Browsing by Author "Mengistie, Desalegn Alemu"

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Direct conversion of multilayer molybdenum trioxide to nanorods as multifunctional electrodes in lithium-ion batteries
(2014)
Ibrahem, Mohammed Aziz
;
Wu, Feng-Yu
;
Mengistie, Desalegn Alemu
;
Chang, Chia-Seng
;
Li, Lain-Jong
In this study we prepared molybdenum trioxide (MoO3) nanorods having average lengths of 0.5-1.5 mu m and widths of approximately 100-200 nm through a one-step mechanical break-down process involving favorable fracturing along the crystal direction. We controlled the dimensions of the as-prepared nanorods by applying various imposing times (15-90 min). The nanorods prepared over a reaction time of 90 min were, on average, much shorter and narrower relative to those obtained over 30 min. Evaluations of lithium-ion storage properties revealed that the electrochemical performance of these nanorods was much better than that of bulk materials. As cathodes, the nanorods could deliver a high specific capacity (>315 mA h g(-1)) with losses of less than 2% in the first cycle at a rate of 30 mA g(-1); as anodes, the specific capacity was 800 mA h g(-1) at a rate of 50 mA g(-1). Relative to alpha-MoO3 microparticles, these nanorods displayed significantly enhanced lithium-ion storage properties with higher reversible capacities and better rate performance, presumably because their much shorter diffusion lengths and higher specific surface areas allowed more-efficient insertion/deinsertion of lithium ions during the charge/discharge process. Accordingly, enhanced physical and/or chemical properties can be obtained through appropriate nanostructuring of materials.
Effect of molecular weight of additives on the conductivity of PEDOT: PSS and efficiency for ITO-free organic solar cells
(2013)
Mengistie, Desalegn Alemu
;
Wang, Pen-Cheng
;
Chu, Chih-Wei
;
光電工程學系
;
Department of Photonics
We systematically investigated the effect of the molecular weight of additives on the conductivity of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) by using different concentrations and molecular weights of polyethylene glycol (PEG) and ethylene glycol (EG). The conductivity enhancement depends on both the molecular weight and concentration of PEG used. The conductivity of PEDOT: PSS was enhanced from 0.3 S cm(-1) to 805 S cm(-1) with 2% PEG but to only 640 S cm(-1) with 6% EG. PEGs with molecular weight higher than 400 have too low mobility to impart the required screening effect, and hence, the conductivity enhancement is less. Through FTIR, XPS and AFM investigations, the mechanism for the conductivity enhancement is found to be charge screening between PEDOT and PSS followed by phase separation and reorientation of PEDOT chains leading to bigger and better connected particles. The molecular weight and concentration of PEG also affect solar cell performances even though the conductivities are the same. Due to their high conductivity and high transmittance, ITO-free organic solar cell devices fabricated using PEDOT: PSS treated with 2% PEG anodes exhibited performance almost equal to that of the ITO counterparts.
Highly Conductive PEDOT:PSS Electrode Treated with Polyethylene Glycol for ITO-free Polymer Solar Cells
(2013)
Mengistie, Desalegn Alemu
;
Wang, Pen-Cheng
;
Chu, Chih-Wei
;
光電工程學系
;
Department of Photonics
We used polyethylene glycol (PEG) and enhanced the conductivity of PEDOT:PSS from 0.3 S cm(-1) to 805 S cm(-1) with only 2% PEG additive. The conductivity was further enhanced to 1100 S cm-1 by combining PEG and methanol treatment. PEG treated PEDOT: PSS films showed better phase separation between PEDOT and PSS with fiber like 3D PEDOT networks. The carrier concentration increased by three orders of magnitude after PEG treatment. ITO-free polymer solar cells with PEG treated PEDOT: PSS anodes were fabricated and optimized. Double layer PEDOT: PSS treated with 2% PEG200 anodes showed power conversion efficiency of 3.71% while and the ITO counterpart showed 3.73%.
Transparent electrodes based on conducting polymers for display applications
(2013-10-01)
Wang, Pen-Cheng
;
Liu, Li-Hung
;
Mengistie, Desalegn Alemu
;
Li, Kuan-Hsun
;
Wen, Bor-Jiunn
The materials science and engineering related to the fabrication of conducting polymer thin films and the progress in the development of devices integrated with organic transparent electrodes based on conducting polymers for display applications are reviewed. Transparent electrodes are essential components for many display modules. With the evolution of display technologies, conducting polymers are recently emerging as important alternative materials for the fabrication of transparent electrodes. Conducting polymers offer some advantages, such as light weight, low cost, mechanical flexibility and excellent compatibility with plastic substrates for the development of next-generation display technologies and, in particular, are expected to play an important role in the development of flexible display technologies. (C) 2013 Elsevier B.V. All rights reserved.