<i>In Situ</i> Investigation of HfO _<i>x</i> Atomic Layer Deposition on (Ag,Cu)(In,Ga)Se ₂ Thin-Film Solar Cell Absorbers: Role of Absorber Bulk Composition and Surface Treatment in HfO _<i>x</i> Growth

Atomic layer deposition (ALD) of HfOx layers has emerged as a promising strategy for interface passivation in chalcopyrite-based thin-film solar cells. However, the nucleation dynamics of (Ag,Cu)(In,Ga)Se2 (ACIGS) absorbers remain insufficiently understood, particularly regarding the interplay between absorber composition and alkali postdeposition treatments (PDTs). Here, we employ in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the initial surface chemistry during HfOx ALD on ACIGS thin-film solar cell absorbers with high [Ga]/([Ga]+[In]) (GGI) ratio subjected to RbF-PDT and compare the findings with low-GGI counterparts (Martin et al., ACS Appl. Energy Mater. 2025, 8, 461–472). The results show that high-GGI ACIGS surfaces are strongly Cu-depleted and enriched in Se and alkali-metal-containing secondary phases, which hinders tetrakisdimethylamido-hafnium (TDMA-Hf) precursor adsorption and delays oxide nucleation, in contrast to low-GGI ACIGS that allow quicker and more efficient HfOx growth. Water pulses are identified as essential for reactivating the RbF-treated high-GGI surface by generating hydroxyl groups, thereby enabling metal precursor activation. The distinct formation of Ga–F and In–O species is only observed on high-GGI ACIGS together with increased Na diffusion, reflecting different surface chemistry for a higher Ga content of ACIGS as compared to the low Ga case. These findings demonstrate that bulk composition and alkali-PDT strongly influence ALD reactivity, surface passivation, and interface formation, with direct implications for composition-specific optimization of ACIGS solar cells.