Browsing by Author "Chang, Chih-Hao"

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An alternative composite electrode for efficient organic light-emitting diodes
(2020-10-01)
Luo, Dian
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Chiang, Tsung-Che
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Chan, Chi-Hao
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Chiu, Po-Chen
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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.
Approach to Fast Screen the Formation of an Exciplex
(2020-05-07)
Luo, Dian
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Liao, Chia-Wei
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Chang, Chih-Hao
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Tsai, Chih-Chieh
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Lu, Chin-Wei
Exciplex formation could be confirmed through photoluminescence (PL) measurement of films consisting of both hole transport material (HTM) and electron transport material (ETM) or by measuring the electroluminescence (EL) of organic light-emitting diodes (OLEDs) with an HTM:ETM mixed emitting layer (EML). However, preparation for solid films or OLEDs is time-consuming, thus reducing the effective identification of exciplex systems in broad material combinations. This study proposes a fast-screening method for exciplex formation by measuring PL spectra in a relatively high polar solvent. Several HTM:ETM combinations were used to assess the feasibility of the proposed fast-screening method. The exciplex was further confirmed by phosphorescence as well as transient PL decay. In addition, pure exciplex OLEDs with an HTM:ETM mixed EML exhibited EL spectra akin to their corresponding PL spectra measured in a high polar solvent, verifying exciplex formation. These results demonstrated that our proposed PL measurement in a high polar solvent could effectively accelerate the screening for exciplex formation, thereby boosting the rapid development of exciplex research.
Combinational Approach To Realize Highly Efficient Light-Emitting Electrochemical Cells
(2020-03-25)
Yi, Rong-Huei
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Lo, Chieh-Liang
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Luo, Dian
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Lin, Chien-Hsiang
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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.
Effects of Long-Term Dharma-Chan Meditation on Cardiorespiratory Synchronization and Heart Rate Variability Behavior
(2013-04-01)
Chang, Chih-Hao
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Lo, Pei-Chen
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電機工程學系
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Department of Electrical and Computer Engineering
Remarkable changes in cardiorespiratory interactions are frequently experienced by Chan meditation practitioners following years of practice. This study compares the results of our study on cardiorespiratory interactions for novice (control group) and experienced (experimental group) Chan meditation practitioners. The effectual coaction between the cardiac and respiratory systems was evaluated by the degree of cardiorespiratory phase synchronization (CRPS). In addition, an adaptive-frequency-range (AFR) scheme to reliably quantify heart rate variability (HRV) was developed for assessing the regulation of sympathetic-parasympathetic activity and the efficiency of pulmonary gas exchange. The enhanced HRV method, named HRVAFR, can resolve the issue of overestimating HRV under the condition of slow respiration rates, which is frequently encountered in studies on Chan meditation practitioners. In the comparison of the three data sets collected from the two groups, our findings resulted in innovative hypotheses to interpret the extraordinary process of the rejuvenation of cardiorespiratory functions through long-term Dharma-Chan meditation practice. Particularly, advanced practitioners exhibit a continuously high degree of cardiorespiratory phase synchronization, even during rapid breathing. Based on our post-experimental interview with advanced practitioners, the activation of inner Chakra energy, during the course of Chan-detachment practice, frequently induces perceptible physiological-mental reformation, including an efficient mechanism for regulating cardiorespiratory interactions.
Enhancing device efficiencies of solid-state white light-emitting electrochemical cells by employing waveguide coupling
(2015-01-01)
Cheng, Chia-Yu
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Wang, Chi-Wei
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Cheng, Jing-Rong
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Chen, Hsiao-Fan
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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.
Enhancing extracted electroluminescence from light-emitting electrochemical cells by employing high-refractive-index substrates
(2017-12-01)
Jang, Yea-Fen
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Lin, Tzu-Chun
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Guo, Jhih-Yan
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Chiang, Chien-Ming Fan
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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.
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
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Hsiang, Chi-Haw
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Yi, Rong-Huei
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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 flexible organic light-emitting diodes based on a high-temperature durable mica substrate
(2019-12-01)
Lai, Yi-Ning
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Chang, Chih-Hao
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Wang, Pei-Chun
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Chu, Ying-Hao
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交大名義發表
Muscovite mica is expected to show great potential in flexible optoelectronics due to its superb temperature tolerance, high transmittance, chemical stability, and mechanical durability. This flexible substrate produces sputtered transparent conducing electrodes (TCEs) with excellent film quality with high transmittance and conductivity. In this study, a designed composite TCE consisting of aluminum-doped zinc oxide (AZO) and indium fin oxide (ITO) is proposed to simultaneously maximize flexibility and conductivity. Blue-, green-, and red-emitting flexible organic light-emitting diodes (FOLEDs) using composite TCEs on mica exhibited satisfactory performance with maximum respective electroluminescence efficiencies of 18.1% (38.7 cd/A), 18.7% (66.2 cd/A), and 13.3% (22.2 cd/A). Furthermore, the green-emitting FOLEDs were modified to construct tandem FOLEDs, giving a higher peak efficiency of 27.9% (93.3 cd/A) and saturated green emission. These results can serve as a useful reference for future work on composite TCEs on mica for FOLEDs in display and lighting applications.
