Analytical estimation of emission zone mean position and width in organic light-emitting diodes from emission pattern image-source interference fringes

Abstract

We present an analytical method for evaluating the first and second moments of the effective exciton spatial distribution in organic light-emitting diodes (OLED) from measured emission patterns. Specifically, the suggested algorithm estimates the emission zone mean position and width, respectively, from two distinct features of the pattern produced by interference between the emission sources and their images (induced by the reflective cathode): the angles in which interference extrema are observed, and the prominence of interference fringes. The relations between these parameters are derived rigorously for a general OLED structure, indicating that extrema angles are related to the mean position of the radiating excitons via Bragg's condition, and the spatial broadening is related to the attenuation of the image-source interference prominence due to an averaging effect. The method is applied successfully both on simulated emission patterns and on experimental data, exhibiting a very good agreement with the results obtained by numerical techniques. We investigate the method performance in detail, showing that it is capable of producing accurate estimations for a wide range of source-cathode separation distances, provided that the measured spectral interval is large enough; guidelines for achieving reliable evaluations are deduced from these results as well. As opposed to numerical fitting tools employed to perform similar tasks to date, our approximate method explicitly utilizes physical intuition and requires far less computational effort (no fitting is involved). Hence, applications that do not require highly resolved estimations, e.g., preliminary design and production-line verification, can benefit substantially from the analytical algorithm, when applicable. This introduces a novel set of efficient tools for OLED engineering, highly important in the view of the crucial role the exciton distribution plays in determining the device performance.

On the relevance of two‐dimensional models for radiation of statistical sources in stratified media

“…lines [Olsen and Wu, 1989], lightning discharge [Valdivia et al, 1998], and thermal radiation [Carter, 1983]. Moreover, as the rigorously derived expressions for the radiation patterns coincide with results obtained by employing the geometrical optics (GO) approximation, this work also provides intuition regarding the possible extension of the present results to other, not necessarily planar, geometries, in which GO is applicable [Epstein et al, 2013].…”

“…Second, our work proves that for certain, not uncommon, statistical source properties, the 2-D model can fully reproduce some of the results of the 3-D realistic scenario. This ability to precisely analyze 3-D systems via a 2-D model is a very powerful tool, which enabled us, for instance, to directly apply techniques developed based on 2-D derivation [Epstein et al, 2010a] to 3-D problems [Epstein et al, 2014] without any change.…”

AZO/Ag/AZO anode for resonant cavity red, blue, and yellow organic light emitting diodes

“…They thus yield a weaker and broader (or low Q) resonance effect as we shall now demonstrate and discuss. We first use the emission zone method presented by Epstein et al 11 to model and optimise the efficiency of OLEDs whose transparent electrode is either the AZO/Ag/AZO/MoOx four layer stack or an ITO/ MoOx bilayer. To understand the efficiency and colour tuning effects, we then use the direction dependent spectral emission OLED device modelling in the range of emission directions from the normal to nearly 690…”

“…coordinates for a particular pixel. The spatial location of the recombination/emission peak within the cavity relative to the cathode also influences the output angular spatial profile and net output, 11 and can be tuned by changing the layer thicknesses although it should be noted that this could cause a decrease in efficiency if charge injection and transport are compromised. The use of cavity resonances for emission tuning is less prominent for thick transparent electrodes with relatively low refractive indices.…”

“…Following a recently proposed model of the charge activated emission zone, 11 we report optimized designs for devices composed of solution processed light-emitting polymers that emit blue, yellow, or red light. The type of electrode was found to have a profound a)…”