Study On Organic Thin Film Transistors And Microcavity Top-emitting Organic Light Emitting Devices

Organic thin film transistor (OTFT) and Microcavity top emitting organic light emitting devices are highlighted nowadays in the research of flat panel display. Compared with the inorganic thin film transistors, OTFT have attracted much interest because of their characteristics of low cost, simple techniques, large area preparation, lower preparation temperature, and compatible with the flexible substrates. Introducing the optical microcavity into OLEDs, we can change the spectra, narrow the spectral width, obtain good homochromatic light-emission and also the light-emitting efficiency can be enhanced. TOLEDs with cavity structure combined with TFT can improve display quality even without sacrificing aperture ratios of pixels. So it will be easy to get large area active-matrix display. In this dissertation, we studied these two types of devices.To carry out the study of the thin film transistors, first we introduce the structure and the theory of OTFT. Then we improve the techniques of preparation and obtain the devices with higher performance. The details of the fabrication techniques of OTFTs and their performances can be described as under:1. We have investigated a double-layer structured gate dielectric for the organic thin films transistor (OTFT) with the purpose of improving the performance of the SiO₂ gate insulator. Copper phthalocyanine(CuPc) was used as an active layer. PMMA/SiO₂ used as organic/inorganic insulator. The structure of device was: Si/SiO₂ (280nm)/PMMA (50nm)/CuPc (40nm)/Au, compared with the device using SiO₂ gate insulator, the results demonstrated that using inorganic/organic compound insulator as the gate dielectric layers is an effective method to fabricate OTFTs with improved electrical characteristics and decreased leakage current and threshold voltage. Electrical parameters such as, carder mobility and decreased leakage current by field effect measurement are 2.0×10^-3cm²/Vs and 2.4×10¹⁰A, respectively. We also introduced the preparation techniques of SiO₂/SiNx/SiO₂ compound insulator. We studied the thickness of the compound films and observed that the carder mobility decreased with increasing thickness. 2. The performance of OTFTs that were modified by silane coupling agentsoctadecyltrichlorosilane (OTS) has been investigated. We introduced the mechanism and process of the modification. The parameters such as field-effect mobility, on/off ratio and threshold voltage of the modified devices are 2.2×10^-3cm²/Vs, 8×10⁴ and -8.1V, respectively. We conclude that silane coupling agents-octadecyltrichlorosilane (OTS) can reduce the surface energy of the SiO2 gate dielectric and significantly improve device performance. This OTS/SiO2 bilayer gate insulator configuration increases the field-effect mobility, reduces the threshold voltage, and improves the on/offratio at the same time. We also investigated the device performance on different thickness of CuPc and found that the best thickness of CuPc was 40nm.3. The influences of different electrodes on the performance of OTFTs were also investigated by using different metal materials. We discussed the work function of different metals matching with CuPc based OTFTs. For reducing effectively the contact resistance and enhancing the charge-injection in OTFTs, a promising approach is the insertion of a layer of organic material with high charge density between source/drain contacts and organic semiconductor. We inserted MoO3 between Al and CuPc and studied the performance of Au, MoO₃/Al, Al electrodes devices. Using this strategy, the injection barrier was lowed effectively and probability of charge tunneling was increased. There is an apparent improvement in the performance of OTFTs can be observed by this approach.To study the TOLEDs with cavity structure, we first introduce the structure and the theory of TOLEDs. The emission intensity and full width at half maxima (FWHM) of spectrum have been studied. RBG tricolor TOLEDS are achieved by adjusting the thickness of ITO. The details of the fabrication of microcavity TOLEDs and their performances can be described as under:1. First, ITO thin film was fabricated. We made aluminum and silver thin films as semi-transparent cathode. The transmittance of the semitransparent cathode was about 30%. The blue and green top-emitting organic light-emitting devices with cavity effect were fabricated. We have used the TBADN: 3% DSAPh and Alq₃ as emitting material for microcavity OLEDs. By changing the thicknesses of ITO, different colors were obtained with Alq3-based devices where as highly saturated colors were obtained on TBADN:3%DSAPh devices. When the thickness of ITO was 155 nm, TBADN: TBPe based TOLED exhibited a narrowed EL peak at 464 nm and (CIEx,y) of (0.141, 0.049). When the thickness of ITO was 210 nm, the Alq3 based TOLED exhibited a narrowed Electroluminescent (EL) peak at 536 nm with a full width at half maximum (FWHM) of 22 nm, (CIEx,y) of (0.229, 0.729) and an efficiency of 1.77cd/A.2. With Alq:DCJTB/TBADN:TBPe/Alq:C545 as white light-emitting layer, RBG tricolor TOLEDS were achieved by adjusting the thickness of ITO. We obtained the peak wavelengths at 603nm, 475nm and 538nm, CIE coordinates of (x=0.513, y=0.360),(x=0.133, y=0.201) and (x=0.335, y=0.567), FWHMs of70nm, 30nm and 48nm for red, blue and green, respectively.