Synthesis of CuIn$S_2$/ZnS core/shell quantum dots and its application to light-emitting diodes

Abstract

In the first part of this research, colloidal CuInS2 quantum dots (QDs) have been studied as an ex-cellent red emitting source for white LED because of its non-toxic, deep red emitting, and large Stokes shifting properties. The CuInS2 QDs are synthesized by a one-pot method, which is one candidate for mass-productive method. In addition, a formation of ZnS shells on QD surfaces has been conducted in order to reduce non-radiative recombination on its surface defects. The photoluminescence (PL) characteristics of core/shell quantum dot show that the maximum PL wavelength blue-shifts by about 80nm with increasing its intensity more than 6 times. Quantum yield (QY) of the CuInS2/ZnS QDs shows about 67% as a maximum value. The dispersion of the quantum dots in polystyrene, which is hydrophobic and transparent, was also conducted by using co-dispersing toluene. Finally, the polystyrene/quantum dot composite was applied on blue LEDs and it show luminous efficacy of 10.7lm/W and (0.4338, 0.1827) CIE coordinates. In second part of this research (sec. 3.2), CuInS2/ZnS(CIS/ZnS) core/shell QDs were used in LEDs as a wavelength converter to investigate degradation characteristics of QD-LEDs. When the CIS/ZnS QDs in silicone encapsulants were applied to LEDs, they exhibited significant fluorescence instability followed by emissive blue-shift of 10nm and decrease of luminescent intensities to 12% of its initial value within 2 hours. To analyze this phenomenon, QDs dispersed in hexane were applied to LEDs, confirming that detachment of surface ligands and oxidation of QDs are main reason for degradation. At last, by using epoxy encapsulant which has much higher gas-barrier property than silicone, photo stability of QDs is highly enhanced. In section 3.3.1, investigation on ligand exchange of QDs was conducted. An aliphatic thiol ligand of CIS/ZnS QDs is replaced with a hydroxyl-terminated thiol ligand by utilizing ‘on-off state’ of ligands during growth stage of the quan...