Alloy core/shell CdxZn1-xS/ZnS quantum dots (QDs) are emerging as robust candidates for light-emitting diodes (LEDs), however the emission range of the current CdxZn1-xS/ZnS is quite limited, ranging from 390 to 470 nm. It still remains a challenging task to construct white LEDs based on current CdxZn1-xS/ZnS system. Here, a versatile ZnSe with a moderate band gap is introduced onto the Cd0.1Zn0.9S core. The ZnSe shell, on one hand, can passivate the core surface which leads to bright emissions. On the other hand, it is essential in extending the emission to red region so that the emission wavelengths of Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs can cover the whole visible region, which is very important for white LED applications. Two- and four-hump QD-based LEDs are computationally and experimentally investigated. Results show that four-hump quantum dot light-emitting diodes (QLED) have better performances than the two-hump one, in the luminous and the vision properties. The fabricated white LEDs (WLEDs) based on Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs exhibits a scotopic/photopic ratio (S/P) ratio as high as 2.52, which exceeds the current limit of 2.50 by common lighting technologies, a color rendering index of 90.3, a luminous efficacy of optical radiation of 460.78 lumen per unit optical power, and a correlated color temperature of 5454 K. These results suggest that CdxZn1-xS/ZnS and CdxZn1-xS/ZnSe quantum dots serving as emitters hold great promise for the next-generation white light source with better S/P ratio.
© 2017 Optical Society of America
Lighting can consume almost 20% of the global electrical energy, and this proportion increases to 30% in some regions of the world [1, 2]. Light emitting diodes (LEDs) have drawn attention from both the research community and the industry because of their energy-saving potential. Moreover, their programmable characteristic makes them easy to combine with intelligent sensors, thus showing great promise in future artificial lightings and displays. Most of the current white LEDs (WLEDs) are realized by integrating color-converting materials with a blue- or near ultraviolet-emitting LED chip. Rare earth ion based phosphors are the conventional color-converting materials. However, WLEDs based on phosphors suffer the problems such as low color rendering index (CRI or Ra) owing to the lack of red component in the emission spectrum, and the difficulty in tuning the spectra distribution because of the broad full width at half maximum (FWHM). Semiconductor quantum dots (QDs) have been exploited as an alternative or additive to rare-earth-ion-based phosphors to solve or improve the performance of the phosphor based WLEDs, owing to their excellent optical properties such as size-tunable emission wavelengths, narrow FWHMs, high photoluminescence (PL) quantum efficiencies (QYs) and etc [3–6]. LEDs combining with intelligent sensor circuit are hopeful to tackle the issue of high energy consumption. Consequently, QD-based WLEDs have become the most attractive candidates for next-generation of artificial light sources. During the past several decades, impressive efforts have been devoted to the chemical synthesis studies, band structure investigations and spectral designs of various types of QDs [7–11].
Future LED design should take the response of human eyes into account to achieve more comfortable lights. In general, human eye is constructed with two types of photoreceptors: cones and rods. Cones and rods have different spectral sensitivities, and their activity differs depending on the ambient lighting levels. Cones are more active at high luminance and rods are primarily responsible for vison at lower luminance. Therefore, when evaluating the luminous efficacy of optical radiation (LER) of a white light illuminant, the sensitivity of the human eye needs to be taken into account. Visions at high or low luminance are called photopic and scotopic vision respectively. A light source with a higher ratio of the scotopic LER to the photonic one (S/P) provides better perception of brightness and a better visual acuity. In other words, this light source is more effective in a dark environment and thereby higher energy efficiency [1, 12, 13]. Therefore, developing white light sources with a high S/P ratio open up a new paradigm for energy-saving in artificial lighting [12, 14–16].
However, there is still a challenge of fabricating a light source offering high CRI together with high S/P and usually one should make a trade-off between them . Many theoretical and experimental works have been carried out to improve the S/P ratio for a white light source [14–16]. However, less effort is paid to attain high S/P and CRI simultaneously. Quantum dot LEDs (QLEDs) are new choices to target this goal. The luminescent and the visual performance of a light source mainly depend upon its spectral power distribution (SPD). QD emitters with different emission characteristics such as peak wavelength, FWHM and intensity participating in white light source is expected to relieve the competition phase between S/P and CRI. In this work, we synthesized Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe core/shell quantum dots with emission wavelength covering the whole visible spectrum by solution method. QD-based WLEDs with high S/P and CRI were fabricated by elaborately choosing QDs with suitable emission wavelengths and ratios. Furthermore, SPDs and parameters such as S/P and CRI of the prepared QD-based WLEDs were simulated based on the double Gaussian model . Theoretical calculations demonstrated a similar variation tendency with the experiments, indicating that our current work was performed on a reliable basis. In this preliminary work, the fabricated QD-based WLEDs exhibited high values of S/P and CRI simultaneously, and showed satisfactory luminescent performance including LER and correlated color temperature (CCT).
