Ultrathin AlOx Films for Back Contact Passivation in Bifacial Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells

Abstract

This work studies the thickness effect of atomic-layer-deposited AlO_x films, acting as back contact passivation layers in bifacial, wide-bandgap (E_G = 1.4–1.5 eV) (Ag,Cu)(In,Ga)Se₂ (ACIGS) solar cells with In₂O₃:W transparent rear electrodes. For each tested AlO_x thickness (1.0, 2.0, and 3.5 nm), several absorber-deposition runs, with varying ACIGS thicknesses, were conducted. It is found that a 3.5 nm-thick AlO_x layer results in strongly impeded hole extraction and thus, severe losses in short-circuit current. As a consequence, carrier collection at front and rear illumination is inferior to reference devices without AlO_x, independent of the absorber thickness. Reducing the AlO_x thickness to 2.0 nm still results in fill factor losses, but the collection of electrons generated close to the back contact can be improved. Finally, 1.0 nm of AlO_x only leads to a minor barrier for carrier transport, but the passivating character vanishes. The experiments confirm that sufficiently thick AlO_x films (≥ 2 nm) can reduce the back contact recombination rate in bifacial wide-gap ACIGS solar cells. However, in order to improve the efficiency at rear illumination, those films need to be locally opened to allow for adequate carrier flow.