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 SiOx 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 SiOx layers remained intact after 550°C deposition of the 700 nm ACIGS absorber. Notably, a MoSe2 interfacial layer formed between the full SiOx layer and Mo contact, likely due to Na and/or Se diffusion through SiOx pores. Combining near-complete passivation and the ohmic MoSe2 contact may explain the improved performance.