Development of thin film photovoltaic cells based on low cost metal oxides

The major market barriers to the use of photovoltaic solar cells are high cost and long payback time of conventional technologies, based largely on the silicon material. In order to overcome the environmental problem resulting from the consumption of fossil fuels, all western countries are required to impose heavy subsidies to encourage the use of solar cells in the reduction of carbon consumption; thereby making them highly unsustainable. Therefore, it is necessary to develop solar cells based on low-cost metal oxides with large natural resources. The objective of this program is to investigate the effects of doping on the structural, optical and electrical properties of low-cost metal oxides, such as doped ZnO and copper oxides (CuO and Cu4O3). These are synthesised via sputter deposition and thermal oxidation method in air. Al doped ZnO is an n-type direct semiconductor with a band gap of around 3.5eV. Its crystalline structure is wurtzite, which is deposited widely by the RF reactive magnetron sputtering technology. In my work, the Al doped ZnO thin films were deposited by sputter with metal and ceramic targets. On the one hand, the influence of RF power on the structural, electrical and optical properties of Al doped ZnO thin films were investigated when they were deposited with metal targets. Conversely, the influence of O2 flow rate on the structural, electrical and optical properties of Al doped ZnO thin films was examined when they were deposited with ceramic targets. CuO is a p-type indirect semiconductor with a narrow band gap of 1.0-1.4eV. Its crystalline structure is monoclinic crystal system. CuO nanowires (NWs) were fabricated by the thermal oxidation method in air. It was found that CuO NWs not only grows on Cu sheets, but also on the Si, FTO, Al doped ZnO and glass substrates. For the growth of CuO NWs, the expanding parameters should meet the following requirements: growing temperature: >390°C and growing duration: ≥6hrs. The peeling-off of the CuO NWs on Cu sheets resulted from the formation of Cu8O and Cu64O between the Cu sheets and Cu2O layer. The electrical properties of a single CuO NW were measured using a nano probe station. The contact behaviour between a CuO NW and metal electrodes (Au and W) was schottky. The electrical resistivity of a CuO NW depended on the diameter of the NW. The contact behaviour between CuO NWs on Cu sheets with silver paste top electrodes was schottky as well. A simple PV cell based on CuO NWs-PCBM p-n heterojunction was fabricated, and the short circuit current, open voltage and fill factor of the PV cell was also measured. It indicated that CuO NWs can be utilized to fabricate diodes and PV cells. Copper oxides thin films were deposited by RF reactive magnetron sputtering technology. The phase structure of copper oxides thin films depended on the sputtering parameters. When the thin film was deposited without a bias power, only CuO was detected in the copper oxide thin films. The electrical properties of CuO thin films depended on the O2 fraction during the sputter process. The current-voltage (I-V) characteristics of CuO thin films with Cu electrodes demonstrated that it was influenced by the O2 fraction during the sputter process. Moreover, Cu4O3 is a p-type indirect semiconductor with narrow band gap of 1.0-1.4eV and its crystalline structure is tetragonal crystal system. When the copper oxide thin films were deposited with a bias power, only Cu4O3 phase was detected. Its structural, optical and electrical properties were studied. The optical band gap of Cu4O3 thin film was 1.37eV. Hall properties of Cu4O3 thin films were 1020cm-3, 10-2cm2·V-1·s-1 and 10-1Ω·cm. The Cu4O3-Al Abstract III doped ZnO p-n heterojunction demonstrated excellent rectifying performance, indicating that Cu4O3 is a good candidate for fabricating diodes and PV cells. In addition, Cu4O3 thin films were annealed at different temperatures in air. Furthermore, I studied the influence of annealing temperature on the structural, optical and electrical properties of Cu4O3 thin films

Thin film engineering for transparent thin film transistors

Zinc oxide (ZnO) and Indium Gallium Zinc Oxide (IGZO) thin films are of interest as oxide semiconductors in thin film transistor (TFT) applications, due to visible light transparency, and low deposition temperature. There is particular interest in ZnO and IGZO based transparent TFT devices fabricated at low temperature on low cost flexible substrates. However, thermal annealing processes are typically required to ensure a good performance, suitable long term stability, and to control the point defects which affect the electrical characteristics. Hence there is interest in post deposition processing techniques, particularly where alternatives to high temperature thermal treatments can be utilised in combination with low temperature substrates. This thesis presents the results of a series of experimental studies as an investigation into photonic (excimer laser) processing of low temperature ZnO and IGZO thin films deposited by RF magnetron sputtering and/or by high target utilisation sputtering (HiTUS), to optimise the microstructure and electrical properties for potential use in thin film electronic applications. ZnO thin films were grown at various deposition parameters by varying oxygen flow rates, RF power, oxygen concentration, and growth temperatures

Integrated optics technology study

The status and near term potential of materials and processes available for the fabrication of single mode integrated electro-optical components are discussed. Issues discussed are host material and orientation, waveguide formation, optical loss mechanisms, wavelength selection, polarization effects and control, laser to integrated optics coupling fiber optic waveguides to integrated optics coupling, sources, and detectors. Recommendations of the best materials, technology, and processes for fabrication of integrated optical components for communications and fiber gyro applications are given

ZnO-Based nanostructures for gas sensing applications.

