High luminescence efficiency white light emitting diodes based on surface functionalized quantum dots dispersed in polymer matrices

• We have functionalized quantum dots with five different chemical ligands.
• We have fabricated quantum dot/polymer films using these particles.
• We visualized three-dimensional dispersion of quantum dots by confocal microscopy.
• Dispersional state of quantum dots is closely related to quantum yield of final films.
• Color balance of white LEDs can also be improved by surface functionalization.

We report enhancement in quantum yield of quantum dot (QD)/polydimethylsiloxane (PDMS) films via surface functionalization of CdSe/ZnS core/shell QDs. Through ligand-exchange processes, five different ligands possessing ethoxy, ester, or alkyl groups of varying chain lengths passivated the surface of QDs. In suspension, the photoluminescence quantum yield (QY) of the QDs remained relatively unaltered regardless of the ligands. On the other hand, the ligands on the surface significantly influenced the photoluminescence QY of the QD/PDMS films: QDs functionalized with the longest alkyl chain exhibited the highest QY, and QDs passivated with ester-containing group the lowest. Three-dimensional confocal microscopy revealed that spatial dispersion of QDs in the films depends on the surface ligands, and explains the QY difference. Stacking red- and green-emitting QD/PDMS films on top of blue light-emitting diodes resulted in white light emission: the longest alkyl chain-functionalized QDs gave rise to higher luminance and better color balance than unfunctionalized QDs.

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