Colorless Polyimide Substrate Research Articles

Dielectric properties of hafnium oxide film prepared by HiPIMS at different O2/Ar ratios and their influences on TFT performance

High-k hafnium oxide (HfO2) film was prepared by high power impulse magnetron sputtering (HiPIMS). The influences of oxygen supply on the plasma state, film properties and TFT performance were investigated. The films are near-stoichiometric and preferentially (−1 1 1)-orientated. When the oxygen supply increased from 1% to 3%, the excitation/ionization rate of the plasma species increased, leading to higher crystallinity, higher density, and lower oxygen vacancy defect concentration of the film, therefore improving the dielectric properties of the film. When the oxygen supply further increased to 5%, the excitation/ionization rate decreased, thereby leading to lower crystallinity, lower density, and higher oxygen vacancy defect concentration of the film, therefore deteriorating the dielectric properties of the film. The film deposited at 3% oxygen supply exhibited the best dielectric properties with the highest k value of 24 and the highest breakdown-electric field (4.7 MV/cm), which should be attributed to the high crystallinity, high density and low oxygen vacancy defect concentration of the film. Finally, transparent thin film transistors (TFTs) with ITO gate electrode, HfO2 gate dielectric layer and indium-gallium-zinc oxide channel were fabricated on flexible colorless polyimide substrate at full room temperature by all HiPIMS process. The fixed positive charges and k value of HfO2 film have significant effects on the TFT performance. The best TFT exhibited good electrical performance, featuring a remarkably low subthreshold swing of 0.13 V/decade. It also exhibited fair stability against bending and gate bias stress.

Flexible and transparent thin-film light-scattering photovoltaics about fabrication and optimization for bifacial operation

Flexible and transparent thin-film silicon solar cells were fabricated and optimized for building-integrated photovoltaics and bifacial operation. A laser lift-off method was developed to avoid thermal damage during the transfer of light-scattering structures onto colorless polyimide substrates and thus enhance front-incidence photocurrent, while a dual n-type rear window layer was introduced to reduce optical losses, facilitate electron transport for rear incidence, and thus enhance performance during bifacial operation. The introduction of the window layer increased the rear-to-front power conversion efficiency ratio to ~86%. The optimized bifacial power conversion efficiency for front and rear irradiances of 1 and 0.3 sun, respectively, equaled 6.15%, and the average transmittance within 500–800 nm equaled 36.9%. Additionally, the flexible and transparent solar cells fabricated using laser lift-off exhibited good mechanical reliability (i.e., sustained 500 cycles at a bending radius of 6 mm) and were therefore suitable for building-integrated photovoltaics.

Flexible, stripe-patterned organic solar cells and modules based on multilayer-printed Ag fibers for smart textile applications

Rapid advancement in the fabrication technologies of stripe/fiber-shaped optoelectronic devices has driven the performance of smart textile electronics to a new height. In the development of high-performance smart textile electronics, indium tin oxide (ITO)-free flexible transparent conductive electrodes (TCEs) are particularly desirable because they offer superior flexibility and lower manufacturing cost than the brittle ITO-based counterparts. Herein, we innovatively combine spin-coating and three-dimensional direct-ink writing (three-dimensional-DIW) techniques to develop large-area (5 × 5 cm2) PEDOT:PSS/Ag-fibers hybrid TCEs (denoted as DIW-TCEs) for application in flexible organic solar cells (OSCs). Through a multilayer printing strategy, high aspect-ratio Ag-fibers are successfully deposited on top of the planar PEDOT:PSS film. The resulting DIW-TCEs, when fully embedded in a colorless polyimide substrate, exhibit excellent electrical conductivity (sheet resistance ∼ 4 Ω/□), superior optical transmittance, and mechanical flexibility. One-dimensional stripe-patterned OSCs (stripe-OSCs) based on DIW-TCEs achieved power conversion efficiency of 9.3% and 8.2% at an active area of 0.11 cm2 and 0.31 cm2, respectively. For real-life application, a flexible power module was constructed using eight stripe-OSCs. When attached on textile, the module successfully lit up a commercial light-emitting diode upon photocharging. The unique DIW-TCE fabricated herein can be the ITO-free alternative for the production of smart textile electronics.

A Bioinspired Ultra Flexible Artificial van der Waals 2D-MoS2 Channel/LiSiOx Solid Electrolyte Synapse Arrays via Laser-Lift Off Process for Wearable Adaptive Neuromorphic Computing.

