Preparation And Device Applications Of CuAlO₂ And AlO_x Thin Films

A novel solvent–free ester elimination technique was designed for the preparation of CuAlO2 ceramic. It involved a direct reaction between aluminium sec-butoxide and copper(II) acetate in the absence of a solvent. Elimination of ester was mainly confirmed by the sharp and intense peak at 1738 cm-1 on the FT-IR spectrum. The formation of Cu-O-Al mixed metal oxide bridge was demonstrated by a comparative XPS study of the as-prepared material and two separately prepared materials, one having pure Al-O bonds and the other having pure Cu-O bonds. Both the observed chemical shifts in O 1s, Al 2p, Cu 2p3/2 and Cu 2p1/2 core levels and the disappearance of the Cu-O satellite peak support this formation. TG-DTA-EGA and XRD analyses revealed the mechanism that led to the formation of a pure phase of CuAlO2 delafossite at a low in-air annealing temperature of 900 oC. The BEI-EDXS and XRD information obtained for ceramic pellets made at different pressures(i.e. 10 MPa, 15 MPa and 20 MPa) also supported this pure delafossite phase formation. The semiconductor behaviors of the ceramics were confirmed from the temperature dependent resistivity curve and their p-type conductivity from Hall Effect measurement.Using a ceramic sputter target derived from the preparation descibed above, bipolar electronic behavior was demonstrated in amorphous Cu-Al-O thin films prepared on a soda-lime glass substrate via radio frequency(r.f.) magnetron sputering deposition technique. The tunable majority charge carrier type was as a result of the intentional adjustment of the percentage chemical composition of the material. Scanning Electron Microscopy(SEM) shows that the prepared films exhibit high uniformity. X-Ray Diffraction(XRD) patterns obtained present a pure phase of a pre-fabricated delafossite CuAlO2 ceramic target. Amorphous Cu-rich and Al-rich Cu-Al-O thin films were deposited by the introduction of pure Cu strip and Al strip respectively on the ceramic target and by the variation of the sputter Ar/O2 gas ratio. Both signs of Hall coefficients from Hall effect measurement and the slopes of Mott-Schottky plots obtained for the films indicate the majority charge carreir type of each film. Negative Hall coefficient and a positive Mott-Shottky slope were obtained for the Al-rich film showing its n-type nature. Positive Hall coefficient and negative Mott-Schottky slope were obtained for the Cu-rich film showing its p-type nature. X-Ray Photoelectron Spectroscopy(XPS) was used to explain the variation in charge carrier type as a result of the adjustments made to the percentage chemical composition of the film. The I-V curves obtaine from fabricated devices show that the homojunction formed from the two films exhibit rectifying and photovoltaic properties. The rectifying behavior was studied between a bias of-1 V to 1 V. In addition, an open circuit voltage(Voc) of 0.33 V, a short circuit current(Jsc) of 8.00 mA/cm2, a fill factor(FF) of 0.30 and a power conversion efficiency(PCE) of 0.79 % were obtained from the fabricated photovoltaic device.The photovoltaic application of the amorphous p-type CuAlO2 was further carried out. The fabrication and device parameters of inverted planar heterojunction(PHJ) organic-inorganic lead mixed-halide(CH3NH3PbI3-xClx) perovskite based solar cells(PSCs) were demonstrated. Amorphous CuAlO2(a:CuAlO2) film was used as a hole selective buffer layer between ITO electrode and PEDOT:PSS in these devices. The thin film of a:CuAlO2 was derived from a pre-fabricated polycrystalline Cu AlO2 ceramic target via direct current(d.c.) magnetron sputtering technique. One-step spin coating method was used to prepare the perovskite layer. A short circuit current density(Jsc) of 21.98 mA/cm2, an open circuit voltage(Voc) of 0.88 V, a fill factor(FF) of 0.75 and a power conversion efficiency(PCE) of 14.52 % were achieved for the optimized device. These improved device parameters were also accompanied with improved stability as a result of sandwiching the ambient stable a:CuAlO2 layer with decent conductivity between the ITO and the PEDOT:PSS layers. The versatility of this material application was also demonstrated as similar improvement in device performance and stability was observed by using the prepared a:CuAlO2 in another perovskite solar cell system based on CH3NH3PbI3 prepared by two-step spin-coating method.Furthermore, an approach to achieve improved performance in pentacene-based organic field effect transistors(OFETs) was demonstrated using high-k AlOx prepared by a low temperature sol-gel technique as a thin buffer layer on SiO2 gate dielectric. The maximum processing temperature for the AlOx thin layer was 150 °C. The resulting all-inorganic SiO2/AlOx bilayer gate dielectric system exhibited a low leakage current density < 1×10-8A/cm2 under an applied electric field strength of 1.8 MV/cm, a smooth surface with an rms of 0.11 nm and an equivalent dielectric constant(k) of 4.13. The OFET fabricated as a result of this surface modification exhibited a significantly improved field effect mobility of 0.81 cm2/Vs when compared with a reference device with SiO2 single layer gate dielectric which had a lower mobility of 0.28 cm2/Vs.