Kyeong K. Lee

Growth of ZnO Nanorod Arrays on Flexible Substrates: Effect of Precursor Solution Concentration

We report a low-temperature aqueous solution growth of uniformly aligned ZnO nanorod arrays on flexible substrates. The substrate is Indium Tin Oxide (ITO) film coated on polyethylene terephthalate (PET). Solutions with five different concentrations of the precursors with equimolar Zinc Nitrate and Hexamethylenetetramine (HMT) in distilled water were prepared to systematically study the effect of precursor solution concentration on the structural and optical properties of ZnO nanorods. It was concluded that the precursor concentration have great influence on the morphology, crystal quality, and optical property of ZnO nanorods. The diameter, density, and orientation of the nanorods are dependent on the precursor solution concentration. X-ray diffraction and micro-Raman spectroscopy showed that the ZnO nanorods with the highest concentration of 50 mM were highly aligned and have the highest level of surface coverage. It was also found that the diameter and length of the nanorods increases upon increasing precursor solution concentration. This is the first systematic investigation of studying the effect of precursor solution concentration on the quality of ZnO nanorods grown on ITO/PET substrates by low-temperature solution method. We believe that our work will contribute to the realization of flexible organic-inorganic hybrid solar cell based on ZnO nanorods and conjugated polymer.

Blue phosphorescent organic light-emitting devices based on carbazole/thioxanthene-S,S-dioxide with a high glass transition temperature

Carbazole/thioxanthene-S,S-dioxide (EBCz-ThX) bipolar molecules were synthesized by incorporating electron-donating carbazole and electron-accepting thioxanthene-S,S-dioxide into one molecule at 85 °C using a solventless green reaction method. The EBCz-ThX bipolar molecule exhibited a high glass transition temperature of 227 °C and a high triplet energy of 2.94 eV. The highest occupied molecular orbital (HOMO, −5.95 eV) and lowest unoccupied molecular orbital (LUMO, −2.63 eV) energy levels of the molecule matched the HOMO energy level of the hole transporting layer and the LUMO energy level of the electron transporting layer, respectively, which facilitated the injection of holes and electrons. Phosphorescent organic light-emitting devices (PHOLEDs) were fabricated with EBCz-ThX as a blue host material and with bis((4,6-difluorophenyl)-pyridinate-N,C2′)picolinate as a blue dopant, and the doping ratio was controlled to optimize the performance of the blue PHOLEDs. Blue PHOLEDs with a 15% doping ratio at 100 cd m−2 showed excellent performances and low operation voltages with quantum and current efficiencies of about 12% and 24 cd A−1, indicating that the EBCz-ThX acted as an excellent host for the blue PHOLEDs. The external quantum efficiency at 1000 cd m−2 was improved up to 30% in comparison with that of a conventional PHOLED with an mCP of 8% doping ratio.

Small-Molecule Vinazene Acceptors for Bulk Heterojunction Organic Photovoltaics

This paper reports the synthesis and characterization of a new vinazene-based small molecule, 4,7-bis(2-(1-(2-ethylhexyl)-4,5-dicyanoimidazol-2-yl) vinyl)diketopyrrolopyrrole (EV-DPP). The known vinazene-based small molecule, 4,7-bis(2-(1-(2-ethylhexyl)-4,5-dicyanoimidazol-2-yl)vinyl)benzo[c][1,2,5]thiadizole (EV-BT), was also prepared for comparison. The benzothiadiazole unit in EV-BT was replaced with a DPP unit in EV-DPP because DPP units are strong absorbing chromospheres. The UV absorption maxima of EV-DPP film (λ max = 386, 573 nm) were greatly red shifted compared to the maxima of EV-BT film (λ max = 337, 444 nm). Our preliminary device work showed the possibility of the use of EV-DPP as an electron acceptor in OPVs. The P3HT: EV-DPP film showed a better power conversion efficiency of 0.02% compared to P3HT: EV-BT due to the preferable UV absorption characteristics of EV-DPP.

Improved OPV Efficiency of Fluorene-Thiophene-Based Copolymers with Hole- and Electron-Transporting Units in the Main Chain

In this paper, we have synthesized a series of fluorene-thiophene-based copolymers by Suzuki coupling polymerization for the use of donor materials in organic photovoltaic cells (OPVs). New conjugated polymers have an electron-deficient moiety such as benzothiadiazole (BT), oxadiazole (OX) and triazole (TZ) or an electron-rich triphenylamine (TPA) moiety or both in the main chains as third or fourth comonomers. The photovoltaic properties of the polymers were varied by introduced electron- or hole-transporting units. The order in the efficiency of the photovoltaic cells was P1 (BT) < P2 (TPA) < P3 (TPA, BT) < P4 (TPA, OX) < P5 (TPA, TZ). The highest power conversion efficiency (PCE) of 0.33% (P5) with an open-circuit voltage (V OC) of 0.76 V and a short circuit current (J SC) of 1.43 mA/cm2 was achieved by device annealing at 80°C. The relationship between molecular structures and photovoltaic properties was systematically established herein.

