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홍기하 한국공업화학회 2018 한국공업화학회 연구논문 초록집 Vol.2018 No.1
Metal halide perovskites (MHPs) is one of a strong candidate for next-generation photovoltaic materials. The material properties of MHPs can be easily manipulated by changing the chemical composition of perovskites, and numerous MHP varieties have been developed through composition engineering. Compositional mixing of MHPs can enhance the stability, modulate the band gap, and make the Pb-free materials. Theoretical analysis and computational screening procedure can be an effective tool to study the underline physics and to develop more efficient and robust materials. Here I will present our recent theoretical works on a compositional mixing of MHPs. The first topic is the phase stabilization by mixing. Theoretical investigations on the black phase stabilization by the dual-site alloying scheme is presented. The second topic is to develop Pb-free materials by mixing cationic metals. Finally, we will discuss the potential problems in the mixing processes.
Effects of Oxygen Plasma Treatment on Vth Uniformity of Recessed-Gate AlGaN/GaN HEMTs
홍기하,최혁순,Injun Hwang,Jongseob Kim 대한금속·재료학회 2014 ELECTRONIC MATERIALS LETTERS Vol.10 No.2
Normally-off AlGaN/GaN HEMTs have been fabricated by employing a recessed-gate structure and oxygen plasma treatment and outstanding improvement of Vth variation is observed. The origin of the observed positive Vth shift and reduced variation window induced by oxygen plasma treatment is investigated by computational methods. Formation energy calculations for oxygen inclusions in III-N reveal that a negatively charged VAl-ON complex in the AlGaN passivation layer can be a major source of Vth variation in AlGaN/GaN hetero-structured devices. Calculated trap energy levels are used as the parameters of a device simulation, which indicated that significant Vth variation can be induced by a small fluctuation in the AlGaN layer thickness and defect densities. Our theoretical investigation shows that normally-off AlGaN/GaN HEMTs having reliable Vth variation can be produced by oxygen inclusions accompanying a recessed-gate structure.
페로브스카이트 태양전지의 최적 정공수송층 물질 및 구조 탐색 시뮬레이션
정지영,송동준,최지혜,정중희,홍기하 대한금속·재료학회 2019 대한금속·재료학회지 Vol.57 No.8
We investigated the effects of materials and the film thickness of hole transport layers (HTLs) on inverted type perovskite solar cells using optics-charge transport coupled simulations. Power conversion efficiencies (PCEs), and the variations in efficiency induced by the film thickness dispersion, were intensively studied, to compare potential HTLs candidates like NiOx, PEDOT:PSS, CuSCN, and CuI. The optimum thickness of the solar cell layers differed based on the chosen combination of HTL and perovskite. It is suggested that the optoelectronic properties of HTLs like band gap, extinction coefficient, and refractive index can be used to determine the best ideal efficiencies, and sensitivity to process fluctuation. CuSCN showed the most promising behaviors, in that it can produce over-25% PCEs, and the lowest efficiency dispersion for various HTL thickness conditions. The best performance by CuSCN can be ascribed to its having a proper refractive index with the perovskite layer, and wide band gap characteristic. NiOx and CuI showed PCEs comparable to the CuSCN, but their efficiencies were sensitive to the varying thickness of the HTL. PEDOT:PSS exhibited the lowest simulated PCEs due to its small band gap. Our study suggests the best HTL candidates for inverted type perovskite solar cells, and demonstrates the importance of sophisticated numerical material studies, and device design, when developing highly efficient and robust perovskite solar cells.
스프레이 코팅법으로 증착된 은나노선의 특성에 미치는 분산용매의 영향
최지혜,장아연,김정희,정중희,홍기하 대한금속·재료학회 2017 대한금속·재료학회지 Vol.55 No.7
Silver nanowire (AgNWs) networks offer a promising approach for producing cost effective transparent electrode materials. However, high temperature annealing is necessary to achieve low sheet resistance, which limits the use of AgNWs for low temperature processing applications such as flexible electronics, and perovskites solar cell fabrication. This study shows that the low temperature processing of efficient AgNW electrodes can be achieved with the choice of an appropriate dispersion solvent. Ethanol shows promising behaviors over IPA by forming uniform percolation networks even without thermal annealing. The key factors affecting percolation uniformity are the solubility of organic contaminants and the viscosity of the solvent which determines the droplet sizes. This study contributes to future studies of transparent conductors and flexible devices.
Shear-coating을 사용한 은 나노와이어 투명 전극 제조 및 특성 분석
조경수(Kyung Soo Cho),홍기하(Ki-Ha Hong),박준식(Joon Sik Park),정중희(Choong-Heui Chung) 한국표면공학회 2020 한국표면공학회지 Vol.53 No.4
Indium tin oxide (ITO) used a transparent electrode of a photoelectric device has a low sheet resistance and a high transmittance. However, ITO is disadvantageous in that the process cost is expensive, and the process time is long. Silver nanowires (AgNWs) transparent electrodes are based on a low cost solution process. In addition, it has attracted attention as a next-generation transparent electrode material that replaces ITO because it has similar electrical and optical characteristic to ITO, it is noted as a. AgNW thin films are mainly produced by spin-coating. However, the spin-coating process has a disadvantage of high material loss. In this study, the material loss was reduced by using about 2∼10 ㎕ of AgNW solution on a (25 × 25) ㎟ substrate using the shear-coating method. It was also possible to align AgNWs in the drag direction by dragging the meniscus of the solution. The electro-optical properties of the AgNW thin film were adjusted by changing the experimental parameters that the amount of AgNWs suspension, the gap between the substrate and the blade, and the coating speed. As a result, AgNW thin films with a transmittance of 90.7 % at a wavelength of 550 ㎚ and a sheet resistance of 15 Ω/□ was deposited and exhibited similar properties to similar AgNW transparent electrodes studied by other researchers.