Quantum Dot Light-Emitting Diodes in the Visible Region: Energy Level of Ligands and Their Role in Controlling lnterdot Spacing and Device Performance

In fabricating quantum dot light-emitting diodes (QDLEDs) based on manganese-doped zinc sulfide (Mn-doped ZnS) nanocrystals, we use a range of dithiolcontaining ligands in forming thin films of nanocrystals to study the role of interdot spacing on the performance of the 2 QDLEDs. Use of dithiols as ligands to the doped nanocrystals is crucial since the redox energy levels of such ligands lie below both the d-states of Mn ions, and since a transition between the d-states yields photoluminescence (PL) emission in the I doped system, the ligands could not quench the PL emission of the nanocrystals. In thin films of nanocrystals, the length of the ligands controls the intensity of the PL emission due to several factors. When the length of the ligands is long, the ligands on one hand increase the PL intensity, and they also reduce the conductivity of the emitting layer due to an increase in the interdot spacing. We observe that the performance of the QDLEDs is optimum at an intermediate length of the ligands. The spectral range of electroluminescence (EL) emission does not depend on the length of the ligands since the PL emission of Mn-doped ZnS nanocrystals, and hence also the EL of the devices, appears to be due to a radiative transition between the d-states of manganese ions.