The photoluminescence properties of Ag(In _x Ga _{1− x} )S ₂ quaternary semiconductor quantum dots are improved by ligand exchange with L-, X-, and Z-type ligands. Analyses of both the quantum dots and byproducts are performed for understanding the mechanism.

Quantum dots (QDs) are expected to be used as an emitting material in wide-color-gamut displays. However, the development of low-toxicity alternatives is necessary because QDs that exhibit high color purity and highly efficient emission contain toxic materials such as Cd. Here, QD light-emitting diodes (QD-LEDs) fabricated using AgInS₂/GaS_x core/shell QDs (AIS core/shell QDs) as low-toxicity QDs were investigated. The photoluminescence (PL) spectrum of an AIS core/shell QD dispersion showed a band edge emission with a peak wavelength of 560 nm and a full-width at half-maximum of 45 nm because the GaS_x shell suppressed the surface defects. Electroluminescence (EL) emission, which mainly comprises the band edge emission, was realized in the AIS-based QD-LED. However, the EL spectra included a large defect emission component, together with the band edge emission. The defect emission was attributed to electrons flowing in the emitting layer (EML) being easily trapped at defect levels in the QDs. The addition of tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB) as an electron transporting material to the EML effectively suppressed the defect emission. The radiation-energy ratio of the band edge emission to the total emission was 50% in the QD-LED without 3TPYMB and was improved to 64% in the QD-LED with 3TPYMB; this ratio was comparable to that in the PL spectrum of the AIS core/shell QD film. The addition of 3TPYMB improved electron injection into the QDs and the carrier balance in the EML. As a result, the defect emission was suppressed and the EL characteristics were improved.