Design and modelling of anisotropic thin film light-emitting devices with the plane wave expansion method

In most emitting materials the dipole moments are distributed in an isotropic way, and light is emitted in all directions (modified by interference in the OLED layers). But the radiation of each individual dipole is directed around the equatorial plane of the dipole moment. By analyzing the photoluminescent decay times in different microcavities, a phosphorescent emitter with an anisotropic distribution of 80% in-plane dipoles is demonstrated. Simulations show that if all dipoles are arranged in-plane with the OLED layers the outcoupling efficiency to air would improve from 20% to 30%. Anisotropic emitters can be combined with known outcoupling techniques like microlens foils to provide even higher outcoupling efficiencies. With a fully oriented emitter nearly 70% of the light is reaches the OLED substrate where it is available for extraction by outcoupling foils. When organic laser dye molecules are dissolved into the CLC, a laser can be made. A CLC film supports a resonant standing wave for wavelengths at the edge of the bandgap. When the dye molecules are pumped above the lasing threshold by a pulsed shorter wavelength pump laser, longer wavelength laser light is emitted perpendicular to the CLC film. The thickness of the CLC film is around 10μm. Laser emission has been shown across the entire visible spectrum employing different dye molecules. The emission wavelength can also be tuned across ranges of about 50nm in different ways. However CLC lasers are still hindered by high lasing thresholds and low output power, as well as bleaching of the dye molecules. In this work the light emitting properties of CLC films are simulated with the anisotropic plane wave expansion method both for spontaneous and stimulated emission. The measured emitted spectrum at different angles and polarization are accurately modelled by the plane wave expansion. A model for estimating the gain threshold and laser wavelength of CLC films was developed and verified by experiment. Optical amplification is treated by introducing gain terms to the refractive index. This model may prove a valuable tool to design more advanced liquid crystal lasers

Light emission from dye-doped cholesteric liquid crystals at oblique angles: simulation and experiment

Dye-doped cholesteric liquid crystals with a helical pitch of the order of a wavelength have a strong effect on the fluorescence properties of dye molecules. This is a promising system for realizing tunable lasers at low cost. We apply a plane wave model to simulate the spontaneous emission from a layer of cholesteric liquid crystal.We simulate the spectral and angle dependence and the polarization of the emitted light as a function of the order parameter of the dye in the liquid crystal.Measurements of the angle dependent emission spectra and polarization are in good agreement with the simulations

Optical anisotropy and light outcoupling in OLEDs

Organic Light Emitting Diodes consist of a stack of thin layers that have a high refractive index and may have anisotropic optical properties. The emission from such a planar structure depends on the layering of the OLED, the anisotropy of the different materials and the orientation of the dipole emitters in the organic layer. OLEDs are used in lighting and display devices and the requirements for both applications are very different

Quantitative analysis of pixel crosstalk in AMOLED displays

The resolution of organic light-emitting diode (OLED) displays is increasing steadily as these displays are adopted for mobile and virtual reality (VR) devices. This leads to a stronger pixel crosstalk effect, where the neighbors of active pixels unintentionally emit light due to a lateral electric current between the pixels. Recently, the crosstalk was quantified by measuring the current flowing through the common hole transport layer between the neighboring pixels and comparing it to the current through the active pixel diode. The measurements showed that the crosstalk is more crucial for low light levels. In such cases, the intended and parasitic currents are similar. The simulations performed in this study validated these measurement results. By simulations, we quantify the crosstalk current through the diode. The luminous intensity can be calculated with the measured current efficiency of the diodes. For low light levels, the unintended luminance can reach up to 40% of the intended luminance. The luminance due to pixel crosstalk is perceivable by humans. This effect should be considered for OLED displays with resolutions higher than 300 PPI

Optical design for efficient light emission in OLEDs and anisotropic layers

The light emission from an electrical dipole antenna is determined by the orientation of the antenna and the optical properties of the materials that surround it. For the outcoupling efficiency of an OLED it is beneficial if the dipole moment of the luminescent transition is parallel with the substrate. In some OLEDs a preferential orientation of the emitting molecules can indeed be observed. In this paper we discuss how anisotropy of the dipole orientation and optical anisotropy of the materials influence the light emitted from a planar structure. Numerical simulations are verified with measurements of OLEDs (for the dipole orientation) and dye doped liquid crystals (for the optical anisotropy)