Correlated Color Temperature Tunable White Electroluminescence from Cadmium-Free ZnS Quantum Dots

We propose correlated color temperature tunable white light-emitting from different sizes cadmium-free quantum dots (QDs) without organic ligand-modulation. A size series of free-standing ZnS QDs were prepared by coprecipitation method. Experimental results show that the broad electroluminescences (EL) spectra from all samples cover almost the entire visible region and the electroluminescence peak is significantly redshifted from 489 to 580 nm with ZnS QDs sizes increasing from 1.1 to 4 nm. Moreover, the chromaticity coordinates calculated from EL spectra are (0.27, 0.36), (0.36, 0.42) and (0.42, 0.46) for QDs with average sizes 1.1, 2 and 4 nm drived at 15 V respectively, correspondingly, white EL spectra with a continuously tunable color changes from bluish white (CCT = 12400 K) to yellowish white (CCT = 3700 K). The differences between the photoluminescence (PL) and EL spectra were observed. Furthermore, the comparison between EL and PL spectra and active defect-levels of ZnS QDs with various sizes are discussed to understand the mechanism of the tunable spectra. The results offer that a convenience method to obtain tunable EL spectra in white color from ZnS QDs defects by controlling the size of the QDs.