Out-coupling Efficiency Of Organic Light-emitting Diodes Research Articles

Improved Light Extraction in Organic Light‐Emitting Diodes via Semiconductor Dilution

AbstractIncreasing the internal light extraction efficiency of organic light‐emitting diodes (OLEDs) is key to improving their performance for solid‐state lighting applications; however, it is challenging to do this in a way that is compatible with high volume manufacturing. Here, it is shown that the outcoupling efficiency of OLEDs can be improved by diluting their hole transport layer (HTL) with the low refractive index material trifluoropropyl oligomeric silsesquioxane (F‐POSS). Specifically, co‐evaporating 40 vol.% F‐POSS in the HTL of single and multi‐stack phosphorescent OLEDs decreases its refractive index by Δn ≈ 0.2, which in turn yields a ≈12% increase in their outcoupling efficiency with no impact on electrical performance or operational lifetime. This result is significant because F‐POSS is a small molecule that sublimes cleanly, does not aggregate, and is compatible with state‐of‐the‐art HTL materials, making it a realistic path to increase light extraction in commercial OLEDs manufactured on existing production lines.

Nearly 40% outcoupling efficiency in OLEDs with all-metal electrodes

Due to its high transparency and low sheet resistance, indium tin oxide (ITO) has been the material of choice for transparent anodes in organic light-emitting diodes (OLEDs). Indium tin oxide, however, is a source of outcoupling loss due to waveguiding and reduced mechanical stability on flexible/stretchable substrates due to its brittle nature. We demonstrate that highly efficient ITO-free OLEDs can be achieved using high quality silver electrodes and horizontally aligned dipole emitters to avoid plasmonic losses. Using an ultrathin Ag/MPTMS anode and a partially aligned phosphorescent emitter, we demonstrate OLEDs with 30% EQE, luminous efficiency exceeding 130 lm/W, and low leakage current. In addition, we demonstrate OLEDs with an optimized structure showing a 36.1% outcoupling efficiency. Theoretical calculations show that our approach can yield up to 48.4% outcoupling efficiency for perfect horizontal alignment, which exceeds the maximum achievable with ITO. The combination of a silver anode and a horizontal phosphorescent emitter is promising for the future design of ultra-efficient flexible OLEDs.

Research progress of light out-coupling in organic light-emitting diodes with non-period micro/nanostructures

Organic light-emitting diodes (OLEDs) possess a number of advantages such as low power consumption, light weight, wide color gamut, high response speed, and high contrast ratio. They have received widespread attention due to their tremendous commercial applications in the fields of full-color flat panel display and solid-state lighting. Although nearly 100% internal quantum efficiency of OLED has been achieved through adopting phosphorescence or thermally activated delayed fluorescence emitters. However, the majority of light generated in an emitting layer is confined within the whole device but does not escape into air due to the induced surface plasmons at the interface between metal and dielectric layers as well as the differences in refractive index between layers of OLED structures including air, glass substrate, transparent electrode as well as organic or inorganic layers. The external quantum efficiency for an OLED with a flat glass substrate is limited to~20%. A low light out-coupling efficiency severely restricts the development and application of OLED. Therefore, enhancing the light out-coupling efficiency of OLED via light extraction technology offers the greatest potential for achieving a substantial increase in the external quantum efficiency of OLED and has been one of the most attractive projects. Up to now, lots of light out-coupling technologies such as micro-lens arrays, photonic crystal, Bragg mirrors and periodic grating have been suggested to enhance the out-coupling efficiency of OLEDs. However, the periodic light out-coupling structures have a limitation that the electroluminescence intensity and spectrum of OLED usually depend on the viewing angle. The angular dependence of the emission characteristic does not hold true for actual display applications due to its deviation from the Lambertian intensity distribution. In this review, we present recent research progress of using non-period micro/nanostructures to improve the light out-coupling efficiency of OLED. In contrast to the emission directionality for OLED using periodic light out-coupling structures, the luminance distribution and spectral stability of OLED based on non-period micro/nanostructures are insensitive to viewing angle. Various light out-coupling techniques such as random micro/nano lens structure, light scattering medium layer, polymer porous scattering films, random concave-convex corrugated structure, and random buckled structure are summarized and discussed. These techniques have the potential applications in displays and solid-state lighting. Finally, summary and prospects regarding to light-coupling techniques of OLEDs are presented.

P-205L:Late-News Poster: Improvement of Viewing Angle Dependence and Out-Coupling Efficiency of OLEDs with Strong Microcavity Structure by Introduction of Nanoporous Polymer Films

To improve a viewing angle characteristic as well as a light extraction effect, we fabricated nanoporous polymer film (NPF) as a scattering medium as well as a light extraction component. We designed two types of organic light emitting diodes (OLEDs) with strong microcavity effect containing two different hole transport layer (HTL) thickness (e.g. 30 nm and 60 nm) to investigate each optical behavior of NPF. Very interestingly, we could observe a significant enhancement of external quantum efficiency (EQE) for each device [30 nm thick HTL : 18.0 %, 60nm thick HTL : 31.6 %] when we attached 125 um thick PET film coated with NPF. Furthermore, NPF successfully suppress the viewing angle dependence to realize ideal angular emission even though it's somehow different from that of Lambertian distribution.