Study And Fabrication Of Transparent Organic Thin-film Transistors

Since the development of transparent electronics, there have been much moreattention on variety of transparent or translucent in the visible range of optoelectronicdevices. Especially for transparent conductive films and transparent thin filmtransistors, significant advancements have been come true in the past two decades.Due to the potential applications of transparent optoelectronic films and devices in theconsumer electronics, new energy and automotive industries, many domestic andinternational companies and research institutions have invested a lot of researchfunding in this area. However, the performance optimizations of the transparentorganic optoelectronic devices suffer from the incompatible preparation technology oforganic semiconductor materials and inorganic transparent conductive thin films,instability and low mobility. N-type organic semiconductor materials, harshpreparation conditions, and the high cost of equipment. These factors impede furtherdevelopment of transparent organic electronics. This thesis focuses on theexisting problems in the process ofpreparation of transparent organic thin filmtransistors (OTFTs). The performance of the transparent OTFTs has been improved byintroducing room temperature-prepared transparent conductive thin films assource-drain top contact electrodes, suitable design of device structure, and combineduse of organic semiconductor materials and inorganic transparent conductive films.The novel results of this thesis are as following:1. High-performance top-contact transparent OTFTs have been developed by using a WO3/Ag/WO3(WAW) multilayer as the source and drain electrodes. TheWAW electrodes were deposited by thermal evaporation at room temperature，and thedevices were fabricated without sputtering damage and breaking the vacuum. WAWthin films deposited on glass substrate show excellent optical and electrical propertiescomparable to ITO. In addition, patterning process of WAW can be made by lithographyand mask. The WAW film has a higher work function that can match the HOMOorbital of organic semiconductor materials very well. High-performance transparentOTFTs based on WAW electrode have been obtained with mobility of8.44×10-2cm2/Vs, an on/off ratio of about1.2×106, and an average visible range transmittanceof81.5%.2. A transparent conductive film of Ag/LiF is designed and prepared, which wasapplied to the OTFTs based on a N-type organic semiconductor of F16CuPc. AN-typetransparent organic thin film transistor was realized for the first time. On the basis ofthe theoretical simulation, combined with experimental results, the parameters oftransparent conductive films were determined and applied on the N-type OTFTs.Experimental results show that OTFTs employed Ag/LiF films as the transparentsource and drain electrodes have a mobility of0.0131cm2/Vs, a on/off ratio of4.2×106, and an average transmittance of55.6%in the visible spectrum range.3. Ambipolar transparent OTFTs were studied using Sb2O3/Ag/Sb2O3(SAS)multilayer as the source and drain electrodes, and Pentacene/PTCDI-C13heterojunctionas the active layer. After the experiment, the parameters of the SAStransparent conductive films were determined with good photoelectric properties.Ambipolar conductivity behavior was observed in the OTFTs with an average visiblerange transmittance is72%, which is one of the best results to our knowledge. Thetwo kinds of carriers have difference performance, the mobility of hole and electronare0.1cm2/Vs and0.042cm2/Vs, respectively. The difference performance resultsfrom the difference contact resistance between active layer and source-drainelectrodes. The LUMO of PTCDI-C13is much higher than the work function oftransparent electrodes leads to the unbalanced carrier mobilities between hole andelectron.