Browsing by Author "Liu, Shun-Wei"

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Achieving high-efficiency non-doped blue organic light-emitting diodes: charge-balance control of bipolar blue fluorescent materials with reduced hole-mobility
(2009)
Chi, Chih-Chin
;
Chiang, Chih-Long
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Liu, Shun-Wei
;
Yueh, Han
;
Chen, Chin-Ti
We found an unusual way in improving electroluminescence efficiency of blue organic light-emitting diodes (OLEDs). Two electron deficient 4,5-diazafluorene- or di(2,2'-pyridyl)-containing blue fluorophores, PhSP(N2)DPV (4,5-diaza-2'-diphenylamino-7'-(2,2"-diphenylvinyl)-9,9'-spirobifluorene) and PhF(py2)DPV (N-[7-(2,2- diphenylvinyl)- 9,9'-di(2,2"-pyridyl)-2-fluorenyl]-N,N-diphenylamine), were synthesized and characterized for non-doped blue OLEDs. Whereas PhF(py2)DPV OLED performs ordinarily, PhSP(N2)DPV OLED outperforms previously known PhSPDPV (2-diphenylamino-7-diphenylvinyl-9,9'-spirobifluorene) OLED significantly: maximum external quantum efficiency of similar to 5% (4.6% at 20 mA cm(-2)) and the peak electroluminance of 60510 cd m(-2) (1810 cd m(-2) at 20 mA cm(-2)) versus 3.4% (2.9% at 20 mA cm(-2)) and 33020 cd m(-2) (910 cd m(-2) at 20 mA cm(-2)) of PhSPDPV OLED. We attribute the superior performance of PhSP(N2)DPV OLED to the good charge balancing, which is in turn due to the very low hole mobility of PhSP(N2)DPV. The experimental results reveal that the electron-deficient moiety, 4,5-diazafluorene or di(2,2'-dipyridyl), increases electron affinity but reduces the hole mobility. Electron mobility, determined by time-of-flight (TOF) method, is 5 x 10(-5) and 5 x 10(-4) cm(2) V(-1) s(-1) (at an electric field of 4.9 x 10(5) V cm(-1)) for PhSP(N2)DPV and PhF(py2)DPV, respectively. Surprisingly, they are not higher than 8 x 10(-4) cm(2) V(-1) s(-1) of nonpolar PhSPDPV. On the other hand, hole mobility is 2 x 10(-6) and 2 x 10(-4) cm(2) V(-1) s(-1) for PhSP(N2)DPV and PhF(py2)DPV, respectively, and they are both significantly lower than 6 x 10(-3) cm(2) V(-1) s(-1) of PhSPDPV. For PhSP(N2)DPV and PhF(py2)DPV bipolar blue fluorophores, we have demonstrated that electron-transporting and light-emitting functions involve different molecular halves. The design of such molecular halves greatly facilitates the optical and electronic optimizations of fluorophores for high-performance OLEDs.
An alternative composite electrode for efficient organic light-emitting diodes
(2020-10-01)
Luo, Dian
;
Chiang, Tsung-Che
;
Chan, Chi-Hao
;
Chiu, Po-Chen
;
Chiu, Hao-Hsuan
Considering a device structure consisting of multi-stacked layers with different refractive indices, we proposed a composite electrode to diminish total internal reflection, thereby improving the out-coupling efficiency of organic light-emitting diodes (OLEDs). The selected transparent conducting oxide materials for the composite electrode were composed of the same main material, gallium-doped zinc oxide (GZO), to avoid lattice mismatch and reduce interfacial strain. Herein, silicon-doped GZO (SGZO) with a relatively low refractive index was used in combination with molybdenum-doped GZO (MGZO) with a high work function to form a multifunctional transparent composite electrode. High transmittance of 94.5% and adequate sheet resistance of 52.3 Ohm/sq were realized through the design of SGZO/MGZO films on a glass substrate. The tested blue phosphorescent OLEDs with SGZO/MGZO composite anode outperformed devices with other selected single-layer electrodes, achieving a high peak efficiency of 29.0% (57.6 cd/A and 47.6 lm/W). These results demonstrate clear advantages of using this composite-electrode concept for realizing high efficiency OLEDs or other flexible optoelectronics.
