Eunhee Lim

Flexible organic/inorganic hybrid solar cells based on conjugated polymer and ZnO nanorod array

We report on the photovoltaic characteristics of organic/inorganic hybrid solar cells fabricated on ‘flexible’ transparent substrates. The solar cell device is composed of ZnO nanorod array and the bulk heterojunction structured organic layer which is the blend of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The ZnO nanorod array was grown on indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates via a low-temperature (85xa0°C) aqueous solution process. The blend solution consisting of conjugated polymer P3HT and fullerene PCBM was spin coated at a low spinning rate of 400xa0rpm on top of the ZnO nanorod array structure and then the photoactive layer was slow dried at room temperature in air to promote its infiltration into the nanorod network. As a top electrode, silver was sputtered on top of the photoactive layer. The flexible solar cell with the structure of PET/ITO/ZnO thin film/ZnO nanorods/P3HT:PCBM/Ag exhibited a photovoltaic performance with an open circuit voltage (VOC) of 0.52xa0V, a short circuit current density (JSC) of 9.82xa0mA cm−2, a fill factor (FF) of 35% and a power conversion efficiency (η) of 1.78%. All the measurements were performed under 100 mW cm−2xa0of illumination with an air mass 1.5xa0G filter. To the best of our knowledge, this is the first presentation of investigation into the fabrication and characterization of organic/inorganic hybrid solar cells based on bulk heterojunction structured conjugated polymer/fullerene photoactive layer and ZnO nanorod array constructed on flexible transparent substrates.

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 50u2009mM 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.

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.

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.

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.