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

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.