Yongsup Park

Energy level alignment at a charge generation interface between 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile

We have determined the electronic energy level alignment at the interface between 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl (NPB) and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) using ultraviolet photoelectron spectroscopy. The highest occupied molecular orbital (HOMO) of 20 nm thick HAT-CN film was located at 3.8 eV below the Fermi level. Thus the lowest unoccupied molecular orbital (LUMO) is very close to the Fermi level. The HOMO position of NPB was only about 0.3 eV below Fermi level at NPB/HAT-CN interface. This enables an easy excitation of electrons from the NPB HOMO to the HAT-CN LUMO, creating electron-hole pairs across this organic-organic interface.

Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes

We report on the fabrication of efficient inverted organic light-emitting diodes (IOLEDs) using pentacene films as an electron injection/transport layer between Al and Alq3 layers. These IOLED devices turn on at 4.7 V and exhibit a luminous efficiency of 9.5 cd/A without any dopants or reactive metals. Analysis using space-charge-limited characteristics of electron-only devices and ultraviolet photoelectron spectroscopy measurement of metal/organic interfaces indicates that the efficient IOLED characteristics can be attributed partly to the electron mobility of pentacene that is 102–104 times larger than that of Alq3 and to the effective reduction in injection barrier at contacts.

Synthesis of regio- and stereoselective alkoxy-substituted spirobifluorene derivatives for blue light emitting materials

Abstract Three new octyloxy substituted spirobifluorenes, 2,7-diphenyl-3′,6′-bis(octyloxy)-9,9′-spirobifluorene (DPBSBF, 1a ), 2,7-dibiphenyl-3′,6′-bis(octyloxy)-9,9′-spirobifluorene (DBBSBF, 1b ) and 2,7-diterphenyl-3′,6′-bis(octyloxy)-9,9′-spirobifluorene (DTBSBF, 1c ) were prepared. All the compounds had been fully characterized by 1H and 13C NMR, UV–Vis, DSC, mass spectrometry and gave satisfactory elemental analyses. They possessed good solubility in common organic solvents and good homogeneous film formation. The optical energy band gap of DBBSBF was 3.27xa0eV between the HOMO energy level, 5.85xa0eV, measured by UPS and the LUMO, 2.58xa0eV, calculated from absorption spectrum. A blue organic light emitting diode (OLED) based on the structure of ITO/TPD (60xa0nm)/DBBSBF (40xa0nm)/Alq3 (20xa0nm)/LiF (1xa0nm)/Al (100xa0nm) showed good performance. The luminance of 3125xa0cd/m2 was observed at a drive voltage of 12.8xa0V and the colour coordinate in CIE chromaticity was (0.14, 0.12). The external quantum efficiency was obtained to be 2.8% at 100xa0cd/m2.

Role of Ultrathin Metal Fluoride Layer in Organic Photovoltaic Cells: Mechanism of Efficiency and Lifetime Enhancement

Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrathin BaF2 layer grows by island growth, so BaF2 layers with a nominal thickness less than that of single-coverage layer (≈3 nm) partially cover the polymeric photoactive layer. As the nominal thickness of the BaF2 layer increased to that of a single-coverage layer, the Voc and power conversion efficiency (PCE) of the organic photovoltaic cells (OPVs) increased but the short-circuit current remained almost constant. The fill factor and the PCE decreased abruptly as the thickness of the BaF2 layer exceeded that of a single-coverage layer, which was ascribed to the insulating nature of BaF2 . We find the major cause of the increased Voc observed in these devices is the lowered work function of the cathode caused by the reaction and release of Ba from thin BaF2 films upon deposition of Al. The OPV device with the BaF2 layer showed a slightly improved maximum PCE (4.0 %) and a greatly (approximately nine times) increased device half-life under continuous simulated solar irradiation at 100 mW cm(-2) as compared with the OPV without an interfacial layer (PCE=2.1 %). We found that the photodegradation of the photoactive layer was not a major cause of the OPV degradation. The hugely improved lifetime with cathode interface modification suggests a significant role of the cathode interfacial layer that can help to prolong device lifetimes.

Doping-Free Inverted Top-Emitting Organic Light-Emitting Diodes With High Power Efficiency and Near-Ideal Emission Characteristics

Inverted top-emitting organic light-emitting diodes (ITOLEDs) with high power efficiency and near-ideal emission characteristics are demonstrated by using the combination of the following: 1) an electron-injection layer composed of Cs2CO3, which lowers the turn-on voltage; 2) an electron-transporting layer with optimal electron mobility, which enhances the electron current and thus improves the carrier balance; and 3) a dielectric/metal/dielectric multilayer electrode that works as a damage-free top transparent anode optimized to achieve high efficiency and ideal emission characteristics. By this approach, ITOLEDs with power efficiency values of 3.8 and 30 lm W-1 are demonstrated in fluorescent and phosphorescent types, respectively, at a luminance value of 1000 cd m-2 with little distortion in spectral/angular characteristics.

