Suppression of the Keto-Emission in Polyfluorene Light-Emitting Diodes: Experiments and Models

The spectral characteristics of polyfluorene (PF)‐based light‐emitting diodes (LEDs) containing a defined low concentration of either keto‐defects or of the polymer poly(9,9‐octylfluorene‐co‐benzothiadiazole) (F8BT) are presented. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green and blue emission components. It is shown that blending hole‐transporting molecules into the emission layer at low concentration or incorporation of a suitable hole‐transporting layer reduces the green emission contribution in the electroluminescence (EL) spectrum of the PF:F8BT blend, which is similar to what is observed for the keto‐containing PF layer. We conclude that the keto‐defects in PF homopolymer layers mainly constitute weakly emissive electron traps, in agreement with the results of quantum‐mechanical calculations. The role of keto‐defects on the charge carrier trapping and recombination in polyfluorene‐based LEDs has been investigated (see Figure). The results strongly support the view that keto‐defects function as weakly emissive electron traps. A simple model has been provided to explain the suppression of green emission upon addition of hole‐trapping molecules.