Wearable electronic devices with next-generation biocompatible, mechanical, ultraflexible, and portable sensors are a fast-growing technology. Hardware systems enabling artificial neural networks while consuming low power and processing massive in situ personal data are essential for adaptive wearable neuromorphic edging computing. Herein, the development of an ultraflexible artificial-synaptic array device with concrete-mechanical cyclic endurance consisting of a novel heterostructure with an all-solid-state 2D MoS2 channel and LiSiOx (lithium silicate) is demonstrated. Enabled by the sequential fabrication process of all layers, by excluding the transfer process, artificial van der Waals devices combined with the 2D-MoS2 channel and LiSiOx solid electrolyte exhibit excellent neuromorphic synaptic characteristics with a nonlinearity of 0.55 and asymmetry ratio of 0.22. Based on the excellent flexibility of colorless polyimide substrates and thin-layered structures, the fabricated flexible neuromorphic synaptic devices exhibit superior long-term potentiation and long-term depression cyclic endurance performance, even when bent over 700 times or on curved surfaces with a diameter of 10mm. Thus, a high classification accuracy of 95% is achieved without any noticeable performance degradation in the Modified National Institute of Standards and Technology. These results are promising for the development of personalized wearable artificial neural systems in the future.

Multifunctional polyimides for resistive switching memory devices based on flexible transparent polyimide–silver nanowires hybrid electrodes

AbstractWith the quick development of flexible memory electronics, multifunctional organic materials have been the necessary for fabricating electronics. In this work, the highly transparent and flexible electrode was successfully prepared by coating the high‐performance silver nanowires (AgNWs) onto the colorless polyimide (PI) substrate. The prepared flexible PI–AgNWs electrodes exhibited a relatively low sheet resistance of 15 Ω/sq with the high transparency of 68% at the wavelength of 400 nm. A novel kind of polyimide TPC6FPI was successfully synthesized and characterized with the excellent thermostability and high glass transition temperature (Tg) above 250°C. Furthermore, a kind of flexible transparent PI–AgNWs/TPC6FPI/Al resistive memory device was prepared and exhibited good SRAM switching behavior with the threshold voltage of around 2.1 V and the ON/OFF current ratio of ˜104, which indicated that multifunctional PI‐based memory device showed the potential to the wearable devices.

Characteristics of the Colorless Polyimide-Based Flexible X-ray Detector with Non-Fullerene Acceptor Polymer.

In this paper, we investigate the characteristics of the colorless polyimide (CPI) film-based flexible organic X-ray detector. The CPI film can be applied to various applications, because it shows excellent visible light transmittance by removing the yellow color of polyimide (PI) film, which has the advantage of physical and chemical stability. In addition, the deformation curvature of the CPI substrate according to temperature showed similar characteristics to that of the glass substrate. For the organic active layer of the proposed detector, PBDB-T was fixed as a donor, and PC71BM and ITIC were used as acceptors. To evaluate the mechanical stability of the flexible detector, the degradation sensitivity was measured as bending curvature and bending cycle. The sensitivity of the detector with ITIC acceptor showed a 46.82% higher result than PC71BM acceptor on bending curvature (R = 10); and at the same curvature, when the bending cycle was 500 times, a 135.85% higher result than PC71BM acceptor.

Ultrathin Flexible Transparent Composite Electrode via Semi-embedding Silver Nanowires in a Colorless Polyimide for High-Performance Ultraflexible Organic Solar Cells.

Ultraflexible organic solar cells (OSCs) with both high power conversion efficiency (PCE) and good mechanical robustness are still challenging, in which flexible transparent composite electrodes (FTCEs, substrate-cum-electrodes) play critical roles. Here, an ultrathin FTCE (∼9 μm) via semi-embedding a silver nanowire electrode in a colorless polyimide (CPI) substrate was developed, which simultaneously possessed outstanding performance such as low square resistance (Rsq ∼ 12.7 Ω sq-1), high optical transmittance (T550 ∼ 86.3%), smooth surface (root-mean-square ∼ 0.32 nm), and excellent thermal, mechanical, and solution producing stability. Prior to the FTCE fabrication, four CPI samples with the number-average molecular weight ranging from 35.9 to 177.5 kDa were prepared and their optical, mechanical, and thermal properties were studied in detail. Moreover, the effect of the molecular weight on the minimum thickness that can withstand the following solution production of ultraflexible OSCs was investigated, which revealed that the molecular weight of CPI here should be above 81.4 kDa. Based on the FTCE, an ultraflexible OSC with a high PCE value of 14.37% and outstanding mechanical robustness was constructed, in which the PCE could still maintain above 96% of its initial value after 1000 bending cycles at a bending radius of 0.5 mm.

