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 postheating 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.

Automated summary

Cu2O/CuO bilayers were successfully fabricated by electrodeposition and photoelectrochemical deposition methods on a copper substrate, and dense and defect-free bilayers with good photovoltaic characteristics could be achieved under specific conditions.