Transparent Polyimide Substrate Research Articles

Anti-reflection Layer-Sputtered Transparent Polyimide Substrate with Reliable Adhesion Strength to the Copper Layer.

Parameters of DC-reactive magnetron sputtering are optimized to deposit anti-reflection (AR) layers on transparent polyimide (PI) substrates, followed by the deposition of the conductive copper layer, to fabricate practically reliable composite films as advanced flexible circuits. When the deposition thickness is controlled and the gas composition during sputtering is adjusted, the resultant AR layer-coated PI film exhibits low reflectance and reveals improved adhesion strength to the copper layer. The adhesion reliability tests confirm that the peel strength between the PI film and the deposited layers could be further improved after thermal processing due to the formation of a worm-like morphology for better mechanical interlocking with layers. The facile sputtering process successfully fabricates a reliable substrate material with low reflectance and sufficient adhesion strength to copper for application as flexible printed circuits.

Sputtering Flexible VO2 Films for Effective Thermal Modulation.

Flexible vanadium dioxide (VO2) thermochromic films show great potential for large-scale fabrication and possess broader applications compared with VO2 coatings on rigid substrates. However, the fabrication of flexible VO2 films remains a challenge so far, leading to the scarcity of research on flexible VO2 films for smart windows. With the aim to obtain a flexible VO2-based films with excellent optical properties and a long service life, we designed and successfully fabricated a flexible ITO/VO2/ITO (IVI) film on the colorless transparent polyimide substrate, which could be directly attached to glasses for indoor temperature modulation. This flexible IVI film effectively enhances the luminous transmittance (Tlum) and solar modulation ability (ΔTsol) (15 and 68% increase relative to a VO2 single layer), reduces the thermal emissivity (εT) (50.7% decrease relative to a VO2 single layer), and exhibits better durability than previously reported structures. Such excellent comprehensive performance offers it great potential in practical applications on smart windows. This work is supposed to provide a new strategy for facile direct fabrication of flexible VO2 films and broaden the applications of flexible VO2 in more coatings and devices.

Flexible Transparent Heater Fabricated from Spray-Coated In:ZnO/Ag-NWs/In:ZnO Multilayers on Polyimide Foil.

A flexible transparent heater is presented, based on an all-sprayed composite architecture of indium-doped zinc oxide (IZO) layers that sandwich a network of silver nanowires, on a polyimide-foil substrate. This architecture could be materialized through the development of a low-temperature (240 °C) spray-pyrolysis process for the IZO layers, which is compatible with the thermal stability of the transparent polyimide substrate and allows for the formation of compact and transparent layers, without precipitates. The IZO layers entirely embed the silver nanowires, offering protection against environmental degradation and decreasing the junction resistance of the nanowire network. The resulting transparent heaters have a high mean transmittance of 0.76 (including the substrate) and sheet resistance of 7.5 Ω/sq. A steady-state temperature of ~130 °C is achieved at an applied bias of 3.5 V, with fast heater response times, with a time constant of ~4 s The heater is mechanically stable, reaching or surpassing 100 °C (at 3.5 V), under tensile, respectively, compressive-bending stress. This work shows that high-performance transparent heaters can be fabricated using all-sprayed oxide/silver-nanowire composite coatings, that are compatible with large-scale and low-cost production.

A flexible transparent organic light‐emitting display with high‐transmittance cathode and high‐contrast electrochromic shutter

AbstractIn this work, we studied a flexible transparent organic light‐emitting display (TOLED) with a stacking magnesium (Mg) and silver (Ag) alloys (Mg:Ag)/1,4,5,8,9,11‐hexaazatriphenylenehexacarbonitrile (HATCN)/indium zinc oxide (IZO) transparent cathode and combined it with a high‐contrast flexible electrochromic shutter (ECS). The transmittance of the segment‐type flexible TOLED based on an 84.5% transparent polyimide substrate was 64.2%. The sheet resistance of the TOLED cathode used in this study was 22.94 Ω/sq, which was close to the value needed in OLED cathode applications. The flexible ECS was composed of two color‐changing layers, namely, a hybrid electrochromic layer and a color‐changing gel electrolyte layer, and the contrast ratio of the flexible ECS was 24.5. The ECS blocked background light and increased the contrast ratio of the flexible TOLED panel. The flexible OLED and flexible ECS were bonded with an optical clear adhesive (OCA) film, and the transmittance of the entire structure was 52.3%. After 2,000 folding repetitions of the integral sample with a folding radius of 3 cm, the ECS maintained its coloration characteristics, and TOLED retained a brightness level of 200 nits.

