Browsing by Author "Chen, Hsiao-Fan"

Now showing 1 - 18 of 18

An ionic terfluorene derivative for saturated deep-blue solid state light-emitting electrochemical cells
(2011)
Chen, Hsiao-Fan
;
Liao, Chih-Teng
;
Chen, Te-Chuan
;
Su, Hai-Ching
;
Wong, Ken-Tsung
In this study, we obtained saturated deep-blue electroluminescence (EL) from solid state light-emitting electrochemical cells (LECs) incorporating the ionic terfluorene derivative 1, which we synthesized through covalent tethering of methylimidazolium moieties as pendent groups of terfluorene. Compound 1 exhibited the deep-blue emission, extremely high quantum yield, and reversible redox behavior that are characteristic of terfluorene derivatives. LEC devices incorporating terfluorene 1 exhibited deep-blue EL with CIE coordinates of (0.159, 0.115) and (0.151, 0.122) in the absence and presence, respectively, of the ionic liquid (IL) BMIM center dot PF(6). The peak external quantum efficiency and peak power efficiency of 1 in the presence of the IL reached 1.14% and 1.24 lmW(-1), respectively. These CIE coordinates are the most saturated blue emissions ever reported for LECs.
Bis(diphenylamino)-9,9 '-spirobifluorene functionalized Ir(III) complex: a conceptual design en route to a three-in-one system possessing emitting core and electron and hole transport peripherals
(2011)
Chen, Hsiao-Fan
;
Wong, Ken-Tsung
;
Liu, Yi-Hung
;
Wang, Yu
;
Cheng, Yi-Ming
Conceptual design of a three-in-one (luminescence chromophore with electron and hole transports) system was demonstrated by a functionalized Ir(III) complex 3, in which 4,5-diazafluorene and bis(diphenylamino) serve as electron and hole transporting sites, respectively. The poor emission quantum yield of 3 was systematically examined via a series of photophysical studies in combination with theoretical approaches. The far lifting of the pi-electron from -NPh(2) renders virtually no (3)MLCT contribution to the lowest transition in the triplet manifold as compared with that of the parent model 2 without amino substituents. With an empirical approach, we conclude that an energy gap law may account for the major deactivation process. A light-emitting electrochemical cell (LEC) device based on 3 shows peak EQE, peak current efficiency and peak power efficiency at 2.4 V of 0.020%, 0.013 cd A(-1) and 0.017 lm/W, respectively. The low device efficiencies are in accordance with the low PL quantum yield, stemming from the ligand-centered radiationless deactivation. The conceptual design presented here should provide valuable information for future progress en route to an ideal three-in-one system suited for OLEDs.
Cationic Iridium Complexes with Intramolecular pi-pi Interaction and Enhanced Steric Hindrance for Solid-State Light-Emitting Electrochemical Cells
(2012-11-19)
Chen, Hsiao-Fan
;
Hung, Wen-Yi
;
Chen, Shou-Wei
;
Wang, Ting-Chih
;
Lin, Shih-Wei
Cationic iridium complexes incorporated 4,5-diaza-9,9'-spirobifluorene as N<^>N ancillary ligands, in which one (2) or two (3) phenyl groups were introduced onto 4,5-diazafluorene to afford intraligand pi-pi interactions. The X-ray crystal structures of complexes 2 and 3 show that the pendant phenyl ring forms strong intramolecular face-to-face pi-stacking with the difluorophenyl ring of the cyclometalated ligand with distances of 3.38 angstrom for complex 2 and 3.40 and 3.46 angstrom for complex 3. This pi-pi stacking interaction minimizes the expansion of the metal ligand bonds in the excited state, resulting in a longer device lifetime in the light-emitting electrochemical cell (LEC) devices.
Efficient and color-stable solid-state white light-emitting electrochemical cells employing red color conversion layers
(2012-03-01)
Wu, Hung-Bao
;
Chen, Hsiao-Fan
;
Liao, Chih-Teng
;
Su, Hai-Ching
;
Wong, Ken-Tsung
We report efficient and color-stable white light-emitting electrochemical cells (LECs) by combining single-layered blue-emitting LECs with red-emitting color conversion layers (CCLs) on the inverse side of the glass substrate. By judicious choosing of the red-emitting dye doped in CCLs, good spectral overlap between the absorption spectrum of the red-emitting dye and the emission spectrum of the blue-emitting emissive material results in efficient energy transfer and thus sufficient down-converted red emission at low doping concentrations of the red-emitting dye in the CCLs. Low doping concentration is beneficial in reducing self-quenching of the red-emitting dye, rendering efficient red emission. Electroluminescent (EL) measurements show that the peak external quantum efficiency and the peak power efficiency of the white LECs employing red CCLs reach 5.93% and 15.34 lmW (1), respectively, which are among the highest reported for white LECs. Furthermore, these devices exhibit bias-insensitive white EL spectra, which are required for practical applications, due to nondoped emissive layers. These results reveal that single-layered blue-emitting LECs combined with red-emitting CCLs are one of the potential candidates for efficient and color-stable white light-emitting devices. (C) 2011 Elsevier B. V. All rights reserved.
