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

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)Se 2 (ACIGS) solar cells with In 2 O 3 :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.