Chang-Min Keum

Chang-Min Keum
University of St Andrews · School of Physics and Astronomy

Ph.D.

About

62
Publications
7,302
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1,073
Citations
Additional affiliations
March 2008 - August 2014
Seoul National University
Position
  • PhD

Publications

Publications (62)
Article
Full-text available
Three novel donor-acceptor molecules comprising the underexplored pyridazine (Pydz) acceptor moiety have been synthesized and their structural, electrochemical and photophysical properties thoroughly characterized. Combining Pydz with two phenoxazine donor units linked via a phenyl bridge in a meta configuration ( dPXZMePydz ) leads to high reverse...
Article
Full-text available
Despite widespread interest, ultrathin and highly flexible light-emitting devices that can be seamlessly integrated and used for flexible displays, wearables, and as bioimplants remain elusive. Organic light-emitting diodes (OLEDs) with µm-scale thickness and exceptional flexibility have been demonstrated but show insufficient stability in air and...
Article
Full-text available
Top‐emitting organic light‐emitting diodes (OLEDs) are of interest for numerous applications, in particular for displays with high fill factors. To maximize efficiency and luminance, molecular p‐doping of the hole transport layer (p‐HTL) and a highly reflective anode contact, for example, made from silver, are used. Atomic layer deposition (ALD) is...
Article
Full-text available
The accurate characterization of thin‐film light emitting diodes (LEDs)—including organic light emitting diodes (OLEDs), perovskite LEDs, and quantum dot LEDs—is crucial to the understanding of the factors that influence their efficiency and thus to the fabrication of LEDs with improved performance and stability. In addition, detailed information a...
Preprint
Despite widespread interest, ultrathin and highly flexible light-emitting devices that can be seamlessly integrated and used for flexible displays, wearables, and as bioimplants remain elusive. Organic light-emitting diodes (OLEDs) with $\mu$m-scale thickness and exceptional flexibility have been demonstrated but show insufficient stability in air...
Article
Full-text available
Liquid crystalline materials possess great potential as emitters in organic light‐emitting diodes (OLEDs) due to their self‐assembling property, which may lead to anisotropic films and improved charge transport. Here, the key photophysical properties of the columnar liquid crystalline emitter perylene‐3,4,9,10‐tetracarboxylic tetraethyl ester (PTCT...
Article
Full-text available
Fluorescent proteins have emerged as an attractive gain material for lasers, especially for devices requiring biocompatibility. However, due to their optical properties, integration with distributed feedback (DFB) resonators is not readily achievable. Here, a DFB laser with enhanced green fluorescent protein (eGFP) as the gain material is demonstra...
Article
Organic light‐emitting diodes (OLEDs) can emit light over much larger areas than their inorganic counterparts, offer mechanical flexibility, and can be readily integrated on various substrates and backplanes. However, the amount of light they emit per unit area is typically lower and the required operating voltage is higher, which can be a limitati...
Conference Paper
Orientation of transition dipoles can strongly improve the outcoupling efficiency of or-ganic LEDs (OLEDs). We present two new classes of OLED emitters that may achieve horizontal alignment: columnar liquid crystals and carbon nanotubes.
Article
Full-text available
Two bipolar host materials 3‐CBPy and 4‐mCBPy are reported. These hosts are structural analogs of the common host materials CBP and mCBP wherein the phenyl rings have been replaced with pyridines. The two materials possess deep highest occupied molecular orbital (HOMO) and shallow lowest unoccupied molecular orbital (LUMO) levels along with suffici...
Article
A series of four 1,3,4-oxadiazole-based thermally activated delayed fluorescence (TADF) derivatives are reported as emitters for organic light emitting diodes (OLEDs). As a function of the nature of the substituent on the weak 1,3,4-oxadiazole acceptor their emission color could be tuned from green-blue to blue. The highly twisted conformation betw...
Article
Full-text available
Fluorescence imaging is an indispensable tool in biology, with applications ranging from single‐cell to whole‐animal studies and with live mapping of neuronal activity currently receiving particular attention. To enable fluorescence imaging at cellular scale in freely moving animals, miniaturized microscopes and lensless imagers are developed that...
Article
Doping organic semiconductors has become a key technology to increase the performance of organic light emitting diodes, solar cells, or field-effect transistors (OFETs). However, doping can not only be used to optimize these devices, but to enable new design principles as well. Here, a novel type of OFET is reported - the vertical organic tunnel fi...
Article
