H2 evolution at Si-based metal–insulator–semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover

Abstract

Photoelectrochemical (PEC) water splitting represents a promising route for renewable production of hydrogen, but trade-offs between photoelectrode stability and efficiency have greatly limited the performance of PEC devices. In this work, we employ a metal-insulator-semiconductor (MIS) photoelectrode architecture that allows for stable and efficient water splitting using narrow bandgap semiconductors. Substantial improvement in the performance of Si-based MIS photocathodes is demonstrated through a combination of a high-quality thermal SiO2 layer and the use of bilayer metal catalysts. Scanning probe techniques were used to simultaneously map the photovoltaic and catalytic properties of the MIS surface and reveal the spillover-assisted evolution of hydrogen off the SiO2 surface and lateral photovoltage driven minority carrier transport over distances that can exceed 2 cm. The latter finding is explained by the photo- and electrolyte-induced formation of an inversion channel immediately beneath the SiO2/Si interface. These findings have important implications for further development of MIS photoelectrodes and offer the possibility of highly efficient PEC water splitting.

Methods of photoelectrode characterization with high spatial and temporal resolution

“…1a). The raw photocurrent or voltage is valuable information by itself, but the method can also been used to determine spatial variation in quantum yield, [29][30][31] minority carrier diffusion length, 29,32 electric field distributions, 32 photoconductivity, dopant concentrations, 32 and more. In PEC systems, the measured photocurrent is strongly dependent on the optical, electronic, and catalytic properties of the photoelectrode material(s).…”

“…For example, in Fig. 2c, a focused laser-beam is combined with a diagonally-mounted conical-shaped UME 31 in a set-up that allows for minimization of shadowing effects and background signal arising from the oxidation/reduction of H 2 or O 2 that would normally diffuse from neighboring illuminated areas. However, this approach is limited to monochromic laser light and could more easily lead to high-level carrier-injection conditions.…”

“…For instance, due to the heterogeneity of catalytic sites on the photoelectrode surface, the photo-generation of carriers and their collection at the interface need not happen at the same location but could occur at sites that are distant from each other. 31 In such studies, the SECM tip and the SPCM probe can be operated independently to gain additional mechanistic information on the operation of the system of interest.…”

Protection of inorganic semiconductors for sustained, efficient photoelectrochemical water oxidation

“…[29] Generally, a tandem 5 structure or triple junction structure is necessary to produce sufficient voltage and current density for efficient water splitting [19,20,30]. In addition to use of semiconductor/liquid interfaces as the voltage-generating junction, buried-junction structures, including planar p-n homojunctions [31,32], semiconductor/metal Schottky barriers [33,34], spherical [35] and radialjunction microwire [36] electrodes, heterojunctions [37,38], metal-insulator-semiconductor contacts [9,15], and emitters derived from in situ inversion layers [21] have all been investigated for use in photoelectrosynthetic or photovoltaic-biased electrosynthetic cells [39]. The efficiencies and main performance characteristics of various reported solar-driven water-splitting systems have been recently compiled [40].…”

“…In contrast, a tandem-device architecture, in which two separate semiconductor materials each provide a substantial contribution towards the needed photovoltage, can produce much higher efficiencies [16][17][18]. Technologically important semiconductors such as silicon [19], Group III-V [20], and the Group II-VI [21] semiconductor families, which are generally regarded as 'small band-gap' materials with the appropriate band gaps of 1.1 to 1.7 eV for optimal efficiency [16,22], provide attractive materials options for the construction of such tandem structures.…”