the relatively low open circuit voltage (VOC) due to intrinsic defect related issues.Here,we propose that alloying CdTe with CdSe could possibly improve the solar cell performance by reducing the "ideal" band gap of CdTe to gain more short-circuit current from long-wavelength absorption without sacrificing much Voc.Using the hybrid functional calculation,we find that the minimum band gap of the CdTe1-xSex alloy can be reduced from 1.48 eV at x =0 to 1.39 eV at x =0.32,and most of the change come from the lowering of the conduction band minimum.We also show that the formation of the alloy can improve the p-type doping of CuCd impurity based on the reduced effective formation energy and nearly constant effective transition energy level,thus possibly enhance Voc,thus PCE.