Research in organic permeable base transistors (OPBTs) has led to a significant increase in their performance. However, despite this progress, understanding of the working mechanism of OPBTs is still limited. Although first numerical models of OPBTs are able to describe the switching mechanism of OPBTs correctly, they neglect currents injected at t...
Article
Herein, the underlying mechanisms for the growth of conductive filaments (CFs) at a metal–polymer electrolyte interface through ion migration in organic electrochemical metallization (ECM) memristor are presented. It is observed that the free volume of voids (nanopores) in the polymer electrolyte serves as the pathways of metal‐cations whereas the...
Article
Full-text available
Optogenetics, photostimulation of neural tissues rendered sensitive to light, is widely used in neuroscience to modulate the electrical excitability of neurons. For effective optical excitation of neurons, light wavelength and power density must fit with the expression levels and biophysical properties of the genetically encoded light‐sensitive ion...
Article
Organic permeable‐base transistors (OPBTs) show potential for high‐speed, flexible electronics. Scaling laws of OPBTs are discussed and it is shown that OPBT performance can be increased by reducing their effective device area. Comparing the performance of optimized OPBTs with state‐of‐the‐art organic field‐effect transistors (OFETs), it is shown t...
Article
Organic electrochemical transistors (OECTs) are becoming a key device in the field of organic bioelectronics. For many applications of OECTs, in particular for enzymatic sensing, a complex mixture of room temperature ionic liquids (RTILs) combined with other electrolytes is used as a gate electrolyte, making the interpretation of experimental trend...
Article
4,7‐Diphenyl‐1,10‐phenanthroline (BPhen) is widely used to create the electron transport layer (ETL) in organic light‐emitting diodes (OLEDs) because of its high electron mobility and good compatibility with alkali metal n‐dopants. However, the morphology of these ETLs is easily altered by heating due to the relatively low glass transition temperat...
Article
Full-text available
Biologically derived fluorescent proteins are attractive candidates for lasing and sensing due to their excellent optical properties, including their high quantum yield, spectral tunability, and robustness against concentration quenching. Here, a time-resolved study of the fluorescence dynamics of protein thin films is reported for the enhanced gre...
Article
Full-text available
Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical pro...
Conference Paper
Here, we demonstrate the successful application of organic light-emitting diodes in optogenetics to control neurons in fruit fly larvae and explore their further use as a light source in fluorescence microscopy.
Article
We demonstrate a flexible multi-level resistive memory device, based on the concept of electrical triggering into an insulating layer, which exhibits a high current ratio between two memory states in the unipolar mode of operation. Due to the presence of a triggering zone in unit memory cell, three resistive memory states having different domain si...
Article
We present a general principle of topography-directed (TD) inkjet printing for functional micro-tracks embedded in a flexible elastomer substrate. The essential features of the TD inkjet printing in a micro-structured substrate with periodic grooves and ridges are described in terms of the topographic parameters for the transformation from a single...
Article
Like most of the vertical transistors, the Patterned Source Vertical Organic Field Effect Transistor (PS-VOFET) does not exhibit saturation in the output characteristics. The importance of achieving a good saturation is demonstrated in a vertical organic light emitting transistor; however, this is critical for any application requiring the transist...
Article
We report the effect of the vapor pressure-dependent solvent evaporation on the electrical performance of solution-processed organic field-effect transistors (OFETs). In our study, a p-type organic semiconductor, 6,13- bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) was used as a soluble active material. For different solvent mixtures of alkane...
Article
Structuring of organic layers is essential for large-scale integration and manufacturing of organic devices. A novel photo-patterning method for organic semiconductors is presented, which allows to use dry-etching to transfer resist patterns into metal layers on top of organic layers. Apart from the general advantages of dry-etching, such as reduce...
Article
We demonstrate a vertical organic light emitting transistor (VOLET) showing a high current on/off ratio through dielectric encapsulation of a finger-like source electrode, which is placed between the bottom gate electrode and the top drain electrode. In such a VOLET configuration, the charge transport occurs largely from the source electrode to an...
Article
