Flexible inorganic vertical microLEDs for wearable systems

Abstract

In modern society, flexible electronic materials have been attracted lots of interests in next generation information technology (IT), such as Internet of Things (IoT), wearable electronic devices and biomedical devices. Previously, flexible electronics were intensively studied and developed by several research groups. However, the critical issues are still remain, which should be solved to apply these technologies to the practical applications, such as wearable sensor, computer, and display. For example, the electronic materials on flexible substrates must have excellent intrinsic characteristics (e.g., thermal, optical, and electrical properties) as well as outstanding mechanical durability under severe bending/twisting motions. Although there have been many impressive studies in this research field, the device properties of flexible materials could not exceed the excellent performance of high-temperature-annealed inorganic materials. Therefore, new and innovative solutions are essential for high performance wearable systems.

Improving the user-interface (UI) with electronic devices requires new technology development in accordance with a flexible and convertible architecture. The flexible display can be expected as the one of main components. Flexible displays can be used to transform space and efficiently improve space usage, because they are thin, lightweight, and not fragile. It can be applied to areas such as wearable smart devices, mobile head-up display and smart window. For developing these next-generation flexible display, the development of flexible components such as operation circuit, energy source, and light-emitting device should take priority.

In chapter 2, Flexible transparent display is a promising candidate to visually communicate with each other in the future Internet of Things era. The flexible oxide thin-film transistors (TFTs) have attracted attention as a component for transparent display by its high performance and high transparency. The critical issue of flexible oxide TFTs for practical display applications, however, is the realization on transparent and flexible substrate without any damage and characteristic degradation. Here, the ultrathin, flexible, and transparent oxide TFTs for skin-like displays are demonstrated on an ultrathin flexible substrate using an inorganic-based laser lift-off process. In this way, skin-like ultrathin oxide TFTs are conformally attached onto various fabrics and human skin surface without any structural damage. Ultrathin flexible transparent oxide TFTs show high optical transparency of 83% and mobility of $40 cm^2 V^(-1) s^(-1)$. The skin-like oxide TFTs show reliable performance under the electrical/optical stress tests and mechanical bending tests due to advanced device materials and systematic mechanical designs. Moreover, skin-like oxide logic inverter circuits composed of n-channel metal oxide semiconductor TFTs on ultrathin, transparent polyethylene terephthalate film have been realized.

In chapter 3. Flexible thermoelectric generators (f-TEGs) are emerging as a semi-permanent power source for self-powered electronic systemsrs, which is an important area of research for the next generation smart network monitoring system in the internet-of-things (IoT) era. We report in this paper a f-TEG produced by a screen-printing technique (SPT) and a laser multi-scanning (LMS) lift-off process. A screen-printed TEG was fabricated on a SiO_2/a-Si/quartz substrate via the SPT process and then the LMS process completely separated the rigid quartz substrate from the original TEG by selective reaction of the XeCl excimer laser with the exfoliation layer (a-Si). Using these techniques, we fabricate a prototype f-TEG composed of an array of 72 TE couples that exhibits high flexibility at various bending radii together with excellent output performance (4.78 mW/$cm^2$ and 20.8 mW/g at a △T = 25 K). There is no significant change in the device performance even under repeated bending of 8,000 cycles.

In chapter 4, a high-performance flexible vertical-structured 30×30 light-emitting didode (LED) array is realized by a new monolithic LED fabrication method without device-transfer process. Free-standing vertical-structured microLEDs (VLEDs) have a perpendicularly electrical interconnection with n- and p-electrodes. The optical characteristics of LED were enhanced by unique structure, based on theoretical and experimental results. The monolithic flexible VLEDs (f-VLEDs) exhibited a low forward voltages (~2.8 V), excellent optical power (~30 mW/$mm^2$) and outstanding mechanical durabilityy during 100,000 bending cycles without severe mechanical damages. The 30×30 monolithic red LED arrays were successfully demonstrated with high flexibility, density and uniformity. Finally, the flexible high-density phototherapy device was inserted under the mouse skin.

In chapter 5, a transparent, and flexible 30×30 GaN VLED arrays are fabricated by simple monolithic LED process. Ultrathin, transparent flexible GaN VLEDs were conformally affixed to human finger nail The lifespan of monolithic GaN LED is calculated over 10 years by Arrhenius equation and the high accelerated stress test (HAST). Wireless monolithic blue f-VLEDs showed a low forward voltages (~2.6 V) and outstanding optical power (~30 mW/$mm^2$) with light linearity. Our f-VLEDs exhibited excellent mechanical durability during the periodic bending/unbending 100,000 cycles. Finally, the biocompatible f-VLEDs were successfully inserted under the mouse skull through, and covered the live mouse brain for optical stimulation.

In chapter 6, a wearable and washable 20×20 microLED arrays are demonstrated by eposy-siloxane molecular hybrid (ESMH) material and transparent elastomeric adhesive (TEA). The wearable VLED array was stably driven on glass fabric and general textile (100 % cotton and cotton with 47% polyester ) in a detergent solution. The transfer medium of ESMH and TEA were deeply investigated by various analysis tools, such as NMR, TGA, TMA, SEM and EDS elemental mapping. Our wearable VLEDs presented outstanding mechanical reliability during severe 100,000 bending/unbending motions.