Interpolative Reconstruction of Millimeter Wave Mobile Channel Responses from Randomly Scattered Snapshots
(2017-01-01)
Chang, Chih-Hao
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
There is strong recent interest in exploiting the millimeter wave (mmWave) frequencies for cellular communication. For signal transmission studies, one needs a channel model that can simulate the spatial variation of mmWave channel impulse responses (CIRs) in treated operating environments. We consider reconstruction of spatially varying CIRs by interpolation over a set of existing CIR snapshots for some randomly scattered geographical locations. Such interpolation, however, is not trivial work, as we need to handle drifting of path delays over different spatial locations as well as deal with appearing and disappearing of paths. We propose a way to address these issues, and we present some numerical results for a typical urban area.
Solid-state light-emitting electrochemical cells employing phosphor-sensitized fluorescence
(2010)
Su, Hai-Ching
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Lin, You-Heng
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Chang, Chih-Hao
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Lin, Hao-Wu
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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
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Cheng, Jing-Rong
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Wang, Chi-Wei
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Sarma, Monima
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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.
Spatially Nonlinear Interdependence of Alpha-Oscillatory Neural Networks under Chan Meditation
(2013)
Lo, Pei-Chen
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Chang, Chih-Hao
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電控工程研究所
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Institute of Electrical and Control Engineering
This paper reports the results of our investigation of the effects of Chan meditation on brain electrophysiological behaviors from the viewpoint of spatially nonlinear interdependence among regional neural networks. Particular emphasis is laid on the alpha-dominated EEG (electroencephalograph). Continuous-time wavelet transform was adopted to detect the epochs containing substantial alpha activities. Nonlinear interdependence quantified by similarity index S(X vertical bar Y), the influence of source signal Y on sink signal X, was applied to the nonlinear dynamical model in phase space reconstructed from multichannel EEG. Experimental group involved ten experienced Chan-Meditation practitioners, while control group included ten healthy subjects within the same age range, yet, without any meditation experience. Nonlinear interdependence among various cortical regions was explored for five local neural-network regions, frontal, posterior, right-temporal, left-temporal, and central regions. In the experimental group, the inter-regional interaction was evaluated for the brain dynamics under three different stages, at rest (stage R, pre-meditation background recording), in Chan meditation (stage M), and the unique Chakra-focusing practice (stage C). Experimental group exhibits stronger interactions among various local neural networks at stages M and C compared with those at stage R. The intergroup comparison demonstrates that Chan-meditation brain possesses better cortical inter-regional interactions than the resting brain of control group.
THERMAL CHARACTERIZATION OF HIGH THERMAL CONDUCTIVE GRAPHITES REINFORCED ALUMINUM MATRIX COMPOSITES
(2009)
Chang, Chih-Jong
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Chang, Chih-Hao
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Hwang, Jen-Dong
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Kuo, Cheng-Tzu
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材料科學與工程學系
Due to the development of high power density and high heat flux of IC and LED components and devices, the thermal management of microelectronics has become a very critical issue in 3C and opto-electronic industries. This has lead to the requirement of high thermal performance materials and thermal modules. To enhance the thermal performance of current thermal modules, it is very important to develop advanced thermal management materials to replace the conventional monolithic metals. In this study some kinds of graphites reinforced aluminum matrix composites were developed. The effects of reinforcement types and its volume fraction on thermal properties such as thermal conductivity as well as the thermal expansion coefficient were studied. Moreover, their thermal performance such as heat spreading resistance and thermal resistance compared to pure aluminum and copper were also conducted. From the results, it showed that the thermal conductivity of graphite/Al composites can reach to 500 similar to 600 W/m.K in X-Y plane and 40 similar to 100 W/m.K in cross plane with thermal expansion less than 10 ppm/K and density less than 2.5 g/cc; The spreading resistance of this composite is 2 similar to 5% lower than the one of pure copper, and 25 % lower than the one of pure aluminum.