Flexible multilayered transparent electrodes with less than 50 nm thickness using nitrogen-doped silver layers for flexible heaters

In this study, Al-doped ZnO/Ag/Al-doped ZnO multilayered electrodes are fabricated on a colorless polyimide substrate through an optimized nitrogen insertion technique, and subsequently applied to transparent flexible thin film heaters. The resulting electrodes have remarkable performance and durability. A completely continuous and smooth Ag thin film with extremely small thickness is fabricated by optimizing the N2 gas flow rates at room temperature. The multilayered films exhibit a high optical transmittance of 96.0% at a wavelength of 550 nm and low sheet resistance of 8.5 Ω sq.−1. The bending test results show that there is no significant change in the sheet resistance even after 5,000 bending cycles, highlighting their mechanical flexibility. For its application in transparent flexible heaters, the performance of the multilayered films exceeds that of conventional ITO-based films, as confirmed by their higher saturation temperatures and flexibility, indicating their outstanding thermal properties and high conductivity with small thickness.

Printed Flexible Transparent Electrodes for Harsh Environments

AbstractFlexible transparent electrodes are widely used in flexible displays, optoelectronic devices, flexible heating/electromagnetic shielding devices, and flexible sensors. However, significant challenges remain to obtain stable service under harsh environments, owing to the limitations of flexible transparent substrates and conductive materials. Herein, a simple and cost‐effective fabrication approach for harsh‐environment flexible transparent electrodes (HFTEs) is demonstrated via the electric field‐driven microscale 3D printing of a silver mesh on a colorless polyimide (CPI) substrate. Vertical jet orientation on the CPI substrate is realized by controlling the printing parameters, which effectively reduce the instability of the cone jet and is conducive to the preparation of the high‐precision silver mesh on the CPI substrate. The HFTE exhibits good optoelectronic properties (resistance of 4.82 Ω sq−1 and transmittance of 92.26%), and excellent extreme environmental stability with a negligible increase in sheet resistance under various cyclic tests (200 000 bending cycles, 100 adhesion tests, high temperature heating capacity of 360 °C, 80 h of chemical attack, and 120 days of atmospheric environment attack). Baking of quail eggs and deicing experiments are also conducted, which successfully demonstrate the practical viability of the HFTE. Thus, the proposed technique offers a simple and efficient solution for the manufacturing of HFTEs.

Thermoelectric Characteristics of Ultrathin Ag₂Se Films with Durability Against Mechanical Stress.

In this study, we investigated thermoelectric materials with durability against mechanical stress using Ag₂Se nanoparticle (NP) thin films and colorless polyimide (CPI) substrates. Ag₂Se NP thin films and CPI substrates were produced by spin-coating, and their thicknesses were 40 nm and 15 μm, respectively. A bendable thermoelectric film with a channel length of 40 μm and a channel area of 1.6 μm² generated a Seebeck voltage of 1.43 mV at a temperature difference of 4.5 K. Owing to the thickness of the extremely thin thermoelectric film and substrate, the mechanical strain was only 0.15% even when the thermoelectric devices were bent with a curvature of 3 mm. Therefore, it was determined that the bendable thermoelectric film was robust against mechanical stress.

Invisible and Flexible Printed Sensors Based on ITO Nanoparticle Ink for Security Applications

Here, we propose a method to create a transparent security system based on printed conductive indium tin oxide (ITO)—the most widely used transparent conducting oxide material integrated into the devices with high transparency. Commonly used solution-processed ITO annealing methods are utilizing temperatures which are limiting the use of flexible polymeric substrates. Our method combines inkjet printing on flexible temperature-stable colorless polyimide (CPI) substrate with fast flash lamp annealing (FLA). In this study, millisecond pulses of visible light from a xenon lamp induce rapid heating of the ITO films up to 650°C through the light-absorbing additional layer of a colored organic dye onto printed ITO, whereas the CPI bulk never exceeds the melting point. Fabricated flexible ITO patterns on CPI film processed with the flash lamp annealing through the dye layer exhibit a transmittance of up to 85% at the wavelength of 550 nm and sheet resistance of 520 Ω/sq for a 70 nm layer thickness. With the proposed technology of our demonstrator realization—transparent glass/window or any other object such as a curved door lock can be used for integrating a touch-enabled transparent security access system, which would be completely invisible.

