Ultrathin layers of hydrogenated amorphous silicon (a-Si:H), passivating the surface of crystalline silicon (c-Si), are key enablers for high-efficiency silicon heterojunction solar cells. In this work, the authors apply highly sensitive attenuated total reflectance Fourier-transform infrared spectroscopy, combined with carrier-lifetime measurements and carrier-lifetime imaging, to resolve several fundamental and technology-related questions related to the a-Si:H/c-Si interface. To gain insight, the a-Si:H/c-Si interfacial morphology is intentionally manipulated by applying different surface, annealing and ageing treatments. Changes are observed in the vibrational modes of hydrides (SiHX), oxides (SiHX(SiYOZ)) together with hydroxyl and hydrocarbon surface groups. The effect of unintentional oxidation and contamination is considered as well. Electronic interfacial properties are reviewed and discussed of hydrogen mono-layer passivation of the c-Si surface and from the perspectives of a-Si:H bulk properties. It is found that both models have severe limitations and suggest that a new physical model of the interface, considering both is required.