A team of researchers led by Li Wan of the Center of Plastic Electronics at Imperial College London (South Kensington Campus, London) are investigating a range of compositions in the materials used to fabricate OLEDs as a means to control the chirality of the light emitted by the materials.
(Note that, in the context of this research, controlling chirality means switching between the emission of light that is right hand circularly polarized and light that is left hand circularly polarized.)
The reason that the development of this capability could be important is that close to half of the light produced by an OLED can be blocked by the circularly polarized anti-glare filter that is typically attached to the uppermost surface of the display. Development of an OLED that directly emits circularly polarized light with a handedness that matches that of the circularly polarizing anti-glare filter should result in a much brighter display. Alternately, an OLED material designed to emit circular polarized light having appropriate handedness could improve energy efficiency by a factor of about two with no reduction in display brightness. In either case, given this approach, the readability of the display should be unchanged even when the display is viewed in an environment having high ambient brightness.
A recent article on the developments at Imperial Collage is entitled “Inverting the Handedness of Circularly Polarized Luminescence from Light-Emitting Polymers Using Film Thickness.” It was posted on June 17th in the on-line version of the American Chemical Society Journal Nano. A copy of the article available for purchase and can be found here.
In their work, the researchers report that “blending achiral polymers with chiral small molecule additives in the new polymer OLEDs chemistry allowed them to control the handedness of the circularly polarized light through absolute stereochemistry.”
Another interesting finding is that “tuning the circularly polarized light output was performed as a function of the active layer thickness, for a fixed optical isomer of the chiral additive.”
The figure below is a graphical representation of the controlled emission of circularly polarized light by the newly developed OLED material.
Changing the handedness of circularly polarized light by adjusting the thickness of the active layer in a film of OLED material.
A few of the details presented in the report include the following: polymer OLEDs were developed that were capable of delivering a brightness of 8000 cd/m² with an efficiency of 4.0 cd/A. In these experiments and using the same additive, varying the film thickness from 110 nm to 160 nm was enough to switch the handedness of the polarized light.
In their article, the researchers describe efforts to “link morphological, spectroscopic, and electronic characterization in thin films and devices with theoretical studies in an effort to determine the factors that underpin these observations.”
Based on these studies, the researchers propose that the observed effect “arises due to an interplay between localized circular polarized emission originating from molecular chirality and circular polarized light amplification or inversion through a chiral medium.”
The article concludes with the observation that “Through the control of active layer thickness and device architecture, this study provides insights into the mechanisms that result in circular polarized luminescence and high performance from circular polarized polymer OLEDs, as well as demonstrating new opportunities in circular polarized photonic device design.” -Arthur Berman