2. Experimental section
Cadmium oxide (CdO, 99.99% powder), zinc acetate (ZnAc, 99.99%), sulfur(S, 99.98%, powder), 1-octadecene (1-ODE, 90%), oleic acid (OA, 90%), and selenium (Se, 99.99%, powder) were purchased from Aldrich. All reagents were used as received without further experimental purification.
2.2 Synthesis of CdxZn1-xS cores
Six millimoles of CdO, 10 mmol ZnAc, and 7 mL OA were loaded in a 100 mL round-bottom flask. After vacuum pumping for 30 min, the mixture was heated to 150 °C under nitrogen. Next, 15 mL 1-ODE was injected into the flask, and then the mixture was cooled and vacuumed for half an hour, followed by heating to 300 °C under nitrogen to obtain a clear mixed solution of Cd(OA)2 and Zn(OA)2. Subsequently, 3 mL sulfur stock solution (sulfur stock solution was prepared by dissolving 2 mmol sulfur powder in 3 mL 1-ODE) was injected quickly into the flask under 300 °C. For the synthesis of CdxZn1-xS core, the injection lasted for 8 min.
2.3 Synthesis of CdxZn1-xS /ZnS QDs
When the synthesis of CdxZn1-xS core was completed, the solution was heated to 310 °C, then 3 mL sulfur stock solution (sulfur stock solution was prepared by dissolving 2 mmol sulfur in 3 mL 1-ODE) as S precursor was injected slowly into the flask. Purple emission CdxZn1-xS/ZnS core/shell QDs was synthesized after reacting for 30 min. For the synthesis of blue emission CdxZn1-xS/ZnS core/shell QDs, S precursor was prepared by dissolving 2 mmol sulfur powder in 1.5 mL 1-ODE. After centrifugation from the reaction solution, followed by drying under 40~50 °C for 2 h in a vacuum chamber, CdxZn1-xS/ZnS QD powder can be obtained.
2.4 Synthesis of CdxZn1-xS /ZnSe QDs
When the synthesis of CdxZn1-xS core was completed, the solution was heated to 310 °C, then 3 mL selenium stock solution (by dissolving selenium powder in 3 mL 1-ODE) as Se precursor solution was injected slowly into the flask. CdxZn1-xS/ZnSe core/shell QDs can be obtained after reacting for 30 min. By changing the stoichiometry of Se, emission wavelength of the prepared CdxZn1-xS/ZnSe QDs can be tuned. The obtained QD colloidal solution was cooled to room temperature and dissolved in 1 mL chloroform and 3 mL ethanol for precipitation. Next the mixture was centrifuged at 8000 rpm for 5 min and the supernatant was decanted. Then the precipitate was dried to a powder under 40~50 °C for 2 h in a vacuum drying chamber.
2.5 Device fabrication
QDs powder for WLEDs fabrication was obtained with the method described above. Different kinds of QD chloroform solutions with concentration 0.12 g/mL and polymethyl methacrylate (PMMA) chloroform solutions with concentration 0.2 g/mL were prepared. For the fabrication of WLEDs, the prepared QD solution was mixed with different volume ratio, followed by stirring. The QD mixture and the PMMA solution were mixed evenly with equal volume, and then the obtained mixture solution was dropped on the blue-emitting GaN LED chips (Real Faith Group Co., Ltd., China). Finally, after the solvent was evaporated and the PMMA cured under room temperature, the QD-based WLEDs were prepared.
The material properties were characterized by high-resolution transmission electron microscopy (JEM-2010, JEOL Ltd.), X-ray diffractometer (Rigaku MiniFlex II X-ray diffractometer), steady-state photoluminescence (PL) spectrophotometer (Cary Eclipse Varian) and UV-vis spectrophotometer (Cary 300, Varian). Time resolved photoluminescence experiments were performed on a spectrometer (Bruker Optics 250IS/SM) with an intensified charge coupled device detector (CCD, Andor, IStar 740). The final concentration of each element was monitored by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Leeman Co., USA, PROFILE SPEC).Absolute PL QYs of the prepared QDs were tested by an absolute QY test system (Quantaurus-QY C11347-11, Hamamatsu Photonics Co., Ltd.). The luminescent and the visual performance of the fabricated QLEDs were measured on a spectrometer (Maya 2000, Ocean Optics) with an integrating sphere (3P-GPS-033-SL, Labsphere).