Design and Performance Investigation of Miniaturized Multi‐Wideband Patch Antenna for Multiple Terahertz Applications

In this paper, a highly efficient multiband optimized slotted pentagonal terahertz patch antenna and its 2 × 2 multi-input multi-output (MIMO) planar array is proposed for multiple terahertz (THz) applications like high-speed indoor communications, explosive detections, arms detection, medical imaging, pharmaceutical analysis, and industrial inspections, etc. Here, the proposed patch antenna is developed from the simple rectangular patch antenna (SRPA) by optimizing the shape of radiating patch and ground plane. Such proposed antenna is designed on the transparent polyimide substrate material having a dielectric constant of ℇr = 3.5 and thickness 21.5 µm. The particle swarm optimization (PSO) technique is used to enhance the performance parameters of proposed antenna. The performance parameters like reflection coefficient, gain, directivity, radiation pattern, and antenna efficiency are computed at the resonating frequencies of proposed antenna. It covers wide bandwidth, which resonates at 3.00 THz, 4.85 THz, 7.02 THz, 8.87 THz, 9.43 THz, and 10.785 THz frequencies having reflection coefficient less than −10 dB. It is also noted that as compared to early reported THz antennas, it has achieved a maximum gain of 13.92 dB and radiation efficiency of 85.77% within the resonating band. Also, a 2 × 2 MIMO antenna array is designed using an optimized antenna element for more effective terahertz communication. The performance of such MIMO array is computed in terms of the transmission coefficient, envelope correlation coefficient (ECC), and Mean effective gain. It has achieved <−15 dB isolation between the antenna elements. The proposed antenna could be a successful choice for multiple terahertz applications.

Characterization of physical and mechanical properties of Al2O3-doped ZnO (AZO) thin films deposited on transparent polyimide supports with various ALD process parameters

Fabrication of Al2O3 doped ZnO (AZO) thin films on transparent polyimide substrates using ex-situ oxygen plasma-assisted atomic layer deposition (ALD) is studied. The alumina composition is controlled by adjusting the ALD cycle ratio in the reaction sequence from 9:1 to 49:1 (diethylzinc/H2O:trimethylaluminum/H2O). Nanoscratch and electromechanical tensile tests are performed to evaluate the mechanical reliability of the AZO thin films fabricated under various processing conditions. Conventionally deposited AZO shows the lowest sheet resistance of 294 Ω/sq at a doping ratio of 19:1 at 150 ℃. However, the resistance of AZO grown on a polymer surface modified by plasma is higher at 429 Ω/sq. Nanoscratch results reveal that the first critical load could be increased by surface hardening and strong adherence as a function of the process temperature and plasma treatment. Under a tensile strain, doping of alumina delayed the strain (%) at crack initiation from 0.58 to 0.82%. The initial elastic modulus of the AZO/transparent polyimide composites showed a transient decrease with an increase in the alumina content from 2.7 to 3.8 at.%. These results demonstrating the mechanical characteristics of thin films as a function of the processing conditions are significant for flexible display manufacturing.

Flexible IGZO thin-film transistors and inverter circuits with diode-connected transistors fabricated on transparent polyimide substrates

Amorphous In2O3–Ga2O3–ZnO (a-IGZO) thin-film-transistors (TFTs) have attracted increasing attention in recent years due to their potential on flexible electronics applications. We investigated low-temperature-processed a-IGZO TFTs and inverters on transparent polyimide. We had successfully implemented flexible top-gate staggered a-IGZO TFTs and inverter circuits on transparent polyimide-based nanocomposite substrates using fully lithographic, etching and PECVD processes that are compatible with existing TFT mass fabrication technologies. The TFTs showed decent performances (with mobility > 20 cm2/V s) as de-bonded from the carrier glass. Furthermore, the IGZO-based inverter also showed decent performances (voltage gain > 1.5) and is considered as a potential pathway toward flexible electronics.

High Operation Stability of Ultraflexible Organic Solar Cells with Ultraviolet-Filtering Substrates.

Ultralightweight and flexible power sources are essential for driving textile or wearable electronic devices and soft robots because they do not induce discomfort or limit movement when they are attached to human skin, textiles, or soft actuators. Organic solar cells (OSCs) are good candidates for developing such power sources because they have the advantages of being lightweight and flexible. However, achieving operational stability and ultrathin shape simultaneously remains difficult because the ultrathin substrate cannot prevent the penetration of ultraviolet (UV) light, which is major a cause for the degradation of OSCs. Here, ultrathin OSCs that show great operational stability and high performance are reported. The 1.3 µm thick transparent polyimide utilized as a substrate can block light of 350 nm wavelength in the UV range by 90%. The ultrathin OSCs with the transparent polyimide substrate produce a power conversion efficiency (PCE) of 9.0% and realize both photostability and operational stability. The PCE was maintained at 90% after 3 h in a maximum power point tracking test, indicating much better operational stability than the reference rigid OSCs.

