Enhancement of Photovoltaic Efficiency in CdSe X Te1−x (where 0 ⩽ X ⩽ 1): Insights from Density Functional Theory

Recent advancements in CdTe photovoltaic efficiency have come from selenium grading, which reduces the band gap and significantly improves carrier lifetimes. In this work, density functional theory calculations were performed to understand the structural and electronic effects of Se alloying. Special quasirandom structures were used to simulate a random distribution of Se anions. Lattice parameters decrease linearly 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 significant changes to band structure around the Gamma point are found. Band offset 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 Cd/Te 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 confirms experimental predictions that Se passivates the non-radiative recombination centres.