Metal oxide chemical sensors based on nanomaterials are gaining popularity and finding extensive use in automotive industries, process control and environmental monitoring. ZnO, a semiconducting metal oxide has attracted great interest over the years for its sensitivity to a variety of gases. Nanostructured sensing materials, such as thin films, nanowires, tetrapods, nanoflackes offer an inherently high surface area, reducing operating temperatures and increasing sensitivity to low concentrations of analytes. In this thesis, ZnO nanostructures have been tested as chemical sensors and a detailed study on the effect of different process parameters such as grain size, roughness, surface-to-volume ratio, depletion layer, temperature, gas concentration and material properties on gas sensitivity is presented. Initially, ZnO nanodevices were prepared with a variety of techniques, such as RF sputtering, electrodeposition, hydrothermal growth, chemical vapour deposition, thermal evaporation and controlled oxidation. The structural characterization of the nanodevices has been done by a FEI QUANTA 3D dual beam SEM/FIB machine and by a Dimension 3100 Atomic Force Microscope (AFM) (Digital Instruments) in tapping mode. X-ray diffraction (XRD) spectra were recorded on an AXS D8 diffractometer (Bruker) with a Cu Kα X-ray tube. The gas sensor substrate based on alumina consisted of Pt grid of 50nm thickness and golden contacts of 200nm thickness creating an alumina patterned substrate. The sensor deposition area was coated with ZnO nanostructures to form the sensing material. Sensing measurements are performed in a closed steel chamber where air and tested gases have been inserted. ZnO based nanostructures’ response was measured in different concentrations of Ethanol, CO and NO2. Initially the role of grain size and roughness has been investigated in several thin film based nanodevices. Grain size is decreasing with increasing RF sputtering power and increasing by post-annealing treatment. Roughness instead is increasing with both the increasing of RF sputtering power and post-annealing treatment. High response was observed for those films with smaller grain size, while the roughness seems to influence very little the response of the sensor. For all thin films, the response is increasing with ii temperature and gas concentration. Recovery time and response time seem to follow a non-linear behavior with the above parameters. Extended studies have investigated the role of surface-to-volume ratio and depletion layer in the sensing performance. It has been observed that the increase of surface-to volume ratio has an important effect on the sensitivity, increasing, more than twice the response of such a device in respect to another that is based on a ZnO thin film. On the other hand, the dimensions of a nanostructure play the most crucial role in the depletion layer width in respect to the sensing properties. The diameter of a nanowire should be comparable with its depletion layer width. In this case the depletion layer has strong effect, which makes the sensor’s response depend also on it. The sensing properties of all fabricated structures have been compared to find the optimum sensor that could face the demands of automotive industries. All fabricated structures have been compared in different configurations to find out which one presents the best sensing performance. To that direction sensors based on thin film, tetrapods, nanowires, nanoflackes have been tested in same environmental conditions. Advanced nanostructures present better sensing properties. Sensing response of every advanced nanostructure presents more than double sensing response than every thin film-based nanostructure. Comparing the advanced nanostructures with each other, tetrapods based sensor has higher response and recovery time, while the sensitivity is slightly higher for the nanowires-based sensor. Theoretical studies have been performed by ab-initio simulations in NO2 environment. They have revealed that the sensing mechanism is driven almost exclusively by competitive adsorption between NO2 and atmospheric oxygen mediated by temperature change. The influence of the NO2 on the electronic properties of ZnO has been assessed and it is in accordance with the experiments. Our future work is the investigation of other materials for the development of sensing nanodevices targeting to develop more sensitive nanosensors in the same or lower cost. Additionally, the investigation of other growth techniques that could develop more complicated structures in low cost is another point of interest for the future

Thin-film transistors fabricated using sputter deposition of zno

Development of thin film transistors (TFTs) with conventional channel layer materials, such as amorphous silicon (a-Si) and polysilicon (poly-Si), has been extensively investigated. A-Si TFT currently serves the large flat panel industry; however advanced display products are demanding better TFT performance because of the associated low electron mobility of a-Si. This has motivated interest in semiconducting metal oxides, such as Zinc Oxide (ZnO), for TFT backplanes. This work involves the fabrication and characterization of TFTs using ZnO deposited by sputtering. An overview of the process details and results from recently fabricated TFTs following a full-factorial designed experiment will be presented. Material characterization and analysis of electrical results will be described. The investigated process variables were the gate dielectric and ZnO sputtering process parameters including power density and oxygen partial pressure. Electrical results showed clear differences in treatment combinations, with certain I-V characteristics demonstrating superior performance to preliminary work. A study of device stability will also be discussed

