Kyuwook Ihm

Effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors

To investigate the effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors, gold was deposited onto pentacene films at various deposition rates. The sharp interface between the gold electrode and the pentacene film that results from a fast deposition rate was found to produce lower contact resistance and an increase in the field-effect mobility.

Number of graphene layers as a modulator of the open-circuit voltage of graphene-based solar cell

Impressive optical properties of graphene have been attracting the interest of researchers, and, recently, the photovoltaic effects of a heterojunction structure embedded with few layer graphene (FLG) have been demonstrated. Here, we show the direct dependence of open-circuit voltage (Voc) on numbers of graphene layers. After unavoidably adsorbed contaminants were removed from the FLGs, by means of in situ annealing, prepared by layer-by-layer transfer of the chemically grown graphene layer, the work functions of FLGs showed a sequential increase as the graphene layers increase, despite of random interlayer-stacking, resulting in the modulation of photovoltaic behaviors of FLGs/Si interfaces.

Molecular orientation dependence of hole-injection barrier in pentacene thin film on the Au surface in organic thin film transistor

We have investigated the effects of a buffer layer insertion on the performance of the pentacene based thin film transistor with a bottom contact structure. When the pentacene molecules have a standing up coordination on the Au surface that is modified by the benzenethiol or methanethiol, the transition region in the pentacene thin film is removed along the boundary between the Au and silicon oxide region, and the hole-injection barrier decreases by 0.4eV. Pentacene on various surfaces showed that the highly occupied molecular level is 0.2–0.4eV lower in the standing up coordination than in the lying down coordination.

Band bending of LiF/Alq3 interface in organic light-emitting diodes

The insertion of LiF for an interlayer material between the Al cathode and tris-(8-hydroxyquinoline) aluminum (Alq3) in the organic light-emitting diodes (OLEDs) provides an improved device performance. The highly occupied molecular orbital (HOMO) level lowering in the Alq3 layer induced by a low-coverage LiF deposition results in the reduction of electron injection barrier height. We investigated the electronic structure of the interface between the ultrathin LiF and the Alq3 layer, using synchrotron x-ray photoelectron emission spectroscopy. The results revealed that the major origin of the HOMO level lowering is not the chemical bonding of dissociated fluorine in the Alq3 layer but the band bending caused by charge redistribution driven by work function difference between LiF and Alq3 layer.

Energy-Level Alignment at Interfaces Between Gold and Poly(3-hexylthiophene) Films with Two Different Molecular Structures

The electronic structures of the interfaces between Au and poly(3-hexylthiophene) (P3HT) films with two different molecular orientations and orderings were investigated using synchrotron radiation photoemission spectroscopy. We found that, depending on whether thermal treatment was used, the P3HT thin film adopts two different molecular orientations, parallel and perpendicular to the silicon substrate, which result in different values of the vacuum level shift and hole-injection barrier. Thus, the molecular orientation and ordering of the P3HT material strongly affect the energy level alignment at the P3HT/Au interface.

Investigation of Thermally Induced Degradation in CH 3 NH 3 PbI 3 Perovskite Solar Cells using In-situ Synchrotron Radiation Analysis

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI3 perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI3 perovskite film changes to an intermediate phase and decomposes to CH3I, NH3, and PbI2 after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH3NH3+ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI3 perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.

Submicro-polymer particles bearing imidazoline-2-selenone: dual mode adsorbents with color-sensing for halogens and mercury ions

Submicron-sized polymer particles (PSE) containing imidazoline selenones were prepared by co-polymerization of styrene derivative (MSE) bearing an imidazoline selenone moiety with 1,4-divinylbenzene (DVB). The size and chemical composition of PSE were controlled by changing the stoichiometric ratios of MSE to DVB. The physical and chemical properties of PSE were characterized by SEM, EDS and elemental analysis. PSE showed an interesting reactivity towards halogens with vivid color-change from white to red-orange, which is attributed to the reaction of selenium in imidazoline-2-selenone with halogens. Acid treatment of PSE generated the hydrophilic red-orange colored particles (PSEA) which showed very selective adsorption properties towards mercury ions with color change to pale yellow. To figure out the origin of color change, model studies were conducted using 1,3-dimethyl-imidazoline-2-selenone. The dimerization of 1,3-dimethyl-imidazoline-2-selenone through Se–Se bond formation by acid-treatment resulted in color change from colorless to red-orange. The coordination-induced cleavage of the Se–Se bond of the dimerized species by mercury ions resulted in color change from red-orange to pale yellow. These observations indicate that hydrophobic PSE and hydrophilic PSEA are efficient systems for adsorption of halogens and mercury ions with a vivid color-detection.

Nitrogen-incorporated multiwalled carbon nanotubes grown by direct current plasma-enhanced chemical vapor deposition

The nitrogen-incorporated multiwalled carbon nanotubes (N-MWCNTs) were synthesized by dc plasma-enhanced chemical vapor deposition with a gas mixture of C2H2, NH3, and N2. Nitrogens in the N-MWCNTs were pyridinic nitrogen and graphitic nitrogen. With increase in the flow rate of N2 gas during the synthesis of MWCNTs, the pyridinic nitrogen increased much more than graphitic nitrogen. The near-edge x-ray absorption fine structure spectra revealed that the density of states such as π*, σ*, and π*+σ* bands of the N-MWCNTs decreased with increase of concentration of pyridinic nitrogen incorporated in the MWCNTs. The intensity ratio of the D band to the G band of Raman spectrum increased with the incorporation of nitrogen into MWCNTs.

Early stages of collapsing pentacene crystal by Au

The characteristic feature of metal contacts with gold on organics is deterioration of the organic crystals during the contact formation. The unveiled key challenge is to probe dynamic details of the microscopic evolution of the organic crystal when the atomic Au is introduced. Here, we report how the collapse of the pentacene crystal is initiated even by a few Au atoms. Our results indicate that the gentle decoupling of intra and intermolecular π-π interactions causes the localization of the lowest unoccupied molecular orbital as well as the removal of cohesive forces between molecules, leading to the subsequent crystal collapse.

The roles of ruthenium nanoparticles decorated on thin multi-walled carbon nanotubes in the enhancement of field emission properties

The roles of metal nanoparticles (NPs) decorated on carbon nanotubes (CNTs) in the enhancement of field emission properties were investigated by measuring the work functions (WF), densities of state (DOS), turn-on fields, current densities, and sizes of NPs. RuO2 was deposited on CNTs and reduced to Ru by annealing at elevated temperatures. The WF decreased during reduction, and integrated DOS increased after the formation of Ru NPs. Consequently, the turn-on field showed strong correlation with the WF, and the shape of the Ru NPs had a more significant impact on current density than the WF and DOS.