Bong Kyun Kang

Reduced Graphene Oxide/Mesoporous TiO2 Nanocomposite Based Perovskite Solar Cells

We report on reduced graphene oxide (rGO)/mesoporous (mp)-TiO2 nanocomposite based mesostructured perovskite solar cells that show an improved electron transport property owing to the reduced interfacial resistance. The amount of rGO added to the TiO2 nanoparticles electron transport layer was optimized, and their impacts on film resistivity, electron diffusion, recombination time, and photovoltaic performance were investigated. The rGO/mp-TiO2 nanocomposite film reduces interfacial resistance when compared to the mp-TiO2 film, and hence, it improves charge collection efficiency. This effect significantly increases the short circuit current density and open circuit voltage. The rGO/mp-TiO2 nanocomposite film with an optimal rGO content of 0.4 vol % shows 18% higher photon conversion efficiency compared with the TiO2 nanoparticles based perovskite solar cells.

Enrichment of Pyrrolic Nitrogen by Hole Defects in Nitrogen and Sulfur Co-Doped Graphene Hydrogel for Flexible Supercapacitors.

The effect of the doping configuration and concentration of nitrogen (N) and sulfur (S) on the electrochemical performance of 3 D N and S co-doped hole defect graphene hydrogel (NS-HGH) electrodes is investigated. Surprisingly, by introducing a hole defect on the graphene surface, the difference in the doping concentrations of N and S can be used to effectively modulate the electrochemical behavior of the NS-HGH. The hole defects provide a rapid ion diffusion path. Finally, we showed that the intriguing specific capacitance (536 F g(-1) ) of the NS-HGH could enhance the overall performance of the pseudocapacitance and electric double layer capacitance. The rational design of the NS-HGH-based flexible solid state supercapacitor results in not only outstanding electrochemical performance with a maximum energy density of 14.8 Wh kg(-1) and power density of 5.2 KW kg(-1) but also in extraordinary mechanical flexibility and excellent cycle stability.

Long-term stable stacked CsPbBr3 quantum dot films for highly efficient white light generation in LEDs

We report highly efficient ethyl cellulose with CsPbBr3 perovskite QD films for white light generation in LED application. Ethyl cellulose with CsPbBr3 quantum dots is applied with Sr2Si5N8 : Eu2+ red phosphor on an InGaN blue chip, achieving a highly efficient luminous efficacy of 67.93 lm W-1 under 20 mA current.

Mesoporous Ni–Fe oxide multi-composite hollow nanocages for efficient electrocatalytic water oxidation reactions

Mesoporous NiO/NiFe2O4 multi-composite hollow nanocages via monodisperse Ni3[Fe(CN)6]2 prussian blue analogue nanocube precursors were successfully synthesized. The three-dimensional (3D) mesoporous and hollow structures provided an efficient electrolyte diffusion path and a high surface area, resulting in the enhancement of electrocatalytic activities for the oxygen evolution reaction. The overpotential and Tafel slope of mesoporous NiO/NiFe2O4 multi-composite hollow nanocages were as low as 303 mV at a current density of 10 mA cm−2 and 58.5 mV dec−1, respectively. In addition, the composite showed excellent durability at approximately 60 mA cm−2 for 12 h.

Design of Advanced MnO/N‐Gr 3D Walls through Polymer Cross‐Linking for High‐Performance Supercapacitor

Three-dimensional, vertically aligned MnO/nitrogen-doped graphene (3D MnO/N-Gr) walls were prepared through facile solution-phase synthesis followed by thermal treatment. Polyvinylpyrrolidone (PVP) was strategically added to generate cross-links to simultaneously form 3D wall structures and to incorporate nitrogen atoms into the graphene network. The unique wall features of the as-prepared 3D MnO/N-Gr hybirdes provide a large surface area (91.516 m(2) g(-1)) and allow for rapid diffusion of the ion electrolyte, resulting in a high specific capacitance of 378 F g(-1) at 0.25 A g(-1) and an excellent charge/discharge stability (93.7% capacity retention after 8000 cycles) in aqueous 1 m Na2 SO4 solution as electrolyte. Moreover, the symmetric supercapacitors that were rationally designed by using 3D MnO/N-Gr hybrids exhibit outstanding electrochemical performance in an organic electrolyte with an energy density of 90.6 Wh kg(-1) and a power density of 437.5 W kg(-1).

Synthesis and Characterization of Monodispersed β-Ga2O3 Nanospheres via Morphology Controlled Ga4(OH)10SO4 Precursors

To our best knowledge, monodispersed β-Ga2O3 nanospheres were successfully synthesized for first time via morphology-controlled gallium precursors using the forced hydrolysis method, followed by thermal calcination processes. The morphology and particle sizes of the gallium precursors were strongly dependent on the varying (R = SO4(2-)/NO3(-)) concentration ratios. As R decreased, the size of the prepared gallium precursors decreased and morphology was altered from sphere to rod. The synthesized S2 (R = 0.33) consists of uniform and monodispersed amorphous nanospheres with diameters of about 200 nm. The monodispersed β-Ga2O3 nanospheres were synthesized using thermal calcination processes at various temperatures ranging from 500 to 1000 °C. Monodispersed β-Ga2O3 nanospheres (200 nm) consist of small particles of approximately 10-20 nm with rough surface at 1000 °C for 1 h. The UV (375 nm) and broad blue (400-450 nm) emission indicate recombination via a self-trapped exciton and the defect band emission. Our approach described here is to show the exploration of β-Ga2O3 nanospheres as an automatic dispersion, three-dimensional support for fabrication of hierarchical materials, which is potentially important for a broad range of optoelectronic applications.

