Abstract
In this work, interactions between different host materials and a blue TADF polymer named P1 are systematically investigated. In photoluminescence, the host can have substantial impact on the photoluminescence quantum yield (PLQY) and the intensity of delayed fluorescence (ΦDF), where more than three orders of magnitude difference of ΦDF in various hosts is observed, resulting from a polarity effect of the host material and energy transfer. Additionally, an intermolecular charge-transfer (CT) emission with pronounced TADF characteristics is observed between P1 and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), with a singlet-triplet splitting of 7 meV. It is noted that the contribution of harvested triplets in monochrome organic light-emitting diodes (OLEDs) correlates with ΦDF. For devices based on intermolecular CT-emission, the harvested triplets contribute ~90% to the internal quantum efficiency. The results demonstrate the vital importance of host materials on improving the PLQY and sensitizing ΦDF of TADF polymers for efficient devices. Solution-processed polychrome OLEDs with a color close to a white emission are presented, with the emission of intramolecular (P1) and intermolecular TADF (PO-T2T:P1).
Highlights
Since the first report of organic light-emitting diodes (OLEDs) by Tang and Vanslyke (1987), great efforts have been dedicated to achieve efficient and cost-effective OLED architectures for display and lighting applications
The photoluminescence quantum yield (PLQY) and the delayed fluorescence for the thermally activated delayed florescence (TADF) polymer P1 are sensitive to the surrounding environment
We report the substantial influence of host materials on the photophysical properties of a TADF polymer P1 and further, the triplet harvesting ability in OLEDs comprising this emitter
Summary
Since the first report of organic light-emitting diodes (OLEDs) by Tang and Vanslyke (1987), great efforts have been dedicated to achieve efficient and cost-effective OLED architectures for display and lighting applications. Even though tremendous efforts have been put on intermolecular charge transfer state (CT-state) TADF (in the OLED community typically referred to as “exciplex”) between small molecules (Goushi et al, 2012; Liu et al, 2015; Wu et al, 2017; Lin et al, 2018a; Ullbrich et al, 2019), the first report of intermolecular CT-emission with TADF characteristics based on a non-TADF polymer (poly(9-vinylcarbazole), PVK) and small molecule (2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5triazine, PO-T2T) was recently reported by Pander et al (2018) In their system, the concentration of small molecule component has only minor influence on the transient PL decay profile of the CT-emission. Combining blue emission from P1 and yellow emission from the PO-T2T:P1 CT-emission, we demonstrate broadband, polychromatic OLEDs with a single emission layer, which holds promise for solutionprocessed white OLEDs based on the collective effect of two distinct CT-states, both giving rise to TADF
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