58‐1: Invited Paper: High Resolution IPS‐LCDs Fabricated with Transparent Polyimide Substrates

Plastic film IPS‐LCDs were fabricated with transparent polyimide (PI) substrates. Conventional transparent PI substrates have a large coefficient of thermal expansion and cause high retardation in thickness direction. We developed a new fabrication process and sheet design that improves upon these characteristics.

Flexible QLED and OPV based on transparent polyimide substrate with rigid alicyclic asymmetric isomer

Abstract A series of isomeric alicyclic-functionalized polyimide with chemical imidization and thermal imidization (CPI-x and TPI-x) were prepared from a rigid alicyclic-functionalized isomerism diamine, 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane (DAPI). The influences of incorporation of the isomeric rigid alicyclic structure onto the backbone of the polymers were systematically investigated in terms of optical, thermal, mechanical, dielectric and surface properties, respectively. Due to the moderate chemical imidization condition, CPI-x series retain higher glass transition temperature (Tg) in the range of 329–429 °C than the Tg of TPI-x (from 321.9 °C to 370.7 °C). Therefore, the CPI-1 film based 5-DAPI was chosen as the flexible substrate. MoO3 was chosen as the interface layer to improve the compatibility between PI substrate and the metal electrode. Then a ultra-thin layer of MoO3 (3 nm)/Au(2 nm)/Ag(4 nm) was utilized to be the transparent electrode. After annealing at 220 °C for 0.5 h, zinc oxide (ZnO) was deposited onto the electrode to maintain the superior electron mobility and improve the transparency of the electrode. Consequently, the flexible quantum dots light emitting diode (QLED) obtained a high luminance of 5230 (cd/m2) and EQE of 5.2%, meanwhile, a device performance of 4.36% was achieved in organic photovoltaic (OPV) devices.

Light extraction of flexible OLEDs based on transparent polyimide substrates with 3-D photonic structure

We report a flexible organic light-emitting diode (OLED) based on transparent polyimide (PI) substrate with 3-D photonic structure, which shows a maximum gain factor of ∼1.7 for current efficiency at large viewing angle. The PI substrate is a replicate from glass carrier with hexagonal closely-packed convex-truncated-cone array. Green OLEDs are fabricated on the planar surface of the PI substrate before being mechanically de-bonded from the glass template. The proposed OLEDs exhibit excellent angular optical properties including stable CIE coordinates with Δx = −0.006 and Δy = 0.002 as the viewing angle varies from 0° to 50°. Surface scattering effect of the 3-D photonic structure eliminates the periodic distortion phenomenon in electroluminescence spectrum of flexible OLEDs.

Flexible Organic Phototransistor Array with Enhanced Responsivity via Metal-Ligand Charge Transfer.

Phototransistors based on organic photoactive materials combine tunable light absorption in the spectral region from ultraviolet to near-infrared with low-temperature processability over large areas on flexible substrates. However, they often exhibit low photoresponsivity because of low molar extinction coefficient of photoactive components. We report a simple, yet highly efficient solution method for enhancing the performance of organic phototransistors using ruthenium complex 1 (Ru-complex 1). An air-stable n-type organic semiconductor, N,N'-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI), has been deposited on a silicon wafer and a transparent polyimide (PI) substrate via thermal evaporation under vacuum. The BPE-PTCDI phototransistors functionalized with Ru-complex 1 exhibit ∼5000 times higher external quantum efficiency (EQE) than that of pristine BPE-PTCDI phototransistors, owing to the metal-ligand charge transfer (MLCT) from Ru-complex 1 to the active component of the device. In addition, a large 10 × 10 phototransistor array (2.5 × 2.5 cm(2)) has been prepared on a transparent PI substrate, showing distinct light mapping. The fabricated phototransistor array is highly flexible and twistable and works well under tensile and compressive strains. We believe that our simple method will pave a viable way for improvements in the photoresponsivity of organic semiconductors for applications in wearable organic optoelectronic devices.

Electromechanical properties of amorphous In–Zn–Sn–O transparent conducting film deposited at various substrate temperatures on polyimide substrate

The electromechanical properties of the amorphous In–Zn–Sn–O (IZTO) film deposited at various substrate temperatures were investigated by bending, stretching, twisting, and cyclic bending fatigue tests. Amorphous IZTO films were grown on a transparent polyimide substrate using a pulsed DC magnetron sputtering system at different substrate temperatures ranging from room temperature to 200 °C. A single oxide alloyed ceramic target (In2O3: 80 wt %, ZnO: 10 wt %, SnO2: 10 wt % composition) was used. The amorphous IZTO film deposited at 150 °C exhibited an optimized electrical resistivity of 5.8 × 10−4 Ω cm, optical transmittance of 87%, and figure of merit of 8.3 × 10−3 Ω−1. The outer bending tests showed that the critical bending radius decreased as substrate temperature increased. On the other hand, in the inner bending tests, the critical bending radius increased with an increase in substrate temperature. The differences in the bendability of IZTO films for the outer and inner bending tests could be attributed to the internal residual stress of the films. The uniaxial stretching tests also showed the effects of the internal stress on the mechanical flexibility of the film. The bending and stretching test results demonstrated that the IZTO film had higher bendability and stretchability than the conventional ITO film. The IZTO film could withstand 10,000 bending cycles at a bending radius of 10 mm. The effect of the surface roughness on the mechanical durability of all IZTO films was very small due to their very smooth surfaces.

