Previous studies on photoelectrode films for photoelectrochemical (PEC) water splitting cells have been mainly focused on synthesizing oxide semiconductors with wide band gaps. Unfortunately, pristine oxide photoanodes without any catalysts have relatively low photocurrent densities because of the inherent limitation of insufficient visible light absorption due to the wide band gap. Unlike typical oxide photoanode materials with wide band gaps and moderate band bending, photoanode materials with relatively narrow band gaps and lower work functions can provide advantageous situations by effectively absorbing visible and infrared light below 2 eV and forming large surface potentials at the same time. In this regard, chalcogenide materials are highly likely to allow high-efficient PEC operation because of their relatively narrow band gap. In essence, it is accepted that the photo-driven electrochemical water splitting reaction process from typical semiconductor photoelectrodes is carried out via the following three steps: 1) light absorption and generation of electron-hole pairs with greater energy than that of band gap of photo-absorbers, 2) subsequent separation and migration of these photo-generated charge carriers in the depletion region of the electrode/electrolyte junction, and 3) water oxidation/reduction reactions of the holes/electrons. Here, the charge separation efficiency was strongly affected by the energy band bending of photoelectrodes at the junction, and the formed energy band bending was dictated by the position of the work function of the photoelectrodes. In the case of the conventional chalcogenides with p-type characteristics, the formation of large band bending is not easy due to the work functions. Alternatively, n-type chalcogenides as photoanodes for plain demonstration of large band bending are inferred to be very attractive, despite the difficulty of obtaining high quality n-type conductivity.

We have strategically synthesized n-type binary chalcogenide photoanodes for applications in PEC water splitting via the suppression of defects causing p-type conductivity to induce n-type conductivity. As intended, the produced compounds showed anodic currents with the absence of dark current; the photocurrent values were remarkably high (approximately 5 mA/cm² at 1.23 V vs. RHE), as compared to those of previous chalcogenide compounds.

In this study, we propose a fabrication design for the synthesis of n-type binary chalcogenide by control of the formation of defects. From electrochemical analyses, the energy band diagram of the n-type compound was established and the mechanisms of such a large improvement in the photocurrent were assumed to be based on the strong inversion model of typical semiconductors. This model makes it possible to understand the abrupt generation in the photocurrent, resembling the current profiles of an electrolysis reaction from metal catalysts rather than that of typical semiconductor photoanodes.