Resolving the Hydrogen Paradox at ALD Al 2 O 3 /Si Interfaces in Dopant‐Free Silicon Photovoltaics

ABSTRACT 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) Al 2 O 3 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 × 10 9 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.