One of the important obstacles on the way to application of Si nanocrystals for development of practical devices is their typically low emissivity. In this study we explore the limits of external quantum yield of photoluminescence of solid-state dispersions of Si nanocrystals in SiO₂. By making use of a low-temperature hydrogen passivation treatment we demonstrate a maximum emission quantum efficiency of approximately 35%. This is the highest value ever reported for this type of material. By cross-correlating PL lifetime with EQE values, we obtain a comprehensive understanding of the efficiency limiting processes induced by P_b-defects. We establish that the observed record efficiency corresponds to an interface density of P_b-centers of 1.3 × 10¹² cm¹², which is 2 orders of magnitude higher than for the best Si/SiO₂ interface. This result implies that Si nanocrystals with up to 100% emission efficiency are feasible.