Optimization of Alkyl Side Chain Length in Polyimide for Gate Dielectrics to Achieve High Mobility and Outstanding Operational Stability in Organic Transistors

Abstract

Alkyl chain modification strategies in both organic semiconductors and inorganic dielectrics play a crucial role in improving the performance of organic thin-film transistors (OTFTs). Polyimide (PI) and its derivatives have received extensive attention as dielectrics for application in OTFTs because of flexibility, high-temperature resistance, and low cost. However, low-temperature solution processing PI-based gate dielectric for flexible OTFTs with high mobility, low operating voltage, and high operational stability remains an enormous challenge. Furthermore, even though di-n-decyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) is known to have very high mobility as an air-stable and high-performance organic semiconductor, the C10-DNTT-based TFTs on the PI gate dielectrics still showed relatively low mobility. Here, inspired by alkyl side chain engineering, we design and synthesize a series of PI materials with different alkyl side chain lengths and systematically investigate the PI surface properties and the evolution of organic semiconductor morphology deposited on PI surfaces during the variation of alkyl side chain lengths. It is found that the alkyl side chain length has a critical influence on the PI surface properties, as well as the grain size and molecular orientation of semiconductors. Good field-effect characteristics are obtained with high mobilities (up to 1.05 and 5.22 cm2/Vs, which are some of the best values reported to date), relatively low operating voltage, hysteresis-free behavior, and high operational stability in OTFTs. These results suggest that the strategy of optimizing alkyl side-chain lengths opens up a new research avenue for tuning semiconductor growth to enable high mobility and outstanding operational stability of PI-based OTFTs.

Double Alkyl Side Chains Functionalized Polyimide as Gate Dielectrics for Application in Organic Transistors

“…The −CH 2 – symmetric and asymmetric stretching vibration peaks near 2850 and 2926 cm –1 in Figure b show a subtle blue shift as the alkyl side chains grew. This trend is also been observed similarly in the previous report, indicating that longer alkyl chains have higher compression pressures …”

“…The −CH 2 − symmetric and asymmetric stretching vibration peaks near 2850 and 2926 cm −1 in Figure 3b show a subtle blue shift as the alkyl side chains grew. This trend is also been observed similarly in the previous report, 21 indicating that longer alkyl chains have higher compression pressures. 27 In addition, thermal stability is one of the important properties to evaluate PI materials, which was investigated with TGA and DSC.…”

“…This property stems from the properties of −CH 2 −/−CH 3 , which features the low polarizability of the alkyl chain. Further, the double alkyl side chains in the API structure tend to distribute the surface of the film, 21 which can be confirmed by the optimized 3D structures of repeating units in Figure S5. Therefore, API dielectric materials with double alkyl side chains can impede trap creation, resulting in good electrical properties and low threshold voltages.…”

“…They found that the modified API can reduce the surface energy and induce a high crystal quality of semiconductors at the interface, thereby enhancing OTFTs performance. Recently, Wang et al synthesized a series of soluble APIs containing different lengths of alkyl side chains and systematically investigated the APIs’ surface properties and the evolution of organic semiconductor morphology deposited on APIs surfaces during the variation of alkyl side chain lengths. As a consequence, optimization and selection of the proper alkyl chain length of the API structure can yield high mobility and outstanding operational stability OTFTs.…”

A review on diverse streams of interface engineering for organic thin-film transistors

“…It is worth noting that the recent years have witnessed remarkable achievements in the electrical performance of OTFTs (Table 1). 9–25…”