Abstract:In this study, several naphthalene tetracarboxylic acid diimide (NTCDI) 2 derivatives substituted at the N and N' positions with long normal alkyl chains of different lengths were evaluated as soluble n-type organic thin-film transistor (TFT) materials. NTCDI derivatives with diundecyl (NTCDI-C11), didodecyl (NTCDI-C12), and ditridecyl (NTCDI-C13) exhibited acceptable solubility in chloroform, and their TFTs showed typical n-type TFT performance with relatively high field effect electron mobility (~0.2 cm 2 /Vs) after annealing at a workable temperature of 150 °C. Although NTCDI with dioctyl (NTCDI-C8) showed good solubility in chloroform, the TFT performance of this material was highly inferior to that of NTCDI-C11, NTCDI-C12, or NTCDI-C13. We could not anneal NTCDI-C8 thin films at workable temperatures in vacuo because of sublimation of the material from the substrates. In contrast, NTCDI with dipentadecyl (NTCDI-C15) and dioctadecyl (NTCDI-C18) exhibited both poor solubility for chloroform and poor TFT performance. In short, these compounds are not suitable as soluble n-type organic TFT materials.

The authors report that thermal treatment effect on various N , NЈ-dialkyl-3,4,9,10-perylene tetracarbxylic diimides ͓PTCDI-Cn, alkyl-dodecyl ͑n=12͒, butadecyl ͑n=14͒, octadecyl ͑n=18͔͒ thin-film transistors ͑TFTs͒ depends on the substituted alkyl chain length. It is clearly demonstrated that there are two kinds of molecular movements during the thermal treatment on PTCDI films; molecular rearrangement in the same layer and molecular migration from the lower layer to the upper layer. The former is directly related to the grain growth and can be controllable by applying an external electric field. The latter is also related not only to the grain growth but also to the formation of cracks between grains. These two movements show opposite dependence on the alkyl chain length during the thermal treatment; the former is more active in longer alkyl chain, but the latter in shorter one. However, they also have opposite effect to TFT performance, and PTCDI films with longer alkyl chains have great advantage on TFT performance for the thermal treatment. Consequently, PTCDI-C18 TFTs show the highest electron mobility as large as 1.2 cm 2 / V s after the thermal treatment at 140°C.

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In this study, vacuum-evaporated thin films of several naphthalene tetracarboxylic acid diimide derivatives substituted at the N and N′ positions with long normal alkyl chains of varying lengths (NTCDI-Cn) were evaluated as active materials for n-type organic thin-film transistors (TFTs). The electron mobility (μe) of the TFTs increased with increasing chain length from octyl (NTCDI-C8) to pentadecyl (NTCDI-C15); those of NTCDI-C15 and C18 TFTs were of 0.262 ± 0.016 and 0.222 ± 0.016 cm2 V−1 s−1, respectively. However, the threshold voltage of the TFTs increased with increasing chain length.

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