The processing ability of organic semiconductors from the liquid phase facilitates the development of large scale, low cost organic electronics on various substrates. An organic thin film transistor (OTFT) is one of the most important building blocks for practical application in organic electronics and is the main focus of this thesis. The OTFT fabricated in this work uses TIPS-Pentacene, a widely studied small molecule semiconductor, as the channel material. With the simple drop-casting technique, the performance of the OTFT is not fully optimized. To improve the device performance in terms of device mobility, operation voltage, sub-threshold swing and on-to-off ration, two fabrication methods are adopted. Firstly, the evaporation rate of the material solution is reduced by changing th...[ Read more ]

The processing ability of organic semiconductors from the liquid phase facilitates the development of large scale, low cost organic electronics on various substrates. An organic thin film transistor (OTFT) is one of the most important building blocks for practical application in organic electronics and is the main focus of this thesis. The OTFT fabricated in this work uses TIPS-Pentacene, a widely studied small molecule semiconductor, as the channel material. With the simple drop-casting technique, the performance of the OTFT is not fully optimized. To improve the device performance in terms of device mobility, operation voltage, sub-threshold swing and on-to-off ration, two fabrication methods are adopted. Firstly, the evaporation rate of the material solution is reduced by changing the tightness of the petri dish. With the decrease of the evaporation rate, the channel material tends to grow into a larger grain with less grain boundaries. The device mobility obtained at a slow evaporation rate is increased about ten times than that in the fast evaporation sample. The second method adopted is to pre-pattern the device substrate using hydrophobic dielectric material. The patterned device shows a lower off current due to the decrease of the parasitic interaction between the neighboring devices. The mobility of the device is increased about five times of that on the unpatterned substrate. The experimental results confirm the crucial importance of material morphology of the channel organic semiconductor for charge transport, and thus for the transistor performance.

Alongside the improvement of device performance, the issues related to the device instability are also investigated. The interface traps between the dielectric and semiconductor is one major factor that causes the device operational instability, for example, hysteresis. The characterization of the trapping and detrapping rates are important for the reliable consecutive DC operation and AC operation. In this thesis, the current hysteresis of organic thin film transistors formed by TIPS-Pentacene is firstly demonstrated by bi-directional gate-voltage scan and the observed phenomenon under different sweep rates is explained using the trapping and detrapping mechanism. The trapping rate is characterized by the gate-voltage sampling and the result is verified by the channel charge pumping method. In addition, the gate-voltage sampling method is also adopted to characterize the detrapping time, and the related equations during the detrapping process have been derived. The detrapping rate is also confirmed by another independent charge pumping method. The two independent methods consistently reveal that the hole trapping and release rates at the interface between the channel of the OTFTs to the gate dielectric are asymmetric.