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Oh, Nam Khen,Lee, Hoon Ju,Choi, Keunsu,Seo, Jihyung,Kim, Ungsoo,Lee, Junghyun,Choi, Yunseong,Jung, Seungon,Lee, Jun Hee,Shin, Hyeon Suk,Park, Hyesung American Chemical Society 2018 Chemistry of materials Vol.30 No.14
<P>Owing to its mass-production capability and simple process, liquid-phase exfoliation (LPE) of layered bulk materials is commonly used for synthesizing two-dimensional materials, such as graphene or transition metal dichalcogenides (TMDs). Surfactants are often used in LPE process to improve the exfoliation efficiency and dispersion stability of target materials. However, during the surfactant-mediated exfoliation process, the as-applied surfactant typically forms strong covalent/noncovalent bonds with target materials, thereby deteriorating the properties of starting materials. Rinsing the surfactants can also be problematic because the decreased dispersion of the exfoliated flakes leads to restacking. Therefore, the contradictory phenomenon between maintaining the dispersion stability and preserving the material property has been a demanding issue in the LPE process of two-dimensional materials. Herein, a surfactant-assisted LPE of molybdenum disulfide (MoS<SUB>2</SUB>) using Nafion as the surfactant in environmentally friendly water solution is introduced. Nafion, which contains both hydrophilic and hydrophobic moieties, not only helps the exfoliation process of MoS<SUB>2</SUB>, but also prevents the exfoliated MoS<SUB>2</SUB> from reaggregation. The Nafion-assisted exfoliated MoS<SUB>2</SUB> is successfully demonstrated as the electrocatalyst in hydrogen evolution reaction without requiring any additional binder and surfactant rinsing process. The dual role of Nafion as the dispersing agent and binder is verified by experimental analyses and theoretical calculations. The proposed approach is expected to provide a useful guide in the application of two-dimensional materials prepared via surfactant-assisted LPE process.</P> [FIG OMISSION]</BR>
Kumar, Sujit,Choi, Yunseong,Kang, So-Huei,Oh, Nam Khen,Lee, Junghyun,Seo, Jihyung,Jeong, Mingyu,Kwon, Hyoung Woo,Seok, Sang Il,Yang, Changduk,Park, Hyesung American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.42
<P>Significant research efforts are currently being devoted to improving both the crystalline quality and stability of lead halide perovskite absorbers to advance the commercial prospects of perovskite-based solar cells. Herein, we report a simple one-step dibutylhydroxytoluene (BHT) additive-based approach for simultaneously improving the crystallinity and resistance of perovskite films under adverse degradation conditions. We found that BHT, commonly known for its antioxidant properties, can considerably improve the performance of methylammonium lead iodide perovskite solar cells by modulating the chemical environment within the precursor medium to form intermediate complexes, and it can also suppress photooxidation, which results in perovskite degradation under environmental operating conditions. Consequently, a device exhibited a significant power conversion efficiency improvement to 18.1% with the BHT-additive-based perovskite absorber, exceeding the 17.1% efficiency achieved for the control device. The BHT additive also improved the perovskite stability by quenching intermediate reactions resulting in perovskite degradation to an undesirable lead iodide phase, as evidenced by detailed analysis of absorption spectra, grazing-incidence wide-angle X-ray scattering, X-ray photoelectron spectra, and photoluminescence measurements.</P> [FIG OMISSION]</BR>
Koo, Donghwan,Jung, Seungon,Oh, Nam Khen,Choi, Yunseong,Seo, Jihyung,Lee, Junghyun,Kim, Ungsoo,Park, Hyesung Institute of Physics 2018 Semiconductor science and technology Vol.33 No.12
<P>Transition metal dichalcogenides (TMDs) have received significant attention because of their potential for replacing or modifying the existing charge transporting materials in organic solar cells (OSCs) with their unique crystalline structure and desirable electrical properties. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been considered as the representative hole transporting material owing to its notable optical transmittance, electrical conductivity, and solution-processability. In this study, we provide a facile method to introduce a liquid-phase exfoliated TMD, tungsten diselenide (WSe<SUB>2</SUB>), as the device performance enhancer in OSCs. Implementation of WSe<SUB>2</SUB> into PEDOT:PSS without significant change to the surface morphology mediates effective charge transport in the completed device. The phase separation of PEDOT and PSS induced by the WSe<SUB>2</SUB> provides a conductivity enhancement in the modified hole transport layer (HTL), which contributes to the increase of hole mobility and decrease of charge recombination loss in the OSCs, resulting in the improvement of power conversion efficiency from 7.3% to 8.5% for pristine and modified HTL devices, respectively. These results provide a simple strategy for the enhancement of device performance in OSCs, demonstrating their promising potential in the application of TMDs for next-generation energy harvesting devices.</P>