3. Results and Discussion
3.1 Emission tunable CdxZn1-xS/ZnS and CdxZn1-xS/ZnSe core/shell QDs
Emission tunable CdxZn1-xS QDs have been achieved by tuning the compositional components or controlling the reaction times or temperatures, and the reported emission wavelengths ranges from 400 to 475 nm with FWHMs of 18-25 nm and PL QYs of about 19-30% . After shelling the CdxZn1-xS QDs with ZnS, the PL QYs of the resulted CdxZn1-xS/ZnS core/Shell QDs approached to 100%. ZnS shell has a relative wider band gap than that of the CdxZn1-xS core QDs, so that both the electrons and the holes are mostly confined to the core region, which enhances the radiative recombination in an efficient way and leads to a great enhancement in PL QYs . However, the emission range of the current CdxZn1-xS/ZnS is quite limited (390-470 nm) by its core-shell system, in which the core and the shell behaves independently, i.e., CdxZn1-xS core decides the emission wavelength while ZnS acts as a passivation layer . Therefore, it is still not easy to deliver highly efficient red or green emission for CdxZn1-xS/ZnS QDs, which restricts their further construction for WLEDs.
The final concentration of each elements of the prepared pure CdxZn1-xS core, CdxZn1-xS/ZnS, and CdxZn1-xS/ZnSe QDs were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). After equivalent mathematical conversion, the composition of the CdxZn1-xS core can be expressed as Cd0.1Zn0.9S. Note that the Zn:Cd ratios and S addition amount were kept unchanged for the core during the synthesis of the CdxZn1-xS/ZnS and CdxZn1-xS/ZnSe QDs. Then the as-prepared QDs are reasonably expressed as Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe unless particularly claimed. Here, a versatile ZnSe with a moderate band gap is introduced onto the Cd0.1Zn0.9S core. Evidence shows that ZnSe shell can act as a passivating layer by consuming of redundant Zn2+ ions by Zn-Se bonding, thus leading to a notable enhancement in emission efficiencies. It also plays an important role in tuning the emission wavelengths. As depicted in Figs. 1(a) and 1(b), the emission wavelengths of Cd0.1Zn0.9S/ZnSe can be effectively tuned from 520 to 680 nm by simply altering the initial adding amounts of Se. This indicates that the Cd0.1Zn0.9S/ZnSe QDs can compensate emission limitation of the Cd0.1Zn0.9S/ZnS QDs (485 nm) to achieve full color display. After introducing the ZnS or ZnSe shell onto the core, the PL QYs are notably enhanced owing to the passivating effects of the shell, as shown in Table 1. It is interesting that the emission wavelength tunability of the Cd0.1Zn0.9S/ZnSe is far beyond its single constituents Cd0.1Zn0.9S and ZnSe. This extraordinary tunability in optical emissions may suggest that it is a type-II structure. Theoretical and experimental results reveal that CdxZn1-xS (x<0.3) has a band gap of ~3.0 eV which is higher than that of ZnSe 2.8 eV. The conduction band of CdxZn1-xS core and ZnSe shell are about −3.6 eV and −2.8 eV, while their valence band are −6.6 eV and −5.7 eV, respectively [20, 21]. When Cd0.1Zn0.9S and ZnSe are combined together, their energy levels are staggered to each other, and therefore one can expect a type-II structure of Cd0.1Zn0.9S/ZnSe. The UV-vis absorption spectra QDs are illustrated in Fig. 1(c). Since the bandgap of ZnS is much larger than that of Cd0.1Zn0.9S, the Cd0.1Zn0.9S/ZnS is a type-I QDs. The first exciton absorption peak is clearly revealed for Cd0.1Zn0.9S/ZnS QDs because of its type-I structure where a wider bandgap material ZnS is coated on Cd0.1Zn0.9S. In contrast, there is no significant first exciton absorption peaks for the spectra of the Cd0.1Zn0.9S/ZnSe QDs. The non-obvious exciton absorption peak is one of the representative properties of type-II QDs, again implying current Cd0.1Zn0.9S/ZnSe has a type-II structure. Because in a type-II QD, the electrons and holes are separately confined in the core and shell or vice versa, and thereby the absorptive oscillator strength is greatly reduced due to the weak coupling between the electrons and holes.
