Sung-Joo Kwon

Efficient Flexible Organic/Inorganic Hybrid Perovskite Light‐Emitting Diodes Based on Graphene Anode

Highly efficient organic/inorganic hybrid perovskite light-emitting diodes (PeLEDs) based on graphene anode are developed for the first time. Chemically inert graphene avoids quenching of excitons by diffused metal atom species from indium tin oxide. The flexible PeLEDs with graphene anode on plastic substrate show good bending stability; they provide an alternative and reliable flexible electrode for highly efficient flexible PeLEDs.

Versatile p-Type Chemical Doping to Achieve Ideal Flexible Graphene Electrodes.

We report effective solution-processed chemical p-type doping of graphene using trifluoromethanesulfonic acid (CF3 SO3 H, TFMS), that can provide essential requirements to approach an ideal flexible graphene anode for practical applications: i) high optical transmittance, ii) low sheet resistance (70 % decrease), iii) high work function (0.83 eV increase), iv) smooth surface, and iv) air-stability at the same time. The TFMS-doped graphene formed nearly ohmic contact with a conventional organic hole transporting layer, and a green phosphorescent organic light-emitting diode with the TFMS-doped graphene anode showed lower operating voltage, and higher device efficiencies (104.1 cd A(-1) , 80.7 lm W(-1) ) than those with conventional ITO (84.8 cd A(-1) , 73.8 lm W(-1) ).

Laminated Graphene Films for Flexible Transparent Thin Film Encapsulation

We introduce a simple, inexpensive, and large-area flexible transparent lamination encapsulation method that uses graphene films with polydimethylsiloxane (PDMS) buffer on polyethylene terephthalate (PET) substrate. The number of stacked graphene layers (nG) was increased from 2 to 6, and 6-layered graphene-encapsulation showed high impermeability to moisture and air. The graphene-encapsulated polymer light emitting diodes (PLEDs) had stable operating characteristics, and the operational lifetime of encapsulated PLEDs increased as nG increased. Calcium oxidation test data confirmed the improved impermeability of graphene-encapsulation with increased nG. As a practical application, we demonstrated large-area flexible organic light emitting diodes (FOLEDs) and transparent FOLEDs that were encapsulated by our polymer/graphene encapsulant.

Flexible transparent electrodes for organic light-emitting diodes

The use of flexible organic light-emitting diodes (OLEDs) for the next-generation displays and solid-state lightings has been considered, but the widely used transparent conducting electrode (TCE), indium–tin-oxide (ITO), should be replaced by flexible electrodes due to its brittleness and increasing cost. Therefore, many kinds of alternative TCEs have been increasingly studied. In this paper, the properties and applications of the candidate transparent flexible electrodes classified into four categories (conducting polymer, silver nanowire, carbon nanotube and graphene) are described. This paper finally suggests how to develop alternative TCEs for replacing the conventional ITO electrode.

Improvement of work function and hole injection efficiency of graphene anode using CHF3 plasma treatment

We report improvement of hole injection efficiency of a graphene anode by tuning its work function (WF) via surface fluorination. We used chemical vapor deposition to synthesize high-quality graphene sheets and then treated them with CHF3 plasma to induce fluorination. We used x-ray photoelectron spectroscopy to examine the fluorine coverage and the kind of chemical bonds in fluorinated graphene (FG). Also, we used ultraviolet photoelectron spectroscopy to systematically study the changes in the WF and sheet resistance of the FG sheets with varying plasma exposure time (0, 10, 30, 60, 90 s) to find an optimum fluorination condition for hole injection. The WF of graphene sheets was increased by up to 0.74 eV, as a result of the formation of carbon-fluorine bonds that function as negative surface dipoles. We fabricated hole-only devices and conducted dark injection space-charge-limited-current transient measurement; the fluorination greatly increased the hole injection efficiency of graphene anodes (from 0.237 to 0.652). The enhanced hole injection efficiency of FG anodes in our study provides wide opportunities for applications in graphene-based flexible/stretchable organic optoelectronics.

Controlled surface oxidation of multi-layered graphene anode to increase hole injection efficiency in organic electronic devices

Ultraviolet ozone (UVO) surface treatment of graphene changes its sp2-hybridized carbons to sp3-bonded carbons, and introduces oxygen-containing components. Oxidized graphene has a finite energy band gap, so UVO modification of the surface of a four-layered graphene anode increases its surface ionization potential up to ~5.2 eV and improves the hole injection efficiency (η) in organic electronic devices by reducing the energy barrier between the graphene anode and overlying organic layers. By controlling the conditions of the UVO treatment, the electrical properties of the graphene can be tuned to improve η. This controlled surface modification of the graphene will provide a way to achieve efficient and stable flexible displays and solid-state lighting.

Solution-Processed n-Type Graphene Doping for Cathode in Inverted Polymer Light-Emitting Diodes

n-Type doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl) dimethylamine (N-DMBI) reduces a work function (WF) of graphene by ∼0.45 eV without significant reduction of optical transmittance. Solution process of N-DMBI on graphene provides effective n-type doping effect and air-stability at the same time. Although neutral N-DMBI act as an electron receptor leaving the graphene p-doped, radical N-DMBI acts as an electron donator leaving the graphene n-doped, which is demonstrated by density functional theory. We also verify the suitability of N-DMBI-doped n-type graphene for use as a cathode in inverted polymer light-emitting diodes (PLEDs) by using various analytical methods. Inverted PLEDs using a graphene cathode doped with N-DMBI radical showed dramatically improved device efficiency (∼13.8 cd/A) than did inverted PLEDs with pristine graphene (∼2.74 cd/A). N-DMBI-doped graphene can provide a practical way to produce graphene cathodes with low WF in various organic optoelectronics.

Effects of temperature variation on PLS Linac SLED-type pulse compressor system

PLS Linac, eleven SLED-type pulse compressors has been operated since 1994. SLED is one of rf pulse compressors, which consists of two high Q-value resonant cavities, 3 dB hybrid coupler and the auxiliary devices such as tuning mechanism and cooling structure. The maximum energy multiplication, klystron pulse width, rf pulse compressor input pulse width, output pulse width and cavity filling time of the SLED-type pulse compressor are 1.676, 4.0 /spl mu/s, 3.17 /spl mu/s, 0.83 /spl mu/s and 1.99 /spl mu/s respectively. The resonant frequency in the cavities should be tuned by tuning needle with maintaining a temperature compensated structural dimensions. The temperature sensitivity was about 50 KHz//spl deg/C. To hold the cavity tuning angle within /spl plusmn/5/spl deg/, the cavity temperature has to be stable to /spl plusmn/0.15/spl deg/C. In this paper, we describe the effect of temperature stabilization of cavities on the rf output waveform, phase and energy gain.