High performance small-molecule organic thin film transistors

by 1971- Kuo, Chung-Chen

iii The roadmap of developing microelectronics has a new branch: organic electronics. Organic electronics, which utilizes the electrical properties of organic materials in the active or passive layers, is an emerging technology that has received much attention. In conjunction with today’s demands for new materials and devices, many technologies have emerged for developing organic electronics and consolidating applications and markets. An organic thin-film transistor is the essential device in this paradigm in addition to organic photodiodes and organic light emitting diodes. This thesis presents advances made in design and fabrication of organic thin-film transistors (OTFTs) using small-molecule organic semiconductors (pentacene, anthradithiophene, and their derivatives) as the active layer with record device performance. In this work OTFT test structures fabricated on oxidized silicon substrates were utilized to provide a convenient substrate, gate contact, and gate insulator for the processing and characterization of vapor-deposited organic materials and their transistors. By developing a gate dielectric treatment using silane coupling agents the performance and yield of pentacene OTFTs was improved and a field-effect mobility of larger than 2 cm2/V-s was achieved. Such device performance is comparable to a-Si:H TFTs and have the potential for electronic applications. In addition, the first direct photolithographic process for top contacts to pentacene OTFTs on oxidized silicon with an acceptable performance (a field-effect mobility of 0.3 cm2/V-s, an on/off current ratio of 107, and a subthreshold slope of 1 V/decade) was developed. The multiple layer photoresist process demonstrated the feasibility of creating source and drain metallic electrodes on vapor-deposited pentacene thin films with a resolution less than 10 µm. Subsequently, solution-processed OTFTs were then investigated and high performance transistors, with field-effect mobilities > 1 cm 2/V-s and an on/off current ratio of 106.5, were developed with a processing-temperature of less than 90 ºC. This record performance was obtained with devices based on triethylsilylethynyl anthradithiophene (TES ADT) and triisopropylsilylethynyl pentacene (TIPS pentacene). iv Chemically modified source and drain contacts and gate dielectric were used to achieve such device performance, a performance that is still limited by the contacts. The studies were extended to several solution deposition methods which included blade coating, bar coating, dip casting, and spin casting. In addition to the above two materials, the design and characterization of four synthesized organic semiconductors with substituted functional groups was carried out. In this work it was found that the stacking of molecules with a large and two-dimensional ?-orbital overlap is crucial in determining its device performance and electron transport. The results of such molecular design approach offer great potential of obtaining suitable materials for large-area, low-cost electronic applications using low-temperature solution-processing technology. It is shown in this work that desirable performance can be achieved from both vapor-deposited and solution-processed OTFTs.