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

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 Forster 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.