| 초록 |
A representative method of forming CuO thin films on Cu2O photoabsorbers was simple annealing oxidation at high temperature in the controlled oxygen atmosphere, but the typical oxidation process is considerably quick and irregular, resulting in the presence of high density of defect sites. To maximize the beneficial effect of CuO/Cu2O heterojunction photocathode, novel criteria on CuO preparation were suggested: 1) in-plane epitaxy with Cu2O, 2) atomically layer-by-layer growth, 3) ultimate-thin and completely conformal coating, and 4) minimized internal defects. As a novel strategy to achieve them, we firstly propose a photo-assisted electrochemical-oxidation (PAEO) method, and the Cu2O surface is precisely phase-changed into the ultimate-thin CuO layer with 4 nm via fine thickness control using photoenergy and external potential at room temperature. The produced CuO crystals are grown on Cu2O without generating structural defects by accommodating the epitaxial relationship below critical thickness. From static and dynamic (photo)electrochemical analyses, the formation of ultimate-thin CuO offers high electrical conductivity and faster charge transport, guarantees sufficient open-circuit voltage (Voc), and substantially retains the initial Voc value by minimizing the contribution of recombination loss. Finally, the PAEO treated photocathodes exhibit the excellent photocurrent density of 15 mA/cm2 (approaching to theoretical maximum current) and 8.3 mA/cm2 at 0 V vs. RHE in the electrolytes with and without scavenger hydrogen peroxide (H2O2), respectively, as well as VOC of 0.78 V, even in use of the suboptimal Al-doped ZnO buffer layers. |
