Browsing by Author "Luo, Dian"

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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.
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
;
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.
Combinational Approach To Realize Highly Efficient Light-Emitting Electrochemical Cells
(2020-03-25)
Yi, Rong-Huei
;
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.
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
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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.
Highly efficient solar-heat shield based on the bipolaron-assisted PEDOT:PSS thin film
(2020-08-01)
Thakur, Diksha
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Wu, Jia-Ren
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Chandel, Anjali
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Liu, Shih-Yao
;
Lin, Pei-Chen
We report on an efficient solar-heat shield based on the highly transparent PEDOT:PSS thin film, which significantly reduces the temperature from 38.2 degrees C to 33.5 degrees C. To understand the superior solar-heat shielding ability, the modified PEDOT:PSS thin films were investigated by using the transmittance spectra, Raman scattering spectra, atomic-force microscopic (AFM) images and broadband refractive index spectra. The experimental results show that the molecular structure of the PEDOT chains can be manipulated by varying the Brownian motion of PEDOT polymers in the PEDOT:PSS solutions and the spin rate. The lower Brownian motion of PEDOT polymers and the higher spin rate both can change the molecular structure of the PEDOT chains from bended structures (de-doping states) to linear structures (doping states), which simultaneously increase the near-infrared absorption strength and the visible transparency due to the formation of bipolarons.