Photoelctrochemically Fabricated and Heated Cu₂O/CuO Bilayers with Enhanced Photovoltaic Characteristics

Cu₂O/CuO bilayers were fabricated by electrodeposition of the CuO layer in a copper(II)-ammonia complex aqueous solution, followed by photoelectrochemical deposition of the Cu₂O layer at potentials ranging from -0.3 to -1.0 V referenced to a Ag/AgCl electrode in a copper(II)-lactate complex aqueous solution under light irradiation, and the effects of varied potentials of the photoelectrochemical Cu₂O depositions and post-heating conditions on their structural, optical, and photovoltaic characteristics were investigated with X-ray diffraction, field emission-scanning electron microscopy, optical absorption measurements, and external quantum efficiency (EQE) measurements with and without applied bias voltage. The Cu₂O layers with a characteristic 2.1 eV band gap energy were adhesively stacked on the thorn-like grains of the CuO layers possessing a characteristic 1.5 eV band gap energy, and dense and defect-free Cu₂O/CuO bilayers could be fabricated at the potentials of -0.4 and -0.5 V, but the grain size of Cu₂O decreased at -0.5 V. In addition, the metallic Cu was deposited simultaneously at potentials less than -0.7 V. The Cu₂O/CuO bilayer fabricated at -0.4 V revealed photovoltaic features at wavelengths ranging from 350 nm to approximately 900 nm, and a maximum EQE value of 56.8% was achieved at 510 nm in wavelength with a bias voltage of -0.1 V. The maximum EQE value, however, decreased to 1.2% accompanied with the peak wavelength shift to 580 nm, and no photovoltaic feature was observed at potentials of -0.3, -0.7, and -1.0 V. The photovoltaic performance for the Cu₂O/CuO bilayer fabricated at -0.4 V was ameliorated by heating at 423 K, and the maximum EQE values were enhanced to 87.7% at 550 nm and 89.8% at 530 nm in an ambient atmosphere and vacuum. Both the Cu₂O and CuO layers acted as photovoltaic layers in the Cu₂O/CuO bilayer fabricated at -0.4 V and heated at 423 K, and the electrical characteristic including the carrier mobility affected the photovoltaic performance. The photovoltaic feature, however, disappeared by heating above 523 K due to the formation of nanopores inside the CuO layer and near the CuO heterointerface to the Cu₂O and fluorine-doped tin oxide substrate.