Narrow bandgap CuIn(Sex, Te1−x)2 — Based solar cells for chalcopyrite tandem photovoltaic devices

We propose CuIn(Se1-_x, Te_x)₂ as a quaternary chalcopyrite material for narrow-bandgap thin films solar cells, and focus on the purpose of realizing tandem photovoltaic devices by using these solar cells combined with a semi-transparent CIGSe solar cell. Numerical simulations are realized to evaluate the potential of such device, and the effect of the acceptor concentration is studied. High quality CuIn(Se_x, Te_1-x)₂ films are grown on Mo-coated SLG substrates by 4 sources co-evaporation and a modified three stage process. The films are characterized by transmission-reflection spectroscopy; a very narrow bandgap of less than 0.9 eV is achieved, much smaller than ternary compounds such as CuInSe₂, CuIn₃Te₅ or CuInTe₂. As a result, both J-V and EQE characterizations show a significant increase in the absorption of low energy photons, leading to an improved short circuit current of 41.5mA.cm^-2 and an efficiency of 7% under standard AM 1.5 illumination. By using a CIGSe layer deposited on glass, we also perform tandem-like J-V and EQE measurements to demonstrate the superiority of the narrow bandgap CuIn(Se_x, Te_1-x)₂ over wider bandgap ternary compounds in such configuration. A CIGSe-filtered efficiency of η=2% is achieved, more than 3 times higher than that of ternary compounds.