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

Abstract

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.

Mind the Interface Gap: Exposing Hidden Interface Defects at the Epitaxial Heterostructure between CuO and Cu2O

“…Izaki et al further stacked Cu 2 O layers on electrodeposited CuO layers and treated the samples at various temperatures. 35 A minimal strain between the layers and maximal PV performance was noticed for samples heated at 423 K, while heating at 473 K and above resulted in deteriorated bilayers and PV features disappearing entirely. Khoo et al created electrodeposited and annealed directly stacked Cu 2 O/CuO layers and measured the conduction band offsets (CBO) to be between 0.80 and 0.96 eV, depending on the annealing temperature (523 and 673 K, respectively).…”

“…Such results reiterate the need to retain high crystallinity, a small lattice mismatch, and low concentration of defects at any interface in order to achieve high efficiency PV performance. Izaki et al further stacked Cu 2 O layers on electrodeposited CuO layers and treated the samples at various temperatures . A minimal strain between the layers and maximal PV performance was noticed for samples heated at 423 K, while heating at 473 K and above resulted in deteriorated bilayers and PV features disappearing entirely.…”

Al:Cu2O n-Type Semiconductors─Electrochemical Fabrication of Heterojunction and Homojunction Photovoltaic Devices

“…The inclusion of two or more p-type semiconductors with different band gap energies is an important criterion to overcome the efficiency limit for single solar cells; multijunction solar cells and quantum dots solar cells have been proposed to realize high conversion efficiencies above 30%, − as demonstrated for the conversion efficiencies of 39.7 and 47.0% for the six-junction solar cell at 1 sun and concentrated conditions . Other types of photovoltaic structures that satisfy such criterion have been proposed for Cu 2 O/CuO bilayer and Cu 2 O–CuO nanocomposite semiconductors . Si tandem solar cells have attracted increasing attention as a high-performance and low-cost solar cell available for conventional use, and several types of top cell materials and devices have been explored including the wide-band gap Cu 2 O semiconductor …”

Photoelectrochemical Fabrication of CuO-Cu2O Nanocomposite Semiconductors by High-Frequency Potential-Switching in Copper(II)-Tartrate Complex Aqueous Solution and the Energy Band Structures

“…The direct fabrication of single layers and bilayers of CuO and Cu 2 O components have been reported by photoelectrochemical reactions in an aqueous solution without any heating process, 23 and the Cu 2 O/CuO bilayer revealed quantum efficiencies of approximately 60% and 90% before and after lowtemperature heating, with both the CuO and Cu 2 O layers acting as photovoltaic layers. 24 Here, we report the direct fabrications of CuO-Cu 2 O nanocomposites by photoelectrochemical high-frequency potential-switching in an aqueous solution containing copper (II) sulfate hydrate, tartaric acid, and sodium hydroxide. Their structural and energy characteristics were investigated with X-ray diffraction, field emissionscanning electron microscopy, X-ray photoelectron spectroscopy, optical absorption spectra measurements, photoelectron yield spectroscopy, Kelvin probe method, and Mott-Schottky plots measurements.…”