Band gap engineering of CuAl1-xInxS2 alloys for photovoltaic applications: a first principles study

The mixed cation alloys CuAl1-xInxS2 span a band gap range that fits the requirement for solar cell light absorbers. However, the materials properties of CuAl1-xInxS2 are not well known as a function of concentration (x). We have used density functional theory calculations to study the structural and electronic properties of these alloys. The most stable configuration out of all possible configurations in the given supercell was obtained with the site occupancy disorder (SOD) program. We find that (i) the structural properties are reproduced to within less than 1% of the measured values; (ii) the CuAl1-xInxS2 alloys are highly miscible with low formation enthalpies; (iii) the band gap of CuAl1-xInxS2 decreases from 3.3 eV to 1.5 eV as In concentration (x) increases from 0 to 1, and shows a small bowing when calculated with hybrid Heyd-Scuseria-Ernzerhof (HSE06) functionals; (iv) the calculated band offset shows that the band gap decreases with the addition of In, resulting mainly from the valence band upshift