Combinational Approach To Realize Highly Efficient Light-Emitting Electrochemical Cells
(2020-03-25)
Yi, Rong-Huei
;
Lo, Chieh-Liang
;
Luo, Dian
;
Lin, Chien-Hsiang
;
Weng, Shu-Wen
Light-emitting electrochemical cells (LECs) show high technical potential for display and lighting utilizations owing to the superior properties of solution processability, low operation voltage, and employing inert cathodes. For maximizing the device efficiency, three approaches including development of efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction have been reported. However, most reported works focused on only one of the three optimization approaches. In this work, a combinational approach is demonstrated to optimize the device efficiency of LECs. A sophisticatedly designed yellow complex exhibiting a superior steric hindrance and a good carrier balance is proposed as the emissive material of light-emitting electrochemical cells and thus the external quantum efficiency (EQE) is up to 13.6%. With an improved carrier balance and reduced self-quenching by employing the host-guest strategy, the device EQE can be enhanced to 16.9%. Finally, a diffusive layer embedded between the glass substrate and the indium-tin-oxide layer is utilized to scatter the light trapped in the layered device structure, and consequently, a high EQE of 23.7% can be obtained. Such an EQE is impressive and consequently proves that the proposed combinational approach including adopting efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction is effective in realizing highly efficient LECs.
Efficiency improvement of organic bifunctional devices by applying omnidirectional antireflection nanopillars
(2014)
Ho, Yu-Hsuan
;
Liang, Hsun
;
Liu, Shun-Wei
;
Tian, Wei-Cheng
;
Chen, Fang-Chung
OLED/OPV tandem devices have attracted great interest due to their various functions in green technology and potential sensing applications. Here we report a novel concept to improve the luminous efficiency and energy conversion efficiency of tandem bifunctional devices by integrating antireflection structures consisting of irregular nanopillars as the capping layer. The nanopillars were made on flexible polycarbonate substrates by using a nanoimprint method with an anodized aluminum oxide template. The average reflectivity of the patterned polycarbonate substrate at visible wavelengths was reduced to 3.3%. With the antireflection structures, the luminous intensity for bifunctional devices was improved by 41.9% compared to a device with a flat capping layer. The power conversion efficiency was improved from 0.48% to 0.61%, a 27.0% enhancement at normal incidence.
Enhancing extracted electroluminescence from light-emitting electrochemical cells by employing high-refractive-index substrates
(2017-12-01)
Jang, Yea-Fen
;
Lin, Tzu-Chun
;
Guo, Jhih-Yan
;
Chiang, Chien-Ming Fan
;
Wu, Ming-Lun
Solidstate lightemitting electrochemical cells (LECs) show several advantages over conventional organic lightemitting devices (OLEDs) such as simple device structure compatible with solution processes, low operation voltage and capability of utilizing inert cathode metals. However, device performance of LECs must be improved, e.g. enhancing light extraction, to meet the requirements for practical applications. Among the optical modes trapped in LECs, light trapped in substrate mode is easier to be extracted, e.g., by simply roughing the output surface. Therefore, increasing the percentage of substrate mode is beneficial in improving light extraction. In this work, the contributions of optical modes in LECs employing substrates with various refractive indices are analyzed. Higherrefractiveindex substrates are shown to trap more light in the substrates. Smaller refractive index difference between higher-refractive-index substrate and indium tin oxide (ITO) layer also increases the cutoff spectral range of light waveguided in ITO layer. Furthermore, light intensity in surface plasmon mode significantly reduces as the refractive index of the substrate increases. Reducing the percentage of surface plasmon mode facilitates light extraction since it requires more complicated methods for outcoupling light in this mode. With commercially available unpolished sapphire substrates, light output of LECs is enhanced by 56%. When a scattering layer was inserted between ITO and sapphire substrate, more light in substrate mode can be extracted and 71% enhancement in light output is realized. High external quantum efficiency up to 5.5% is consequently obtained in LECs based on a ruthenium complex. Such device efficiency is among the highest reported values for redemitting LECs and thus confirms that utilizing higher-refractive-index substrates would offer a simple and feasible approach to improve light output of LECs. In comparison to OLEDs, increased EL trapped in substrates of LECs mainly comes from surface plasmon mode rather than waveguide mode. (C) 2017 Elsevier B.V. All rights reserved.
