Browsing by Author "Lu, Chin-Wei"

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Achieving highly saturated single-color and high color-rendering-index white light-emitting electrochemical cells by CsPbX3 perovskite color conversion layers
(2018-12-21)
Wang, Cheng-Ming
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Su, Yong-Ming
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Shih, Ting-An
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Chen, Guan-Yu
;
Chen, Yan-Zhi
Solid-state light-emitting electrochemical cells (LECs) have been thought to be applicable for display and lighting due to their advantages such as simple device architecture, low-voltage operation and compatibility with inert cathode metals. Among the emissive materials of LECs, ionic transition metal complexes (iTMCs) exhibit relatively higher emission efficiency but they show relatively broader emission spectra, resulting in lower color saturation for display application. Here, we demonstrated that by integrating perovskite-based color conversion layers (CCLs) with blue iTMC-LECs, highly saturated green, yellow, red and deep-red and high color-rendering-index (CRI) white LECs can be obtained. The Commission Internationale de l'E'clairage (CIE) 1931 coordinates of the highly saturated single-color LECs are almost on the boundary of the National Television System Committee (NTSC) color gamut, showing their potential application in display. By mixing perovskites of different colors at an appropriate ratio in the CCLs, the white LECs show a CRI >90, which is among the highest reported values, and a correlated color temperature of 2649 K. Hence the proposed technique can also be one of the good candidates for lighting applications. In addition, both the highly saturated single-color and white LECs still exhibit efficient emission for display and lighting applications.
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
;
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.
Cationic Ir-III Emitters with Near-Infrared Emission Beyond 800 nm and Their Use in Light-Emitting Electrochemical Cells
(2019-04-11)
Chen, Guan-Yu
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Chang, Bo-Ren
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Shih, Ting-An
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Lin, Chien-Hsiang
;
Lo, Chieh-Liang
Solid-state near-infrared (NIR) light-emitting devices have recently received considerable attention as MR light sources that can penetrate deep into human tissue and are suitable for bioimaging and labeling. in addition, solidstate NIR light-emitting electrochemical cells (LECs) have shown several promising advantages over NIR organic light-emitting devices (OLEDs). However, among the reported NIR LECs based on ionic transition-metal complexes (iTMCs), there is currently no iridium-based LEC that displays NIR electroluminescence (EL) peaks near to or above 800 nm. In this report we demonstrate a simple method for adjusting the energy gap between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) of iridium-based iTMCs to generate NIR emission. We describe a series of novel ionic iridium complexes with very small energy gaps, namely NIR1-NIR6, in which 2,3-diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO energy levels and 2,2'-biquinoline, 2-(quinolin-2-yl)quinazoline, and 2,2'-bibenzo[d]thiazole moieties mainly control the LUMO energy levels. All the complexes exhibited NIR phosphorescence, with emission maxima up to 850 nm, and have been applied as components in LECs, showing a maximum external quantum efficiency (EQE) of 0.05% in the EL devices. By using a host-guest emissive system, with the iridium complex RED as the host and the complex NIR3 or NIR6 as guest, the highest EQE of the LECs can be further enhanced to above 0.1%.
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
;
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 tuning the applied voltage pulse periods on the electroluminescence spectra of host-guest white light-emitting electrochemical cells
(2018-07-21)
Wu, Ming-Lun
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Chen, Guan-Yu
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Shih, Ting-An
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Lu, Chin-Wei
;
Su, Hai-Ching
Solid-state white light-emitting electrochemical cells (LECs) are potential candidates in solid-state lighting due to their promising advantages of simple device structure, low-voltage operation and compatibility with inert cathode metals. Adjusting the correlated color temperature (CCT) of background illumination is highly desired for modern smart lighting systems. In this work, a novel technique to tune the CCT of electroluminescence (EL) from white LECs is proposed. Color tuning is based on adjusting the applied voltage pulse period on the host-guest white LECs and the working mechanism is illustrated. A shorter voltage pulse period is insufficient to completely charge the capacitive LEC device and thus the effective voltage applied on the device is lower. Since the host-guest energy level offsets favor carrier trapping, a lower effective applied voltage results in a more pronounced guest emission, rendering redder white EL with a lower CCT. On the other hand, a longer voltage pulse period facilitates more complete charging and the effective voltage applied on the white LEC is higher. A higher bias facilitates direct exciton formation on the host molecule and subsequent partial host-guest energy transfer generates bluer white EL with a higher CCT. By tuning the voltage pulse period from 0.2 to 20 ms, the CCT of EL resulting from white LECs ranges from 2482 to 5723 K. The CCT tuning range is sufficient for general lighting applications. In contrast to color tuning of white LECs under constant-voltage driving, in which >10 x brightness enhancement is accompanied by higher-CCT white EL, the discharging halfperiod in pulse-voltage driving provides relaxation time to turn off the device and reduces the average brightness of the white LECs driven under a longer voltage pulse period. Therefore, similar brightness can be achieved for white EL with different CCTs. No additional optical filtering device is needed for this novel color tuning technique and it has potential for use in solid-state lighting.
Efficient and Saturated Red Light-Emitting Electrochemical Cells Based on Cationic Iridium(III) Complexes with EQE up to 9.4 %
(1970-01-01)
Yu, Guang-Xiang
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Lin, Chien-Hsiang
;
Liu, You-Xuan
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Yi, Rong-Huei
;
Chen, Guan-Yu
Solid-state light-emitting electrochemical cells (LECs) have several advantages, such as low-voltage operation, compatibility with inert metal electrodes, large-area flexible substrates, and simple solution-processable device architectures. However, most of the studies on saturated red LECs show low or moderate device efficiencies (external quantum efficiency (EQE) <3.3 %). In this work, we demonstrate a series of five red-emitting cationic iridium complexes (RED1--RED5) with 2,2 '-biquinoline ligands and test their electroluminescence (EL) characteristics in LECs. The Commission Internationale de l ' Eclairage (CIE) 1931 coordinates for the LECs based on these complexes are all beyond the National Television System Committee (NTSC) red standard point (0.67, 0.33). The maximal EQE of the neat-film RED1-based LECs reaches 7.4 %. The reddest complex, RED3, is doped in the blue-emitting host complex, BG, to fabricate host-guest LECs. The maximal EQE of the host-guest LECs (1 wt % complex RED3) reaches 9.4 %, which is among the highest reported for the saturated red LECs.
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
;
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.