Jeonghun Kwak

We report highly bright and efficient inverted structure quantum dot (QD) based light-emitting diodes (QLEDs) by using solution-processed ZnO nanoparticles as the electron injection/transport layer and by optimizing energy levels with the organic hole transport layer. We have successfully demonstrated highly bright red, green, and blue QLEDs showing maximum luminances up to 23,040, 218,800, and 2250 cd/m(2), and external quantum efficiencies of 7.3, 5.8, and 1.7%, respectively. It is also notice

Thin-film ultraviolet (UV) light-emitting diodes (LEDs) with emission wavelengths below 400 nm are emerging as promising light sources for various purposes, from our daily lives to industrial applications. However, current thin-film UV-emitting devices radiate not only UV light but also visible light. Here, we introduce genuine UV-emitting colloidal nanocrystal quantum dot (NQD) LEDs (QLEDs) using precisely controlled NQDs consisting of a 2.5-nm-sized CdZnS ternary core and a ZnS shell. The effe

Abstract Inkjet printing of colloidal quantum dots (QDs) is considered a promising technology for application in full‐color quantum dot light‐emitting diode (QLED) displays. However, QLEDs that are inkjet printed in a pixel‐defining bank structure generally exhibit a low performance, mainly due to the nonuniformity in its QD morphology. In this study, an enhanced performance of inkjet‐printing‐based pixelated QLEDs is achieved by introducing small amounts of poly(methyl methacrylate) (PMMA) of d

A semicrystalline p-type thermoelectric conjugated polymer based on a polymer backbone of cyclopentadithiophene and benzothiadiazole, poly[(4,4′-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT), is designed and synthesized by replacing normal alkyl side-chains with bis(alkylsulfanyl)methylene substituents. The sp2-hybridized olefinic bis(alkylsulfanyl)methylene side-chains and the sulfur–sulfur (S–S) chalcogen interactions extend

Scientific and technological advances in transparent conductive electrodes improve the heating performance of flexible transparent film heaters (TFHs), which can be utilized for various applications as defrosters and heaters. To achieve high performance as well as practical TFHs, several conditions, such as high optical transmittance, low electrical resistance, heating uniformity, and operational stability in various environmental conditions should be satisfied. However, due to the trade-offs be

Wide interest in quantum dot (QD) light-emitting diodes (QLEDs) for potential application to display devices and light sources has led to their rapid advancement in device performance. Despite such progress, detailed operation mechanisms of QLEDs, which are necessary for the fundamental understanding and further improvements, have been still uncertain because of the intricate interaction between charges and excitons in electrical operation. In this work, the transient electroluminescence (TREL)

Abstract Thermoelectric (TE) energy conversion in conjugated polymers is considered a promising approach for low‐energy harvesting and self‐powered temperature sensing. To enhance the TE performance, it is necessary to understand the relationship between the Seebeck coefficient (α) and electrical conductivity (σ). Typical doped polymers exhibit α–σ relationship that is distinct from that of inorganic materials due to their large structural and energetic disorder, which prevents them from achievi

A new carbazole-based host material for red emitters, BBTC, was designed, synthesized and characterized with the phosphorescent organic light-emitting diodes (PHOLEDs). With the molecular design strategy of maintaining the large triplet energy and a good hole transporting ability of carbazole while increasing the morphological and electrochemical stability, the C3 and C6 positions of carbazole are blocked with biphenyl groups and the C9 position is terminated with a terphenyl group. Red PHOLEDs

Two types of p-type thermoelectric (TE) polymers with alkyl (PCPDTSBT) and oligoethylene glycol (OEG) side chains (PCPDTSBT-A) on an sp2-hybridized olefinic bis(alkylsulfanyl)methylene-substituted cyclopentadithiophene backbone are synthesized. Interestingly, the OEG-substituted polymer, PCPDTSBT-A, exhibits significant self-doping compared to PCPDTSBT, where the polaron density of the former is 2.3 × 1016 mm–3 (vs 7.9 × 1014 mm–3 for PCPDTSBT) without external doping. Changing the side chains a

Thermoelectric (TE) generation with solution-processable conducting polymers offers substantial potential in low-temperature energy harvesting based on high tunability in materials, processes, and form-factors. However, manipulating the TE and charge transport properties accompanies structural and energetic disorders, restricting the enhancement of thermoelectric power factor (PF). Here, solution-based strong acid-base treatment techniques are introduced to modulate the doping level of poly(3,4-

A series of ionically interconnected polypyrrole (PPy) films are fabricated through two-monomer-connected-precursor polymerization by varying diacid linkers, thereby significantly influencing the crystalline morphology and electrical properties. The structure obtained using 1,5-napthalenedisulfonic acid (PPy-Nap) as a fused aromatic linker exhibits a higher electrical conductivity (∼78 S cm^-1) than that (6.7 S cm^-1) without a linker (PPy-ref). Cryogenic conductivity measure

Conjugated polymer-based energy-harvesting devices hold distinctive advantages in terms of low toxicity, high flexibility, and capability of large-area integration at low cost for sustainable development. An organic thermoelectric (OTE) device has been considered one of the promising energy-harvesting candidates in recent years because it can efficiently convert low-temperature waste heat into electricity over its inorganic counterparts. However, a cruel irony is that environmentally toxic solve

In this study, a temperature-insensitive strain sensor that detects only the strain without responding to the temperature was designed. The transport mechanism and associated temperature coefficient of resistance (TCR) of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film were modified through secondary doping with dimethyl sulfoxide (DMSO). Upon DMSO-doping, the carrier transport mechanism of the PEDOT:PSS thin film transitioned from hopping to band-like transport,