DOI: https://doi.org/10.29363/nanoge.inform.2019.058
Publication date: 8th January 2019
Quantum dot (QD) based light-emitting diodes (QLEDs) are competitive alternatives to purely organic light-emitting diodes (OLEDs) in terms of color purity, luminescence intensities, and external quantum efficiencies (EQEs). Hybrid QD/organic polymer light-emitting diodes combine the advantageous emitting properties of the QDs with the flexibility in device construction of polymeric materials. In QD/polymer hybrid LEDs, adding QD emitters in the polymer host usually leads to strong charge trapping or charge transfer from the polymer host to the QDs, which adversely affects device performance. In a host-guest system comprising a blue-emitting donor polymer and a red-emitting QD acceptor, the emission process should preferably occur via long-range Förster energy transfer (FRET), without charge trapping or charge transfer. Therefore, the design challenge for such a system is to optimize energy transfer, while at the same time separating donor and acceptor sufficiently far to avoid short-range processes such as charge transfer and trapping.1,2
To address this challenge, we designed a novel QD/semiconducting polymer hybrid material in which the surface of the QDs is covered with an electrically insulating shell with an optimized thickness. We separate donor and acceptor far enough to prevent charge transfer and trapping, while still allowing FRET by i) shielding the emitting CdSe core of the QDs with a ZnS shell having a wider bandgap than the core and ii) grafting polystyrene (PS) chains onto the ZnS surface. The PS layer furthermore enhances miscibility with the polymeric host. We studied the performance of QLEDs and single charge carrier devices based on the red-emitting CdSe/ZnS/PS core shell QDs (acceptor) blended with the blue emitter poly(dioctylfluorene) (PFO) as the host (donor). For optimal QD size, PS molecular weight, grafting density and particle loading, FRET from PFO to the QDs is the dominant process, with charge trapping being only marginal compared to active layers containing non-PS-grafted QDs.
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