Effect of NaF pre-cursor on alumina and hafnia rear contact passivation layers in ultra-thin Cu(In,Ga)Se2 solar cells

In this work, we evaluate the effect of NaF layers on the properties of Al2O3 and HfO2 rear contact passivation layers in ultra-thin Cu(In,Ga)Se-2 solar cells. The 6 nm thin passivation layers were deposited by atomic layer deposition and neither intentionally opened nor nano-patterned in any extra-fabrication step. NaF layers, 7.5 or 15 nm thin, were deposited as precursors prior to CIGS absorber co-evaporation. The 215 nm thick absorbers were co-evaporated with constant evaporation rates for all elements. Directly thereafter, a 70 nm thick cadmium sulfide layer was deposited. Photoluminescence measurements indicate a strongly reduced recombination at the rear contact for all passivated samples compared to an unpassivated reference. Although the sample with Al2O3 passivation and a 15 nm NaF precursor layer luminesces by far the least of the passivated samples, solar cells made from this sample show the highest efficiency (8.6% compared with 5.6% for the reference with no passivation). The current-voltage curves of the solar cells fabricated from the sample with 7.5 nm NaF on top of the Al2O3 layer and both samples with HfO2 exhibit blocking behavior to various degrees, but a high photoluminescence response. We conclude that NaF precursor layers increase conduction through the Al2O3 layer, but also reduce its effectiveness as a passivation layer. In contrast, conduction through the HfO2 passivation layers seem to not be influenced by NaF precursor layers, and thus requires nano-patterning or thinning for conduction.