Sean M. Garner

Gravure Printing of Conductive Inks on Glass Substrates for Applications in Printed Electronics

In graphics, gravure printing is the preferred method for printing high quality, fine dimension graphics using high-speed roll-to-roll or sheet fed presses. Gravure printing typically employs flexible and compressible substrates such as various papers and polymer films. In electronics, glass substrates are a common, if not preferred, substrate in many applications, particularly displays and photovoltaics. In combining printing with glass substrates, challenges exist in adapting contact-based printing methods such as gravure to the mechanical properties of the more rigid substrates. In this work, sheet-fed gravure printing has been successfully used to print silver-based conductive inks on glass substrates. Various features were designed and printed to evaluate conductive layers in terms of their printability and electrical performance. The independent variables include gravure cell dimensions, trace orientation with respect to printing direction and ink type. Results from this work provide an insight into the science of gravure printing on glass by correlating the independent variables to printed feature quality and electrical performance.

14%-efficient flexible CdTe solar cells on ultra-thin glass substrates

Flexible glass enables high-temperature, roll-to-roll processing of superstrate devices with higher photocurrents than flexible polymer foils because of its higher optical transmission. Using flexible glass in our high-temperature CdTe process, we achieved a certified record conversion efficiency of 14.05% for a flexible CdTe solar cell. Little has been reported on the flexibility of CdTe devices, so we investigated the effects of three different static bending conditions on device performance. We observed a consistent trend of increased short-circuit current and fill factor, whereas the open-circuit voltage consistently dropped. The quantum efficiency under the same static bend condition showed no change in the response. After storage in a flexed state for 24 h, there was very little change in device efficiency relative to its unflexed state. This indicates that flexible glass is a suitable replacement for rigid glass substrates, and that CdTe solar cells can tolerate bending without a decrease in device performance.

Variable optical attenuator for large-scale integration

A polymer thermooptic variable optical attenuator (VOA) was designed and demonstrated for dense waveguide device integration. The waveguide bend design is compatible with photolithography fabrication techniques and operates by controlling waveguide bend radiation loss. The design optimizes the compromise between integration capability, integration cost, and attenuation efficiency. For a 5 mm bend length, optical attenuation of >40 dB has been achieved with an applied electrical power of 250 mW. The fiber-to-fiber insertion loss was 1.5 dB for a 20-mm total waveguide length. The designs compact size makes it practical for both VOA arrays and dense integrated optical circuits.

An Indium Tin Oxide-Free Polymer Solar Cell on Flexible Glass

Future optoelectronic devices and their low-cost roll-to-roll production require mechanically flexible transparent electrodes (TEs) and substrate materials. Indium tin oxide (ITO) is the most widely used TE because of its high optical transmission and low electrical sheet resistance. However, ITO, besides being expensive, has very poor performance under mechanical stress because of its fragile oxide nature. Alternative TE materials have thus been sought. Here we report the development of a multilayer TiO2/Ag/Al-doped ZnO TE structure and an ITO-free polymer solar cell (PSC) incorporating it. Electro-optical performances close to those of ITO can be achieved for the proposed TE and corresponding PSC with an additional advantage in their mechanical flexibility, as demonstrated by the fact that the cell efficiency maintains 94% of its initial value (6.6%) after 400 cycles of bending, with 6 and 3 cm maximum and minimum radii, respectively. Instead of common plastic materials, our work uses a very thin (0.14 mm) flexible glass substrate with several benefits, such as the possibility of high-temperature processes, superior antipermeation properties against oxygen and moisture, and improved film adhesion.

