Additive Enhancements for Solution Processed Metal Oxide Thin Film Transistors
Solution processed metal oxide semiconductors have attracted much attention as a promising class of materials to be used as the channel material in thin film transistors due to its transparency, high mobility, scalability, and low cost of manufacturing. Nevertheless, there are still major challenges in terms of processing, device performance and stability that need to be overcome before this process can be implemented for large scale production. In this thesis, several chemical additives and their effects on film formation, processing temperature and electrical parameters such as field effect mobility, on-off ratio and threshold voltage shifts under PBS stress tests were investigated. In particular, the addition of ethylene glycol, acetylacetone, and acetic acid were investigated in a metal oxide precursor solution. It was demonstrated that ethylene glycol significantly improved the wettability of the concentrated IGZO solution and resulted in a minimal contact angle of 3.8 degrees. This allowed for coating a concentrated metal
oxide precursor solution (0.5M) five times the baseline concentration while still maintaining high film formation quality. The addition of acetylacetone allowed for low annealing temperatures (less than 300ï¿½C) through the combustion synthesis route and serves as a protection group to prevent premature formation of metal oxide network in solution. Finally, the addition of acetic acid improved the solubility of the metal precursor in the solution and allowed for higher concentrations of metal precursor to be dissolved in solution, which becomes important if higher viscosity precursors for thick films are needed. The addition of these additives produced devices with near zero turn on voltage, excellent on-off ratio (>107), and superior stability (less than eight volts of threshold voltage shift at 10,000 seconds of PBS). The findings in this thesis present improved synthesis routes for solution processed semiconductors and open new possibilities for the fabrication of flexible electronic devices and next generation large scale consumer electronics.