This thesis presents the development of low-voltage organic thin-film transistors with high transconductance. This was achieved by employing ultra-thin bi-layer gate dielectric consisting of aluminium oxide (AlOx) and a self-assembled monolayer of octadecyl phosphonic acid (C18PA) and by increasing the channel width of the transistors through the implementation of the multi-finger source/drain contacts. The transistors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) exhibited low turn-on voltage and a.c. transconductance around 30 to 60 µS. Transistor amplifiers based on such transistors exhibited voltage gain approaching 10 V/V and a gain of about 2 V/V when the supply voltage was limited to 5 V. Next, a series of [n]phenacenes ([n] = 5, 6, or 7) was used for the first time in combination with the thin AlOx/C18PA dielectric bi-layer. Regardless of the substrate and the source-drain contact geometry, the field-effect mobility of such transistors was found to increase with increasing length of the conjugated [n]phenacene core, leading to the best performance for [7]phenacene with the largest average field-effect mobility of 0.27 cm2/V⋅s for transistors on glass and 0.092 cm2/V⋅s for transistors on flexible PEN. The highest transconductance of 12.2 µS was achieved for [7]phenacene transistors on glass, which was lower than that achieved for DNTT transistors. In addition, nearly hysteresis-free behaviour, improved charge carrier injection/extraction properties, and reduced threshold voltage were achieved. Finally, a semi-empirical transistor model was developed in Matlab. The model was validated using d.c. and a.c. measurements obtained on DNTT transistors with high transconductance. Four fitting parameters were extracted by optimising a fitting function using genetic algorithm. The model reproduces the d.c. transistor measurements with high accuracy. The error between the measured and simulated peak-to-peak a.c. transconductance values ranged from 1.7% to 11.6%.
| Date of Award | 6 Feb 2020 |
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| Original language | English |
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| Awarding Institution | - University Of Strathclyde
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| Sponsors | University of Strathclyde |
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| Supervisor | Helena Gleskova (Supervisor) & John Soraghan (Supervisor) |
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