Shi-Woo Rhee

Sub-micrometer-sized graphite as a conducting and catalytic counter electrode for dye-sensitized solar cells.

Sub-micrometer-sized colloidal graphite (CG) was tested as a conducting electrode to replace transparent conducting oxide (TCO) electrodes and as a catalytic material to replace platinum (Pt) for I(3)(-) reduction in dye-sensitized solar cell (DSSC). CG paste was used to make a film via the doctor-blade process. The 9 μm thick CG film showed a lower resistivity (7 Ω/◻) than the widely used fluorine-doped tin oxide TCO (8-15 Ω/◻). The catalytic activity of this graphite film was measured and compared with the corresponding properties of Pt. Cyclic voltammetry and electrochemical impedance spectroscopy studies clearly showed a decrease in the charge transfer resistance with the increase in the thickness of the graphite layer from 3 to 9 μm. Under 1 sun illumination (100 mW cm(-2), AM 1.5), DSSCs with submicrometer-sized graphite as a catalyst on fluorine-doped tin oxide TCO showed an energy conversion efficiency greater than 6.0%, comparable to the conversion efficiency of Pt. DSSCs with a graphite counter electrode (CE) on TCO-free bare glass showed an energy conversion efficiency greater than 5.0%, which demonstrated that the graphite layer could be used both as a conducting layer and as a catalytic layer.

Electroluminescence from Graphene Quantum Dots Prepared by Amidative Cutting of Tattered Graphite

Size-controlled graphene quantum dots (GQDs) are prepared via amidative cutting of tattered graphite. The power of this method is that the size of the GQDs could be varied from 2 to over 10 nm by simply regulating the amine concentration. The energy gaps in such GQDs are narrowed down with increasing their size, showing colorful photoluminescence from blue to brown. We also reveal the roles of defect sites in photoluminescence, developing long-wavelength emission and reducing exciton lifetime. To assess the viability of the present method, organic light-emitting diodes employing our GQDs as a dopant are first demonstrated with the thorough studies in their energy levels. This is to our best knowledge the first meaningful report on the electroluminescence of GQDs, successfully rendering white light with the external quantum efficiency of ca. 0.1%.

Size‐Controlled Soft‐Template Synthesis of Carbon Nanodots toward Versatile Photoactive Materials

Size-controlled soft-template synthesis of carbon nanodots (CNDs) as novel photoactive materials is reported. The size of the CNDs can be controlled by regulating the amount of an emulsifier. As the size increases, the CNDs exhibit blue-shifted photoluminescence (PL) or so-called an inverse PL shift. Using time-correlated single photon counting, ultraviolet photoelectron spectroscopy, and low-temperature PL measurements, it is revealed that the CNDs are composed of sp² clusters with certain energy gaps and their oleylamine ligands act as auxochromes to reduce the energy gaps. This insight can provide a plausible explanation on the origin of the inverse PL shift which has been debatable over a past decade. To explore the potential of the CNDs as photoactive materials, several prototypes of CND-based optoelectronic devices, including multicolored light-emitting diodes and air-stable organic solar cells, are demonstrated. This study could shed light on future applications of the CNDs and further expedite the development of other related fields.

Control of Photoluminescence of Carbon Nanodots via Surface Functionalization using Para-substituted Anilines

Carbon nanodots (C-dots) are a kind of fluorescent carbon nanomaterials, composed of polyaromatic carbon domains surrounded by amorphous carbon frames, and have attracted a great deal of attention because of their interesting properties. There are still, however, challenges ahead such as blue-biased photoluminescence, spectral broadness, undefined energy gaps and etc. In this report, we chemically modify the surface of C-dots with a series of para-substituted anilines to control their photoluminescence. Our surface functionalization endows our C-dots with new energy levels, exhibiting long-wavelength (up to 650 nm) photoluminescence of very narrow spectral widths. The roles of para-substituted anilines and their substituents in developing such energy levels are thoroughly studied by using transient absorption spectroscopy. We finally demonstrate light-emitting devices exploiting our C-dots as a phosphor, converting UV light to a variety of colors with internal quantum yields of ca. 20%.

Multiwall Carbon Nanotube and Poly(3,4-ethylenedioxythiophene): Polystyrene Sulfonate (PEDOT:PSS) Composite Films for Transistor and Inverter Devices

Highly conductive multiwalled carbon nanotube (MWNT)/Poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) films were prepared by spin coating a mixture solution. The solution was prepared by dispersing MWNT in the PEDOT:PSS solution in water using ultrasonication without any oxidation process. The effect of the MWNT loading in the solution on the film properties such as surface roughness, work function, surface energy, optical transparency, and conductivity was studied. The conductivity of MWNT/PEDOT:PSS composite film was increased with higher MWNT loading and the high conductivity of MWNT/PEDOT:PSS films enabled them to be used as a source/drain electrode in organic thin film transistor (OTFT). The pentacene TFT with MWNT/PEDOT:PSS S/D electrode showed much higher performance with mobility about 0.2 cm²/(V s) and on/off ratio about 5 × 10⁵ compared to that with PEDOT:PSS S/D electrode (∼0.05 cm²/(V s), 1 × 10⁵). The complementary inverters exhibited excellent characteristics, including high gain value of about 30.