High morphology stability and ambipolar transporting host for use in blue phosphorescent single-layer organic light-emitting diodes
(2014-11-01)
Hsu, Fang-Ming
;
Chien, Liang-Jung
;
Chen, Kuan-Ting
;
Li, Ya-Ze
;
Liu, Shun-Wei
The authors report a small molecule host of 2,7-bis(diphenylphosphoryl)-9-[4-(N, N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) doped with 8 wt% iridium(III)-bis[(4,6-difluorophenyl)pyridinato-N,C-2\']picolinate (FIrpic) for use in efficient and single-layer blue phosphorescent organic light-emitting diodes (PHOLEDs) exhibiting a maximum external quantum efficiency of similar to 20.3% at brightness of 100 cd/m(2). The high performance of such single layer PHOLEDs is attributed to the POAPF host\'s high morphological stability, suitable triplet energy level, and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, thus reducing the triplet-triplet annihilation and resulting in a slight efficiency roll off of 0.5% from the brightness of 1 and 1000 cd/m(2). This work also systematically investigated the arrangement of the POAPF:FIrpic recombination zone for optimizing the performance of the single layer PHOLED. (C) 2014 Elsevier B.V. All rights reserved.
Highly efficient blue and white light-emitting electrochemical cells employing substrates containing embedded diffusive layers
(2020-02-01)
Chen, Yan-Zhi
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Luo, Dian
;
Hsiang, Chi-Haw
;
Yi, Rong-Huei
;
Lin, Ching-Hsuan
Enhancing device efficiencies of the blue and white light-emitting electrochemical cells (LECs) is realized by employing substrates with embedded diffusive layers containing scattering TiO2 nanoparticles (NPs). The diffusive layers can eliminate the influence of microcavity effect on the output electroluminescence (EL) spectrum and recover the intrinsic EL spectrum of the emissive layer. The emission zone positions of the blue and white LECs are estimated by fitting the measured EL spectra with the simulated EL spectra based on precisely tuned emission zone positions. Incorporating red-emitting guest dopant in the white LEC results in shifted emission zone toward the cathode due to enhanced electron trapping. The external quantum efficiency (EQE) of the blue and white LECs employing proper diffusive substrates can be enhanced by 260 and 210%, respectively. The maximal EQE (power efficiency) of the blue and white LECs reach 35.4% (83.4 lm W-1) and 22.0% (41.6 lm W-1), respectively. These promising device efficiencies confirm the potential applications for the proposed diffusive substrates in enhancing light extraction from LECs. In addition, adjusting the effective refractive index of the diffusive layer has distinct effect on different EL emission color. With well confined optical field of blue EL in indium tin oxide layer, lower effective refractive index of the diffusive layer results in higher refractive index difference between the scattering TiO2 NPs and the host medium, rendering higher scattering efficiency and more light extraction. In contrast, higher effective refractive index of the diffusive layer leads to unguided red EL extending into the diffusive layer and more light can be outcoupled. These results show that the effective refractive index of the diffusive layer should be judiciously chosen according to the emission color of EL to be extracted.
Highly efficient red electrophosphorescent device incorporating a bipolar triphenylamine/bisphenylsulfonyl-substituted fluorene hybrid as the host
(2009)
Hsu, Fang-Ming
;
Chien, Chen-Han
;
Hsieh, Ya-Jou
;
Wu, Chen-Hao
;
Shu, Ching-Fong
We have fabricated highly efficient red phosphorescent organic light-emitting diodes (PHOLEDs) incorporating a bipolar host material, 2,7-bis(phenylsulfonyl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (SAF), doped with 7 wt% tris(1-phenylisoquinolinolato-C(2),N)iridium(III) [Ir(piq)(3)]. Attaching the electron-donating (p-type) triphenylamine group onto the electron-accepting (n-type) 2,7-bis(phenylsulfonyl)fluorene segment (through the C9 position of the fluorene unit) imparts SAF with good morphological stability, high triplet energy gap (ET), bipolar transporting ability, and matching energy levels with adjacent carrier-transporting layers. Consequently, the SAF-based red-PHOLED exhibited a very low turn-on voltage (2.4 V) and high electroluminescence efficiencies of 15.8% and 22.0 lm W(-1), superior to those of the corresponding device incorporating a conventional host material, 4,4'-N,N'-dicarbazolbiphenyl (CBP; 3.2 V, 8.5%, and 8.4 lm W(-1), respectively). At a practical brightness of 1000 cd m(-2), the efficiencies of the SAF-based red-PHOLED remained high (13.1%, 14.4 lm W(-1)).
