Study On Performance Improvement Mechanism Of Pentacene Based On Organic Thin Film Transistors

In this doctoral dissertation,the ways to enhance and improve the performance of pentacene-based OTFTs(organic thin film transistors) are investigated by using different kinds of gate insulators and different interface modified-layers,and by controlling the phase-transformation and crystallization of pentacene polymorphic film and the particle size of pentacene grains.The mechanism of enhancements is explained. In addition,the effect of bending-strain on structure and electrical characteritics of the flexible devices,and the operation and improvement of ambipolar OTFTs with C₆₀/pentacene active layer at room temperature and atmosphere are studied too.The details as follows:Firstly,in order to investigate the effect of defect trapping at the interface between the semiconductor active layer and the gate insulator layer of OTFTs,the surface of gate insulator is treated by using different interface modified-layers,such as octadecyltrichlorosilane(OTS) and phenyltrimethoxysilane(PhTMS).The microstructure phase-transformation mechanism of the pentacene polymorphic film induced by modified-layers is investigated,and then the carrier transport mechanism in trap states under gate-source voltage is explained.It is the good way to improve the performance of OTFTs.Secondly,in order to study further the characteristics of OTFTs,we also investigate the effect of dielectric constant and kinds of gate insulator layers on the performance of OTFTs.The works mainly focus on OTFTs with different gate insulators including inorganic,organic and organic/inorganic complex insulator.The effect of different insulators on the growth mechanism of pentacene thin film is compared.Inorganic gate insulator layers,such as SiO₂,SiNx and SiOxNy,of OTFTs are fabricated by ratio frequency(rf) magnetron sputtering,electron beam evaporation and thermal oxidation methods,respectively.The organic gate insulator layers such as poly(4-vinylphenyl) (PVP) are fabricated by spin coating method.It is found that the performance of OTFTs with PVP/SiO₂ complex gate insulator is improved greatly.The hot dispute about the effect of pentacene particle size on the performance of OTFTs is explained through the comparation of OTFTs with treated PVP/SiO₂ gate insulator and with single SiO₂gate insulator.Thirdly,in order to investigate the effect of bending-strain on the performance of flexible OTFT devices and the carrier transport mechanism in bending-strained devices, the flexible pentacene-based OTFTs with bottom gate top conduct structure are fabricated on the flexible ITO-PET substrate.PVP,pentacene and Au are used as the gate insulator,the active layer and the source-drain(S-D) electrode,respectively.When OTFT devices are repeatedly bended with small radius of curvature about 0.9-1.1 cm, the device performance has been reduced.The main reason is that bending-strain induces the microstructure phase-transformation of the pentacene polymorphic film from "thin-film phase" to "triclinic bulk phase",which results in the performance degradation of OTFTs.These results for the application of OTFT device at the flexible display field provide a good theoretical and experimental basis.Finally,in order to study the carrier transport mechanism of OTFTs and the application of OTFTs at digital integrated circuit complementary inverter,the ambipolar characteristics of complex C₆₀/pentacene-based OTFTs are investigated.A good am-bipolar performance of OTFTs is achieved at room temperature and atmosphere by using an ultra-thin electrode modified-layer(LiF) between pentacene and S-D electrodes to adjust the injection characteristics of charge carriers.Then in order to further optimize the ambipolar characteristics of devices and improve the carrier transport of OTFT devices,an ultra-thin electron blocking-layer N, N'-bis-(1-naphthyl)-N,N'-biphenyl-1,1'-biphenyl-4,4'diamine(NPB) is inserted between C₆₀ and pentacene at the basis of the above mentioned devices to adjust the distribution of charge carriers and to limit charge carriers captured by different type semiconductor materials.Therefore,the carrier transport in the am-bipolar devices is improved and results in the good performance of OTFT device.