Blue light-emitting-diodes from poly (N-vinylcarbazole) doped with colloidal quantum dots encapsulated with carbazole terminated ligand: spectroscopic studies and devices

Abstract

Blue emitting CdSe/ZnS quantum dots (QDs) were encapsulated with the ligand 11-(N-carbazolyl) undecanoic acid (C11). Steady-state photoluminescence (PL) experiments show an enhancement of the QD emission upon the excitation of the carbazole ligand in solution compared to the situation where a solution with the same concentration of QDs capped with oleic acid (OA) were excited at the same wavelength. This suggests energy transfer from the carbazole moiety to the QD cores. When incorporating the QDs in a poly (N-vinylcarbazole) (PVK) matrix, a significant enhancement of the QD emission upon the excitation of PVK was also observed indicating an efficient energy transfer from PVK to the QDs in the case of C11 capped ligands. Confocal microscopy images of the doped PVK films show clearly better miscibility of PVK and QDs capped with C11 compared with those capped with OA. Nanosecond time-resolved PL experiment shows evidence of singlet transfer with Förster resonance energy transfer (FRET) efficiency of 39% for the QDs in solution, while the efficiency of this process amounted to 15.6% for a PVK film doped with 30 wt% of the QDs. The smaller efficiency of the singlet transfer compared to the overall efficiency of energy transfer, suggested by the stationary PL spectra suggests an important role for triplet energy transfer. Electroluminescent devices were prepared with the structure; ITO/PEDOT:PSS/doped PVK with C11 capped QDs/Butyl PBD/Aluminum. Upon applying voltage, the devices show pure blue electroluminescence at low concentration of QDs (10 wt%) with a turn on voltage close to 6V.Keywords: Colloidal quantum dots, organic light emitting diodes, electroluminescence, blue OLEDs, energy transfer, triplet transfer, organic electronics, PVK INTRODUCTIONColloidal semiconductor quantum dots (QDs) are a relatively a new class of materials for optoelectronics. Due to their unique properties, e.g., tunable emission wavelength, they are attractive for many applications such as light emitting diodes, solar cells, and lasers [1-2]. Among them, cadmium selenide (CdSe) QDs have become a bench mark material for research on colloidal nanomaterials for (electro)luminesncence applications due to the tunability of their emission over the visible range between 475 and 670 nm upon increasing QD size [3]. To use QDs as a light source however requires also a the absence of (photo)chemical degradation and a high luminescence quantum yield (QY). These can be fulfilled by surface passivation with inorganic compounds which have broader band gap that encompasses the bandgap of the QD core, so-called core/shell nanostructure (type I) [2,[4][5].

Silver Zeolite Composites‐Based LEDs: A Novel Solid‐State Lighting Approach

“…Therefore it was not possible to determine the behavior of the reference OLED under reverse bias. Under forward bias the EL of the reference OLED has a maximum at 420 nm, which corresponds to the typical PL of the full overlap excimer of PVK [44] and is stable up to an applied voltage of 20 V, after which breakdown occurs. In a following step a PVK OLED containing 10 wt% of silver exchanged zeolites, with respect to PVK, was fabricated (ZEOLED sample).…”

“…PVK can be considered a benchmark material due to the extensive amount of research performed using this conductive polymer. [43][44][45] Finally, the photoluminescence (PL) and electroluminescence (EL) of these devices are studied and compared with a reference OLED.…”

Silver Zeolite Composite-Based LEDs: Origin of Electroluminescence and Charge Transport

“…Few-atom silver clusters have been shown to possess well-defined and intense photoluminescence characterized with high external quantum efficiencies. − Different strategies have been utilized to solve the problem of stabilization and geometry control, of which template-mediated strategies, such as zeolite incorporation, are perhaps the most successful strategies. , By tuning the framework topology, chemical composition, and the nature and mixing ratio of the counterions, it is possible to vary the geometry and nuclearity of metal clusters. Optimization of these parameters has recently led to the generation of silver clusters with external quantum efficiencies close to 100% .…”