Monolithic Flexible Vertical GaN Light‐Emitting Diodes for a Transparent Wireless Brain Optical Stimulator

Abstract

Flexible inorganic-based micro light-emitting diodes (µLEDs) are emerging as a significant technology for flexible displays, which is an important area for bilateral visual communication in the upcoming Internet of Things era. Conventional flexible lateral µLEDs have been investigated by several researchers, but still have significant issues of power consumption, thermal stability, lifetime, and light-extraction efficiency on plastics. Here, high-performance flexible vertical GaN light-emitting diodes (LEDs) are demonstrated by silver nanowire networks and monolithic fabrication. Transparent, ultrathin GaN LED arrays adhere to a human fingernail and stably glow without any mechanical deformation. Experimental studies provide outstanding characteristics of the flexible vertical μLEDs (f-VLEDs) with high optical power (30 mW mm ), long lifetime (≈12 years), and good thermal/mechanical stability (100 000 bending/unbending cycles). The wireless light-emitting system on the human skin is successfully realized by transferring the electrical power f-VLED. Finally, the high-density GaN f-VLED arrays are inserted onto a living mouse cortex and operated without significant histological damage of brain.

Micro Light‐Emitting Diodes for Display and Flexible Biomedical Applications

“…Comparison of liquid crystal display (LCD), organic light‐emitting diode (OLED) and inorganic‐based micro light‐emitting diode (µLED). Reproduced with permission . Copyright 2018, Wiley‐VCH.…”

“…µLED transfer from a mother substrate to a target substrate is regarded as one of the most important technologies for large‐scale and high‐pixel full‐color displays . As shown in Figure , several transfer methods such as electrostatic transfer, elastomer stamping, laser‐assisted transfer, electromagnetic transfer, pressure‐based transfer, and ACF adhesive transfer methods have been developed to assemble µLEDs . Electrostatic transfer utilizes voltage‐induced adhesion to pick up µLEDs from a mother substrate and subsequently release them onto a target substrate .…”

“…The µLED fields are currently expanding to various wearable, internet of things (IoT), and biomedical applications such as virtual reality (VR) applications, smart watches, and medical sensors for hyperconnected society . In addition, flexible µLEDs have attracted a lot of attentions, because it can be applied to curvilinear or dynamic skin surfaces, clothes, and automobiles to provide user‐friendly and biocompatible optoelectronics based on human–machine interface …”

“…The structure and the packaging technology of the thin‐film µLEDs are the important factor for the device performance, reliability, and productivity. The device structure determines the overall LED characteristics including power efficiency, brightness, and thermal stability, which are critical factors for display and flexible biomedical applications . Various types of thin‐film µLEDs (e.g., lateral structured µLED, vertical structured µLEDs, and flip chip lateral structured µLED) have been exploited for strategies of enhancing µLED performance .…”

“…The device structure determines the overall LED characteristics including power efficiency, brightness, and thermal stability, which are critical factors for display and flexible biomedical applications . Various types of thin‐film µLEDs (e.g., lateral structured µLED, vertical structured µLEDs, and flip chip lateral structured µLED) have been exploited for strategies of enhancing µLED performance . Packaging technology, related to the interconnection of transferred µLEDs to other electronic components and their protection from the external stress, is also crucial for fully functional optoelectronic systems .…”