The large photovoltaic response from homogeneous silicon wafer obtained upon deposition of a molecular layer on its surface remained an unexplained phenomenon so far. Here, we show by X-ray and ultraviolet photo-electron spectroscopy that deposition of species containing acidic groups on the surface of n-type silicon with native silicon oxide overlayer always results in increased work function of the hybrid interface. This effect is shown to originate due to the surface band bending of the silicon crystal upward, which is accompanied by a negative surface dipole formed. This effect is assigned to protonation of the silicon oxide film by molecular acidic groups, which in turn facilitates accumulation of a mirror negative charge at the Si-SiO2 interface, thus increasing the depth of the depletion region and height of the Schottky barrier in the silicon semiconductor, respectively. Comparison of the work functions of the samples in the dark and under illumination confirms formation of a depletion region at the silicon surface upon molecular adsorption.