Research Field

Wearable electronics have significant potential to shift personal computing toward new wearable and healthcare applications, such as robotic skin, conformal photovoltaics, and wireless communication systems. For instance, they can be mounted on curved and dynamic surfaces of the skin to monitor the body’s vital signs through real-time/bilateral data transmissions based on a human–machine interface (HMI), thus realizing a hyperconnected society. Commercial wearable devices exist predominantly in the form of patches, bands, and types of textiles that yield strains associated with motions and detaching stresses in the wrist, elbow, and waist, which requires wide range of stretchability over 20%.

Persistent progress of the photonic technology from the 1960s not only has fulfilled the Moore’s Law-driven processing time limit (one second) but also provided significant impacts in a wide range of fields, including academic research, material processing, and biomedical applications. Over the past few years, these light-material interaction technologies have earned even more attention for wearable electronics due to its exceptional capability to stimulate spatiotemporally controlled, transient, multi-physical, and non-equilibrium photon reactions without polymer substrate damage. For example, flexible active-matrix organic light-emitting diode (f-AMOLED) enabled via excimer laser annealing (ELA) was commercialized in 2019 as a foldable phone display by Samsung Electronics.

Therefore, we aim to commercialize various light-induced wearable systems such as flexible displays, solar cells, healthcare sensors, communication devices, and etc. For this purpose will perform collaborative studies with a variety of research fields, including precise optical design, product quality optimization, manufacturing automation technology, and equipment and production system. Through this efforts, we intend to contribute to the Korean IT industry by replacing numerous conventional technology for future wearable platform.

High-performance wearable materials (e.g., metal electrodes, two-dimensional materials, and inorganic semiconductors) can be implemented on plastic substrates without thermal damage by rapid light interaction due to its exceptional capability to stimulate physical/chemical reactions accompanying extremely high spatial and temporal control. Nonetheless, experimental and theoretical research has not been extensively performed due to its high degree of freedom (e.g., ultrashort reaction time, non-equilibrium photon reaction, narrow processing window, material related variables), which requires numerous trials and errors for searching optimized processing conditions. Therefore, fundamental investigation on photothermal interaction is needed to demonstrate high-performance flexible materials and devices for optimum light-induced reaction that can significantly improve physical material properties.

Therefore, we will perform fundamental investigations on the photo-thermo-chemical interaction mechanism of materials based on multiscale simulations. The light-material interaction process undergoes four (a → d) sequential stages (a: light absorption of material, b: conversion of light energy to thermal energy, c: diffusion of thermal energy, d: physical/chemical change in material properties). By applying a suitable simulation tool at each stage, trial and error for the experimental optimization process can be significantly reduced, as follows.

1) ​​In the light-matter interaction, it is required to calculate the absorption rate according to the wavelength, intensity, pulse width, absorbance and refractive index of the light. To do this, we want to find the absorbance speed of a material through a discrete dipole approximation method (DDA).

2) Secondly, the calculated absorbance rate will be applied to molecular dynamics (MD) and phase field model (PFM) to predict the material behavior. We will verify the simulation results based on experimental measurement data such as reflectivity and refractive index of the changed material.

3) Nextly, ideal photothermal energy transfer process will be calculated through finite element method (FEM) by controlling the thickness, thermal conductivity, and heat capacity of the materials.

4) Based on the above simulation results and experimental feedback process, we will establish a model for the phtotothermal annealing of various wearable materials.

The conventional photolithographic processes have been well developed for silicon based electronics since the second half of the 20th century. However, previous IC fabrication methods are subjected to limitations since they require multi-step process, high processing temperatures, and toxic etchant. In addition, the increasing size of electronic devices such as displays or solar cells astronomically raise manufacturing cost especially due to expensive vacuum chamber and photo-mask.