In order to further confirm the type-II properties of the as-prepared QDs, time-resolved PL experiments were performed. Figure 1(d) shows the PL decay curves of Cd0.1Zn0.9S core, Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe core/shell QDs All decay curves were well fitted by a bi-exponential based formula [22, 23]:Eq. (2) given asFig. 1(d). The prepared Cd0.1Zn0.9S/ZnSe QDs exhibits an extremely long PL lifetime of 126.954 ns, which is over 10 times longer than that of Cd0.1Zn0.9S/ZnS QDs 11.588 ns. The interactions between the electrons and holes can be greatly suppressed in a type-II QD and the PL lifetime will be notably lengthened, owing to the decreased overlap of the wave functions between electrons and holes. The great tunable emission, non-obvious exciton peak together with extremely long PL lifetime suggest that current Cd0.1Zn0.9S/ZnSe is a type-II QD. The as-prepared QDs show narrow FWHMs<35 nm and moderate PL QYs 50-60%, which may guarantee their applications in WLEDs.
3.2 Structural characterization
TEM images of Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs and their corresponding size distribution histograms are shown in Figs. 2(a) and 2(b) respectively. Both the TEM images show reasonable narrow size distributions with average diameters of 7.31 nm Fig. 2(a) and 7.51 nm 2(b) for Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs respectively. To further characterize the structures of Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs, their crystallographic properties were determined by XRD. According to the XRD patterns shown in Fig. 2(c), the characteristic peaks of Cd0.1Zn0.9S locate between the zinc blende ZnS (JCPDS NO. 65-0309) and CdS bulk materials (JCPDS NO. 65-3414). This suggests a compositional homogeneity of Cd0.1Zn0.9S rather than a mixture of CdS and ZnS. When ZnS shell were coated onto the Cd0.1Zn0.9S cores, obvious peak shift to the position of the standard zinc blende phase ZnS (JCPDS NO. 65-0309) has been observed, which is consistent with the related system . However, after overcoat ZnSe on to the Cd0.1Zn0.9S cores by adding Se precursor the corresponding XRD patterns of the Cd0.1Zn0.9S/ZnSe reveal a ZnSe phase only. This suggests that ZnSe tends to coat onto the surface of the core and form a core/shell heterostructure rather than an alloy compound. Similar results were observed in some related core/shell systems [5, 12]. In detail, the diffraction peaks of this Cd0.1Zn0.9S/ZnSe QDs can be indexed as the cubic ZnSe, which is consistent with the standard pattern of JCPDS NO. 65-7409.
3.3 Performances of the QD-based WLEDs with Cd0.1Zn0.9S/ZnSe QDs
The S/P ratio of a light source is dominated by values of the scotopic LER (LERs) and the photopic LER (LERp), which are described by Eq. (3), where S(λ) is the emission spectrum of the as-prepared QLED, V(λ) and V’(λ) are the photonic eye sensitivity function under photonic and scotopic lighting conditions respectively. The LER is in units of lumen per unit optical power (). The expression of S/P ratio is given by Eq. (4) .
Actually, LER denotes the overlap between the human eye sensitivity curve and the SPD of the light source . Several spectral models have been proposed for studying the luminescence properties of LEDs, such as Gaussian model (G-model) , Yoshihiro’s model (Y-model) , double Gaussian model , He’s model (H-model) . Thereinto, double Gaussian model has been proved to be effective in simulating the narrow-band SPD of chips or QDs .
