Organic light emitting diodes (OLEDs) are developed to be used in  next-generation flat panel displays like TV sets, and more generally in  display devices like cell phones, PDA... In a multi-layer OLED device (Fig.1), multiple reflections of emitted electroluminescence (EL) light from interfaces generate  optical interferences that can strongly reduce the external quantum efficiency of EL. Determination of both thickness and optical constants of each individual layer is compulsory for optimizing the display performance.

To comply with the requirement of on-line optical characterization and monitoring of individual organic layers in the 190-1000 nm spectral range, we developed the following procedure on a Woollam M2000 rotating-compensator spectroscopic ellipsometer.

First, the analysis was limited to a spectral region where the layer is optically transparent (500nm-1000nm typically) to determine its thickness and refractive index in this limited range, using a Cauchy model. Then, at a fixed thickness, both refractive index and extinction coefficient were calculated at each wavelength of the investigated spectral range. Finally, based on this mathematical approximation, an oscillator-based model, Kramers-Kronig consistent, was built to fully describe the optical dispersion of the layer, including position and shape of the numerous absorption peaks in the UV range (Fig.2).

Figure 1 : Schematic view of multilayer OLED stack

Figure 2 : Optical dispersion of Alq3 organic layer in the 190-1000nm spectral range

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