Inkjet printed metal oxide thin film transistors incorporating polyethyleneimine

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Kemian tekniikan korkeakoulu | Master's thesis
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Master's Programme in Chemical, Biochemical and Materials Engineering
Fully inkjet printed fabrication of thin film transistors (TFTs) is desirable to enable reproducible, high throughput, low cost production of electronics under mild conditions. However, TFT devices fabricated from printable materials typically exhibit inferior performance to those of non-printed. For example, there is a lack of well performing source-drain electrode materials for metal oxide semiconductors. In contrast to vacuum-deposited Al, printed Ag has a high contact resistance and work function, with poor charge carrier injection to the semiconductor. Therefore, there is a requirement to improve electrical performance of TFTs incorporating printed electrode material such as Ag. The approach to achieve this through this work is by inclusion of an inkjet printed thin film of polyethyleneimine (PEI) at the interface between semiconductor and source-drain contacts. PEI contains tertiary amine groups, which possess lone pairs of electrons available to assist charge injection and lower the interfacial resistance. Two sets of reference devices were prepared, both with inkjet printed In2O3 semiconductor. One set was fabricated by vacuum deposition of Al for source drain electrodes, the other set with inkjet printed Ag source drain contact electrodes. The solution-based processing method limits the thermal budget to 300°C. Devices with Al electrodes provided charge carrier saturation mobility (μsat) of 4.3 ± 0.93 cm2 V-1 s-1, whereas those with Ag contacts exhibited an expected lower μsat of 8.0·10-3 ± 3.9·10-3 cm2 V-1 s-1. Addition of an inkjet printed interfacial thin film containing PEI between the semiconductor and Ag contact electrodes significantly increased the μsat to 3.1 ± 0.53 cm2 V-1 s-1. Interfacial engineering in this work yielded TFTs possessing printed Ag contacts that display electrical performance comparable with devices incorporating vacuum-deposited Al contacts. The impact of this result is the possibility for high performance fully printed TFTs. Development of this fabrication route might facilitate low temperature solution based roll-to-roll production of electrical components containing fully inkjet printed TFT devices.
Karttunen, Antti
Thesis advisor
Alastalo, Ari
polyethyleneimine, indium oxide, thin film transistor, inkjet printing
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