Efficient solid-state white light-emitting electrochemical cells based on phosphorescent sensitization
(2012-11-21)
Su, Hai-Ching
;
Chen, Hsiao-Fan
;
Chen, Po-Hsien
;
Lin, Shih-Wei
;
Liao, Chih-Teng
Efficient phosphorescent sensitized white light-emitting electrochemical cells (LECs) based on a blue-emitting phosphorescent cationic transition metal complex (CTMC) doped with a red-emitting fluorescent dye are demonstrated. Blue phosphorescence and phosphorescence-sensitized red fluorescence contribute to white emission. Furthermore, the microcavity effect of the device structure is utilized to suppress the green part of the electroluminescence (EL) spectrum of the blue-emitting phosphorescent metal complex (1) via destructive interference. Hence, more saturated blue EL emission can be obtained. When combined with saturated red emission, white EL emission with Commission Internationale de l'Eclairage coordinates approaching (0.33, 0.33) can be obtained without the need for saturated deep-blue emitting CTMCs. Peak external quantum efficiency and power efficiency of the phosphorescence sensitized white LECs are up to 7.9% and 15.6 lm W-1, respectively. These efficiencies are among the highest reported for white LECs and reveal that phosphorescent sensitization is useful for improving device efficiencies of white LECs.
Efficient solid-state white light-emitting electrochemical cells employing embedded red color conversion layers
(2015-01-01)
Lu, Jung-Sheng
;
Chen, Hsiao-Fan
;
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 double-doped solid-state white light-emitting electrochemical cells
(2011)
Su, Hai-Ching
;
Chen, Hsiao-Fan
;
Shen, Yu-Chun
;
Liao, Chih-Teng
;
Wong, Ken-Tsung
We report highly efficient, solid-state, white light-emitting electrochemical cells (LECs) based on a double-doped strategy, which judiciously introduces an orange-emitting guest, [Ir(ppy)(2)(dasb)](+)(PF(6)(-)), into a single-doped emissive layer comprised of an efficient blue-green emitting host, [Ir(dfppz)(2)(dtb-bpy)](+)(PF(6)(-)), and a red-emitting guest, [Ir(ppy)(2)(biq)](+)(PF(6)(-)), to improve the balance of carrier mobilities and, thus, to enhance the device efficiency. Photoluminescence (PL) measurements show that the single-doped (red guest) and the double-doped (red and orange guests) host-guest films exhibit similar white PL spectra and comparable photoluminescence quantum yields, while the device efficiencies of the double-doped white LECs are twofold higher than those of the single-doped white LECs. Therefore, such enhancement of the device efficiency is rationally attributed to the improved balance of carrier mobilities of the double-doped emissive layer. Peak external quantum efficiency and peak power efficiency of the double-doped white LECs reached 7.4% and 15 lm W(-1), respectively. These efficiencies are amongst the highest reported for solid-state white LECs and, thus, confirm that the double-doping strategy is useful for achieving highly efficient white LECs.
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
;
Liao, Chih-Teng
;
Su, Hai-Ching
;
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.
Improving the balance of carrier mobilities of host-guest solid-state light-emitting electrochemical cells
(2012)
Liao, Chih-Teng
;
Chen, Hsiao-Fan
;
Su, Hai-Ching
;
Wong, Ken-Tsung
;
照明與能源光電研究所
We report efficient host-guest solid-state light-emitting electrochemical cells (LECs) utilizing a cationic terfluorene derivative as the host and a red-emitting cationic transition metal complex as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the host-guest films and thus improves the balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting characteristics. Photoluminescence measurements show efficient energy transfer in this host-guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host-guest LECs reaches 3.62% (7.36 lm W(-1)), which approaches the upper limit that one would expect from the photoluminescence quantum yield of the emissive layer (similar to 0.2) and an optical out-coupling efficiency of similar to 20% and consequently indicates superior balance of carrier mobilities in such a host-guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host-guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.
