Interfacial properties play a critical role in the dynamic process of carrier transport in dopant-free silicon (Si) heterojunction solar cells (HSCs), based on the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). In this study, we use quinhydrone (QHY) to engineer the interfacial properties by grafting the semiquinone (QH) on Si surface at low temperature. The QH monolayer provides effective chemical and fieldeffect passivation by the surface dangling-bond saturation and its interface dipole, respectively, and results in a large minority carrier lifetime of 477 mu s. At the front Si/PEDOT:PSS interface, the QH-terminated Si surface presents higher wettability for the improved contact at the Si/PEDOT:PSS junction. At the rear Al/Si interface, the work function of Al film is reduced significantly to form ohmic contact for electron-selective transport. The dark current-voltage and capacitance-voltage measurements show the improved electric characteristics with a higher carrier collection efficiency. Furthermore, the silicon band bending generated by the QH dipoles enhances the overall built-in potential of Si/PEDOT:PSS HSCs for a larger open-circuit voltage. As a result, the QHY modified Si/PEDOT:PSS HSC yields a power conversion efficiency of 13.29%. This approach demonstrates that the organic grafting is a simple, effective and low-cost method for the interface engineering to achieve high efficiency HSCs.