Engineering Si-compatible materials based on transparent nitrides and conductive oxides (TNCOs) for broadband active plasmonic and metamaterials applications

Alternative plasmonic materials of Transparent Nitrides and Conductive Oxides (TNCOs) including Indium Tin Oxide (ITO), Al-doped ZnO (AZO) and Titanium Nitride (TiN), have been proposed as novel material platforms for Si-compatible plasmonics and metamaterials, showing enhanced light-matter interaction over a broad spectral range. It has been recently shown that these materials feature reduced optical losses compared with conventional noble metals such as Au and Ag in the visible and near-infrared spectral range. However, it is still an open challenge to tailor the structural and optical properties of these materials, and to further reduce their optical losses, in order to effectively utilize them in photonic devices. In this thesis work, I demonstrate wide tunability of the optical and structural properties of ITO, AZO and TiN thin films, by using post-deposition annealing treatments, enabling significant reduction of their optical losses. By measuring the optical bandgaps of the investigated materials, I show that the tunability of the optical properties originates from the modulation of the free carrier concentration induced by the annealing treatment. Moreover, I perform XRD characterization of the fabricated films, indicating that the annealing also effectively tunes the grain size, which is consistent with the change of the optical properties. Eventually, I investigate the role of the annealing gases for ITO and AZO, demonstrating that free-carrier modulation in ITO and AZO is due to the change in the density of oxygen vacancies after post-deposition annealing. In particular, TNCOs possess epsilon-near-zero (ENZ) condition in near-infrared range with optical loss ε^"<1, thus providing enhanced internal fields in the medium at the ENZ condition. In collaboration with Prof. Nader Engheta and the previous post-doc in our group Dr. Antonio Capretti, we demonstrate enhanced second-harmonic generation (SHG) and third-harmonic generation (THG) from ITO thin films driven by ENZ condition. It results that the SHG generation efficiency is comparable with that of a crystalline quartz plate of thickness 0.5 mm, and that the THG generation efficiency is ∼600 times larger than crystalline silicon. As an application for the fabricated TiN material, I investigate PL intensity and lifetime in Hyperbolic Metamaterials (HMMs) coupled with emitting Si Quantum Dots (QDs). In collaboration with Hiroshi Sugimoto in Prof. Minoru Fujii’s group and the previous post-doc in our group Dr. Sandeep Inampudi, we demonstrate up to 1.6-times enhanced decay rate of QDs emission. Photonic devices based on TNCO plasmonic materials offer an effective approach for the engineering of novel Si-based photonic devices with enhanced light-matter coupling over a broad spectral range. As an application for the fabricated ITO, in collaboration with Hongwei Zhao in Prof. Jonathan Klamkin’s group, electro-absorption modulators are numerically investigated to show high extinction ration of greater than 6dB, while insertion loss is less than 1.3dB for wavelength range from 1.25 µm to 1.42 µm. Additionally, we demonstrate tunable optical properties of ITO thin films in mid-infrared spectrum by thermal annealing of ITO in oxygen environment. In collaboration with Sajan Shrestha and Adam Overvig in Prof. NanFang Yu’s group, we fabricate 2D periodic arrays of ITO and show wide tuning of plasmonic resonances of ITO nanostructure from 4 µm to 10 µm. Combining with the tunability of ITO thin films in near-infrared, the ITO material platform provides a promising method for the control and engineering of Si-based tunable plasmonic and metamaterial devices in the infrared spectrum. Finally, in collaboration with my colleague Ren Wang, we experimentally demonstrate silicon nanodisk arrays with tunable anapole mode excitation in the visible spectrum. The proposed high index nanostructures can be used to enhance absorption rate for applications in semiconductor photodetector

Effects of Hydrogen Plasma on the Electrical Properties of F-Doped ZnO Thin Films and p-i-n -Si:H Thin Film Solar Cells

1.5 wt% zinc fluoride (ZnF2) was mixed with zinc oxide powder to form the F-doped ZnO (FZO) composition. At first, the FZO thin films were deposited at room temperature and 5×10-3 Torr in pure Ar under different deposition power. Hall measurements of the as-deposited FZO thin films were investigated, and then the electrical properties were used to find the deposition power causing the FZO thin films with minimum resistance. The FZO thin films with minimum resistance were further treated by H2 plasma and then found their variations in the electrical properties by Hall measurements. Hydrochloric (HCl) acid solutions with different concentrations (0.1%, 0.2%, and 0.5%) were used to etch the surfaces of the FZO thin films. Finally, the as-deposited, HCl-etched as-deposited, and HCl-etched H2-plasma-treated FZO thin films were used as transparent electrodes to fabricate the p-i-n α-Si:H thin film solar cells and their characteristics were compared in this study. We would show that using H2-plasma-treated and HCl-etched FZO thin films as transparent electrodes would improve the efficiency of the fabricated thin film solar cells