Synthesis and characteristics of ZnGa2O4 hollow nanostructures via carbon@Ga(OH)CO3@Zn(OH)2 by a hydrothermal method

ZnGa2O4 hollow nanostructures were synthesized by a two step hydrothermal and calcination process using carbon spheres as a template. We observed that the carbon@Ga(OH)CO3 core–shell nanostructures were covered by a uniform shell of Zn(OH)2 nanoparticles when prepared using 2.5 mmol of ZnAc, and ZnGa2O4 hollow nanostructures with diameters of approximately 200 nm and shell thicknesses of around 15 nm could then be obtained. After calcination at 900 °C for 1 h, the amorphous core–shell–shell nanostructures yielded highly crystalline ZnGa2O4 hollow nanostructures. The shell possesses a single-crystal structure and a lattice spacing of around 0.253 nm which corresponds to the d spacing of (311) crystal planes of cubic ZnGa2O4. The composition and surface electron state of the ZnGa2O4 hollow nanostructures prepared at 900 °C were confirmed by X-ray photoelectron spectroscopy. UV and blue emissions of the ZnGa2O4 hollow nanostructures were found to result from self-activation centers of the octahedral Ga–O groups in the spinel structures and symmetry distortion of the octahedral sites by oxygen vacancies, respectively.

Enhanced electrochemical performance of lamellar structured Co–Ni(OH)2/reduced graphene oxide (rGO) via hydrothermal synthesis

Lamellar Co–Ni(OH)2/rGO structures were prepared by co-precipitation followed by hydrothermal synthesis. Lamellar and crystalline Co–Ni(OH)2/rGO delivered better specific capacitance (Cs) and rate capability than low-crystallinity composites due to improved ionic and electronic conductivity. A high value of the Cs was achieved with Co–Ni(OH)2/rGO prepared at 150 °C for 12 h with a graphite oxide (GO)-to-metal precursor ratio of 1 : 5. This condition gave a Cs of 617 F g−1 at 2 mV s−1 and 811 F g−1 at 0.5 A g−1 with the highest capacitance retention of 90% at 2 A g−1 and good cycling stability (∼77% at 70 mV s−1) at 5000 cycles. Furthermore, high mass loading, ∼4.5–5.5 mg cm−2, was used in the fabrication of the electrodes, which is higher than that of most previous reports.

Synergistically Active NiCo2S4 Nanoparticles Coupled with Holey Defect Graphene Hydrogel for High-Performance Solid-State Supercapacitors

Nickel cobalt sulfide nanoparticles embedded in holey defect graphene hydrogel (HGH) that exhibit highly porous structures and uniform nickel cobalt sulfide nanoparticle sizes are successfully prepared by a facile solvothermal-hydrothermal method. As an electrode material for supercapacitors, the as-prepared NiCo2 S4 @HGH shows ultra-high specific capacitances of 1000 F g-1 and 800 F g-1 at 0.5 and 6 A g-1 , respectively, owing to the outstanding electrical conductivity of HGH and high specific capacitance of NiCo2 S4 . After 2100 charge/discharge cycles at a current density of 6 A g-1 , 96.6 % of the specific capacitance was retained, signifying the superb durability of NiCo2 S4 @HGH. Moreover, remarkable specific capacitance (312.6 F g-1 ) and capacity retention (87 % after 5000 cycles) at 6 A g-1 were displayed by the symmetric solid-state supercapacitor fabricated by using NiCo2 S4 @HGH electrodes. These auspicious supercapacitor performances demonstrate that the as-developed solvothermal-hydrothermal approach can be widely used to prepare graphene-coupled binary metal sulfides for high-performance supercapacitor applications.

Highly Stable Perovskite Solar Cells in Humid and Hot Environment

An organic–inorganic perovskite solar cell (PSC) is a very promising candidate for a next-generation photovoltaic system. For the last three years, the power conversion efficiency (PCE) of PSCs has been dramatically improved from 9.7% to 22.1%; however, a poor long-term stability still limits the commercialization of PSCs. In this study, we explore the effect of poly(methyl methacrylate) (PMMA)/reduced graphene oxide (rGO) composite (PRC) passivation layer on the chemical and thermal stability of PSCs. The PRC passivation layer shows superior protection performance due to improved hydrophobicity and increased complexity of the O2 or H2O diffusion pathway. Moreover, the excellent thermal conductivity of rGO facilitates heat dissipation through the PRC layer. When the PRC layer is coated, the aging of PSCs is significantly prevented even under extreme conditions of humidity (>75%) and temperature (∼85 °C). Consequently, the PCE of PRC-passivated PSCs exhibits a negligible change in air, temperature of 35 °C, and humidity of 40% for 1000 h. Our study offers a simple and robust way to fabricate long-term stable and highly efficient PSCs, thus providing a path to PSC commercialization.