Browsing by Author "Yi, Rong-Huei"

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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
;
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.
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
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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.