Bifacial passivation of n-silicon metal–insulator–semiconductor photoelectrodes for efficient oxygen and hydrogen evolution reactions

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 the 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 the photovoltage extracted from the band offset between the metal and semiconductor will be counteracted by the parasitic light absorption of the metal layer, sacrificing the saturation photocurrent. This paper describes the design and realization of a bifacial passivation strategy for the metal/Si interface of an MIS photoelectrode, featuring a bi-layer stack consisting of amorphous silicon (a-Si) for passivating the silicon surface and a metal oxide (TiO2) for passivating the metal surface. Upon the bifacial passivation of both a-Si and TiO2, the minority carrier lifetime of the Si MIS photoanode was significantly improved from 18 to 2360 μs. Enabled by this extremely long minority carrier lifetime, it becomes possible to place the MIS junction on the back side of a 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 the OER and HER, respectively. Eventually, unprecedented applied bias photon-to-current efficiencies (ABPE) of 3.91% and 12.66% were obtained by Si MIS and Si I-MIS, which are the highest 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.