As displays and electronics evolve to become lighter, thinner, and more flexible, the choice of substrate continues to be critical to their overall optimization. The substrate directly affects improvements in the designs, materials, fabrication processes, and performance of advanced electronics. With their inherent benefits such as surface quality, optical transmission, hermeticity, and thermal and dimensional stability, glass substrates enable high-quality and long-life devices. As substrate thicknesses are reduced below 200 μm, ultra-slim flexible glass continues to provide these inherent benefits to high-performance flexible electronics such as displays, touch sensors, photovoltaics, and lighting. In addition, the reduction in glass thickness also allows for new device designs and high-throughput, continuous manufacturing enabled by R2R processes. This paper provides an overview of ultra-slim flexible glass substrates and how they enable flexible electronic device optimization. Specific focus is put on flexible glass’ mechanical reliability. For this, a combination of substrate design and process optimizations has been demonstrated that enables R2R device fabrication on flexible glass. Demonstrations of R2R flexible glass processes such as vacuum deposition, photolithography, laser patterning, screen printing, slot die coating, and lamination have been made. Compatibility with these key process steps has resulted in the first demonstration of a fully functional flexible glass device fabricated completely using R2R processes.

Ultra-slim flexible glass for roll-to-roll electronic device fabrication

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.

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

Flexible glass substrate based dye sensitized solar cells

In today's society, displays are indispensable for digital interaction and personal contact. Therefore, the development of displays is an urgent need for the next generation. Leading the commercialization trends is the development of industrial, high-resolution, portable, and multifunctional displays. Moreover, for small-size portable displays, the most critical factor affecting future applications is their flexibility. Among the various materials that can act as the channel layer, low-temperature polycrystalline silicon (LTPS) is a potential candidate for next-generation portable displays, which require high resolution and stable reliability, and which can achieve virtual reality applications. This article is a review of the development of the LTPS thin-film transistors (TFTs) on soft and flexible electronics, especially the effect of mechanical strain. This article starts by providing an overview of the difficulties in fabricating LTPS TFTs on flexible substrates. The physical mechanism corresponding to each degradation caused by mechanical stress is presented next. Finally, to support the development of the flexible technologies and realize their commercialization, methods to overcome these mechanical strain-induced degradations are reported in three different aspects by understanding the physical degradation model.

Flexible low-temperature polycrystalline silicon thin-film transistors

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.

Bendable transparent ZnO thin film surface acoustic wave strain sensors on ultra-thin flexible glass substrates

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.

O. Kuhls

Papers

Active Matrix Color-LCD on 75 $\mu$ m Thick Flexible Glass Substrates