Power Efficiency Of Organic Light-emitting Diodes Research Articles

Outcoupling-enhanced organic light-emitting diodes using simple phase-separated polymer films

A high-performance external light outcoupling film for organic light-emitting diodes (OLEDs) is developed using a novel phase-separated polymer (PSP) which is spontaneously formed from a homogeneous solution of poly(amic acid) and polyimide in dimethylacetamide. The phase separation during the spin-coating process leads to complex morphologies of film with random microdent and nanobump patterns by a self-assembly of polymers. The PSP film is utilized on the backside of a glass substrate in green OLEDs as a light outcoupling film, leading to a considerable enhancement of light outcoupling efficiency. Both the current efficiency and power efficiency of OLEDs with the PSP film are improved by a factor of 1.45 and 1.53, respectively, in comparison with those of the reference device. The results demonstrate that the PSP film could serve as a promising outcoupling film for OLEDs.

Enhanced outcoupling in down-conversion white organic light-emitting diodes using imprinted microlens array films with breath figure patterns

ABSTRACT We demonstrate high-performance down-conversion microlens array (DC-MLA) films for white organic light-emitting diodes (OLEDs). The DC-MLA films are readily fabricated by an imprinting method based on breath figure patterns, which are directly formed on the polymer substrate with a novel concept. The DC-MLA films result in high-quality white light as well as enhanced light outcoupling efficiency for white OLEDs. The external quantum efficiency and power efficiency of OLEDs with DC-MLA films are increased by a factor of 1.35 and 1.86, respectively, compared to OLEDs without outcoupling films. Moreover, the white OLEDs with DC-MLA films achieve a high color-rendering index of 84.3. It is anticipated that the novel DC-MLA films fabricated by the simple imprinting process with breath figure patterns can contribute to the development of efficient white OLEDs.

Enhanced light extraction of organic light-emitting diodes using recessed anodes

Abstract Recessed indium tin oxide films were prepared to serve as anodes for enhancing light extraction efficiency of organic light-emitting diodes (OLEDs). The power efficiency of OLEDs with the proposed films was enhanced by up to 28%. This improvement can be attributed to enhanced light extraction due to the wall effect and the bottom scattering effect of the recesses. The power efficiency increased with recess depth but was limited by the accompanying high electrical resistivity and large optical loss.

Review paper: Recent developments in light extraction technologies of organic light emitting diodes

Organic light emitting diodes (OLEDs) have rapidly progressed in recent years due to their potential applications in flat panel displays and solid-state lighting. In spite of the commercialization of OLEDs, they still have a low out-coupling efficiency of about 20% due to factors such as the total internal reflection, absorption, and surface plasmon coupling. This light out-coupling efficiency is a major limitation on the high efficiency levels of OLEDs. Hence, enhancing the light out-coupling efficiency of OLEDs offers the greatest potential for achieving a substantial increase in the external quantum efficiency and power efficiency of OLEDs. Accordingly, significant advancements in OLEDs have driven the development of light extraction technologies as well as highly transparent conducting electrode materials. Recent efforts to combine light extraction structures with the improved out-coupling efficiency of OLEDs have produced OLEDs with an efficiency level that matches the efficiency of a fluorescent tube (>100 lm/W). This paper reviews the technical issues and recent progress in light extraction technologies and discusses ways of enhancing the out-coupling efficiency of OLEDs.

Improvement of highly efficient organic light-emitting diodes using Mg-doped ZnO buffer layers

In this paper, we report on the improvement of the anode buffer layer of Mg-doped ZnO (MZO) on the electrical and optical properties of organic light-emitting diodes (OLEDs). The MZO layers with different thickness were prepared by thermal-evaporating the MZO powders and then treated by ultraviolet (UV) ozone exposure. The turn-on voltage of OLEDs decreased from 4 V (4.2 cd/m 2) to 3 V (4.7 cd/m 2), the maximum luminance value increased from 16780 cd/m 2 to 35400 cd/m 2 and the power efficiency increased from 2.74 lm/W to 5.77 lm/W as the 1-nm-thick MZO layer was inserted between indium tin oxide (ITO) anodes and α-naphthylphenylbiphenyl diamine (NPB) hole-transporting layers. The surface of the MZO/ITO became smoother with the UV-ozone treatment. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) studies were performed to show that the formation of the MZO layer and the work function increased as the MZO/ITO layer was treated by UV-ozone for 20 min. Thus, the hole-injection energy barrier was lowered by inserting MZO buffer layer, consequently resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs.

Improved Hole-Injection and Power Efficiency of Organic Light-Emitting Diodes Using an Ultrathin Li-Doped ZnO Buffer Layer

We report on the advantages of an anode buffer layer of Li-doped ZnO (LZO) on the electro-optical properties of organic light-emitting diodes (OLEDs). LZO layers with different thicknesses were prepared by thermally evaporating the LZO powders and then treating them with ultraviolet (UV) ozone exposure. The turn-on voltage of OLEDs decreased from , the maximum luminance value increased from and the power efficiency increased from when a thick LZO layer was inserted between indium-tin oxide (ITO) anodes and β-naphthylphenylbiphenyl diamine hole-transporting layers. X-ray and ultraviolet photoelectron spectroscopy were performed to show that the formation of the LZO layer and the work function increased by when the LZO/ITO layer was treated by UV-ozone for . The surface of the LZO/ITO film became smoother after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an LZO buffer layer, resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs.

Improvement in Power Efficiency in Organic Light Emitting Diodes Through Intermediate Mg:Ag Layer in LiF∕Mg:Ag∕Al Cathodes

Effects of Mg:Ag interlayer between LiF and Al on electron injection and device performances of organic light-emitting diodes (OLEDs) were investigated. Thickness of Mg:Ag layer was changed from 0 to 10 nm and the relationship between interlayer thickness and device performances was studied. Current density of OLEDs was increased due to efficient electron injection from cathode to organic layer by Mg:Ag interlayer. In addition, power efficiency of OLEDs was improved by more than 50% due to low driving voltage and high recombination efficiency and it showed a maximum value at a Mg:Ag thickness of 1.0 nm.

Ohmic cathode for low-voltage organic light-emitting diodes

It is found that voltage-current characteristics of Al∕C60 interfaces are fundamentally altered from a strong rectifying junction to an Ohmic junction where a LiF interlayer is used. Al∕LiF∕C60 has been used as an Ohmic cathode for organic light-emitting diode (OLED). The Ohmic cathode together with a high electron mobility of the fullerene layer have lead to a drastic reduction in the driving voltage and consequently a significant increase in the power efficiency of OLED.