Bundle-type columnar Cu₂O photoabsorbers with vertical grain boundaries fabricated using instant strike-processed metallic seeds and their enhanced photoelectrochemical efficiency

The design to control grain boundaries (GBs) causing recombination losses as planar defects in the absorbing layers is an essential strategy for the development of highly efficient photoelectrochemical (PEC) reaction systems. We propose a method to design a progressive bundle-type columnar structure as an alternative to single crystals with the least defects, where the GBs are parallel to the charge transport movement and the electric field direction. An instant strike bias (0.01 s and −1 V vs Ag/AgCl) in the same electrolytes induces the formation of island-shaped metallic Cu nanoparticles in the initial stage. These act as seed crystals for controlling Cu₂O growth evolution, resulting in dramatically high-density Cu₂O nuclei. This is followed by the deposition of bundle-type columnar Cu₂O with longitudinal GBs, contrary to the typical randomly crystallized Cu₂O. Metallic Cu seeds with a stronger electric field than that of the exposed indium tin oxide (ITO) region provide selective crystallization sites for Cu₂O growth along the <111> ionic bonding. Despite the instant strike interval, the p-type Cu₂O photoelectrodes retained an outstanding photocurrent density of 5.2 mA cm⁻² at 0 V_RHE and an onset potential of 0.7 V_RHE because of the highly improved charge transport and transfer efficiencies inside Cu₂O induced by effectively suppressing charge scattering in the GBs. The impedance measurements depict the overall decrease in the total resistance, charge-transfer resistance, and the photoabsorber/electrode interface resistance of the PEC systems with the introduction of the instant strike process.