Resolving the Hydrogen Paradox at ALD Al₂O₃/Si Interfaces in Dopant-Free Silicon Photovoltaics

Dopant-free silicon solar cells based on field-induced junctions offer a pathway beyond the limitations of diffusion doping, yet the atomic-level chemistry governing junction quality remains poorly understood. Here, we resolve the "hydrogen paradox" of atomic-layer-deposited (ALD) Al₂O₃ passivation, demonstrating that interfacial chemical purity, rather than hydrogen content, determines device performance. Standardless ion-beam quantification establishes that thermal ALD films contain more hydrogen than PE-ALD films (5.0 vs. 3.7 at%) yet passivate worse, and photo-induced force microscopy reveals signatures consistent with hydrogen-bonded hydroxyl networks in an incompletely oxidized matrix, with residual methyl species observed only in thermal ALD. PE-ALD leaves no detectable methyl-related signature, indicating effective ligand combustion, while injecting excess oxygen (oxygen-to-aluminum ratio of 1.83, invariant upon annealing) that enhances the negative fixed charge density to -6.6 × 10^{1 2} cm^{- 2}, nearly threefold higher than thermal ALD. This dual optimization yields minority-carrier lifetimes of 2780 µs and interface trap densities of 4.3 × 10⁹ eV^{- 1}cm^{- 2}. Dopant-free planar cells achieve efficiencies of up to 9.15% with near-unity internal quantum efficiency at short wavelengths, and simulations suggest that three-dimensional (3D) microstructures could raise efficiencies toward 20%. The completeness of interfacial oxidation thus emerges as a generalizable design principle for carrier-selective contacts in dopant-free photovoltaics.