Lin-Ann Hong

Efficient Solution-Processed Green Phosphorescent Organic Light-Emitting Diodes Using Bipolar Host Material

Solution-based processing was applied to fabricate green phosphorescent organic light-emitting diodes (OLEDs). EPH31 was used as a phosphorescent host, doped with guest dopant green phosphorescent Ir(ppy)3, and dissolved in chlorobenzene solvent to form the emitting layer. Device structural parameters were controlled by changing the spin coating speed of the emitting layer and hole injection layer [poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), PDOT:PSS] to adjust the thickness of the electron transport layer [tris(8-hydroxyquinolinato)aluminum, Alq3]. In addition, the differences in using CsF and LiF materials as the electron injection layer were investigated. A maximum current efficiency of 13.6 cdA-1 was obtained at a high emitting layer spin coating speed. Despite the close resemblance in both the luminance intensity and current efficiency when using CsF and LiF as the electron injection layer, CsF devices had a low driving voltage. Smooth and stable films resulting from the spin coated hole injection layer, along with the control of the thickness of the electron transport layer (Alq3) and electron injection layer (CsF), effectively improved the performance of green OLEDs. The emitting layer host material (CBP) and three guest dopants [Firpic, Ir(ppy)3, and Ir(piq)2] were dissolved in toluene solvent during solution preparation to fabricate white OLEDs. The properties of the resulting solution-processed white PHOLEDs are a current efficiency of 2.4 cdA-1 at 20 mAcm-2 and CIE coordinates of (0.33, 0.33) at 9 V. Results of these experiments demonstrate that solution processing can be used as an alternative to and in conjunction with thermal evaporation.

Power Efficiency Improvement of White Phosphorescent Organic Light-Emitting Diode with Thin Double-Emitting Layers and Hole-Trapping Mechanism

This study is carried out to discuss how to reduce the driving voltage of blue phosphorescent organic light-emitting diodes (PHOLEDs) by using a thin double-emission layer. A hole transport-type host (TCTA) is inserted between the hole transport layer (TAPC) and the emitting layer (EML), constituting a buffer layer between them with the aim of improving charge carrier balance. Furthermore, in this study, we also utilize the interface between double light-emitting layers of devices by codoping them with a red phosphorescent dopant [Os(bpftz)2(PPh2Me)2]. An Os complex with a high-lying highest occupied molecular orbital (HOMO) energy level (trapping holes) is codoped at the interface between emitting layers and an exciton-formation zone is expanded to obtain a white PHOLED with high efficiency. From the results, the optimal structure of the white device exhibits a yield of 35 cd A-1, a power efficiency of 22 lm W-1, and CIE coordinates of (0.33,0.38) at a luminance of 1000 cd m-2. Furthermore, the power efficiency can be improved to 30 lm W-1 by attaching the outcoupling enhancement film.

Material structure selection of blue organic light emitting diodes utilizing a hybrid experimental design

A hybrid approach integrating a Taguchi orthogonal table with an irregular design of experiments (DOE) method was proposed for selecting an optimal material structure of blue organic light emitting diodes in order to achieve multi-objective quality characteristics of devices. Analysis of variance (ANOVA) was adopted to identify significant factors before regression models were obtained. Finally, an optimal material structure was determined by maximizing a desirability function relating to selected critical quality characteristics including the driving voltage, the luminance, and luminance efficiency.

Material Structure Selection of Solution Blue OLEDs Using a Design of Experiment

A blue small-molecular organic light-emitting diode (SM-OLEDs) based on a solution-process is investigated in this study. Design of experiment (DOE) with response surface methodology (RSM) was applied to optimize the driving voltage and current efficiency of blue SM-OLED devices. The spin-coating speed of the PEDOT: PSS as hole injection layer and the 26DCzPPy: FIrpic as emitting layer were chosen as two main process input factors. Analysis of variance (ANOVA) was adopted to identify significant factors before regression models were obtained. The optimal material structure was determined by minimizing and maximizing a desirability function relating to selected critical quality characteristics including the driving voltage and current efficiency, respectively.

Current–Voltage Numerical Simulation of Organic Light Emitting Diodes with Dual-Layer Structures

The use of numerical simulation method to study the current–voltage (I–V) of organic light emitting diode (OLED) has always been an effective method to upgrade the luminous efficiency of OLED. As the I–V theoretical simulation equations are based on injected carrier passing through Schottky barrier, and considering that carrier capturing defect and carrier mobility rate might be generated within the inner organic layer of Pool–Frenkel model, the study had made a comparison between the I–V theoretical model with a double-layer device and the experimental data, and proposed the best parameters for the theoretical model after careful adjustment and comparison to establish an optimal simulated numerical model with a double-layer OLED current–voltage. Finally, a study was made on the carrier capturing defect and mobility rate affected by electric field and temperature.

Blue Fluorescent Organic Light-Emitting Diodes with Optimized Electron Transportation Layer

In this research, the optimization of device structures of blue fluorescent organic light emitting diode (OLED) with WBH-301 doped with WBD-701 was fabricated. By adjusting the thickness of each layer in OLED structure as well as total thickness of device, the position of recombination zone was controlled and located in the central region of emitting layer (EML) that significantly increases device efficiency. The device showed the current efficiency of 8.7 cd/A at current density 50 mA/cm2 and with Commission Internationale de L’Eclairage (CIE) coordinates (x = 0.17, y = 0.34). This efficiency enhancement is important for understanding and further improving high-performance fluorescent OLEDs.