Red, green and blue are the three primary colors of light. By means of combining these colors with different ratios, all types of color light including white one can be obtained. Hence there exist red and green component for yellow light. Theoretically, it is expected that white light can be composed by mixing blue and yellow light. By using of the double Gaussian model, the SPD for a monochromatic LED chip or the emission spectrum of QD component is written asFig. 3(a). Figure-of-merits such as S/P, Ra, LER and CCT of the two-hump QLEDs are also calculated and the results are shown in Table 2. Given that the emission wavelength and FWHM of the chip are about 390 and 10 nm respectively, the overlap between the spectrum of the chip and V’(λ) is expected to be small, implying that the chip emission is difficult for human eye to percept. In short, the chip emission contributes very little to the luminous and the vision properties of the QLED. Therefore, for this two-hump QLED, these figure-of-merits are dominated by the blue and the yellow components, whose peak wavelengths are 490 and 590 nm respectively. However, V(λ) has a maximum value at 555 nm, and V’(λ) 507 nm. Consequently, with the increase of FWHMs of the blue component, the overlap between the SPD of the two-hump QLED and V’(λ) expands. As a result the S/P increases accordingly, as given in Fig. 3(a). However, lack of green emission restricts the further extension of the overlap between the SPD and V’(λ) so that the value of S/P ratio is somewhat restrained. Red emission component contributes much to the CRI of a light source [6, 27], that is why these two-hump QLEDs present low Ra values. And therefore, it is also hard to achieve high S/P ratios for these two-hump QLEDs. Theoretically, wider and may aid in improving the S/P ratio and Ra. Unfortunately, the FWHM of QDs is usually about 30 nm or even less than this value, which is not enough for achieving notable enhancement. Consequently, the use of green and red emission should be taken into account for the construction of QD-based WLEDs.
The emission spectrums of several four-hump QLEDs with different red FWHMs and their figure-of-merits are simulated, and the corresponding results are shown in Fig. 3(b), where = 390 nm, = 10 nm, = 10 mW/nm, = 485 nm, = 10 nm, = 20 mW/nm, = 550 nm, = 30 nm, = 11 mW/nm, = 580 nm, = 30 nm, = 10 mW/nm, = 640 nm, = 20 mW/nm. The four-hump QLEDs incorporated with optimized green and red emission components show higher S/P ratios and Ra values than that of the two-hump one, as shown in Table 3. The participation of the blue emission lead to the addition of the overlap between the QLED spectrum and V’(λ), and this addition underlies the increase for S/P of the four-hump QLED. On the other hand, red emission contributes to the increase of Ra. With = 30 nm, S/P ratio can achieve 2.80 accompanied with Ra exceeding 80. While green emission affects much on LER, and the short wavelength radiation in the emission spectrum affects the CCT of a light source. More short-wavelength radiation will lead to a higher CCT, conversely, a lower one . As a white light source, this four-hump QLED with LER about 215 and CCT about 4300 K is also acceptable.
Two-hump QLED was fabricated by coating blue and yellow QDs on a 390-nm violet chip. The optical properties of the prepared two-hump QLED were measured and shown in Fig. 4. The S/P ratio, Ra, LER and CCT values are given listed in Table 4. The evolution of electroluminescence (EL) spectra combined with V(λ) and V’(λ) curves are shown in Fig. 5(a). The blue and the yellow QDs are peaked at 490 and 590 nm, respectively. Obviously, emission intensity of the two-hump QLED enhances with the increasing forward bias current, hence the overlap between the SPD and V’(λ) expands, so does that between the SPD and V(λ). However, the growth of the blue emission intensity is not much as that of the yellow one, which leads to the decrease of S/P ratio with current increasing from 25 to 40 mA, as shown in Fig. 4(b) and Table 4. When current exceeds 40 mA, it is observed that the overlap between SPD and V(λ) remains unchanged even though the current increases gradually. This is why S/P ratio begins to raise after 40 mA. Actually, it can be seen from Table 4 that S/P ratio, Ra, LER and CCT do not change significantly with current increasing from 25 to 50 mA. That is also the reason why the color coordinates gather too close to distinguish them, as shown in Fig. 4(c). During the process of current increasing, S/P ratio maintains about 0.695, and Ra 46.5. Generally, neither of them is good for a white light source. Furthermore, as shown in Table 4, with current increasing, LER of this QLED is always kept about 175 or so, which is expected to be promoted to a higher value. CCT of this QLED being about 1850 K with different operation current indicates that it is a yellow white light source, and its color coordinates locate exactly at the yellow region on a CIE 1931 chromaticity diagram as shown in Fig. 4(c). The triangle on the CIE 1931 chromaticity diagram defines the gamut of the stand Red Green Blue (sRGB) color space commonly used for imaging. Moreover, the emission intensity of the blue QD is always lower relatively than that of the yellow one. This may be resulted from the low PL QY of the employed blue QD or reabsorption between the blue and the yellow QDs. Both the simulation and the experiment results indicate that two-hump QLEDs have difficult in achieve high enough S/P and Ra simultaneously.
