Thinnest npn homojunction for inspired photoelectrochemical water splitting

Tin monosulfide semiconductors have great potential for a wide range of applications related to solar energy conversion, however, the photoelectrochemical water splitting performance and stability of tin monosulfide are still inferior. To bridge the gap, effective strategies should be developed to optimize tin monosulfide semiconductors. Here, we report for the first time the thinnest sandwiched SnS n-p-n homojunction regulated by Sb doping and Sn vacancies (npn-SnS). Experimental observations and density functional theory calculations reveal that Sb doping and Sn vacancies form a bidirectional built-in electric field that can enhance the separation and transport efficiency of carriers. Due to the interaction between Sb and S, the thickness of npn-SnS is only 1.69 nm, and defect-related recombination is suppressed after Sb doping. As a result, the photocurrent density of the ultrathin npn-SnS photocathode is 3.3 mA cm⁻², which is 4 times higher than that of p-SnS (containing Sn vacancies) and surpasses all SnS-based photocathodes reported so far. This work synthesizes the thinnest sandwiched n-p-n homojunction, which can serve as a guide for improving the carrier separation efficiency and stability of other semiconductors.