Flexible polymeric films are not only widely used in conventional packaging as substitute for glass and aluminum foil packaging but are also proposed as encapsulation for novel products, like flexible solar cells or organic light-emitting devices. The two essential properties of the polymeric packaging are flexibility and good permeation barrier properties against gases and vapors. This article deals with vacuum web coating as a common way of increasing barrier properties of polymeric films and the problems related to this procedure. Defects caused by particles and surface imperfections are found to dominate the permeation rate for such coated polymeric films. Atomic force microscopy, electron and also optical microscopy was used for analysis of the coating layer. Three-dimensional numerical simulations were performed for modeling of the influence of defect size, spacing and film thickness. Results of numerical modeling and of many practical experiments show that the permeability is almost independent of the substrate film thickness when a critical thickness is exceeded. In most cases the defects can be treated as independent of each other. The gas permeability of vacuum web-coated polymeric films can be quantitatively predicted by a simple formula. For gases, like oxygen, it is shown that a statistic analysis of the defect sizes by optical microscopy is sufficient. For water vapor transmission, however, the structure of the coating layer itself has also to be taken into account.
An amorphous indium gallium zinc oxide (a‐IGZO) layer is deposited on very thin conductive amorphous indium zinc oxide (a‐IZO) thin film to demonstrate high‐performance, coplanar thin‐film transistors (TFTs) with dual‐channel oxide semiconductor architecture. Based on material properties, a conduction band offset (∆EC) of ≈0.28 eV between a‐IZO and a‐IGZO layers and a conduction band bending of ≈0.3 eV at a‐IGZO/gate insulator (GI) interface exist. Through the electrical characterization, high field‐effect mobility (μFE) of ≈50 cm2 V−1 s−1, a positive threshold voltage (VTh) of ≈2.3 V, and low off‐current (IOFF) of <1 pA in coplanar a‐IZO/a‐IGZO TFT are demonstrated. The electron accumulation (>5 × 1018 cm−3) at both the a‐IZO/a‐IGZO and a‐IGZO/GI interfaces confirm the dual‐channel conduction. The bottom a‐IZO channel significantly contributes to increasing drain current (ID) due to large electron density (≈1019 cm−3). The dual‐channel coplanar TFT with a‐IGZO/IZO provides a guideline for overcoming the trade‐off between high μFE and positive VTh control for stable enhancement mode operation with increased ID.
Self‐aligned coplanar thin‐film transistors (TFTs) with a novel dual channel architecture comprising of low mobility and high mobility oxide semiconductors show high field‐effect mobility of > 50 cm2/Vs with positive threshold voltage of > 0 V and low off‐leakage current of < 1 pA. The TFTs with dual channel allow higher mobility than TFTs with a single high mobility channel because the TFTs with dual channel allow strong electron accumulation due to high electron densities both at the interface between gate insulator and 1st oxide semiconductor and at the hetero‐junction interface between 1st and 2nd oxide semiconductors.
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