Electrophoretic Displays Fabricated on Ultra-Slim Flexible Glass Substrates

Ultra-slim flexible glass substrates enable high performance displays and electronic devices through their inherent benefits of a high-quality surface, process compatibility, thermal and dimensional stability, optical transmission, and barrier properties. This study demonstrates use of flexible glass as a backplane substrate for both active matrix displays with organic thin-film transistors (TFTs) as well as segmented displays. The demonstrated 4.7-inch active matrix VGA displays have a resolution of 170 dpi (640×480 pixels), pixel size of 150 μm × 150 μm, and aperture ratio of 40%. The dimensional stability benefit of flexible glass was clearly observed when compared to the registration of polymer substrate devices. This study demonstrates the capability and benefits of flexible glass substrates in devices fabricated with solution-based processing as a step toward roll-to-roll flexible electronic fabrication.

Active Matrix Color-LCD on 75

We have demonstrated a full color 4-in quarter-VGA amorphous silicon active-matrix (AM) LCD (AM-LCD) with 75 μm thick flexible glass backplane and frontplane substrates. The device was built directly on the flexible glass without using a processing carrier or additional protective layers. The overall thickness of the LC cell is <; 170 μ m. Process modifications were made to accommodate the flexibility and reduced thickness of the substrate. These process changes were related to: thin film deposition, photolithography, cell assembly and filling, glass scribing, and driver bonding. The achieved results indicate that the incorporated flexible glass substrates are compatible with the fabrication of high quality color AM-LCDs.

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The practicality of using polysulfone as a host material for electro-optic polymer devices is demonstrated. For loadings of 15–25 weight %, r33 values of 53–55 pm/V were obtained at λ=1.06 μm. This corresponds to chromophore alignment efficiencies of up to 27%. Also, Mach–Zehnder devices demonstrated the implementation of a polysulfone host with a Vπ of 6.9 v, optical loss of 1.8 dB/cm, and thermal stability >100 °C.

m Thick Flexible Glass Substrates

Aluminum-doped zinc oxide (AZO) thin films have been used in low cost transparent conductive oxide (TCO) applications. For flexible electronics, the devices are subjected to cyclic bending during manufacturing and usage, which may lead to both electrical and optical degradation of TCO thin films. This paper was designed to investigate the effect of the strained growth and normal growth methods on the electrical and optical degradation under diverse cyclic bending conditions. The AZO thin films were deposited on a 100 μm thick Corning Willow Glass flexible substrate by using an RF-magnetron sputtering technique. The design of experiments technique was applied to analyze the significant factors that can affect the electrical and optical performance of AZO thin films. The experimental factors include growth methods, bending radius, and tension. From the analysis of the X-ray diffraction technique, the AZO thin films grown by the normal method have dominant (0 0 2) orientation, but the AZO thin films prepared by the strained growth method show other orientations, including (0 0 2) orientation. Although the strained growth method does change the AZO thin film properties, the strained growth method does not significantly improve the reliability of the AZO thin film after a 2000 cycle bending fatigue test.

Polysulfone as an electro-optic polymer host material

Flexible and transparent (FT) ZnO thin film based surface acoustic wave (SAW) devices using indium tin oxide (ITO) electrodes were fabricated on ultrathin flexible glass substrates. The influence of the annealing process and ITO thickness on the optical properties and acoustic wave power transmission properties of the devices was investigated. The performance of the devices improved significantly when the annealing temperature was raised up to 300 °C. The flexible glass based SAW devices exhibited similar power transmission performance, but have a better optical transmittance than those on rigid glass. These FT strain sensors worked well under various applied strains up to ±3000 μe with fast response time, and showed excellent linearity of resonant frequency with the change of strain with a sensitivity of ∼34 Hz μe−1. The strain sensors demonstrated excellent stability and reliability under cyclic bending. The results demonstrated great potential of applications of the FT-SAW device based strain sensors on flexible glass substrates.

Strained Growth of Aluminum-Doped Zinc Oxide on Flexible Glass Substrate and Degradation Studies Under Cyclic Bending Conditions

Flexible glass substrates enable high-quality device fabrication through unique properties such as very low surface roughness and thermal and dimensional stability. As an initial step toward roll-to-roll manufacturing of active matrix displays, this paper describes the structure and performance of organic TFT backplane electrophoretic displays fabricated on flexible glass substrates.