2. Flexible Micro Light-Emitting Diodes
The μLED has been spotlighted as a novel fusion technology of information technology (IT) and nanotechnology (NT) and is proposed as the next-generation light source to solve numerous problems of existing OLED and QLED displays (e.g., afterimage, burn-in effect, heat/water instability, and slow response time). Figure 2 shows a comparison table among QLED, OLED, and μLED displays in terms of their emission type, thickness, optical property, lifetime, and cost. The μLEDs are self-emissive light sources with high brightness and a longer lifetime than OLED and QLED, and display excellent image quality. In particular, μLEDs of extremely small size (width from 10 to 100 μm and thickness under 5 μm) have superior optical/electrical properties, such as excellent luminescence efficiency over 100 lm W−1, strong illumination over 105 cd m−2, high color contrast, and superior power efficiency.
Figure 2. Comparison of QLED, OLED, and μLED display in terms of structure, optical/electrical properties, lifetime, and cost (red: disadvantages, blue: advantages).
Therefore, in the display industry, it is predicted that high-performance μLEDs will even enter fine-pitch display fields that are dominated by OLEDs and QLEDs, as shown in Figure 3. Despite several advantages of μLEDs, there are still some challenges in realizing commercialization, such as efficient packaging, chip structure innovation, and development of a new transfer methodology.
Figure 3. Development prospect of light sources as display components.
First, flexible μLEDs are fabricated as follows 
: III-V compound semiconductor layers are grown by metalorganic chemical vapor deposition (MOCVD), metalorganic vapor-phase epitaxy (MOVPE), or molecular beam epitaxy (MBE) on the mother substrate (e.g., silicon, GaAs, and sapphire), forming a thin-film LED structure with a p-type material, a multi quantum well (MQW), and an n-type material. The epitaxial layers are etched into numerous microscale chips through micro-electromechanical systems (MEMS) processes. The LEDs on the mother substrate are selectively peeled off and placed into a specific position on a target substrate. Finally, the μLEDs are electrically connected and packaged for passivating the device from environmental stresses.
The μLEDs can typically be divided into three types (Figure 4). There are fully packaged bulk LEDs, as well as lateral- and vertical-structured μLEDs, which are made by III-V or III-N compound semiconductors on epitaxial substrate (e.g., III-V: GaAs; III-N: Si, SiC, and sapphire). They have different characteristics in thermal, electrical, and optical properties, resulting from structural differences of bulk/thin-film and electrode location. Lateral μLEDs have a long current path of about 50~100 μm between electrodes, whereas vertical μLEDs have a short current path under 5 μm, inducing negligible Joule heating during device operation. Thanks to this structural advantage, vertical-structured microLEDs have various superiorities compared to others, including a small size, a fast/cheap process, superior electrical/thermal properties, and simple wiring. Particularly, vertical-structured μLEDs with low heating are most suitable to realize biomedical applications due to their thermally noninvasive property.
Figure 4. Comparison of bulk LED, lateral-, and vertical-structured μLEDs; red: disadvantages, blue: advantages).
As mentioned above, the processes, i.e., the transfer technology of numerous μLED chips from the mother wafer to the target substrate, are the most important factors for developing μLED-based electronic systems, such as displays, biomedical therapeutic devices, and healthcare sensors. Since the transfer efficiency is directly related to the device yield, manufacturing time, and unit cost, several researchers around the world have attempted to transfer thin-film μLED chips through various methods. The polydimethylsiloxane (PDMS)-based pick-and-place method was the most widely used for μLED transfer. The μLED chips were attached to sticky PDMS by van der Waals adhesive force and released onto the adhesive-coated target substrate. Furthermore, although various approaches, such as electrostatic or electromagnetic force-based transfer, were suggested as new transfer methods, they still had many problems, including thermal/electrical damage to the LED layers, slow transfer speed, and difficulty in large-area processing. Recently, novel technology was proposed by using an anisotropic conductive film (ACF) 
. Thin-film μLEDs were simultaneously attached and packaged with the target substrate through ACF by applying heat and pressure. The ACF-based transfer was performed with a rapid process speed, high yield, and large-area transfer, enabling electrical connection with complicated electrodes at the same time as the μLED transfer.
