Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (b-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution (a-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm 2 V À 1 s À 1 (a-phase) and up to 3.5 cm 2 V À 1 s À 1 (b-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on b-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm 2 V À 1 s À 1 . The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps.
Despite their importance for future applications, the operational electrical stability of organic thin-film transistors is far from being understood. Even in the most stable organic field-effect transistors (OFETs) operated under vacuum, a hitherto unknown source leads to bias stress. Here, we investigate the electrical characteristics and operational stability of a high-performance diketopyrrolopyrrole-alt-terthiophene organic semiconductor. Even though the OFETs are characterized by a high mobility of 3 cm2 V–1 s–1 and trap-free transport, the threshold voltage shift in all stress modes remains sensitive to the presence of water even when operating devices in high vacuum. Exponential fitting from current bias-stress measurement up to 500 000 s showed a bias-voltage shift of <1 V, which corresponds to the density of the bias-induced trap states at infinite time N T ∞ = 7.6 × 1010 cm–2. We have surprisingly found that electrical stress could be completely suppressed when devices are cooled to below 273 K. We present evidence that H3O+ and OH– stemming from the autoionization of liquid water is the hitherto unidentified universal trap (i.e., an extrinsic trap not stemming from the semiconductor itself) causing threshold voltage shift even in the otherwise stable devices. This interpretation would also clarify why in the literature similar N T have been reported in various semiconductors, suggesting that this number is independent of the organic semiconductor, processing and measurement environment but only dependent on residual contaminantsmost notably water.
with a new generation of technology. Currently, a few technologies are in competition of which vacuum-processed lowtemperature poly silicon (LTPS) and metal oxide (MO) are taking the lead in the commercial launch (e.g., Samsung Galaxy S7, LG flex 2), since they offer high mobility and are easy to integrate with the current display fabrication process.Despite the advantage of flexibility and low-temperature processing, the development of organic TFT technology has still not reached a commercial breakthrough. While recent scientific publications show that reliable mobilities of up to 10 cm 2 V −1 s −1 [1,2] for p-channel organic semiconductors and 4 cm 2 V −1 s −1[3] for n-channel semiconductors can be reached, most of these examples still require dedicated processing steps, which are incompatible with the industrial requirements of fast, large-scale fabrication processes. In parallel with the mobility requirements, the electrical stability of the organic TFTs is another critical concern and likely even more important for the product development. The requirements in terms of electrical stability are especially challenging for TFTs for use in active-matrix OLED displays compared to technologies such as liquid crystal or electrophoretic displays. The most simple driving circuit design is based on a 2-transistor-1-capacitor (2T1C) circuit, with one switch-TFT and one drive-TFT, as demonstrated in a number of prototypes. [4] More complicated circuit design is required if there is a need to compensate for decreased OLED luminescence over time, or to compensate for inhomogeneity in the electrical parameters of the TFTs. The requirements for the two different transistors in the driving circuit are distinct. The switch TFT controls the charging of the storage capacitor and is only active during this charging. It therefore requires an excellent off-bias stability (in other words an excellent positivebias stress (PBS) for a p-type transistor). On the other hand, the requirement for the drive-TFT is an excellent stability with respect to current-bias stress (CBS) (equivalent to a negativebias stress (NBS) with applied negative drain-source voltage V DS for a p-type transistor). A shift of the threshold voltage ∆V th during operation will strongly affect the magnitude of the TFT current and therefore the pixel brightness. A typical requirement, e.g., is put forward by Sirringhaus, e.g., ∆V th < 1 V for 100 h of constant current bias stress at room temperature. [5] It is well known that the electrical stability of TFTs is a system property. The density of charge carrier traps and defects in anThe application of organic transistors in backplane technology is strongly hindered by their perceived instability to current and bias stress. Highperformance polymer organic semiconductors with unprecedented electrical stability under ambient conditions are demonstrated. Remarkably, these unencapsulated, solution-processed organic transistors show slightly better bias stress stability than even the the current benchmarks for flat panel d...
A bistable ferroelectric liquid-crystal display (FLCD) for application in a smart card has been developed together with Infineon Technologies Business Unit Security and Chip Card ICs. The manufacturing process of the display will be explained. To further improve the functionality and reliability of the display, its layout has been modified. Barrier layers to reduce water permeation have been introduced. Defects in the FLC have been observed around the spacers. A change in the process order can help to avoid them. To reduce the sensitivity of the smectic layers of the FLC to shear forces, a special display layout with enlarged spacers has been developed together with an appropriate vacuum-free filling process.
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