Model for birefringence in OLEDs


Since many years, OLEDs are known for producing the highest quality displays. OLED displays are still steadily improving with more and more prototypes of flexible displays. A next hurdle is to bring these prototypes to the mass market. OLEDs consist of a stack of organic semiconductor layers that have to be carefully designed to display perfect colours in all circumstances.

The OLED industry is constantly inventing and testing new organic semiconductor materials. Many of these materials have birefingent optical properties. In past projects PlanOpSim's team has developed a thin film modelling method for light emission from these materials. This dipole emission model for birefringent materials and micro-cavities places no restrictions on the extra-ordinary optical axis of the organic material.
This method was developed as a research project within the Liquid Crystals and Photonics group of Ghent University.

Multi-projector setup

This is the Philips TV that grew out of this foundation prototype.

This is the Philips TV that grew out of this foundation prototype.

The  resulting image from several picoprojectors at the back of the TV.

The  resulting image from several picoprojectors at the back of the TV.

Projectors work best when they project onto a wall in front of them. But that wasn't an option in this project, where an array of pico projectors had to be combined to form a single image on a wall parallel to them and about 30cm away.

In this project source videos were specially processed to be projected by 5 different projectors each at a different angle from the back of a television.

The key steps were:

  • Mapping and calibrating the deformation by the non-standard projection
  • Correcting the source video for the deforamtion by the projector. This creates an "inverse-warped"video
  • Splitting the new video stream and sending it to individual projectors

The prototype from this project was developed further into the Philips Ambilux TV.

Optical film characterization


PlanOpSim has used a number of optical techniques for characterization and parameter extraction from thin film materials. The most important optical parameters extracted are:

  • Refractive index and absorption coefficient

  • Haze and transmission

  • Optical retardation of birefringent materials

  • Film thickness

  • Transmission and reflection

Thin film characterization techniques recquire parameter fitting to extract material parameters from measured transmitted and reflected signals. We build a parametrized model of the wavelength-dependendent refractive index, extra-ordinairy axis, layers and their thickness. The parameters are then varied to find the best fit to measured data. The fit can be performed on multiple measurements and/or multiple samples. The more data, the more reliable the extracted parameters will be.

PlanOpSim can help you with these techniques:

  • Spectroscopic Ellipsometry: is the reference method for determining the refractive index of thin films.

  • Optical retardation measurements using a Babinet compensator: the most accurate method to determine the retardation factor of a birefringent film.

  • Transmission and reflection spectrometry: is an alternative method to determine the refractive index and asbsorption coefficient. Generally thicker films are needed than in ellipsometric measurements

  • Haze & Transmission: Haze is an important experimental parameter for scattering and structured films. The measurement must be carefully conducted with an integrating sphere.