The most important aspect in the development of Si-based photovoltaic technologies is the passivation of the Si wafer surface. Here, we show that Poly(3,4-ethylthiophene):polystyrenesulfonate (PEDOT:PSS) thin films, frequently used in organic/inorganic hybrid solar cells as emitter layers, provide an excellent surface passivation with a millisecond (ms) level effective minority carrier lifetime (tau(eff)). More importantly, we find that this passivation effect is dominated by the PSS species in PEDOT:PSS rather than PEDOT ones. The PSS thin film passivation yields a high tau(eff) of 9.4 ms on a high-resistivity Si wafer. The interface nature of PSS/Si is unveiled by the X-ray photoelectron spectroscopy and first-principles total-nergy calculations together. An electrochemical passivation mechanism is clarified:the Si sub-oxides form at PSS/Si interface by the electrochemical grafting O groups in PSS onto Si surface, which can be controlled by injecting holes or electrons into the Si surface, leading to a switchable interface state between oxidation and deoxidation. To examine the application of the passivation strategy in real devices, we develop a polymer-passivated both-sides Si heterojunction solar cell with light-induced enhancement of photovoltaic performances, which is consistent with the light-induced enhancement of tau(eff). (C) 2017 Elsevier Ltd. All rights reserved.