Y. Park

University of Rochester, Rochester, NY, United States

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Publications (10)26.76 Total impact

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    ABSTRACT: Molecular quantum chemical calculations were performed both at the ab initio and at the semi-empirical level to model the molecular conformations and electronic structure of oligomers of poly(phenylene vinylene) during the early stages of interface formation with Al, Ca and Mg. We found that the divalent metals, Mg and Ca, disrupt the conformation of the oligomers less than Al does. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) move into the energy gap both for Ca- and for Mg-doped systems, resulting in gap-state formation. This is consistent with the polaron/bipolaron picture. The electron density plots indicate that the de-localization of electrons is reduced more significantly by Al than it is by Ca and Mg. Our simulation results have been confirmed experimentally via XPS and NEXAFS.
    Journal of Physics D Applied Physics 12/1998; 30(10):1421. · 2.53 Impact Factor
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    ABSTRACT: The importance of the interfacial properties in organic light emitting devices (OLED) is well recognized. We will describe our recent efforts to understand interfaces in OLEDs using surface/interface analytical techniques in a well controlled ultrahigh vacuum environment. We observed severe photoluminescence quenching of organic thin films comprising of a model OLED material, namely tris-(8-hydroxyquinoline) aluminum (Alq3), upon deposition of a number of metals. Such quenching may severely affect the electroluminescence device efficiency. We have investigated the interfaces using x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy, and studied the intriguing process of the interface formation at an atomic/molecular level. We will show that microscopic surface and interface properties are intimately related to the device characteristics and performance. © 1998 American Vacuum Society.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/1998; 16(3):1838-1841. · 1.43 Impact Factor
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    ABSTRACT: Poly(p-phenylenevinylene) and its derivatives (DP-PPV and labeled R-7) were carefully studied using x-ray photoelectron spectroscopy. The C 1s peaks taken from spin-coated thin films of these materials were fitted with the consideration of the bonding state differences (–C=CH and C=C–, etc). The intensity of these C 1s peaks agrees well with the value estimated from the bonding state consideration. © 1997 American Institute of Physics.
    Journal of Applied Physics 11/1997; 82(10):4962-4965. · 2.21 Impact Factor
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    ABSTRACT: Ca, Ag, and Ge atoms quench the photoluminescence (PL) of an organic thin film, tris-(8-hydroxyquinoline) aluminum (Alq3), a model organic material for organic light emitting diodes. The observed PL quenching behavior was similar for all three elements, independent of their metallic nature. Due to strong interactions at the Ca/Alq3 interface, the quenching was much less effective at submonolayer coverages. We have also observed that proper oxidation of Ca at the Ca/Alq3 interface can significantly recover the quenched luminescence of Alq3. © 1997 American Institute of Physics.
    Applied Physics Letters 08/1997; 71(8):1005-1007. · 3.52 Impact Factor
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    ABSTRACT: Organic materials have been demonstrated to have the necessary attributes for display applications. In typical organic light emitting devices, metallic electrodes are used to inject charged carriers into the organic electroluminescent (EL) medium. We report severe photoluminescence (PL) quenching of organic thin films comprising the most useful materials, namely 1,4-bis[4-(3,5-di-tert-butylstyryl)styryl]benzene (4PV), upon sub-monolayer deposition of Al, Ag, and Ca in an ultrahigh vacuum environment. The severity of the luminescence quenching, which depends on the type of metal used, can greatly affect the EL device performance. For example, a sub-monolayer coverage of the various metals on a 300 Å 4PV thin film can reduce the PL by as much as 50%. Depositing the 4PV layer onto a metal substrate also exhibits PL quenching. An exciton diffusion length of 200 Å can be estimated from the quenching data. © 1997 American Vacuum Society. 
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 06/1997; · 1.43 Impact Factor
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    ABSTRACT: We have studied the radiation-induced photoluminescence (PL) quenching and the associated electronic structure modification of a light-emitting conjugated oligomer, namely, 1,4-bis[4-(3,5-di-tert-butylstyryl) styryl]benzene (4PV), in the absence of oxygen. We found that a short period of exposure to the radiation sources used in UV or x-ray photoelectron spectroscopies (UPS or XPS) resulted in total PL quenching in 4PV. Although there was no detectable change in the XPS C 1s core-level spectra UPS valence spectra indicated the disappearance of the vinylene peak upon exposure to UV radiation. This suggests the destruction of the vinylene groups, and the possible creation of radical species, which may serve as PL quenching sites. We also found that the modification in the electronic structure by Ca deposition masks that by radiation, although both quenches the PL effectively.
    Physical Review B 06/1997; 55(23). · 3.66 Impact Factor
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    ABSTRACT: Using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), we have studied the formation of metal/organic interfaces in organic electroluminescent devices. Oligo(p-phenylenevinylenes) (OPV) and tris-(8-hydroxyquinoline)aluminum (Alq3) were used as the organic materials and Ca was used as the metallic layer. Interfaces are formed differently by depositing organic layer on Ca and Ca on organic substrate. For Ca/OPV, UPS revealed a clear evidence for interface state formation upon Ca deposition. The evolution of XPS core level peak as a function of Ca layer thickness was consistent with the energy level bending picture. The XPS and UPS spectra for OPV/Ca as a function of organic layer thickness also confirmed the energy level bending. The data obtained allowed us to deduce the energy level diagram near the interface. Similar data for Alq3/Ca indicated that no electron injection barrier exists at this interface if the Alq3 optical band gap in the literature was used for estimating the energy position of the lowest unoccupied state. © 1997 American Vacuum Society.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/1997; 15(5):2574-2578. · 1.43 Impact Factor
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    ABSTRACT: We report that Ca atoms severely quench the photoluminescence (PL) of an organic thin film 1,4‐bis[4‐(3,5‐di‐tert‐butylstyryl)stryyl]benzene (4PV). A submonolayer coverage (0.035 monolayer or 0.1 Å) of Ca on the 4PV surface reduced the PL of a 300 Å layer of 4PV by as much as 50%. Three distinct quenching rates were observed throughout the 200 Å Ca deposition. An exciton diffusion length of 200 Å in 4PV is estimated assuming that the PL quenching is diffusion controlled. © 1996 American Institute of Physics.
    Applied Physics Letters 09/1996; 69(10):1492-1494. · 3.52 Impact Factor
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    ABSTRACT: We have studied the interface formation of a vinylene phenylene oligomer with a Ca substrate using photoemission spectroscopy. The evolution of core and valence spectra during the deposition of the oligomer on Ca has indicated the molecular energy level bending. The total energy level bending was 0.5 eV and the thickness of the level bending region was about 100 Å. We propose an energy level diagram of the oligomer‐Ca interface based on the information obtained from the photoemission spectra. © 1996 American Institute of Physics.
    Applied Physics Letters 08/1996; 69(8):1080-1082. · 3.52 Impact Factor
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    ABSTRACT: We used ultraviolet and x‐ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering. © 1996 American Institute of Physics.
    Applied Physics Letters 05/1996; 68(19):2699-2701. · 3.52 Impact Factor