Precursor Chemistry and the CVD of Transparent Conducting Oxides

This project will involve the investigation of alternative precursors and deposition technologies in order to improve performance and enable next generation transparent conducting oxide (TCO) films to be developed.

Indium tin oxide (ITO) is the current TCO of choice for most industrial applications but it has many limitations, such as modest conductivity (2000-4000 S/cm), a relatively low work function and some optical absorption in the blue-green spectral region. In addition, indium is expensive since it is in relatively short supply, which presents a significant challenge for larger-scale production of next generation photovolatic technologies and flat panel displays. It is therefore crucial to develop alternative TCO materials with no indium with improved optical and electrical properties. Alternatives to ITO include doped ZnO (ZnO:Al, ZnO:Ga, ZnO:SnO2) and doped SnO2 (SnO2:Sb, SnO2:F, ZnO-SnO2) and many of these have been investigated in their bulk form. However, studies of some of these materials as thin films is limited and for many of the applications thin films are required. An ideal method for preparing thin fims for large scale applications is chemical vapour deposition (CVD) given that films with good uniformity and compositional control, large area growth and step coverage can be achieved. However, for a successful CVD process, a volatile precursor is necessary which is prefereably a liquid or low metling solid for atmospheric pressure CVD or highly soluble for liquid based (aerosol assisted) CVD. Current precursors to TCO materials, particularly indium and zinc still suffer from chemical instability, poor reproducibility in the growth process and less than favourable vapour pressures and reactivity for film growth.

This work aims to develop highly volatile and soluble precursors based on metal ketoiminates. The advantages of using the ketoiminate ligand include:
- reactive complexes can be formed in high yield
- complexes with a hign vapour pressure can be formed as monomeric species are isolated
- thermal stability of the metal complexes can be increased by tuning the groups attached to the nitrogen atoms
- the surface reaction between the metal precursor and the surface of the substrate can be enhanced due to the high chemical reactivity of the complexes.

TCO materials to be investigated include doped-ZnO and doped-SnO2. We have the ability to lay down thin films using a new combinatorial aerosol-assisted (AA)CVD reactor for solution based and also a combinatorial APCVD reactor to make films of graded composition. This new reactor enables upto 400 different compositions to be made on a single plate in one CVD experiment. This is important as it will enable us to rapidly screen composition space make idealised and optimised compositions for TCO applications. This combined approach will enable us to investigate different combinations and go towards achieving the next generation TCO materials.

This project will have impact through the development of alternative precursors and deposition of thin films of new transparent conducting oxides (TCOs). Solar energy panels offer alternative solutions for a range of energy requirements, for example, Pilkington TCO glass is used in the manufacture of thin plate panels used in the direct conversion of solar radiation to electricity. The thin film solar market is predicted to grow six-fold over the next 5 years to 2GW by 2015. As such, there is significant pressure to supply large volumes of high quality transparent conducting oxides with a range of properties suitable to meet the differing requirements that thin film solar cell manufacturers demand. Solar energy panels offer alternative solutions for a range of energy requirements, from small-scale domestic applications to large-scale solar power stations, from cloudy northern rooftops to hot sunny deserts. TCO films are a key component of these applications and the development of improved TCO films will benefit the public, for example by supplying electricity with a very low environmental footprint. The reduction of fuel costs and the development of improved TCOs will have significant impact on the public sector. With increasing legislation in place via international agreements to reduce greenhouse gases to acceptable levels the development of alternative energy, such as solar-power is clearly of great importance and will reduce the need for fossil fuels.