Hydrogenation of polycrystalline silicon (poly-Si) passivating contacts is crucial for maximizing their passivation performance. This work presents the application of Al2O3 prepared by atomic layer deposition as a hydrogenating capping layer. Several important questions related to this application of Al2O3 are addressed by comparing results from Al2O3 single layers, SiNx single layers, and Al2O3/SiNx double layers to different poly-Si types. We investigate the effect of the Al2O3 thickness, the poly-Si thickness, the poly-Si doping type, and the postdeposition annealing treatment on the passivation quality of poly-Si passivating contacts. Especially, the Al2O3/SiNx stack greatly enhances the passivation quality of both n+ and p+ doped as well as intrinsic poly-Si layers. The Al2O3 layer thickness is crucial for the single-layer approach, whereas the Al2O3/SiNx stack is less sensitive to the thickness of the Al2O3 layer. A thicker Al2O3 layer is needed for effectively hydrogenating p+ compared to n+ poly-Si passivating contact. The capping layers can hydrogenate poly-Si layers with thicknesses up to at least 600 nm. The hydrogenation-enhanced passivation for n+ poly-Si is found to be more thermally stable in comparison to p+ poly-Si. These results provide guidelines on the use of Al2O3 capping layers for poly-Si contacts to significantly improve their passivation performance.

The degradation of hybrid perovskite films when exposed to ambient air is a major challenge for the development of perovskite-based photovoltaics at large scale. At present, little is known about the environmental degradation of perovskite films associated with the development of structural defects or open volumes (such as atomic vacancies, voids, crystallographic defects and grain boundary defects) in the lattice, and about the depth dependence of the structural degradation. Therefore, in this work, we use Doppler broadening-positron annihilation spectroscopy (DB-PAS) depth-profiling to gain insight into the structural degradation of CH ₃ NH ₃ PbI _3-x Cl _x perovskite when exposed to ambient air. In parallel, we investigate the effect of ultrathin (<1 nm) atomic layer deposited (ALD) Al ₂ O ₃ processed directly on top of the perovskite as a means to suppress the degradation process. Specifically, we infer that the perovskite degradation involves changes in open volumes in its crystal lattice. This could be caused by the ingress of H ₂ O molecules into the cation vacancies. In parallel, chemical changes in the perovskite films upon decomposition are observed, accompanied by a decrease in the film thickness as a function of air exposure time. When the perovskite films are decorated with ALD Al ₂ O ₃ , the latter delays the thickness reduction of the perovskite layer during air exposure and also suppresses the changes in its open volumes and chemical transformations. Our findings illustrate that an improved understanding of the perovskite degradation process can be obtained using DB-PAS, especially when combined with other thin film characterization techniques, such as X-ray diffraction and X-ray photoelectron spectroscopy.