Understanding charge trapping in organic semiconductors is key to the development of highly efficient organic devices. Here, charge trapping in organic p-i-n Zener diodes is studied and an analytical model able to describe the influence of the trapped charges on the electrical performance is proposed. The model shows a good agreement with the exper...
Article
The development of low-frequency-driven liquid crystal displays (LCDs) has recently received intense attention to open up low-power consumption display devices, such as portable displays, advertising panels and price tags. In fringe-field switching (FFS) LCD mode, a unidirectional electric field gives rise to head-tail symmetry breaking in liquid c...
Article
Doping allows to control the majority and minority charge carrier concentration in organic field-effect transistors. However, the precise mechanism of minority charge carrier generation and transport in organic semiconductors is largely unknown. Here, the injection of minority charge carriers into n-doped organic field-effect transistors is studied...
Article
Organic field-effect transistors hold the promise of enabling low-cost and flexible electronics. Following its success in organic optoelectronics, the organic doping technology is also used increasingly in organic field-effect transistors. Doping not only increases device performance, but it also provides a way to fine-control the transistor behavi...
Article
We investigate the effect of the interfacial properties between a polymer dielectric and an organic semiconductor (OS) layers on the photoresponse properties of an organic phototransistor (OPT). Three different polymer materials having different interfacial properties, poly(methyl methacrylate), amorphous fluoropolymer CYTOP, and poly(4-vinylphenol...
Article
Organic p-i-n diodes enable the development of highly efficient organic devices such as organic light-emitting diodes. Understanding charge carrier trapping in these diodes is essential to comprehensively describe their electrical behaviors and increase their efficiency further. Here, a new bias stress protocol is developed to study charge trapping...
Article
Injection at the source contact critically determines the behavior of depletion-type organic electrochemical transistors (OETs). The contact resistance of OETs increases exponentially with the gate voltage and strongly influences the modulation of the drain current by the gate voltage over a wide voltage range. A modified standard model accounting...
Article
We demonstrated a new architecture of an electrowetting-on-dielectric (EWOD) device to transport a liquid droplet by the spatial modulation of an electric field produced using an embedded undulating electrode. The undulating electrode was constructed on an array of dielectric microstructures with different periods in region by region to generate a...
Article
We develop a simple and biocompatible method of patterning proteins on a wettability gradient surface by thermo-transfer printing. The wettability gradient is produced on a poly(dimethylsiloxane) (PDMS)-modified glass substrate through the temperature gradient during thermo-transfer printing. The water contact angle on the PDMS-modified surface is...
Article
We demonstrated an ambipolar-type organic field-effect transistor (OFET) in a dual-gate configuration where two different layers of unipolar organic semiconductors (OSC) are stacked. In our OFET, the hole-channel for the p-type operation depends primarily on the corresponding gate insulator-OSC interface and the electron-channel for the n-type oper...
Article
We demonstrate a vertical-type organic light-emitting transistor (VOLET) with a network electrode of closed topology for quasi-surface emission. In our VOLET, the spatial distribution of the surface emission depends primarily on the relative scale of the aperture in the network electrode to the characteristic length for the charge carrier recombina...
Article
We demonstrate two types of combinatorial color arrays based on the Fabry-Perot (FP) micro-resonators in monolithic architecture. Optical micro-resonators corresponding to color elements are constructed using a soluble dielectric material between two transreflective layers by transfer-printing in either a pattern-by-pattern or a pattern-on-pattern...
Article
We develop miniature high-voltage sources from polymer solar cells (PSCs) with charge-transporting molybdenum oxide (MoOx) integrated in a serial architecture through sacrificial layer (SL)-assisted patterning. The MoOx layer, being patterned by the lift-off process of the SL of a hydrophobic fluorinated-polymer, as a hole transporting layer plays...
Article
The progress of organic field-effect transistors (OFETs) has led to the advent of a new area of printed and/or flexible electronics. In organic transistors and circuits, the interface between a gate insulator (GI) and an organic semiconductor (OS) plays a critical role on the electrical performance together with the functionality, the reliability a...
Article
We propose a concept of rewritable ferroelectric random access memory (RAM) with two lateral organic transistors-in-one cell architecture. Lateral integration of a paraelectric organic field-effect transistor (OFET), being a selection transistor, and a ferroelectric OFET as a memory transistor is realized using a paraelectric depolarizing layer (PD...