In this regard, photon-associated thermal processing technologies have earned significant attention as an alternative to lithographic process. Its exceptional capability to stimulate spatiotemporally controlled, transient, multi-physical, and non-equilibrium photon reactions enables simple, scalable, eco-friendly, and low temperature fabrication of novel materials on large-area substrates without damaging adjacent electronic components. For example, crystallization of amorphous silicon using excimer laser annealing (ELA) reported in 1994 has been broadly used to fabricate high-quality polycrystalline silicon and to change its microstructure, enabling the commercialization of thin-film transistors (TFT) with enhanced electron mobility for active matrix flat-panel displays in both mobile phones and flat-screen televisions.

Transfer technology can demonstrate soft electronics with excellent electrical performance, exceeding those previously demonstrated on plastic substrates. Through this process, devices initially fabricated on a bulk wafer can be transferred onto a flexible substrate, eliminating additional problematic multi-alignment and nanofabrication steps on plastics. In recent years, there have been novel approaches for transferring entire devices that have been fully fabricated on rigid substrates at high temperature to flexible substrates. Several clever methodologies such as chemical/mechanical thinning of the wafer, epitaxial layer transfer, and stress-controlled exfoliation have been explored to achieve mechanical flexibility, high performance, nanoscale features, nanoscale alignment, and multi-functionality. Although these works have shed a positive light on high-performance flexible electronics, major issues such as the sophisticated process, limited applicability, high cost, and unpredictability of the transfer still remain to be resolved.

Recently, light-induced lift-off technology has been developed to transfer entire devices onto plastic films to realize flexible, high-performance, and multifunctional electronics with excellent complementary metal– oxide–semiconductor (CMOS) compatibility. The representative mechanisms of the photon-associated lift-off processes are as follows. When a light (e.g., excimer (XeCl) laser) is irradiated to the backside of the rigid substrate with a high bandgap (larger than input photon energy), light can pass through the transparent substrate, reaching to the light absorbing material. Depending on light interacting films (e.g., lead zirconate titanate, gallium nitride (GaN), and hydrogenated amorphous Si), various photothermal interactions such as melting/vaporizing, dissociating (e.g., 2GaN(solid) → 2Ga(liquid) + N2 (gas)), and explosive gas releasing occurred in the sacrificial layer, which weaken the interfacial adhesion between the substrate and active device. This low adhesion enables ultrathin electronics to be safely peeled off from the mother substrate without any mechanical deformations, cracks, or wrinkles.

Three-dimensional (3D) multimaterials allow the rapid design and fabrication of materials that has tunable mechanical, electrical and other functional properties, enabling the programming of their composition and architecture across various length scales. The expanding range of 3D multimaterials coupled with low-temperature photonic annealing technologies can offer novel paradigm toward myriad wearable applications, including flexible internet of things (IoTs), wireless communication systems on plastics, conformal biomedical devices, soft robotics, and wearable metamaterials.

To achieve the final research goals, we will hybridize photonic annealing technologies and 3D printing process to demonstrate rapid structuring of seamless 3D multimaterials. Ultra-precision laser, or large area flash light will be sequentially irradiated in milliseconds during the successive layer by layer printing process based on the comprehensive understanding of the photothermal interaction model. A variety of flash light parameters such as wavelength, intensity, pulse shape, pulse duration, and pulse repetition rate will be precisely tuned to induce optimal photonic annealing effect, including sintering, crystallization, grain growth, densification, doping, and synthesis. For the demonstration of specific applications such as IoTs, healthcare devices, and metamaterials, 3D printer will programmably control the material types and arrangements by computer software. And then, the ideal photothermal annealing process with designed experimental variables will be applied to discover core creative materials and systems on wearable substrates.

The high-performance wearable platform enables the realization of various technologies for the 4th industrial revolution era such as internet of things (IoTs), artificial intelligence (AI), bio electronics, augmented reality (AR), and etc. For this purpose, it is necessary to connect not only the field of bendable/stretchable materials and devices, but also various research areas including heat/fluid engineering, AI-based human-machine interface, production and control systems, and reliability evaluation technologies.

Therefore, we will try to merge wide range of research fields from light-induced processing to mass/intelligent production systems (e.g., roll-to-roll manufacturing process, smart factory system) for developing source technology that can be practically applied in future wearable platform industry.