Silicon-based (Si-based) junctions have been widely investigated in recent years as photoelectrochemical (PEC) water splitting photoelectrodes, including buried junctions and metal-insulator-semiconductor (MIS) Schottky junctions. However, Si-based MIS photoelectrodes suffer from low performance for PEC oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) because of the dilemma that a thin insulator cannot provide enough interfacial passivation while a thick insulator will block the transport of charge carriers. Another trade-off is the fact that photovoltage extracted from the band offset between metal and semiconductor will be counteracted by the parasitic light absorption of metal layer, sacrificing the saturated photocurrent. This paper describes the design and realization of a bifacial passivation strategy for metal/Si interface of the MIS photoelectrode, featured by a bi-layer stack consisted of an amorphous silicon (a-Si) for passivating the silicon surface and a metal oxide (TiO2) for passivating the metal surface, respectively. Upon the bifacial passivation of both a-Si and TiO2, the minority carrier life time of Si MIS photoanode was significantly improved from 18 to 2360 μs. Enabled by this exremely long minority carrier life time, it becomes possible to place the MIS junction to the back side of Si substrate to construct an inverted-MIS (I-MIS) structure to eliminate the parasitic light absorption of traditional Si MIS photoelectrodes. The obtained photoelectrode exhibits an excellent onset potential of 0.85 V and 0.62 V vs. reversible hydrogen electrode (RHE) for OER and HER, respectively. Eventually, an unprecedented applied bias photon-to-current efficiency (ABPE) of 3.91% and 12.66% were obtained by Si MIS and Si I-MIS, which are the higest among MIS-based photoanodes and photocathodes, with 30 h and 108 h stable operation. When pairing the Si I-MIS photocathode with a BiVO4 photoanode to form a PEC membrane-free tandem cell, an unbiased solar-to-hydrogen conversion efficiency of 1.9% is achieved.
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