Recent advancements in CdTe photovoltaic eciency have come from selenium grading, which reduces the band gap and signicantly improves carrier lifetimes. In this work, density functional theory calculations were performed to understand the structural and electronic eects of Se alloying. Special quasirandom structures were used to simulate a random distribution of Se anions. Lattice parameters decrease lin- early as Se concentration increases in line with Vegard's Law. The simulated band gap bowing shows strong agreement with experimental values. Selenium, by itself does not introduce any defect states in the band gap and no signicant changes to band structure around the Γ point are found. Band oset values suggest a reduction of recombination across the CdSeTe/MgZnO interface at x 0:1875, which corresponds with the Se concentration used experimentally. Band structure analysis of two cases x=0.03125 and x=0.4375, shows a change from dominant Te/Se contributions in the conduction band minimum to Cd/Se contributions as Se concentration is increased, hinting at a change in optical transition characteristics. Further calculations of optical absorption spectra suggest a reduced transition probability particularly at higher energies, which conrms experimental predictions that Se passivates the non-radiative recombination centres
