Abstract
Circular polarizer (CP)-free, high-contrast inverted top-emitting organic light-emitting diodes (ITOLEDs) are demonstrated by using a semitransparent Ag electrode capped with a single dielectric layer as a top anode and Cs(2)CO(3) (1.5 nm)/Al (1.5 nm)/Cr (100nm) as a partially absorbing bottom cathode. Low luminous reflectance is achieved by combining the broadband absorption of Cr, the weak but inherent cavity structure, and the optimal thickness of the capping layer yielding a high transmittance of a top electrode. With the optimized organic capping layer, contrast-enhanced ITOLEDs exhibit a luminous reflectance as low as 3.6% with a large thickness margin. Their luminous efficiency is shown to be comparable to or even higher than that of CP-based conventional OLEDs.
Highlights
There has been a great deal of interest in organic light-emitting diodes (OLEDs) due to their various advantages in displays and lighting [1] When used for displays, OLEDs must be recognizable under ambient illumination
Our work proposes a high-contrast ratio (CR) structure for an inverted top-emitting OLED (ITOLED) configuration; this type of configuration is important because of its benefits when used with emerging backplane technologies that are based on n-type amorphous oxide thin-film transistors [8]
It is unlikely to obtain a large enhancement of luminous efficiency in the proposed ITOLED devices due to the low Rbot( + ), which tends to induce a relatively low cavity enhancement effect, a part of the light emitted toward the bottom electrode still survives the reflection at the Cr electrode
Summary
There has been a great deal of interest in organic light-emitting diodes (OLEDs) due to their various advantages in displays and lighting [1] When used for displays, OLEDs must be recognizable under ambient illumination. Under conditions of bright illumination, the reflected light can be comparable to the light emitted from OLEDs and the resultant level of contrast can become poor This problem can be circumvented by attaching a circular polarizer (CP), which consists of a quarter-wave plate and a linear polarizer, to the opposite side of a substrate. The work of Py et al exemplified the second approach: they proposed a high-CR OLED structure in which a multilayer filter consisting of a partially absorbing metal layer and a distributed Bragg reflector is stacked on a conventional microcavity-type OLED to strike a balance between the microcavity effect for emitted light and the low reflection for incident ambient light [7]. A particular attention is paid to a proper choice of the capping layer with a suitable thickness and to a right combination of the bottom cathode assembly yielding an efficient electron injection so that an optimal structure ensuring a low reflectance and high brightness can be established
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