a shows a self-powered flexible μLED array on a flexible plastic film. Jeong et al. made a flexible μLED array by transferring from a gallium arsenide (GaAs) wafer to a polyimide (PI) substrate by using the ACF transfer/packaging method 
. Finally, μLEDs were integrated with a flexible piezoelectric energy harvester to complete a self-powered display system. Figure 5
b indicates a flexible 3 × 3 μLED array in a bent state (bending curvature radius of 5 mm) illuminating brilliant light. Figure 5
c is a luminance–current–voltage (L–I–V) graph of the fabricated flexible μLED. As shown in the bottom inset image, the 3 × 3 μLED arrays were driven by a ~3 V, emitting red light with a wavelength of 653 nm. When mechanical force was applied to the thin-film piezoelectric material (lead zirconate titanate) of the device, the energy harvester generated an electrical power for operating a flexible μLED array without any external power source. The μLEDs were repeatedly turned on and off by the pulsed electrical power from the energy generator, as depicted in the upper inset graph of Figure 5
) Schematic image of self-powered flexible μLEDs that were composed of flexible red μLEDs and a thin-film piezoelectric power generator. (b
) Photograph of flexible red μLED array. The inset image is a top view of the device. (c
) I-V curve of flexible μLED arrays. The top inset shows output current and voltage from a thin-film energy harvester. The bottom inset indicates flexible red μLEDs, operated by a piezoelectric energy harvester. Reproduced with permission 
. Copyright 2014, Royal Society of Chemistry.
a displays a monolithic flexible GaN μLED array with a 30 × 30 passive-matrix circuit 
. Lee et al. developed the monolithic fabrication process for flexible, vertical-structured μLEDs (f-VLEDs). The GaN μLED array was fabricated on a rigid sapphire wafer and then exfoliated with a laser lift-off process. The freestanding GaN μLED array was isolated with epoxy-based polymer. After the electrical interconnection of μLED chips by a silver nanowire (AgNW)-based electrode, the device was passivated by the biocompatible parylene-C layer. After flipping over the device, the AgNW top electrode was made on the device. As shown in the inset images of Figure 6
a, the high-density blue μLED array with passive-matrix structure was operated at a bending curvature radius of 5 mm. Since the device was electrically connected by a transparent AgNW network, the μLEDs in off-state were invisible on the human fingernail (Figure 6
b and its inset image). According to heat flux simulation, flexible vertical μLEDs had superior thermal stability compared to flexible lateral μLEDs due to efficient heat dissipation through the bottom electrode. The GaN f-VLED displayed excellent mechanical durability during 105
bending/unbending cycles because its mechanical neutral plane was placed at the center of μLED chips (Figure 6
c). Furthermore, the device lifetime was expected to last 11.9 years. In accordance with these results, monolithic f-VLEDs are anticipated to commercialize in the display and biomedical device fields.
) Photograph of monolithic flexible blue μLEDs with 30 × 30 array. The top inset is a magnified optical image of a passive-matrix μLED array. Scale bar: 1 cm. The bottom inset is an optical image of monolithic blue μLEDs in a bent state. (b
) Transparent flexible blue μLEDs on human fingernail. The inset image shows fingernail-attached transparent device in off-state. (c
) Fatigue test results of monolithic flexible μLEDs during 106
bending/unbending cycles. Reproduced with permission 
. Copyright 2018, Wiley-VCH. (d
) Normalized forward voltage of 50 × 50 μLED arrays with flash-induced Cu electrodes. Reproduced with permission 
. Copyright 2021, Wiley-VCH. (e
) Optical image of wearable red μLED on fabric substrate. The inset is a cross-sectional SEM image of wearable μLED. (f
) I-V characteristics of wearable μLEDs with various size. Reproduced with permission 
. Copyright 2019, Elsevier.
Shin et al. investigated robust Cu electrodes for μLED display applications 
. Although Cu-based electrodes were spotlighted for application in display circuits due their cheap price, robustness, and high conductivity, Cu had a critical delamination issue on rigid glass substrate. Shin et al. resolved this issue through flashlight-induced Cu-glass interlocking. By radiating a high-powered flashlight on CuO nanoparticles, the CuO was reduced to a conductive Cu layer, simultaneously forming strong adhesion with glass. The 50 × 50 μLED arrays were interconnected with a flash-inducted robust Cu electrode by a thermo-compressive ACF bonding process. The developed device showed thermal/humid stability and high uniformity in large-scale μLED arrays, as shown in Figure 6
d. Lee et al. developed wearable μLEDs (WμLEDs) on a 100% cotton fabric 
. WμLEDs were fabricated using the same protocol with monolithic f-VLED and transferred through transparent elastomeric adhesive by the thermo-compressive process. Figure 6
e displays WμLEDs emitting red light at a bent state. The attachment stability between μLED chips and fabric was sophisticatedly investigated by finite element method (FEM) simulation, tensile tests, peel-off tests, digital image correlation (DIC) analysis, and bending fatigue tests. As shown in Figure 6
f, WμLEDs with various chip sizes were operated at a forward voltage of ~2.8 V. The WμLEDs showed potential for outdoor application through chemical, thermal, humid, and artificial sunlight tests.