Unveiling a Near-Total Rear Passivation for High Performance of Ultra-Thin ACIGS

Thin-film Cu(In,Ga)Se₂ solar cells are a key photovoltaic technology for addressing global energy and environmental challenges. Reducing absorber thickness lowers manufacturing cost and critical raw material consumption but introduces optical and electrical losses due to incomplete light absorption and increased rear interface recombination. Rear recombination can be mitigated through Ga grading and/or rear contact passivation, for example using tunnel oxide as in c-Si TOPCon technology. In this work, a full coverage plasma enhanced chemically vapor deposited 8 nm porous SiO_x passivation layer was implemented in ultra-thin (Ag,Cu)(In,Ga)Se₂ (ACIGS) solar cells and compared with patterned passivation layers (∼70% coverage) and an unpassivated reference. Standard current-voltage and external quantum efficiency characterization methods show that the complete passivation yielded the highest performance, achieving 16.4% efficiency, a 2% absolute improvement over the reference. Patterned samples showed enhanced performance to a lower extent. Elemental and structural analysis showed that the SiO_x layers remained intact after 550°C deposition of the 700 nm ACIGS absorber. Notably, a MoSe₂ interfacial layer formed between the full SiO_x layer and Mo contact, likely due to Na and/or Se diffusion through SiO_x pores. Combining near-complete passivation and the ohmic MoSe₂ contact may explain the improved performance.