On the Origin of the Color Shift in White‐Emitting OLEDs

Abstract

White‐emitting organic light‐emitting diodes (OLEDs) notoriously suffer from brightness‐dependent color shifts. Based on experimental data and fundamental considerations about charge‐transport and trapping, a quantitative model is developed which gives insight into the origin of this shift and describes the experimental data extremely well. Having understood the major contribution to the spectral instability, a device architecture is designed that shows excellent color stability over a wide range of brightness levels.

Energy and charge transfer in blends of dendronized perylenes with polyfluorene

“…We finally like to note that Gather et al recently provided an alternative model to explain the color shift in a white-emitting copolymer comprising an electron-trapping red dye. 65 In this model, the emission color is determined by the competition between electron trapping on the red-emitting site and the unper- turbed charge transport through the organic layer. A major drawback of the approach is that it essentially considers the kinetics of electrons in the active layer in the absence of holes.…”

White organic light-emitting diodes with an ultra-thin premixed emitting layer

“…They should be stable during the device lifetime (no differential aging of the luminophores), and at last the OLED manufacturing process should allow for a high reproducibility and a precise tuning of the color. The color stability issue is prominent for all OLED devices but is of particular importance for WOLED as only a shift of more than 0.005-0.01 for the x-and the y-values of CIE coordinates, respectively, results in an appreciable visible shift in the rendered color [5]. This correspond to a four-step MacAdam ellipse, which is a typical measure of an acceptable color shift (even if the lower sensitivity of the human eye in this part of the colorimetric diagram make this definition maybe rather stringent for white emitters).…”

Get it white: color-tunable AC/DC OLEDs

“…5 Voltage-dependent color shifts are the result of a variety of mechanisms, e.g., voltagedependent charge trapping, a spatial shift of the recombination zone, a modified exciton distribution, or exciton quenching at high current densities. [5][6][7] However, this approach has several drawbacks: not only are the mechanisms that lead to voltage-dependent color-shifts difficult to control, but adjusting the driving voltage also unavoidably results in a dramatic and undesired change in device brightness. The second concept overcomes the disadvantages of the voltage-controlled approach by using a stacked tandem OLED structure with two (or more) independently addressable units emitting light of different colors.…”