Non-doped solid-state white light-emitting electrochemical cells employing the microcavity effect
(2015-03-14)
Lin, Guan-Rung
;
Chen, Hsiao-Fan
;
Shih, Hsien-Chang
;
Hsu, Jia-Hong
;
Chang, Yi
Solid-state white light-emitting electrochemical cells (LECs) have attracted research attention owing to their advantages of simple device structure, low operation voltage and compatibility with solution processes. In this work, we demonstrate a simple approach to obtain white electroluminescence (EL) from non-doped LECs based on a blue-emitting complex. With a relatively thicker emissive layer, red emission can be additionally enhanced by the microcavity effect when the recombination zone moves to appropriate positions. Hence, white EL can be harvested by combining blue emission from the complex and red emission from the microcavity effect. These non-doped white LECs show external quantum efficiencies and power efficiencies up to 5% and 12 lm W-1, respectively. These results show that efficient white EL can be obtained in simple non-doped LECs.
Phosphorescent sensitized fluorescent solid-state near-infrared light-emitting electrochemical cells
(2011)
Ho, Chien-Cheng
;
Chen, Hsiao-Fan
;
Ho, Yu-Che
;
Liao, Chih-Teng
;
Su, Hai-Ching
We report phosphorescent sensitized fluorescent near-infrared (NIR) light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic transition metal complex [Ir(ppy)(2)(dasb)](+) (PF(6)(-)) (where ppy is 2-phenylpyridine and dasb is 4,5-diaza-9,9'-spirobifluorene) as the host and two fluorescent ionic NIR emitting dyes 3,3'-diethyl-2,2'-oxathiacarbocyanine iodide (DOTCI) and 3,3'-diethylthiatricarbocyanine iodide (DTTCI) as the guests. Photoluminescence measurements show that the host-guest films containing low guest concentrations effectively quench host emission due to efficient host-guest energy transfer. Electroluminescence (EL) measurements reveal that the EL spectra of the NIR LECs doped with DOTCI and DTTCI center at ca. 730 and 810 nm, respectively. Moreover, the DOTCI and DTTCI doped NIR LECs achieve peak EQE (power efficiency) up to 0.80% (5.65 mW W(-1)) and 1.24% (7.84 mW W(-1)), respectively. The device efficiencies achieved are among the highest reported for NIR LECs and thus confirm that phosphorescent sensitized fluorescence is useful for achieving efficient NIR LECs.
Solid-state light-emitting electrochemical cells employing phosphor-sensitized fluorescence
(2010)
Su, Hai-Ching
;
Lin, You-Heng
;
Chang, Chih-Hao
;
Lin, Hao-Wu
;
Wu, Chung-Chih
We report highly efficient phosphor-sensitized solid-state light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic iridium complex [Ir(dFppy)(2)(SB)](+)(PF(6)(-)) as the host and a fluorescent cationic dye (R6G) as the guest. Photophysical studies show that R6G retains a high photoluminescence quantum yield (PLQY) in highly polar media, revealing its suitable use as an emitting guest in an ionic host matrix. Such solid-state LECs achieve quantum efficiency (cd A(-1)) efficiency, and power efficiency up to 5.5% photon/electron, 19 cd A(-1) and 21.3 lm W(-1), respectively. The device quantum efficiency achieved is among the highest reported for fluorescent LECs and is higher than one would expect from the PLQY of the R6G fluorescent dye in the host film, thus indicating that phosphor-sensitization is useful for achieving highly efficient fluorescent LECs. Moreover, using narrow-band fluorescent emitters, such as R6G (FWHM, similar to 50 nm), is effective in improving the color saturation of solid-state LECs based on cationic complexes.
Solid-state white light-emitting electrochemical cells based on scattering red color conversion layers
(2015-01-01)
Lin, Guan-Rung
;
Cheng, Jing-Rong
;
Wang, Chi-Wei
;
Sarma, Monima
;
Chen, Hsiao-Fan
Solid-state white light-emitting electrochemical cells (LECs) based on ionic transition metal complexes are capable of generating efficient white electroluminescence (EL). Recently, blue-emitting LECs combined with embedded red color conversion layers (CCLs), which were inserted between indium tin oxide (ITO) and a glass substrate, have been reported to offer an external quantum efficiency (EQE) >12%. However, the output spectrum of EL from such white LECs changed with time due to the altered microcavity effect when the recombination zone was moving. Furthermore, the device efficiency should be further improved to realize practical applications. In this work, TiO2 nanoparticles (NPs) of two sizes are incorporated into red CCLs to improve device performance. Large NPs (250 nm) can scatter and redirect the light passing through red CCLs and further enhance the extracted light. Small NPs (25 nm) increase the refractive index of red CCLs and raise the amount of light intensity in the evanescent tail of the waveguide mode near the ITO layer. Therefore, more trapped light in the evanescent tail of the waveguide mode can be recycled to the external mode by scattering. The peak EQE and power efficiency of white LECs employing scattering red CCLs reach 20.0% and 39.5 lm W-1, respectively. Furthermore, with scattering NPs to average the EL in different directions, white EL is almost invariant with time. These results show that blue-emitting LECs combined with scattering red CCLs would be effective to provide efficient and stable white EL.
