Among solid-state lighting technology, phosphor-converted white light-emitting diodes (pc-WLEDs) are excellent candidates to replace incandescent lamps for their merit of high energy conservation, long lifetime, high luminous efficiency as well as polarized emissions. Semiconductor quantum dots (QDs) are emerging color tunable emissive light converters. They have shown significant promise as light emitters, as solar cells, and in biological imaging. It has been demonstrated that the pc-WLED devices integrated with red emissive ZnCdSe QDs show improved color rendering index of device. However, cadmium-based QDs have limited future owing to the well-known toxicity. Recently, non-cadmium luminescence materials, i.e. CuInS2-based QDs, are investigated as desirable low toxic alternatives. Particularly, CuInS2-based QDs exhibit very broad emissions spectra with full width at half maximum (FWHM) of 100-120 nm, large Stokes shifts of 200~300 meV and finely-tunable emissions. In order to adjust emission wavelengths and improved quantum yield (QY), CuInS2/ZnS (CIS/ZnS) core/shell structure was introduced. Therefore, CIS/ZnS QDs have been extensively investigated and be used as color converter in solid-state lighting. Synthesis and application of CuInS2/ZnS core/shell QDs are conducted using a hot injection route. CIS/ZnS core/shell QDs with molar ratio of Cu:In equal to 1:4 are prepared. For WLED fabrication, the CIS/ZnS QD is dispersed in toluene first, and then it is blended with transparent acrylic-based UV resin. Subsequently, the commercial green-emitting Lu3Al5O12: Ce3+ (LuAG) phosphors are mixed with QDs-resin mixture. After that, the QDs-phosphors-resin mixtures are put in the oven at 140 °C for 1 h to evaporate the toluene. Subsequently, the homogeneous QDs-phosphors-resin mixture is dropped on the top of a blue LED chip (InGaN). Then, the device is cured by 400 W UV light to form WLED. The emission wavelength of CIS/ZnS QD exhibits yellow region of 552 nm with QY of 76 %, and with relatively broad bandwidth of 86 nm. The structure of CIS/ZnS belongs to chalcopyrite phase and its average particle size is 3.2 nm. The luminous efficacy, color rendering index (CRI), correlated color temperature (CCT), and CIE chromaticity coordinate of WLED is 47 lm/W, 89, 5661 K, and (0.33, 0.29), respectively.
In this study, we have demonstrated a facile chemical route to prepare CdSe QDs with white light emission, and the performance of white CdSe-based white light emitting diode (WLED) is also exploded. An organic oleic acid (OA) is used to form Cd-OA complex first and hexadecylamine (HDA) and 1-octadecene (ODE) is used as surfactants. Meanwhile, by varying the reaction time from 1 s to 60 min, CdSe QDs with white light can be obtained. The result shows that the luminescence spectra compose two obvious emission peaks and entire visible light from 400 to 700 nm, when the reaction time less than 10 min. The wide emission wavelength combine two particle sizes of CdSe, magic and normal, and the magic-CdSe has band-edge and surface-state emission, while normal size only possess band-edge emission. The TEM characterization shows that the two different sizes with diameter of 1.5 nm and 2.7 nm for magic and normal size CdSe QDs can be obtained when the reaction time is 4 min. We can find that the magic size of CdSe is produced when the reaction time is less than 3 min. In the time ranges from 3 to 10 min, two sizes of CdSe QDs are formed, and with QY from 20 to 60 %. Prolong the reaction time to 60 min, only normal size of CdSe QD can be observed due to the Ostwald repining, and its QYs is 8 %. Based on the results we can conclude that the two emission peaks are generated from the coexistence of magic size and normal size CdSe to form the white light QDs, and the QY and emission wavelength of CdSe QDs can be increased with prolonging reaction time. The sample reacts for 2 (QY 30 %), 4 (QY 32 %) and 60 min (QY 8 %) are choosing to mixes with transparent acrylic-based UV curable resin for WLED fabrication. The Commission International d’Eclairage (CIE) chromaticity, color rendering index (CRI), and luminous efficacy for magic, mix, and normal size CdSe are (0.49, 0.44), 81, 1.5 lm/W, (0.35, 0.30), 86, 1.9 lm/W, and (0.39, 0.25), 40, 0.3 lm/W, respectively.
A white light emission CdSe quantum dots (QDs) can be prepared by chemical route under 180°C. An organic oleic acid
(OA) is used to react with CdO to form Cd-OA complex. Hexadecylamine (HDA) and 1-Octadecene (ODE) were used
as co-surfactants. By controlling the reaction time, a white light emission CdSe QDs can be obtained after reacts for 3 to
10 min. The luminescence spectra compose two obvious emission peaks and entire visible light ranges from 400 to 650
nm. Based on TEM measurement result, spherical morphologies with particle size 2.39±0.27 nm can be obtained. The
quantum yields (QYs) of white CdSe QD are between 20 and 60 %, which depends on reaction time. A white CdSe QDs
were mixed with UV cured gel (OPAS-226) with weight ratios 50.0 wt. %, and putted the mixture into reflective cup
(3020, 13 mil) as convert type. The white LEDs have controllable CIE coordinates and correlated color temperature
(CCT). The luminous efficacy of the device is less than 3 lm/W, but the color rendering index (CRI) for all devices are
higher than 80. Since the luminous efficacy of hybrid devices has a direct dependence on the external QY of the
UV-LED as well, the luminous efficacy can be improved by well dispersion of CdSe QDs in UV gel matrix and using
optimized LED chips. Therefore, in this study, we provide a new and simple method to prepare high QY of white CdSe
QDs and its have a potential to applicate in solid-state lighting.