Nanostructured Copper Oxides: Production and Applications

Cuprite (Cu2O) and tenorite (CuO) have been extensively studied because of their potential use in several electronic applications, which include solar cells and gas sensors, just to mention the most appealing ones. Both materials are p-type semiconductors, the one with a wide bandgap (Cu2O, 2.0 eV-2.2 eV), the other with a much narrower one (CuO, 1.2 eV-1.8 eV), and both show interesting optical properties in the visible and near-visible range. This Thesis work is devoted to the synthesis, characterisation and application of nanostructured copper oxides in the field of renewable energies. Within this broad scope the Thesis focuses on: • production of defect-free nanocrystals (Cu2O & CuO) and investigation of the correlation between experimental parameters and resulting microstructure; • production of highly defective nanocrystalline Cu2O powders, with the estimation of the effect of milling on microstructure and phase transformations; • production of inks for photonic applications in photovoltaic cells. Reverse micelle microemulsions (a bottom-up approach) have been employed for the production of the defect-free nanocrystals. Models have been proposed for the nanocrystal formation and growth, validated by means of several techniques such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), UV-Visible and Fourier Transform InfraRed spectroscopy (UV-Vis and FTIR). The produced nanocrystals show good crystallinity with Cu2O and CuO exhibiting cuboidal and rod-like structures, respectively. The nanometric nature of the primary domains (20 nm – 4 nm) leads to quantum confinement phenomena highlighted by photoluminescence measurements. A top-down approach has been exploited to produce highly defective particles to be possibly employed in new-generation intermediate-band solar cells. A high-energy mill, suitably modified to work in controlled temperature and environment, allowed the production of highly defective copper oxides with little or no phase transformation and contamination from the mill. Finely dispersed powders with a high density of line defects (ρ ≈ 4×10-16 m-2) were ultimately obtained. The effect of milling on the microstructure evolution was investigated using both traditional and synchrotron radiation XRD line profile analysis supported by High Resolution TEM and SEM. The synthesised powders were employed for the production of copper oxide inks for photonic applications. Those inks would allow solar cells to be directly printed on a substrate, with a dramatic reduction of production costs and the possibility of coating objects of any shape. Sprayed films usually need high consolidation temperatures: the proposed formulation, on the contrary, allows sintering of the ink-derived films at a relatively low temperature (below 600 °C), thus making possible the deposition on inexpensive substrates such as aluminium. Prototype solar cells based on the copper oxide inks have been fabricated using simple coating techniques. Results can be considered as a first step towards the production of fully recyclable solar cells, made of low-cost raw materials and realized by cost-effective deposition techniques.