Browsing by Author "Yeh, Yun-Shiuan"

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Efficient solid-state white light-emitting electrochemical cells employing embedded red color conversion layers
(2015-01-01)
Lu, Jung-Sheng
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Chen, Hsiao-Fan
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Kuo, Jung-Chin
;
Sun, Ray
;
Cheng, Chia-Yu
Recently, white light-emitting electrochemical cells (LECs) have attracted scientific attention due to their advantages of simple device structure, low operation voltage and compatibility with solution processes. In this work, efficient white emission from blue-emitting LECs with embedded red color conversion layers (CCLs) is reported. The red CCL is embedded between an indium tin oxide (ITO) layer and a glass substrate. The blue electroluminescence (EL) trapped in the waveguided mode, i.e. in the ITO layer, shows an evanescent tail in the CCL layer and excites the red-emitting dye, generating red photoluminescence (PL). Because the trapped blue EL can be partially transferred to the out-coupled red PL, the doping concentration of the red dye in the CCL can be reduced, and thus the absorption loss of the blue EL in the forward direction (air mode) can be mitigated, rendering enhanced device efficiency. The peak external quantum efficiency and power efficiency obtained in the blue-emitting LECs with embedded red CCLs are up to 12.5% and 27 lm W-1, respectively. These values are among the highest reported for white LECs, and thus confirm that blue-emitting LECs containing embedded red CCLs are capable of providing efficient white emission.
Enhancing device efficiencies of solid-state white light-emitting electrochemical cells by employing waveguide coupling
(2015-01-01)
Cheng, Chia-Yu
;
Wang, Chi-Wei
;
Cheng, Jing-Rong
;
Chen, Hsiao-Fan
;
Yeh, Yun-Shiuan
Solid-state white light-emitting electrochemical cells (LECs) have received intense scientific attention owing to their potential applications in display and lighting. Although device efficiencies of white LECs have been significantly improved in recent years, further improvements are still required for their practical applications. In this work, we demonstrate the enhancement of device efficiencies of white LECs by employing waveguide coupling. Two transparent photoresist (TPR) layers doped with TiO2 nanoparticles (NPs) are inserted between the indium tin oxide (ITO) layer and the glass substrate. By tuning the doping concentration of 25 nm TiO2 NPs in the upper TPR layer to adjust the refractive index, effective waveguide coupling between the ITO layer and the lower TPR layer can be achieved. Since the lower TPR layer contains 250 nm TiO2 NPs, electroluminescence (EL) outcoupled from the ITO layer can be scattered and redirected into the forward direction. Furthermore, the EL trapped in the glass substrate can also transmit into the lower TPR layer and then is scattered to the forward direction. When the EL trapped in the ITO layer and the glass substrate can be effectively recycled into the forward direction, the peak external quantum efficiency and power efficiency obtained in white LECs employing waveguide coupling are up to 19.4% and 34.1 lm W-1, respectively. These efficiencies are among the highest reported for white LECs and thus confirm that waveguide coupling would be useful for realizing highly efficient white LECs. In addition to the enhanced device efficiencies, improved color migration of EL spectra, which is desired for lighting applications, can be obtained in white LECs with scattering waveguide layers since EL of different angles can be mixed in the forward direction.
Highly efficient exciplex emission in solid-state light-emitting electrochemical cells based on mixed ionic hole-transport triarylamine and ionic electron-transport 1,3,5-triazine derivatives
(2013)
Chen, Hsiao-Fan
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Liao, Chih-Teng
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Su, Hai-Ching
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Yeh, Yun-Shiuan
;
Wong, Ken-Tsung
Highly efficient exciplex emission in solid-state light-emitting electrochemical cells (LECs) was achieved by mixing two ionic fluorescent emitters with opposite bipolarity. A triarylamine derivative as a hole-transport material (HTM) and a 1,3,5-triazine derivative as an electron-transport material (ETM) were selected due to their superior carrier mobilities. The ionic character of HTM and ETM was achieved by chemically attaching three pendant methylimidazolium groups as peripheries, rendering them amenable to LEC applications. Strong exciplex green emission can be observed either from photoluminescence in solid states or electroluminescence in LEC devices when HTM and ETM are mixed. By optimizing the mixing ratio, the external quantum efficiency and power efficiency can be achieved up to 3.04% and 10.29 lm W-1, respectively. These results are among the highest reported for fluorescent LECs.
Improving device efficiencies of solid-state white light-emitting electrochemical cells by adjusting the emissive-layer thickness
(2013-10-01)
Jhang, Yuan-Pei
;
Chen, Hsiao-Fan
;
Wu, Hung-Bao
;
Yeh, Yun-Shiuan
;
Su, Hai-Ching
Exciton quenching in the recombination zone close to electrochemically doped regions would be one of the bottlenecks for improving device efficiencies of solid-state white light-emitting electrochemical cells (LECs). To further enhance device efficiencies of white LECs for practical applications, we adjust the emissive-layer thickness to reduce exciton quenching. In white LECs with properly thickened emissive-layer thickness, the recombination zone can be situated near the center of the emissive layer, rendering mitigated exciton quenching and thus enhanced device efficiencies. High external quantum efficiencies and power efficiencies of optimized devices reach ca. 11% and 20 lm/W, respectively, which are among the highest reported for white LECs. These results confirm that tailoring the thickness of the emissive layer to avoid exciton quenching would be a feasible approach to improve device efficiencies of white LECs. (C) 2013 Elsevier B.V. All rights reserved.
UV light-emitting electrochemical cells based on an ionic 2,2 '-bifluorene derivative
(2012-10-01)
Chen, Hsiao-Fan
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Liao, Chih-Teng
;
Kuo, Ming-Cheng
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Yeh, Yun-Shiuan
;
Su, Hai-Ching
UV light-emitting electrochemical cells (LECs) were, for the first time, achieved by the ionic 2,2'-bifluorene derivative, 1, which was synthesized through covalent tethering of methylimidazolium moieties as pendent groups. LEC devices incorporating ionic bifluorene 1 without (Device I) and with (Device III) the presence of poly(methyl methacrylate) (PMMA) exhibited UV EL emissions centered at 388 and 386 nm with maximum external quantum efficiencies and power efficiencies of 1.06% and 7.44 mW W-1 for Device III and 0.15% and 1.06 mW W-1 for Device I, respectively. Transmission electron microscopy (TEM) images showed that 1 tends to form nanospheres due to amphiphilic nature. The presence of PMMA unified the size of nanospheres which greatly reduced the void area in films, suppressing the current leakage and enhancing the device efficiency. Furthermore, thicker thickness of the emissive layer of LECs increases the distance between carrier recombination zone and electrodes to avoid exciton quenching. Thus, a sevenfold increase in device efficiency was obtained in thicker UV LECs containing PMMA (Device III) as compared to thinner UV LECs based on neat films of 1 (Device I). The EL emissions in the UV region are successfully achieved by LECs based on 1, which are so far the shortest emission wavelength achieved in LECs. (c) 2012 Elsevier B.V. All rights reserved.