Deposition rate dependent mobility of an organic transistor with an anisotropic polymeric insulator
Graphical abstract
Highlights
► Deposition rate dependent mobility was systematically studied on an anisotropic insulator. ► Initial growth of organic semiconductor was examined as a function of deposition rate. ► Optical anisotropy of organic semiconductor was examined as a function of deposition rate. ► The physical origin why the mobility is dependent on the deposition rate was described.
Introduction
Organic thin-film transistors (TFTs) are in the spotlight as a possible substitute for typical silicon-based TFTs due to many comparative advantages such as mechanical flexibility, including compatibility to plastic substrates, a low processing temperature, and wide areas of applicability with a low cost [1], [2], [3], [4], [5]. At present, the electrical performance of organic TFTs have reached a level comparable to, or surpassed, those of silicon TFTs [6], [7], thanks to the recent progress in the field of organic electronics. Among many organic semiconductors, it is known that pentacene shows the highest mobility.
Since the mobility is directly related to the interface interaction between an organic semiconductor and a gate insulator, much effort has been devoted to improve the electrical performance of a pentacene-based TFT by modifying the interface [8], [9], [10], [11], [12], [13], [14], [15]. These efforts, focused on the interfacial phenomenon, would be macroscopically classified into two types, one of which is a physico-chemical modification and the other methodological (or technical) modifications.
In physico-chemical approaches, due to the fact that a pentacene molecule has a rod-like shape similar to the core structure of liquid crystals (LCs) [17], [18], the structural order in the pentacene film has been vigorously studied. For example, using an additional alignment layer and/or adapting various alignment techniques onto the gate insulator, the π–π stacking in a pentacene film has been improved. These techniques are considered a common method to increase the mobility in the organic TFT [8], [9], [10], [11]. In addition, since the ultra-thin pentacene layer interfaced with the gate insulator has a dominant effect on the charge transport [19], [20], an enhancement of the surface interaction was studied to increase the early stage packing density of pentacene molecules [13]. In methodological (or technical) modifications, on the other hand, controlling the deposition rate of pentacene molecules is considered as a simple and effective method to improve the mobility [14], [15], [16]. Although each effect on the device performance has been previously investigated, a systematic study on the mobility of the pentacene TFT has not been fully investigated from the viewpoint of the relationships between the deposition rate, structural orientation, and molecular growth. Specifically, no study has been carried out for a topologically modified polymeric insulator such as a rubbed layer.
In this work, we describe how the mobility in a pentacene TFT with a rubbed buffer layer stacked onto a polymeric insulator is affected by the deposition rate from the viewpoint of both structural orientation and the growth of pentacene molecules at an early stage. From a morphological study and an optical phase examination, it was found that the alignment and early stage packing densities of pentacene increased with increasing the deposition rate up to 2 Å/s. Here, a sufficient diffusion process of pentacene molecules gives rise to an increase of the packing density and thus the corresponding mobility increases with increasing the deposition rate. In a deposition rate above 2 Å/s, however, the growth of the pre-deposited and pre-aligned pentacene molecules is interrupted by pentacene evaporating onto the pre-deposited pentacene molecules. Thus, the optical retardation is decreased, and the early stage packing density is saturated with increasing the deposition rate. As a result, the resultant mobility is found to decrease with an increase of the deposition rate. This gives a more systematic understanding of how the deposition rate of pentacene molecules affects the mobility in relation with the alignment and diffusive growth of pentacene molecules.
Section snippets
Experimental
A bottom gate, top contact pentacene TFT was fabricated in this study as shown in Fig. 1. Indium-tin-oxide (ITO) was deposited on a glass substrate as a gate electrode. The ITO patterned glass substrate was cleaned with acetone, iso-propyl alcohol, methanol, and de-ionized (DI) water in sequence. Poly(vinyl cinnamate) (PVCi), dissolved in cyclopentanone in 10 wt.%, was spin-coated on the top of the ITO layer as a polymeric gate insulator, which exhibits a small hysteretic behavior and high
Optical phase retardation of pentacene as a function of deposition rate
Due to the rigid rod-like structure of pentacene molecules, it is generally expected that the structural or directional ordering of the pentacene molecules can be induced on an alignment layer for the LCs due to the anisotropic surface interaction. The rubbing process is typically used for the alignment of LCs on polymeric insulators. Recently, the alignment of pentacene molecules along the rubbed direction of polymers was reported [8], [9], [22]. In order to verify the aligned features of
Electrical properties of pentacene TFTs
Based on the earlier results, we now describe how the mobility of pentacene TFTs is affected by the deposition rate in relation with the structural orientation and early stage molecular growth in the pentacene film. Fig. 4a shows the corresponding transfer characteristic curves of pentacene TFTs under the various deposition rates of 0.5, 2, and 5 Å/s. From the transfer characteristic curves, the mobility of each pentacene TFT can be determined using following equation:where ID
Conclusions
We report how the mobility of pentacene TFTs with an anisotropic rubbed polymeric insulator is affected by the deposition rate of pentacene molecules from the view point of both the structural alignment, examined by the optical phase retardation, and morphological early stage growth, examined by AFM. As the deposition rate increases up to an optimum value of 2.0 Å/s (Region I), pentacene molecules are more aligned and more densely packed following the scaling law, and thus the magnitude of the
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) Grant (No. 20110016968) funded by the Ministry of Education, Science, and Technology of Korean Government (MEST) and Smasung Display Co. Ltd.
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