Deposition of transparent conducting oxide coatings via ultrasonic spray pyrolysis
Creators: name: Kim, Jaewon
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 401807
subject: Nanomaterials

description: Over the past few decades, the demand for transparent conducting oxide (TCO) thin- film electrodes has significantly increased due to the widespread diffusion of optoelectronic devices such as smartphones, touchscreen displays, and solar panels. In addition, the market for smart windows, of which TCOs are a key component, has expanded as well due to an increased demand for energy efficient buildings. Tin-doped indium oxide (known as ITO) is the most used material for transparent electrodes within optoelectronic devices. However, the manufacturing of ITO thin films is very expensive because of the cost of raw materials (indium) and the cost of fabrication processes (sputtering). This has crucially limited the mass production of large-area and inexpensive devices. Therefore, many researchers have developed TCO materials for commercial applications using alternative precursors and also liquid-based deposition methods. In addition to reducing the cost of the raw materials, wet-chemistry processes can potentially be simpler, more cost efficient, and still produce TCO thin films of high quality in terms of electrical and optical properties.

This PhD project has focused on the development of protocols to deposit high quality TCO thin films using wet-chemistry methods, based specifically on the sol-gel technique. My research targets are the development of highly conductive, highly transparent coatings composed of widely available and possibly non-toxic elements, through a fabrication method called ultrasonic spray pyrolysis, which is operatively simple, and applicable to large area coatings (and therefore scalable). With these aims in mind, I have decided to focus initially on tin oxide (SnO2), a well-known and well-studied material for application in transparent electrodes. Undoped SnO2 is a wide-band gap n-type oxide with moderate electrical conductivity. When appropriately doped, its outstanding conductivity makes it the most interesting alternative to traditional indium-based oxides also because of its chemical inertness and high temperature resistance.

Another goal for this project is to fabricate and understand the properties of a p-type material, CuCrO2, which is an emerging TCO. Notably, the electrical and optical properties of p-type TCOs are generally lower than those of n-type TCOs, and the low electrical conductivity leads to their poor performance within optoelectronic devices. If appropriately fabricated, p- type TCO are the key to unlock a large variety of devices including all-oxide transparent electronics.
Therefore, in this doctoral project, I have developed a detailed investigation on the synthesis of thin films of both n-type (doped SnO2) and p-type (CuCrO2) materials using spray pyrolysis. Previous studies on doped SnO2 and CuCrO2 coatings (including a few selected results on films deposited by ultrasonic spray pyrolysis) have individually and unsystematically explored the structural, electrical and optical properties of the deposited films by altering some of the key parameters including dopant concentration, film thickness and deposition temperature. However, a comprehensive investigation of these parameters and a deep understanding of their role is still required to generate reproducible, high performance SnO2 and CuCrO2 films.

In the first experimental part of this thesis, the fabrication of antimony-doped tin oxide (ATO) films is highlighted. Spray deposited ATO films with high conductivity and optical transparency were achieved. Higher deposition temperatures favour the formation of coatings with higher crystallinity, and better electrical and optical properties. The best conductivity is achieved for antimony concentration of ~1%. This is the result of the combination of optimal carrier concentration and carrier mobility. Higher antimony content leads to reduced transparency, which is detrimental for the optoelectronic performance of ATO films. Champion films achieved excellent optical transparency in the visible region (transmittance of 80-90%), high conductivity (sheet resistance of the order of 10-20 Ω/sq) and a very high Haacke figure of merit of ~16.5 (mΩ/sq)-1.

The second experimental part is based on the study of doping fluorine into SnO2 films. The deposition temperature was again found to have a key role in controlling the properties of fluorine-doped SnO2 (FTO), specifically its optoelectrical and nanomechanical properties. The higher deposition temperature leads to the formation of larger crystalline grains which enhance carrier mobility and also improve the mechanical properties. Notably, fluorine concentration in SnO2 did not greatly influence the structural and mechanical properties, however it has a marked effect on the electrical and optical properties. I achieved the record electrical resistivity of 7.3 x 10-4 Ω cm, excellent visible transparency ~ 93.7 % (peak visible transmittance), an outstanding Haacke figure of merit of 35 (mΩ/sq)-1, and improved mechanical properties (elastic modulus of 168.8 GPa and hardness of 14.6 GPa, both higher than reported values for similar FTO coatings).
The last part of the thesis is focused on studying the optoelectronic properties of p- type delafossite CuCrO2 coatings fabricated by spray pyrolysis. CuCrO2 has been fabricated using a variety of methods such as sputtering, pulsed laser deposition (PLD), and chemical vapour deposition (CVD) methods. However, these methods are slow, require an additional post-annealing, and involve expensive equipment. Here I take advantage of the flexibility and versatility of the ultrasonic spray pyrolysis method to fabricate high-quality CuCrO2 films. Structural, optical, and electrical properties of CuCrO2 films were investigated by varying the deposition conditions and the Cu/Cr ratio. The crystallinity of CuCrO2 films depends again upon the deposition temperature, which has a strong effect on the crystal orientation and on the overall optical and electrical properties of the films. In addition, the optical and electrical properties of CuCrO2 coatings can also be influenced and tailored to the desired application by varying Cu and Cr amount within the sprayed precursor solution. In particular, the concentration of copper in the precursor was directly linked to the increase in carrier concentration. As a result, an increase in the copper content resulted in an increase in electrical conductivity. Through the work conducted in this part of my PhD project I obtained CuCrO2 coatings with the highest values of conductivity for solution-processed films (103 S/cm), very high carrier concentration (~1022 cm-3), good transparency up to 68%, and excellent figure of merit values for p-type TCOs (Haacke figure of merit: ~2.8 x 10-7 (Ω/sq)-1 and Gordon figure of merit: 2040 (µS)).

Overall, this thesis has demonstrated the deposition of high quality n-type and p-TCO thin films using ultrasonic spray pyrolysis from sol-gel precursor solutions. The structural, electrical, and optical performance of sprayed TCO coatings have demonstrated competitive advantages over vacuum-based deposition techniques. The coatings prepared here have applications in various optoelectronic devices as n- and p-type transparent conductive oxides, including solar cells, LEDs, smart windows and touchscreen panels.