Stacked metal cathode for high-contrast-ratio polymeric light-emitting devices

Abstract

An extremely high optical absorbing film made of alternating aluminum-silver layers was used as cathode in polymeric light-emitting devices ͑PLEDs͒. Physical properties of the cathodes were characterized by I-V measurement, atomic force microscopy, and x-ray photoemission spectroscopy. As a result of the slow evaporation rate, each pair of the aluminum-silver layer was shown to be in the form of aluminum-aluminum oxide nanoclusters embedded in an amorphous charge conducting network of silver. The nanoclusters helped to absorb and scatter the ambient light effectively. The use of four alternating layers structure in conventional PLEDs demonstrated 126% enhancement of contrast under 1000 lx ambient illumination. The I-V characteristics of the black cathode PLEDs remained intact when compared with reference PLEDs. This technology offers precise control of the cathode quality in terms of its reflectivity and conductivity.

High-contrast top-emitting organic light-emitting diodes with AlO 1.086 dark-and-conductive electrodes

“…3(b), the reflectance of the devices with AlO 1.086 is significantly lower than that with pure Al, especially at the wavelength of 470-620 nm, where human eye is most sensitive to the reflective photons as reflected in the human photopic curve. The calculated luminous reflectance (R L ) to human eyes under a light source D65 is 1.4% for the devices with AlO 1.086 , which is remarkably lower than 46.7% for the devices with pure Al, and is among the lowest reported values [5][6][7][8][9][10][11][12][14][15][16][17][18][19]. Due to the low R L , devices with AlO 1.086 exhibit extremely high CR even under strong illumination, as shown in Fig.…”

“…The binding energy for Al 2p electrons in pure Al and Al 2 O 3 is 72.9 eV and 76.4 eV, respectively [14]. As shown in Fig.…”

“…However, if Al is partially oxidized, the resultant oxygen-deficient AlO x films, with x ranging from 0 to 1.5, become optically absorptive and electrically conductive. With this understanding, Yang's group had fabricated a dark electrode by evaporating Al with a very slow rate of 0.2 Å/s at a pressure of $10 À6 Torr [14]. At such slow rate, a part of the Al atoms are oxidized by the residue oxygen.…”

“…According to this principle, a contrast-enhancing stack (trademarked Black Layer™) has also been invented by Luxell Technologies Co., Ltd [13] In this work, we propose a very simple and cost-effective method that reduce the ambient reflection effectively and thus improve the CR of top-emitting OLEDs significantly. It is widely recognized that energetic Al is very easily oxidized to AlO x even in a high vacuum environment [14]. If Al is fully oxidized, the resultant oxide film (Al 2 O 3 ) is optically transparent and electrically insulated.…”

Numerical simulation of top-emitting organic light-emitting diodes with electron and hole blocking layers

“…1,2 The few reported high-contrast OLED structures all deal with bottom-emitting OLEDs and may not be readily adapted for top-emitting OLEDs (TOLEDs), which have a few technical merits (e.g., larger aperture ratios) over bottom-emitting devices for high-performance active-matrix OLED (AMOLED) displays. [3][4][5] Although the cost is relatively low, the passive-matrix (PM) architecture requires high peak current to provide high luminance, resulting in high power consumption along with adverse effects on the OLEDs reliability. 6 On the other hand, the AM technique is more suitable for large size, high quality, and long lifetime application.…”