Theoretical exploration of Cd-free CIGS solar cells with a charged tunneling electron collector layer

Beyond the common cadmium-free copper indium gallium (di) selenide (CIGS) solar cell made with a physical buffer layer of zinc sulfide (ZnS) or indium sulfide (In₂S₃), a newly devised Cd-free cell using a charged tunneling layer is explored theoretically in this work. In principle, applying an ultra-thin tunnel layer (for example by atomic layer deposition) with a high positive fixed interface charge density, the conventional buffer layer can be substituted. Assuming a fixed interface charge density of 1013 cm^-2 of the tunneling layer, the simulated CIGS efficiency is over 15%, being close to the reference CIGS solar cell with CdS buffer layer. The benefits and limitations of the Cd-free tunneling layer CIGS solar cells are discussed. Potential processes suitable for realizing the tunneling layer are also discussed.