The passivation quality at the interface between dielectric interlayer (IL) stacks and liquid-phase crystallized silicon (LPC-Si) was investigated by means of high-frequency capacitance–voltage (C–V) measurements. The developed device structure was based on a molybdenum layer sandwiched between the glass substrate and the IL/LPC-Si stack. C–V curves were discussed in terms of hysteresis formation and capacitance relaxation. We varied the nitrogen and hydrogen content in the a-SiOxNy:H layer adjacent to the LPC-Si and studied the effects on the defect state density at the IL/LPC-Si interface (Dit) as well as on the effective charge density in the IL (QIL,eff). Both parameters are crucial for the analysis of chemical and field-effect passivation. Furthermore, the effect of an additional hydrogen plasma treatment (HPT) on Dit and QIL,eff was investigated. A Gaussian-like defect distribution at around 0.1 eV above the mid gap energy is significantly reduced by the additional HPT. With additional HPT, the lowest Dit and highest QIL,eff at mid gap, i.e., Dit = (3.5 ± 0.7) × 1011 eV−1 cm−2 and QIL,eff = (1.6 ± 0.3) × 1012 cm−2, correspond to the passivation by an a-SiOxNy:H layer with a low nitrogen and high hydrogen content.