Cheng-Chieh Hou

Enhancing Efficiency of Organic Light-Emitting Diodes Using Lithium-Doped Electron Transport Layer

In this work, we study the electrical and optical properties of organic light-emitting diodes (OLEDs) with a lithium (Li)-doped tris(8-hydroxyquinolino)-aluminum (Alq3) electron transport layer (ETL). When the Li:Alq3 doping ratio in a Li-doped Alq3 ETL is 2:1, the luminous efficiency of OLEDs is 5.25 cd/A; that of an OLED without a Li-doped Alq3 ETL is only 0.7 cd/A. X-ray photoelectron (XPS) and UV–vis absorption spectra of Li-doped Alq3 films reveal that the Li-doped Alq3 ETL has an improved electron conductivity. However, heavy Li doping in a Li-doped Alq3 ETL reduces the optoelectric performance of OLEDs. Excess Li atoms or cations quench light-emitting excitons in an Alq3 emitting layer (EML). Additionally, annealing improves the morphological stability of Li-doped Alq3 films. An OLED, comprising a Li-doped Alq3 ETL, requires no extrinsic thin LiF film, meeting commercial requirements, improving reproducibility and ensuring uniformality in a large area.

Temperature Effect on the Optoelectronic Properties of GaN-based Light-Emitting Diodes with ITO p-Contacts

In this study, indium-tin-oxide (ITO) films were used as p-contacts in GaN-based light-emitting diodes (LEDs). The dependence of the optoelectronic performance of LEDs on various annealing temperatures of ITO films was studied. At an annealing temperature of 600°C the transmittance of ITO films can reach 98.6% at a wavelength of 470 nm. The specific contact resistance characterized by the circular transmission line model is as low as 7.73 X 10 -3 Ω cm 2 . Furthermore, GaN-based LEDs employing an ITO p-contact annealed at 600°C have high luminescence intensities of 243.2 mcd at a 20 mA injection current. Under the same injection current, the proposed LED also has the largest electroluminescence intensity with the ITO p-contact annealed at 600°C.

Nitride-Based Blue-Light-Emitting Diodes with ITO-Covered Ni ∕ Au Mesh p-Contacts

This investigation presents a novel indium-tin oxide (ITO) covered Ni/Au mesh p-contact to improve the light transmittance and output efficiency of InGaN/GaN multiquantum well light-emitting diodes. The presented ITO-covered Ni/Au mesh p-contact blocks less light than the conventional Ni/Au film p-contact but makes a good ohmic contact. Experimental results indicate that to light with a wavelength of 470 nm, the ITO-covered Ni/Au mesh p-contact exhibits 94% transmittance and 12-mW output power at a 20-mA injection current. In contrast, at the same wavelength the conventionally adopted Ni/Au film p-contact exhibits 72% light transmittance and 8.12-mW output power at a 20-mA injection current.

Reduction of Ohmic Contact Resistance on n-GaN by Surface Treatment Using Cl2 Inductively Coupled Plasma Following Laser Lift-Off

The Cr/Pt/Au ohmic contact resistance on n-type gallium nitrogen (GaN) is reduced by the Cl2 inductively coupled plasma (ICP) surface treatment of n-type GaN films following laser lift-off (LLO). X-ray photoelectron spectroscopy (XPS) shows the modified atomic ratio of the n-type GaN surface following the Cl2 ICP treatment. The Cl2 ICP treatment increases the atomic ratio of gallium to nitrogen. GaClx and NClx are suggested to be generated and then removed using a boiling HCl solution. Nitrogen vacancies at the n-type GaN surface are therefore produced and act as donors for electrons, reducing ohmic contact resistance induced by reducing the resistivity of electrons to conduction.

Enhancing Efficiency of Organic Light-Emitting Diodes Using a Carbon-Nanotube-Doped Hole Injection Layer

A nanocomposite layer consisting of multiwalled carbon nanotube (MWCNT)-doped poly(3,4,-ethylene dioxythiphene):poly(styrene sulfonic acid) (PEDOT:PSS) was employed as a hole injection layer (HIL) in an organic light-emitting diode (OLED). The structure of the OLED is glass/indium-tin oxide (ITO)/MWCNT-doped PEDOT:PSS/PEDOT:PSS/tris(8-hydroxyquinolinato)aluminum (Alq3)/LiF/Al. The luminous efficiency of the OLED is as high as 2.1 cd/A, which is 70% higher than that of a conventional device without an MWCNTs-doped HIL. The MWCNTs in the PEDOT:PSS act as a hole-blocking material that results in a reduction in holes transport ability, and therefore, in a balance between electron and hole mobilities, leading to enhanced luminescent performance. The mechanism was well demonstrated by through investigations by atomic force microscopy (AFM) of the film surface morphology and carrier injection properties of hole-only devices.

High Efficiency White Organic Light-Emitting Diodes with Double-Doped in a Single Emissive Layer

Organic light-emitting diodes (OLEDs) have been widely recognized as a technology for flat panel display (FPD) products today and for potential future use in the lighting industry. Several approaches for developing full-color organic displays have been proposed: patterned RGB subpixels, produced by precise shadow masking [1], conversion of blue emissions with fluorescent dyes (CCM) method [2], and white emissions combined with color filters and stacked RGB cells [3]. The phosphorescent materials used were iridium (Ir) and platinum (Pt) complexes with organic ligands, and they were doped as an emissive guest into a charge-transport host material of the emissive layer [4]. In the study, we report the white phosphorescent OLEDs (PHOLEDs) with a single emissive layer doped with a blue phosphorescent material and a red material.