Digitally Patterned Mesoporous Carbon Nanostructures of Colorless Polyimide for Transparent and Flexible Micro-Supercapacitor

Here, we demonstrate the fabrication of a flexible and transparent micro-supercapacitor (MSC), using colorless polyimide (CPI) via a direct laser writing carbonization (DLWC) process. The focused laser beam directly carbonizes the CPI substrate and generates a porous carbon structure on the surface of the CPI substrate. Fluorine, which is one of the chemical compositions of CPI, can enhance the specific area and the conductivity of the carbon electrode by creating micropores in carbon structures during carbonization. Thus, the fabricated carbonized CPI-based MSC shows enhanced specific capacitance (1.20 mF at 10 mV s−1) and better transmittance (44.9%) compared to the conventional PI-based MSC. Additionally, the fabricated carbonized CPI-based MSC shows excellent cyclic performance with minimal reduction (<~10%) in 3000 cycles and high capacitance retention under mechanical bending test conditions. Due to its high flexibility, transparency, and capacitance, we expect that carbonized CPI-based MSC can be further applied to various flexible and transparent applications.

Controlling Neutral Plane of Flexible Substrates by Asymmetric Impregnation of Glass Fabric for Protecting Brittle Films on Foldable Electronics

Neutral plane strategies have been studied to reduce the tensile strain of brittle films in foldable electronics under bending. However, those strategies depend on the dimensions of components in devices, implying that they could be limited to application to various designs of devices. Herein, the glass fabric's position in a glass fabric–reinforced siloxane hybrid substrate is shifted to control the neutral plane of the substrate. Due to the stiffness difference between the glass fabric and epoxy siloxane hybrid matrix, the neutral plane of the substrate is affected by the position of the glass fabric. Therefore, the glass fabric is asymmetrically impregnated to be located at the substrate surface so as to shift the neutral plane of the substrate toward the surface. The effectiveness of the newly developed substrate is proved by comparison with a conventional colorless polyimide substrate. The neutral plane position and maximum tensile strain of the substrates are investigated by analytical calculation, digital image correlation analysis, bending test, and finite element method simulation. The results clearly show that the newly developed substrate–based structure can reduce the maximum tensile strain under bending. Consequently, the cracking of the brittle hard coating is effectively prevented.

Transparent and Flexible SiOC Films on Colorless Polyimide Substrate for Flexible Cover Window.

We fabricated transparent and flexible silicon oxycarbide (SiOC) hard coating (HC) films on a colorless polyimide substrate to use as cover window films for flexible and foldable displays using a reactive roll-to-roll (R2R) sputtering system at room temperature. At a SiOC thickness of 100 nm, the R2R-sputtered SiOC film showed a high optical transmittance of 87.43% at a visible range of 400 to 800 nm. The R2R-sputtered SiOC films also demonstrated outstanding flexibility, which is a key requirement of foldable and flexible displays. There were no cracks or surface defects on the SiOC films, even after bending (static folding), folding (dynamic folding), twisting, and rolling tests. Furthermore, the R2R-sputtered SiOC film showed good scratch resistance in a pencil hardness test (550 g) and steel wool test under a load of 250 g. To test the impact protection ability, we compared the performance of thin-film heaters (TFHs) and oxide-semiconductor-based thin-film transistors (TFTs) with and without SiOC cover films. The similar performance of the TFHs and TFTs with the SiOC cover window films demonstrate that the R2R-sputtered SiOC films offer promising cover window films for the next generation of flexible or foldable displays.

Electrical characteristics of bendable a-ITGZO TFTs on colorless polyimide substrates

In this study, we fabricate amorphous indium tin gallium zinc oxide (a-ITGZO) thin-film transistors (TFTs) on colorless polyimide (CPI) substrates and examine their electrical characteristics as functions of the bending radius. A representative a-ITGZO TFT shows typical n-type behavior and operates with a mobility of 34.32 cm2 V−1 s−1, threshold voltage of −0.71 V, subthreshold swing of 169 mV, and on/off ratio of 2 × 107. The a-ITGZO TFT operates stably even under a bending radius of 2 mm, regardless of the upward or downward bending. The thickness of the thin CPI substrate contributes to the stable operation of the bent TFT.