Improving the functionality of carbon nanodots: doping and surface functionalization

Distinct from conventional carbon nanostructures, such as fullerene, graphene, and carbon nanotubes, carbon nanodots (C-dots) exhibit unique properties such as strong fluorescence, high photostability, chemical inertness, low toxicity, and biocompatibility. Various synthesis routes for C-dots have been developed in the last few years, and now intense research efforts have been focused on improving their functionality. In this aspect, doping and surface functionalization are two major ways to control the chemical, optical, and electrical properties of C-dots. Doping introduces atomic impurities into C-dots to modulate their electronic structure, and surface functionalization modifies the C-dot surface with functional molecules or polymers. In this review, we summarize recent progress in doping and surface functionalization of C-dots for improving their functionality, and offer insight into controlling the properties of C-dots for a variety of applications ranging from biomedicine to optoelectronics to energy.

Chemical vapor deposition of nickel oxide films from Ni(C5H5)2/O2

Nickel oxide films were deposited with Ni(C5H5)2(bis-cyclopentadienyl nickel)/O2 at various temperatures and O2 flow rates. Gas phase reaction of Ni(C5H5)2/O2 was observed above the temperature of 300°C and a peak from CO2, the product of the Ni(C5H5)2 oxidation, was observed. Pure oxide films were not obtained but a mixed phase of nickel, NiO and Ni2O3 was observed and the amount of each phase in the film depended on the deposition condition. Films deposited at a high deposition temperature region (>275°C) had a high nickel content, and NiO(111) preferred crystal orientation. Films deposited at a low deposition temperature region (<275°C) had low nickel content and NiO(200) preferred orientation.

Bulk and interface properties of low-temperature silicon nitride films deposited by remote plasma enhanced chemical vapor deposition

Hydrogenated silicon nitride (a-SiNx:H) films were deposited at temperatures ranging from 50 to 300 °C with remote plasma enhanced chemical vapor deposition (RPECVD) from NH3 and SiH4. The effect of the operating variables, such as deposition temperature and especially the partial pressure ratio of reactant (R=NH3/SiH4) on the properties of the Sa-SiNx:H interface was investigated. The H* radical was dominantly observed and the deposition rate was proportional to the NH* radical concentration. The density of highly energetic N2* radicals increased in the high plasma power regime in which the film surface was roughened, but they promote surface reactions even at low temperature. The refractive index was more closely related to the film stoichiometry than film density. The interface trap density is related to the amount of silicon intermediate species and Si–NH bonds at the Si/SiNx:H interface and it can be minimized by reducing the intermediate Si species and Si–NH bonding state. The films showed a midgap interface trap density of 2 × 1011 - 2 × 1012cm-2.

Deposition of La2 O 3 Films by Direct Liquid Injection Metallorganic Chemical Vapor Deposition

A novel lanthanum precursor [La(tmhd) 3 -TETEA] where tmhd is 2,2,6,6-tetramethyl-3,5-heptanedione, and TETEA is triethoxy-triethyleneamine, has been characterized by thermogravimetric analysis and differential scanning calorimetry. Lanthanum oxide thin films deposited at 325-450°C by direct liquid injection metallorganic chemical vapor deposition process had a dense and smooth morphology. La 2 O 3 film deposited at 325°C had a cubic phase with a low X-ray diffraction (XRD) intensity, while at deposition temperatures above 375°C. XRD patterns indicated that both hexagonal and cubic phases of La 2 O 3 were formed. The measured capacitance of La 2 O 3 films deposited at 350°C was 70 pF that gave an effective dielectric constant of 25. La 2 O 3 film annealed at 500°C showed a low leakage current of 4.45 X 10 8 A/cm 2 at 5 V.

Electrocatalytic carbonaceous materials for counter electrodes in dye-sensitized solar cells

Carbonaceous materials have received much attention as alternative catalysts for platinum in dye-sensitized solar cells. Recently, various forms of carbon materials have been intensely investigated due to their low cost, excellent electrochemical stability, reasonable catalytic activity, and process adaptability. In this review, we introduce the current research issues in carbon counter electrodes including the equivalent circuit analysis, the electrochemical properties, the photovoltaic performances, and the research outlook. In this regard, the electrochemical properties of selected carbon materials are compared with each other by means of the impedance spectroscopy and equivalent circuit analysis. Also, fabrication methods and related photovoltaic performance are discussed. This knowledge will offer valuable insight to inspire investigation of carbon materials and encourage a multitude of applications ranging from all-carbon electrodes to flexible devices.