Electroluminescence from isolated CdSe∕ZnS quantum dots in multilayered light-emitting diodes

Electro- and photoluminescence spectra of the $\mathrm{CdSe}∕\mathrm{ZnS}$ core-shell quantum dots (QDs) covered by various organic ligands and incorporated into multilayered light-emitting diodes (LEDs) were recorded by a confocal optical microscope. The QDs were dispersed in a hole transporting material, N,N’-Diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD), to investigate the LED performance at different QD concentrations and the effect of different surface modifications on the isolated QDs. No wavelength shift was observed in the electroluminescence spectra from the QD LEDs with or without the TPD. The peak energies of the electro- and photoluminescence showed only small spectral shifts (several nanometer) for the diluted QDs and no dependence on the QD-concentration, surface ligands, or conductive polymers that were used. This suggests that the relative peak shifts are related to the different filling processes in the $\mathrm{CdSe}$ QDs under photo excitation and electric injection, rather than to the “chemical” effects on the surface of the $\mathrm{CdSe}∕\mathrm{ZnS}$ QDs.