Conductivity Enhancement of PEDOT/PSS Films with Ionic Liquids as Dopants

The ionic materials were added to PEDOT/PSS solution as secondary dopants. The conductivity of PEDOT/PSS film improved with adding ionic materials. The film of PEDOT/PSS with 1% pyridinium p-toluene-sulfonate showed the conductivity of 23S/cm, which is increased about three orders than the film of origin PEDOT/PSS with 0.028S/cm. The surface morphology of films of PEDOT/PSS mixture is investigated by atomic force microscope. The AFM showed the increasing of grain size with the addition of pyridinium p-toluene-sulfonate.

Poly(3,4-Ethylenedioxythiophene)-Indium Tin Oxide Nanocomposites: Improved Electrochromic Response and Efficiency

Poly(3,4-ethylenedioxythiophene) (PEDOT)-indium tin oxide (ITO) nanocomposites were synthesized via a solution route. The electrochromic behavior of the devices was investigated in terms of particle sizes, dispersibility and electrical conductivity of the nanocomposites. The electrical conductivity of PEDOT-ITO composite thin films were in the range of 0.1∼15 S/cm with transparency changes from 25% to 85%. The electrochromic properties of the nanocomposite as an active material for the devices (ECDs) are discussed in terms of conductivity and transparency. The electrochromic behavior was confirmed with colored state and blanched state at +3.1 V∼−2.9 V which transparency difference about 50∼60% at 600 nm. The switching speeds of the ECD are closely correlated with the changes in the electrical conductivity and the morphology of the nanocomposite.

Synthesis and Characterization of Carbazole-Thiophene-based Conjugated Polymers for Organic Photovoltaic Cells

In this paper, we have synthesized a series of carbazole-based copoly- mers, poly(N-9″-heptadecanyl-2,7-carbazole-alt-bithiophene) (PCzT2), poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5′-bis(3″-hexyl-2″thienyl)-2,2′-bithiophenyl-5″,5″- diyl] (PCzT4), and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCz3TB), by Suzuki coupling polymerization. The optical and electrochemical properties of copolymers were effectively tuned by changing conjugation lengths of comonomer units from bithiophene to quaterthiophene or by introducing the electron-accepting benzothiadiazole into the polymer main chain. Finally, the organic photovoltaic cells (OPVs) were fabricated by blending of the synthesized polymers as donor and PCBM as acceptor, and the photovoltaic properties of the copolymers such as a power conversion efficiency (PCE), a short-circuit current (J SC), an open-circuit voltage (V OC), and a fill factor (FF) were evaluated. The relationship between molecular structures and device performances could be explained on the basis of the optical and electrochemical properties of the copolymers.

Preparation of Indium Tin Oxide Inks for Electrically Conductive Transparent Oxide Film with Ink-Jet Printing Method

The ink-jet printing method is an important process in display industry due to the low-cost production of large-scale electronic devices. In this work, we prepared the nanoparticle indium tin oxide (ITO) solutions for ink-jet printing and characterized the electrical, structural and morphological properties of the film. The improvement of conductivity after reduction process was correlated with the sharpening of the diffraction peak as proved by X-ray diffraction measurement. The ITO inks was coated on a glass by spin-coating process and patterned by the ink-jet printing technique using a 50 μm diameter ink-jet nozzle. The sheet resistance and optical transmission of ITO thin film was ca. 50∼300Ω/sq and ca. 75∼95%, respectively. The electrical resistance of patterned ITO lines showed about 5∼30 * 103Ω. The detailed relationship between microstructure and properties are discussed.

Improved Photovoltaic Efficiency of Polymer Photovoltaic Cells by Microwave Irradiation

In this study, microwave annealing treatment was introduced into poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61butyric acid methyl ester (PCBM) and poly(9,9‘-dioctylfluorene-cobithiophene (PFT2):PCBM systems instead of thermal annealing treatment. In both systems, microwave annealing showed photovoltaic performane comparable to that of conventional thermal annealing. Through the UV-vis absorption, atomic force microscopy (AFM) and X-ray diffraction (XRD) studies, we were able to confirm that the microwave annealing increases the crystallization of the P3HT polymer chains.

Preparation and Characterization of Silicon Nanoparticles for Solution-Processed Thin Film

We describe the synthesis of the alkoxy-capped silicon nanoparticles (Si NPs) via solution route without an explicit reduction process. Based on the UV–visible absorption and photoluminescence (PL) emission spectra of the alkoxy-capped Si NPs, the average size of Si NPs is estimated to be ca. 3 nm. Fourier transform infrared (FT-IR) spectroscopy on the sintered Si thin film confirms elimination of the alkyl groups from the nanoparticles as evidenced by the shrinkage of the absorption peaks corresponding to –CHn and Si–OR. Although the silicon film was highly resistive, we have consistently observed the electrical conduction from films coated with the doped NPs.