Hydroxynaphthyridine-Derived Group III Metal Chelates: Wide Band Gap and Deep Blue Analogues of Green Alq(3) (Tris(8-hydroxyquinolate)aluminum) and Their Versatile Applications for Organic Light-Emitting Diodes
(2009-01-21)
Liao, Szu-Hung
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Shiu, Jin-Ruei
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Liu, Shun-Wei
;
Yeh, Shi-Jay
;
Chen, Yu-Hung
A series of group III metal chelates have been synthesized and characterized for the versatile application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5-naphthyridine derivates as chelating ligands, and they are the blue version analogues of well-known green fluorophore Alq(3) (tris(8-hydroxyquinolinato)aluminum). These chelating ligands and their metal chelates were easily prepared with an improved synthetic method, and they were facially purified by a sublimation process, which enables the materials to be readily available in bulk quantity and facilitates their usage in OLEDs. Unlike most currently known blue analogues of Alq(3) or other deep blue materials, metal chelates of 4-hydroxy-1,5-naphthyridine exhibit very deep blue fluorescence, wide band gap energy, high charge carrier mobility, and superior thermal stability. Using a vacuum-thermal-deposition process in the fabrication of OLEDs, we have successfully demonstrated that the application of these unusal hydroxynaphthyridine metal chelates can be very versatile and effective. First, we have solved or alleviated the problem of exciplex formation that took place between the hole-transporting layer and hydroxynaphthyridine metal chelates, of which OLED application has been prohibited to date. Second, these deep blue materials can play various roles in OILED application. They can be a highly efficient nondopant deep blue emitter: maximum external quantum efficiency (eta ext) of 4.2%; Commision Internationale de L'Eclairage x, y coordinates, CIE(x,y) = 0.15, 0.07. Compared with Alq(3), Bebqp(2) (beryllium bis(benzoquinolin-10-olate)), or TPBI (2,2',2 ''-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole), they are a good electron-transporting material: low HOMO energy level of 6.4-6.5 eV and not so high LUMO energy level of 3.0 3.3 eV. They can be ambipolar and possess a high electron mobility of 10(-4) cm(2)/V s at an electric field of 6.4 x 10(5) V/cm. They are a qualified wide band gap host material for efficient blue perylene (CIE(x,y) = 0.14, 0.17 and maximum (eta ext) 3.8%) or deep blue 9,10-diphenylanthracene (CIE(x,y) = 0.15, 0.06 and maximum (eta ext) 2.8%). For solid state lighting application, they are desirable as a host material for yellow dopant (rubrene) in achieving high efficiency ((eta ext) 4.3% and (eta P) 8.7 lm/W at an electroluminance of 100 cd/m(2) or (eta ext) 3.9% and (eta P) 5.1 lm/W at an electroluminance of 1000 cd/m(2)) white electroluminescence (CIE(x,y) = 0.30, 0.35).
Unified assay of adverse effects from the varied nanoparticle hybrid in polymer fullerene organic photovoltaics
(2013-09-01)
Huang, Yu-Jui
;
Lo, Wei-Chun
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Liu, Shun-Wei
;
Cheng, Chao-Han
;
Chen, Chin-Ti
Nanoparticles (NPs) having different surface capping agent, variant electrical conductivity and sunlight absorption have been studied for the ternary hybrid containing poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) in bulk heterojunction (BHJ) organic photovoltaics (OPVs). These NPs are composed of conducting gold, semiconducting CdS or PbS, or insulating cage-like molecular silica (FOSS). We use a series of microscopic methods including TEM, AFM, SEM, optical, and fluorescence microscopy to estimate NP size and to probe the agglomeration and/or aggregation of NPs in P3HT/PCBM blends. Surface capping agent aromatic thiophenol (SPh) was found to be poor in the dispersion of NPs in P3HT/PCBM blends. The light harvesting of these NPs ranges from transparent (POSS NP), near transparent (CdS NP), visible light absorbing (Au NP), to near-infrared absorbing (PbS NP)., Nevertheless, the absorbance of these NPs is all too small relative to that of P3HT polymer. Concerning the charge separation of P3HT exciton, the LUMO energy levels of these NPs have been determined by the combination of optical band-gap energy and HOMO energy levels. By the transient photocurrent time-of-flight method, charge carrier mobility of P3HT/PCBM/NP(CdS-SPh) ternary hybrid was found to be improved, although fluorescence quenching studies imply insufficient or ineffective contact between P3HT and all NPs in the present study. NPs hybrid P3HT/PCBM BHJ OPVs were fabricated by solution process. Regardless of conductivity or sunlight absorbance, all NPs show no improvement on power conversion efficiency of ternary hybrid OPVs, which is 3.03-3.91% less than 4.0-4.1% of P3HT/PCBM OPVs without NPs. Based on the present study, a few problems that associate with the inferior performance of NPs hybrid P3HT/PCBM BHJ OPVs are delineated. (C) 2013 Elsevier B.V. All rights reserved.