Graphene interlayer for current spreading enhancement by engineering of barrier height in GaN-based light-emitting diodes

Pristine graphene and a graphene interlayer inserted between indium tin oxide (ITO) and p-GaN have been analyzed and compared with ITO, which is a typical current spreading layer in lateral GaN LEDs. Beyond a certain current injection, the pristine graphene current spreading layer (CSL) malfunctioned due to Joule heat that originated from the high sheet resistance and low work function of the CSL. However, by combining the graphene and the ITO to improve the sheet resistance, it was found to be possible to solve the malfunctioning phenomenon. Moreover, the light output power of an LED with a graphene interlayer was stronger than that of an LED using ITO or graphene CSL. We were able to identify that the improvement originated from the enhanced current spreading by inspecting the contact and conducting the simulation.

Surface Reaction of Sulfur-Containing Amino Acids on Cu(110)

Adsorption behaviors of sulfur-containing amino acids, cysteine, methionine, and cystine molecules on Cu(110) surface were studied by core level photoelectron spectroscopy using synchrotron radiation. We found the following through the systematic comparisons of core level peaks such as S 2p, N 1s, and O 1s from different amino acids. At low coverage regimes, all the molecules form two distinct thiolate species, and their S 2p binding energy difference was about 0.9 eV. The relative populations of the two thiolates were different for different molecules and their coverage, which is due to the different bond strength of the sulfur-containing functional groups. At high coverage regimes, only cysteine molecules form zwitterionic state, which is related to the molecular ordering on Cu(110) surface.

Direct electron injection into an oxide insulator using a cathode buffer layer

Injecting charge carriers into the mobile bands of an inorganic oxide insulator (for example, SiO2, HfO2) is a highly complicated task, or even impossible without external energy sources such as photons. This is because oxide insulators exhibit very low electron affinity and high ionization energy levels. Here we show that a ZnO layer acting as a cathode buffer layer permits direct electron injection into the conduction bands of various oxide insulators (for example, SiO2, Ta2O5, HfO2, Al2O3) from a metal cathode. Studies of current–voltage characteristics reveal that the current ohmically passes through the ZnO/oxide-insulator interface. Our findings suggests that the oxide insulators could be used for simply fabricated, transparent and highly stable electronic valves. With this strategy, we demonstrate an electrostatic discharging diode that uses 100-nm SiO2 as an active layer exhibiting an on/off ratio of ∼107, and protects the ZnO thin-film transistors from high electrical stresses.

Energy level alignment in polymer organic solar cells at donor-acceptor planar junction formed by electrospray vacuum deposition

Using ultraviolet photoelectron spectroscopy (UPS), we have measured the energy level offset at the planar interface between poly(3-hexylthiophene) (P3HT) and C61-butyric acid methylester (PCBM). Gradual deposition of PCBM onto spin-coated P3HT in high vacuum was made possible by using electrospray vacuum deposition (EVD). The UPS measurement of EVD-prepared planar interface resulted in the energy level offset of 0.91u2009eV between P3HT HOMO and PCBM LUMO, which is considered as the upper limit of Voc of the organic photovoltaic cells.Using ultraviolet photoelectron spectroscopy (UPS), we have measured the energy level offset at the planar interface between poly(3-hexylthiophene) (P3HT) and C61-butyric acid methylester (PCBM). Gradual deposition of PCBM onto spin-coated P3HT in high vacuum was made possible by using electrospray vacuum deposition (EVD). The UPS measurement of EVD-prepared planar interface resulted in the energy level offset of 0.91u2009eV between P3HT HOMO and PCBM LUMO, which is considered as the upper limit of Voc of the organic photovoltaic cells.

Deposition sequence dependent variation in interfacial chemical reactions between 8-hydroxyquinolatolithium and Al

The chemical reactions between 8-hydroxyquinolatolithium (Liq) and Al were investigated by using high resolution synchrotron radiation photoelectron spectroscopy. Unlike the LiF/Al case, two opposite deposition sequences (Al/Liq versus Liq/Al) give different interface reactions. When Al is deposited on a Liq layer, there occurs a strong reaction between Liq and Al, which accounts for a clear peak shift in the Li 1s core level. On the other hand, an interface-localized charge transfer without Li 1s splitting occurs with the reversed deposition sequence. The former strong interface reaction can generate ionic Li as a dopant material in Liq layer, causing band bending.