In Situ Investigation of HfO x 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 x Growth

High Resolution Image Download MS PowerPoint Slide Atomic layer deposition (ALD) of HfO x 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)Se 2 (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 HfO x 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 HfO x 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.