Tailoring balance of carrier mobilities in solid-state light-emitting electrochemical cells by doping a carrier trapper to enhance device efficiencies
(2011)
Liao, Chih-Teng
;
Chen, Hsiao-Fan
;
Su, Hai-Ching
;
Wong, Ken-Tsung
;
照明與能源光電研究所
We demonstrate the improving balance of carrier mobilities in neat-film light-emitting electrochemical cells (LECs) utilizing a cationic transition metal complex (CTMC) as the emissive material and a cationic near-infrared laser dye as the carrier trapper. This low-gap carrier trapper is judiciously chosen such that a significant energy offset in the highest occupied molecular orbital (HOMO) levels between the CTMC and the carrier trapper impedes hole transport in the emissive layers while similar lowest unoccupied molecular orbital (LUMO) levels of these two materials result in relatively unaffected electron transport. Since the CTMC neat films would intrinsically exhibit characteristics of preferred transport of holes, the balance of carrier mobilities would be improved by doping such carrier trapper. Electroluminescent measurements show that the peak external quantum efficiency (EQE) and the peak power efficiency of the neat-film LECs doped with the carrier trapper reach 12.75% and 28.70 lm W(-1), respectively. These device efficiencies represent a 1.4 times enhancement as compared to those of the undoped neat-film LECs and approach the upper limit of EQE (similar to 15%) that one would expect from the photoluminescence quantum yield of the emissive layer (similar to 0.75) and an optical out-coupling efficiency of similar to 20% from a typical layered device structure, consequently indicating superior balance of carrier mobilities in such a doped emissive layer. These results confirm that the balance of carrier mobilities in the CTMC neat films would be improved by doping a proper carrier trapper and this technique offers a general approach for optimizing device efficiencies of CTMC-based neat-film LECs.
Tailoring carrier injection efficiency to improve the carrier balance of solid-state light-emitting electrochemical cells
(2012)
Liao, Chih-Teng
;
Chen, Hsiao-Fan
;
Su, Hai-Ching
;
Wong, Ken-Tsung
;
照明與能源光電研究所
We study the influence of the carrier injection efficiency on the performance of light-emitting electrochemical cells (LECs) based on a hole-preferred transporting cationic transition metal complex (CTMC) [Ir(dfppz)(2)(dtb-bpy)](+)(PF6) (complex 1) and an electron-preferred transporting CTMC [Ir(ppy)(2)(dasb)](+)(PF6) (complex 2) (where dfppz is 1-(2,4-difluorophenyl) pyrazole, dtb-bpy is 4,4'-di(tert-butyl)-2,2'-bipyridine, ppy is 2-phenylpyridine and dasb is 4,5-diaza-9,9'-spirobifluorene). Experimental results show that even with electrochemically doped layers, the ohmic contacts for carrier injection could be formed only when the carrier injection barriers were relatively low. Thus, adding carrier injection layers in LECs with relatively high carrier injection barriers would affect carrier balance and thus would result in altered device efficiency. Comparison of the device characteristics of LECs based on complex 1 and 2 in various device structures suggests that the carrier injection efficiency of CTMC-based LECs should be modified according to the carrier transporting characteristics of CTMCs to optimize device efficiency. Hole-preferred transporting CTMCs should be combined with an LEC structure with a relatively high electron injection efficiency, while a relatively high hole injection efficiency would be required for LECs based on electron-preferred transporting CTMCs. Since the tailored carrier injection efficiency compensates for the unbalanced carrier transporting properties of the emissive layer, the carrier recombination zone would be located near the center of the emissive layer and exciton quenching near the electrodes would be significantly mitigated, rendering an improved device efficiency approaching the upper limit expected from the photoluminescence quantum yield of the emissive layer and the optical outcoupling efficiency from a typical layered light-emitting device structure.
UV light-emitting electrochemical cells based on an ionic 2,2 '-bifluorene derivative
(2012-10-01)
Chen, Hsiao-Fan
;
Liao, Chih-Teng
;
Kuo, Ming-Cheng
;
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.