Study of indium tin oxide films deposited on colorless polyimide film by magnetron sputtering

Polyimide attracted most interest for its high performance as a flexible substrate of touch panels. Experimental models were built to evaluate production feasibility. The solution with monomer was coated on a glass substrate, cured at 250 °C to form a solid colorless polyimide (CPI) film. Then, a supporting layer with silicon oxide and a conducting layer with indium tin oxide (ITO) were deposited on the CPI film by magnetron sputtering. Composite with a CPI substrate and ITO patterns were prepared via a series of procedures. Test results indicated ITO film thickness and sheet resistance could be controlled by adjusting sputtering power, and the film showed smooth curves on the contour line chart. The transmittance of CPI/ITO film composite was up to 80%. The bending durability of the film was evaluated by two-point bending method, and it could be significantly improved by shielding the ITO pattern with ultraviolet cured resin as an external buffer layer. Samples with buffer layers passed 64,000 cycles bend test, and the minimum fracture radius reached 4 mm. The increase of ITO film thickness had no significant negative impact on the performance of flexible devices.

Highly flexible and transparent film heaters based on colorless polyimide substrate with a GZO/AgNW/GZO sandwich structure

In the past years, transparent film heaters (TFHs) based on silver nanowires (AgNWs) have attracted much attention because of their high transmittance, excellent flexibility, and great thermal response. In this paper, we fabricated flexible TFHs with a sandwich structure composed of Ga-doped ZnO (GZO) and AgNWs on the colorless polyimide (cPI) substrate. The optimized hybrid film exhibited a sheet resistance of 14.6 Ω sq−1 with a transmittance of 79%. Excellent thermal uniformity and heating stability of TFHs were demonstrated. The thermal response tests of the GZO/AgNW/GZO/cPI TFHs showed high saturation temperature (~ 176 ℃) with low input voltage (~ 6 V), fast response time (~ 15 s), and stable heating performance. These results indicate that the flexible TFHs with a GZO/AgNW/GZO sandwich structure have broad potential applications in flexible electronic devices.

Ultra-high transparent sandwich structure with a silicon dioxide passivation layer prepared on a colorless polyimide substrate for a flexible capacitive touch screen panel

Indium tin oxide (ITO)-based multilayer structures with ultrathin silver (Ag) deposition on colorless polyimide (CPI) were investigated as touch sensor electrode materials in this study. The electrical, optical, structural, and morphological properties of the ITO-based multilayer structures were compared. The figure of merit of the ITO/Ag/ITO(160)/CPI multilayer structure (bottom ITO film deposited at 160 °C), in which the bottom ITO layer was deposited at 160 °C, was calculated using Haacke's formula, and the value was found to be 63.6 × 10−3 Ω−1. Moreover, the sheet resistance and optical transmittance at 550 nm were 6.4 Ω/□ and 91.4%, respectively. Transmission electron microscopy measurements demonstrated that the continuous Ag layer had a uniform film of 10.9 nm. When an ITO/Ag/ITO(160) multilayer structure was used as a substitute for an X–Y ITO layer electrode for fabricating a flexible capacitive-type touch screen panel (FCTSP) and a low-refractive-index silicon dioxide (SiO2) layer was used as the passivation layer, the ultra-high optical transmittance of the SiO2/ITO/Ag/ITO(160) multilayer structure was measured to be 94.6% (at 550 nm), and the average optical transmittance over the visual region (400–700 nm) was measured to be 92.2%. An FCTSP with multiple touch points was successfully fabricated using an X–Y ITO/Ag/ITO(160) multilayer electrode and SiO2 passivation layer. Finally, we also derive the related theoretical formulas to fix our experimental data. Moreover, based on the theory, we can predict the optimized thickness to support the desirable ultra-high transparency of multilayer structures. This auxiliary simulation also paves an alternative route to explore the possibilities of optimized multilayer flexible devices.

Highly Robust Flexible Vertical-Channel Thin-Film Transistors Using Atomic-Layer-Deposited Oxide Channels and Zeocoat Spacers on Ultrathin Polyimide Substrates

Mechanically flexible vertical-channel-structured thin-film transistors (VTFTs) with a channel length of 200 nm were fabricated on 1.2 μm thick colorless polyimide (CPI) substrates. All layers comp...

P‐10.3: Research and Analysis of Polymer Walls Technologies for Flexible Display

We studied the effects of photomask design, illumination time, and UV intensity on the formation of polymer walls, and speculated the related mechanism. According to the optimized condition, we make a flexible LCD cell with colorless polyimide substrate.