The Role of Keto Defect Sites for the Emission Properties of Polyfluorene-Type Materials
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B9.11.1
The Role of Keto Defect Sites for the Emission Properties of Polyfluorene-Type Materials Emil J.W. List1 Roland Guentner2, Patricia Scanducci de Freitas2, and Ullrich Scherf2 1
Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria & Institute of Nanostructured Materials and Photonics, Franz-Pichler-Strasse 30, A-8160 Weiz, Austria. 2 Department of Chemistry, BUGH Wuppertal, Gauss-Str. 20 D-42097 Wuppertal, Germany. ABSTRACT In this paper we have identified a dominant degradation mechanism of blue light emitting polyfluorene (PF) type polymers, which turns the desired blue emission colour of these polymers into an undesired blue-green emission. As shown low energy emission band at 2.2-2.3 eV can be identified as the emission from exciton and/or charge trapping keto defect sites (9-fluorenone sites), which in fact can be regarded as emission of a guest (defect) accidentally incorporated into the π-conjugated PF backbone. We present spectroscopic evidence for the formation of keto defect sites in 9-monoalkylated polyfluorenes (PF-a) which are identified as the source of the strong low energy emission band. Furthermore we show that the keto defects can be generated directly during polymer synthesis as show for the PF-a or can be formed as main channel of an (photo- or electro-) oxidative degradation process as show for the 9-bisalkylated PF. INTRODUCTION Over the last years polyfluorenes (PF) [1] have emerged as a promising class of conjugated polymers, which can be utilized as the blue light emitting active layers in polymer light emitting diodes (PLEDs) [2,3,4], as the host material for internal colour conversion techniques [5,6] or for PLEDs with polarized light emission [7] exploiting the liquid crystalline nature of most of the PFs. Yet, despite the ongoing improvements in terms of stability and colour purity of PLEDs fabricated from PF type materials, most of these PLEDs suffer from a degradation of the device under operation documented in the formation of a low energy emission band at 2.2-2.3 eV which turns the desired blue emission colour into an undesired blue-green emission. This band, which can also be found in photoluminescence (PL) emission upon photooxidation of the polymer, has been attributed to aggregation and/or excimer formation in the material [8]. As presented here as well as elsewhere in more detail [9] we have identified this strong low energy emission band at 2.2-2.3 eV using UV-VIS, PL emission and IR spectroscopy as the emission from exciton and/or charge trapping keto defect sites. We show that such keto defects, leading to the unwanted low energy emission band in polyfluorenes, can be formed already during synthesis of a 9-monoalkylated polyfluorenes (PF-a), or as a result of photo- (or electro-) oxidative degradation processes of a 9.9-dialkylated polyfluorene (PF-b). The presented data demonstrate that the keto defects act as low energy trapping sites for singlet excitons, b
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