Ultra-thin transparent electrodes for energy applications

Abstract

Due to the unique feature of being contemporarily optically transparent and electrically conducting, Transparent Conductive Oxides (TCOs) play a fundamental role in many technologies: communications, information, energy, buildings. Up to now, the most diffused material in the TCO s family was indium tin oxide (ITO), especially for large-area applications such as flat panel displays. Recently, the increasing expansion of the display market and, even more, of photovoltaics, are endangered by the scarcity and rising price of indium. This is one of the reasons and a strong motivation for searching alternative transparent electrodes, not necessarily oxides, to replace ITO. Moreover, in order to meet the expectations for the growing demand and lower production costs for photovoltaic and electronic applications, new TCOs, or equivalent materials, must be abundant, not expensive and very thin, so to be suitable for flexible electronics. Among new transparent electrodes candidates, Aluminium-doped zinc oxide (AZO) films and very thin multilayers of AZO/Ag/AZO have emerged as a very promising alternative . In particular, AZO films are Indium-free and show electro-optical properties comparable to ITO films of the same thickness (700-900 nm for industrial applications), especially after thermal annealing at 250 °C. On the other hand, AZO/Ag/AZO multilayer structures, 10 times thinner than ITO or AZO single layers, show very high transparency and low resistivity even at room temperature. Replacing thick TCO layers with thin TCO/Ag/TCO multilayers would produce great benefits in terms of material consumption, cost, toxicity and flexibility (a mandatory point for the development of the electronics on plastic). In this context, it is important the study and understanding of the fundamental properties of these materials, the process conditions and post fabrication treatments to optimize their application to different fields. Transparent conductive materials are known since about 100 years and represent one of the strategic topics for the actual industrial research, but still many fundamental properties and mechanisms need to be clarified and explained. Aim of this work is the fabrication, processing and characterization of ultra-thin AZO and TCO/Ag/TCO transparent electrodes. The study focused on the optimization of structural, optical and electrical properties for application in photovoltaics. The thesis is organized as follows. Chapter 1 introduces TCOs and covers the conventional and non materials history, properties, applications and market. Chapter 2 starts with a comprehensive and detailed study about thick AZO grown by RF magnetron sputtering films on glass substrates, focusing on the influence of sputtering process parameters, i.e. power, temperature substrate, and thermal treatment (during or after the deposition) on the film properties. After the work on thick AZO, we report the modification of optical, electrical and structural properties of very thin films (60 nm) upon ion irradiation with different ion type (O+ or Ar+ ions) and energy (30 and 350 keV) at different ion doses (3, 10, 30E15ions/cm-2), before and after thermal treatments up to 400°C. Chapter 3 treats of very thin TCO/Ag/TCO multilayer structures grown by RF magnetron sputtering. Synthesis and properties of AZO/Ag/AZO multilayers as a function of Ag film thickness, with a fixed AZO thickness (~20 nm), are investigated. Then, we studied multilayers with fixed Ag thickness(~10 nm), but different combinations of AZO and ITO as top and bottom TCO layers. Chapter 4 describes the compatibility of the AZO/Ag/AZO multilayers with one of the most important steps for the implementation in thin film photovoltaic technology: laser scribing. AZO/Ag/AZO multilayers must be able to guarantee the same level of TCO reliability under laser scribing processes. In this study, we used a single nanosecond laser pulse to irradiate AZO/Ag/AZO deposited on glass.