The nanostructure of hydrogenated amorphous silicon (a-Si:H) is studied by means of doppler broadening positron annihilation spectroscopy (DB-PAS) and Fourier transform infrared (FTIR) spectroscopy. The evolution of open volume deficiencies is monitored during annealing, demonstrating that small vacancies and other small vacancy clusters that are initially present in the a-Si:H nanostructure agglomerate into larger vacancy clusters. The migration of open volume deficiencies is less pronounced for a-Si:H deposited at higher hydrogen-to-silane gas flow rate ratio, R. FTIR spectroscopy reveals the presence of a peculiar peak in the refractive index in the infrared - and hence the calculated mass density - which occurs just before H effusion from the films starts. The combined results of DB-PAS and FTIR spectroscopy indicate that a stress buildup caused by the accumulation of H2 in agglomerating vacancies during annealing can explain the sudden mass density increase. At higher temperatures, stress is released with the onset of H effusion. The H effusion consists of a two-stage process involving small open volume deficiencies and nanosized voids, contrasting earlier interpretations. The reduced amount of hydrogen migration and enhanced hydrogen passivation degree are suggested as key factors to the reduced light-induced degradation associated with increased R values.@en

Electron beam (E-beam) deposited molybdenum oxide (MoOx) has been investigated for its potential to replace p-type hydrogenated amorphous silicon (a-Si:H) in Si heterojunction (SHJ) solar cells. Excellent passivation was achieved for our best MoOx/c-Si junction based device, reaching an average implied Voc (iVoc) of 734 mV on textured, commercially available 6-inch Cz wafers. This confirms the compatibility of MoOx as a hole selective layer with industrial SHJ cell processing. A hole barrier was, however, observed for our MoOx-based solar cells due to inefficient hole extraction. The formation of this hole barrier can be related to annealing of MoOx and the presence of a native oxide grown on the intrinsic a-Si:H interface layer below. Pre-annealing, followed by an HF treatment on the a-Si:H(i) layer prior to MoOx deposition, proved to be useful to mitigate the formed barrier, while making it more stable under standard SHJ annealing conditions.@en