Study of temperature effects on thin-film microstructures and electrical properties of organic-based thin-film transistors

• Main Authors: Chuan-chun Tai, 戴川鈞
• Other Authors: Horng-Long Cheng
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/77237955359317803939

碩士 === 國立成功大學 === 光電科學與工程研究所 === 96 === This study investigates the influence of environmental temperature on the electronic characteristics of pentacene-based organic thin-film transistors (OTFTs). The measuring temperature varied from room temperature (RT) up to 400 K. During the measurement processes of OTFTs, the following issues are discussed in detail, including (i) the influence of the thermal energy from measuring environments on the molecular hopping transport parameters, that is, intermolecular interactions and molecular reorganization energy; (ii) the defect states generated from the changing measuring temperature; (iii) long-term gate-bias stress due to continuous and repeated operation of the devices; (iv) annealing effects due to the long period of operation at high temperatures and the subsequent structural rearrangement of pentacene film. The corresponding microstructural changes of pentacene films as a function of temperature were investigated by variable-temperature Raman and absorption spectroscopy. Additionally, we studied the capacitance-voltage behavior, which can give information about mobile charges and trapping processes in the devices. The results reveal that the measuring temperature has a significant influence on electronic characteristics of OTFT devices. The charge mobilities increased with increasing temperature until 360 K and then became saturated. At the same time, the threshold voltage, output on-current, and off-current all increased with increasing temperature. It was found that the hysteresis effects in transfer curves were enhanced with increasing temperature, thus suggesting that more defect states were generated at high temperature, which then decreased the charge-transport efficiency. From varying temperature capacitance-voltage measurements of metal-silicon dioxide-metal (MIM) and metal-silicon dioxide-pentacene-metal (MISM) diodes, the MIM diodes showed no hysteresis between forward and reverse bias sweeps, even at high temperature. To the contrary, the MISM diodes showed significant hysteresis at higher temperature. The results indicated that the defect states are located in pentacene itself and in the interface between pentacene/dielectric and act as a dominating factor in high-temperature-transfer electrical characteristics of pentacene OTFTs. Additionally, the effects from annealing and gate-bias stress on pentacene OTFTs were not obvious as compared to that of environment temperature. Raman and absorption spectra indicated that the intermolecular interactions between pentacene molecules in films decreased with increasing temperature and are thus unfavorable for charge transport. In summary, we still obtained increased charge mobilities at higher temperatures, thus suggesting the thermal energy from the environment governs the electric transport characteristics in pentacene-based OTFTs. Furthermore, we have obtained similar experimental results in n-type copper hexadecafluorophthalocyanine- based OTFTs.