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


  • We acknowledge Lazar Kulikovsky (University of Potsdam) for fruitful discussions. This work was funded by the German Ministry of Science and Education. Further, financial support by the Fond der Chemischen Industrie is acknowledged. The work at Georgia Tech is partly supported by the US National Science Foundation (through the STC for Materials and Devices for Information Technology Research and through CHE-0342321) and the Georgia Tech Center for Organic Photonics and Electronics.


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.