Article
We describe a control scheme of the molecular order and cracks in a solution-processed 6,13-bis(triiso-propylsilylethynyl)-pentacene (TIPS-pentacene) film on a polymeric insulator by the anisotropic temper-ature-gradient in the process of solvent drying. The solvent for TIPS-pentacene sequentially evaporates from the high temperature region to the...
Article
We report on the enhancement of the field-effect mobility of solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) by unidirectional topography (UT) of an inkjet-printed polymer insulator. The UT leads to anisotropic spreading and drying of the TIPS-pentacene droplet and enables to spontaneously develop the ordered struct...
Article
This paper reports a viable method for enhancing mobility anisotropy in solution-processed organic thin-film transistors (TFTs) by irradiating linearly polarized ultraviolet (LPUV) light onto a 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/poly (vinyl cinnamate) (PVCi) blended semiconductor film. The needle-like crystalline domains...
Article
We investigated the bias voltage effect on the electrical properties of n-type polymer field-effect transistors (FETs) with dual gate electrodes. The electrical characteristics of such polymer FETs were measured as a function of the sweeping voltage at one gate electrode when the other gate electrode was floated or biased. The hysteresis was neglig...
Article
We present the control mechanism for interfacial charges in an organic field-effect transistor (OFET) by the introduction of a surface polarized layer (SPL), interfaced with a p-type organic semiconductor (p-OS), which generates a transverse dipolar field. The concept of such SPL enables to develop a high noise-margin full-swing inverter of the p-O...
Article
We demonstrated a new type of a solution-processed organic field-effect transistor (OFET) in a bottom-gate, top-contact geometry where low leakage current and self-pattern registration were achieved using a patterned dielectric barrier (PDB). The PDB of a hydrophobic fluorinated-polymer was produced on the top of a polymeric gate insulator of poly(...
Article
We describe the effect of thermo-gradient-assisted solvent evaporation of an organic semiconductor, 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN), on the enhancement of the electrical properties of TIPS-PEN thin-film transistors (TFTs). The TIPS-PEN molecules are found to align themselves along the direction of the temperature gradient, f...
Article
We demonstrate a chevron-type gate configuration of a short channel top-contact organic thin-film transistor (OTFT) showing large saturated drain current with the help of a polymeric edge support. The channel length (L) depends on the geometrical shape of the edge support which can be controlled by thermal treatment. Oblique deposition of Au onto t...
Article
We demonstrate that the sequential surface modification of a ferroelectric polymer insulator plays an essential role in both the enhancement of the carrier mobility and the shift in the turn-on voltage (V<sub> on </sub>) in an organic ferroelectric field-effect transistor (FeFET) for nonvolatile memory. The surface of a ferroelectric polymer insula...
Article
Full-text available
We investigated the electrical stabilities of two types of pentacene-based organic thin-film transistors (OTFTs) with two different polymeric dielectrics: polystyrene (PS) and poly(4-vinyl phenol) (PVP), in terms of the interfacial charge depletion. Under a short-term bias stress condition, the OTFT with the PVP layer showed a substantial increase...
Article
We report the thermal annealing effect on the mobility enhancement, the crack development, and the stability of 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene field-effect transistors (FETs) with a solution-processed polymeric insulator. A high value of the field-effect mobility (0.401 cm2/V s) is achieved by thermally annealing the TIPS-penta...
Article
We report on the optical anisotropy of a pentacene film on a rubbed (poly)vinylalcohol (PVA) layer related to the electrical performances of the pentacene organic field effect transistors (OFETs) depending on the direction of a current flow. The optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In...
Article
We report on a low driving voltage VA-DHFLC display by using micro-confinement channels structure. The low voltage operation was achieved by the confinement structures. Particularly, the alignment of FLCs is extremely stable in the confinement structure, so that the excellent electro-optic performance was obtained in the regime of high electric fie...
Article
We report the effect of temperature gradient curing of an organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS PEN), on the performance of organic thin film transistors (OTFTs). The crystalline domains in the TIPS PEN film are found to be aligned along the direction of temperature gradient in the substrate. The TIPS PEN OTFT fa...

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