To fabricate QD-based WLEDs with high S/P ratio values, blue, green, yellow, and red Cd0.1Zn0.9S/ZnS or Cd0.1Zn0.9S/ZnSe QDs were coated layer by layer on a violet chip of emission wavelength 390 nm. The optical properties of the as prepared QLEDs are presented in Fig. 5. The evolution of EL spectra curves of the QLED are shown in Fig. 5(a). During the current increasing, relative amplitude of the green component remain weak than those of the blue, yellow and red ones. Both lower PL QY of the employed green QDs and reabsorption among these QDs may be the reasons for the weak green emission. As shown in Fig. 5(a), the luminescence intensity of QLED increases continuously, while the forward bias current increases from 10 to 100 mA. It is found that all the four QDs can be well excited by 390 nm. EL spectra of the QLED obviously consist of four emission bands peaked at about 485, 550, 580, and 640 nm, respectively. With increasing forward bias current, S/P, Ra, LER and CCT of the prepared QLED are measured, and the evolution of these parameter are presented in Fig. 5(b) and Table 5. When the QLED was operated at 10 mA, the values of S/P and Ra were 1.85 and 60.3, respectively. Although these two values advanced much compared with that of the two-hump QLED, they were unfavorable as a white light source. However, with forward bias current increasing to 20 mA, these two values approach quickly to 2.34 and 78.9 respectively. In our measurement, when the current is 60 mA, S/P ratio reaches a satisfactory value 2.52 accompanied a surprising maximum Ra 90.3. Here, the S/P ratio 2.52 surpasses the upper limit value 2.5 form most phosphor-based LEDs and traditional white light source . As can be seen from Fig. 5(a), the growth of the green emission is slowly than that of the blue one, which underlies the reason for the enhancement of the S/P ratio. Red emission component is benefit for the enhancement of Ra. The going down of Ra after 60 mA may result from the ascend of the emission of other color [6, 27]. Actually, with current surpassing 20 mA, the values of S/P and Ra shown in Table 5 are acceptable for a general white light source. As shown in Fig. 5(c), for different currents, most of the color coordinates of this four-hump QLED locate at the white light region on a CIE 1931 chromaticity diagram. Furthermore, with current increasing from 10 to 100 mA, the QLED always maintain a white shade with the CCT values ranging from 4000 to 6200 K as shown in Table 5. In addition, with current exceeds 20 mA, values of LER surpass 250 , which appears certain improvements compared to that of the two-hump QLED. Especially at 60 mA, the LER value of the QLED is 460.78 , and the CCT is 5454 K, which is agree to that of the sunlight. In some extent, this device exhibited a good vision performance accompanied by satisfied luminous properties. Moreover, with S/P value beyond 2.5, this device may offer benefits of energy-saving. Both theoretical and experiment results indicate that by using of Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs, white light source with high S/P ratio companied by good Ra can be achieved.
In short, the optical properties of Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs, including emission wavelength, FWHM and PL QYs, are related to their synthesis condition such as temperature, reaction time, stoichiometric and so on. Therefore, these Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs provide much choice for the fabrication of efficient white light source with ideal visual and luminescent properties.
In summary, we have synthesized emission tunable Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs for WLED applications with high S/P ratio. We demonstrate that the versatile ZnSe shell does not only serve as a passivation layer, but also contributes to the tunable emission. After incorporating ZnSe onto the Cd0.1Zn0.9S core, bright visible emissions are observed. The blue, green, yellow and blue QDs as novel color converters are applied to fabricate QLEDs. Comparing with the two-hump QLED based on blue and yellow QDs, four-hump QLEDs based on blue, green, yellow and red QDs emit white light similar to the sunlight. To our surprise, luminescent and visual properties of the four-hump QLEDs are even comparable to the commercial triphosphor white light source. Their good luminescent and visual properties in this preliminary work shows a S/P ratio 2.52 which exceeds the current limit of 2.50 by common lighting technologies, a color rendering index of 90.3, a luminous efficacy of optical radiation of 460.78 , and a correlated color temperature of 5454 K. The visual and the luminescent properties of the QD-based WLEDs can be further improved, providing much more in-depth studies on the QDs. These results suggest that CdxZn1-xS/ZnS and CdxZn1-xS/ZnSe quantum dots show great promise for the next generation energy-efficient and vision-friendly displays.
We gratefully acknowledge the financial support of the Natural Science Foundation of China (61366003 and 11564026), Outstanding Youth Funds of Jiangxi Province (20171BCB23051 and 20171BCB23052), Natural Science Foundation of Jiangxi Province (20151BAB212001, 20151BBE50114, 20171BAB202036 and 20161BAB212035), Science and Technology Project of the education department of Jiangxi Province, China (GJJ150727 and GJJ160681).
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