All-Wet Metallization Process for Transparent Polyimide Films

Electrically conductive Ag thin films were successfully formed onto optically transparent polyimide films via all-wet chemical process. The process involves alkaline-induced surface modification of polyimide films, subsequent incorporation of Ag+ ions through ion exchange reaction followed by reduction of incorporated Ag+ ions. Initial alkaline treatment time determines thickness (depth) of surface-modified layer and amount of Ag+ ions loading consequently. Higher Ag+ ions loading result in forming electrically conductive Ag thin films and its granular interfacial structure. Cu electroplating is able to be conducted on the resulting Ag thin films directly, and adhesive strength between the Cu(/Ag) film and transparent polyimide substrate reached up to 2 kN m−1.

Electroosmotic Flow Pump on Transparent Polyimide Substrate Fabricated Using Hot Embossing

Improved fabrication processes of an all-polyimide micro electroosmotic flow pump using hot embossing are described. Microchannels in the micropump were fabricated by hot embossing on a transparent polyimide substrate. A silicon micromachined mold was pressed into the transparent polyimide substrate at a temperature of 300 oC to form microchannel patterns on the substrate. The depth and width of the microchannels were 25 μm and 50 μm, respectively. A UV ozone treatment was performed to improve adhesion between the transparent polyimide substrate and film capping layer. This UV ozone treatment enhanced adhesion and resulted in the reduction of the adhesion temperature as low as 100 oC, and nearly no deformation of the microchannels was observed. As a result, the electroosmotic flow pump exhibited the flow rate of 0.7 μl/min when a voltage of 50 V was given between the electrodes separated 20 mm each other.

Transparent Polyimides and their ITO Flexible Conductive Film

The preparation and properties of indium tin oxide (ITO) coated transparent polyimide substrates were reported in the paper. The transparent polyimide films were prepared through thermal imidization of poly(amic acid) films. Transparent conducting ITO films were prepared on polyimide substrates by magnetron-sputtering technique at room temperature. The optical and electrical properties of the obtained films have been investigated. Polycrystalline ITO films were deposited with resistivity as low as 10-4 Ω.cm on polyimide substrates. The average transmittance exceeded 78% in the visible spectrum for 200nm thick films deposited on polyimide.

The Influence of Electromechanical Stress on the Stability of Nanocrystalline Silicon Thin Film Transistors Made on Colorless Polyimide Foil

Staggered bottom-gate hydrogenated nanocrystalline silicon (nc-Si:H) thin film transistors (TFTs) fabricated on flexible transparent polyimide substrates were investigated. The saturation electron field-effect mobility and electrical bias-stress stability of these TFTs were evaluated under applied tensile mechanical strain parallel to the source-drain direction. The mobilities increased accompanying with deteriorated electrical stabilities as the applied tensile strain increased. The power-law dependence between threshold voltage shift and gate-bias stressing time indicates that the instability is mainly caused by the state creation at the interface between nc-Si:H channel and gate dielectric.

61.4: High‐Performance and Highly Rollable a‐IGZO TFTs Adopting Composite Electrodes and Transparent Polyimide Substrates

AbstractWe have demonstrated high‐performance and highly rollable flexible oxide TFTs on fully transparent/colorless polyimide substrates by adopting lithographic and etching processes and composite electrodes. The TFTs exhibited a field‐effect mobility of up to 15.9 cm2/Vs, a sub‐threshold swing of 0.43 V/decade, and an on/off ratio of &gt; 108. The devices could be bent down to a radius of 3 mm and yet remained normally functional.

Effective extra-cellular recording from vertebrate neurons in culture using a new type of micro-electrode array

We describe the fabrication and use of a new type of extracellular micro-electrode array mounted on a flexible transparent polyimide substrate that can be rapidly moved from one part of a culture of vertebrate neurons (rat nodose) to another, which permits co-culture of glia under the neurons and is easily and rapidly replaceable in the event of damage. The array can be mounted on a micromanipulator and moved into place whenever and wherever recordings with or without stimulation are needed. The basic electrode system consists of 20–30 μm diameter gold electrodes, with or without platinisation, exposed to the cells through openings in the polyimide and joined to the recording or stimulating circuitry through gold tracks embedded in the polyimide. If rigid control over neuron placement has been achieved the patterns of electrodes can be matched to the neuron positions.