Pentacene Thin-film Transistors Fabricated with a Simple Solution Process
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0937-M10-32
Pentacene Thin-film Transistors Fabricated with a Simple Solution Process Yutaka Natsume1,2, and Takashi Minakata1,2 1 Asahi-KASEI corporation, 2-1 Samejima, Fuji, Shizuoka, 4168501, Japan 2 Optoelectric Industry and Technology Development Association, AIST Central 4th, 1-1-1 Higashi, Tsukuba, Ibaraki, 3058562, Japan
ABSTRACT We have succeeded in developing a simple solution process of pentacene thin films without particular precursor materials. The solution-processed pentacene thin-film transistors (TFTs) were fabricated and exhibited high transfer characteristics with the carrier mobility of 0.8 cm2/Vs and on/off ratio of exceeding 105. The maximum carrier mobility above 1 cm2/Vs was also confirmed by improving the quality of the film. We observed the surface morphology and the crystal structure of the films by several analyses. The solution-processed thin films have a molecular orientation similar to the single crystal, and high crystallinity determined with both inplane and out-of-plane X-ray diffraction methods. The surface morphology observed with scanning electron microscopy and atomic force microscopy shows a large plate-like structure. We have measured the transfer characteristics of the pentacene TFTs. The solution-processed TFTs indicated a steeper subthreshold swing than the vacuum-deposited films. The trap states in the films were evaluated from the transfer characteristics by using the analytical model. It was revealed that the trap density of the solution-processed thin film was lower than that of the vacuum-deposited film. In addition, the solution-processed thin-film transistors indicated high stability of the threshold voltage during the storage for a couple of months under dark condition in the air, while the threshold voltage of the vacuum-deposited films exhibited large negative shifts. These shifts were assumed to relate with the decrease of the bulk carrier in the films. We considered that the steep subthreshold swing and the air stability of the solution-processed films could be attributed to the high crystallinity and the formation of large grain in the films. INTRODUCTION Organic thin-film transistors (OTFTs) have been expected various applications such as flexible displays [1], smart cards [2], and gas sensors [3]. Pentacene was considered as one of the most promising candidate among organic semiconductors. However, vacuum deposition has been used as a fabrication method of OTFTs of small molecules such as pentacene until now. On the other hand, solution processes with pentacene precursors have been presented [4,5]. These methods require molecular conversion under 200 °C after coating on the substrate. OTFTs fabricated by precursor method exhibit relatively good performances. In contrast, we have prepared OTFTs by direct solution process without precursors [6]. It is known that pentacene is soluble in trichlorobenzene. But if pentacene is dissolved in the solvent in the air, pentacene is oxidized to form pentacenequinone. The π-conjugated system is broken by formation of C=O
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