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Abstract
In this study, direct bi-color CdSe quantum dots (QDs) have been successfully synthesized and used as luminescent materials to fabricate the QDs-based white light-emitting diodes (WLEDs). We demonstrate a one-pot preparation of direct bi-color CdSe QDs-based WLEDs with a high color gamut. For the application of the backlight source, CdSe QDs with a CIE coordinate of (0.34, 0.25) were mixed with 30% UV resin, which provides a color gamut of 89 and 126% in National Television Standard Committee (NTSC) and sRGB standards, respectively. We expects that full color generation by bi-color QDs-based WLED will lead to further technological advancements in the area of efficient and facile display.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The liquid crystal display (LCD) has become the most popular displays in our daily lives. An essential requirement for display devices is an accurate representation of colors, and the white backlight source plays an important role accordingly. The cold cathode fluorescent lamp (CCFL) was once the most prevalent backlight source in LCD, but its color gamut is only ~75% of National Television Standard Committee (NTSC) standard. In order to achieve wider color gamut, higher brightness, and lower power consumption, mercury-free white light-emitting diodes (WLED) has rapidly replaced CCFL as the major backlight source [1–4]. Until now the LCD backlights have evolved from blue-pumped yellow YAG:Ce3+ phosphor-converted WLED (1pc-WLED) to green and red phosphors converted WLEDs (2pc-WLED) and to cadmium-based quantum dots (QDs-WLED) [1, 5–8]. Although the NTSC of phosphor-based WLED can be improved from 68.3 (1pc) to 82.1% (2pc) [5], the emission spectrum of commercial YAG-based and green/red phosphor-based WLEDs are broad and do not fit to color filters, resulting in that the NTSC is not high enough for High Definition Television (HDTV) application. Therefore, choosing a material with narrow emission width and tunable emission wavelength is the total solution for extending the color gamut of display. Quantum dots (QDs), especially cadmium-based compounds are one of the most promising candidates because they have narrow bandwidth, tunable emission wavelength and solution processed functional applications [9, 10]. Meanwhile, in order to achieve good stability and high photoluminescence (PL) quantum yield (QY), surface passivation (shelling) is used inevitably [11–15].
Usually, single wavelength emissions cannot only be obtained through quantum confinement effect but also composition related control with a very wide wavelength range for QDs [16-17]. Among various preparation methods to synthesize QDs, the hot injection method is still the most popular and efficient strategy [18] and only single emission wavelength can be obtained in the most literatures [19–22]. Although the nucleation and growth rate of QDs can be controlled by the reaction temperature and injection times, the luminescence intensity between green and red light is still not controlled very well [23-24]. Although full color display can be achieved by mixing blue (blue-LED), green and red color to achieve larger color gamut due to the more narrow emission peak, it is not easy to adjust the emission intensity of green and red light and package method is also a key issues for QD-based WLED [25, 26]. Although the color rendering index (CRI) and correlated color temperature (CCT) can be adjusted by manipulating the concentrations between different colors of QDs [28-29], the effect of emission intensity between green and red QDs on their color gamut is still not clear. Moreover, in order to compensate the self-absorption between green and red -QDs, the luminescence intensity of green-QD is expected to be higher than that of red-QDs [27]. In our previous study we have demonstrated the preparation methods of monochromatic light ZnCdSe QDs with high quality [30-31]. We found that single capping reagent is beneficial for growing red QDs [30] and co-capping reagents are suitable for preparing green QDs [31]. Therefore, in this paper, we demonstrate a one-pot method to prepare three kinds of direct bi-color CdSe QDs with different emission area between green and red QDs and the color gamut of the direct bi-color CdSe QDs-based WLED is studied.
2. Experimental
2.1 Preparation of direct bi-color CdSe QDs
Bi-color CdSe QDs were prepared directly based on our previous work [32]. Briefly, TOP-Se solution with different concentrations (Se = 1.5, 1.1, and 0.75 mmol) was injected into three-necked flask which contains Cd-SA-HDA precursor solution to form red QDs (Rx solution, x = 1.5, 1.1, and 0.75). In another three-necked flask, Cd-SA-TOPO-HDA precursor was prepared and added into previous Rx solution and another TOP-Se solution (1.5 mmole) was injected into Rx solution to form GRx QDs (GR1.5, GR1.1, and GR0.75). GRx solution was purified by hot methanol and dispersed in hexane. The emission area ratio between green and red light was defined as AG/AR.
2.2 Preparation of bi-color CdSe QDs-based WLEDs
All of the devices were fabricated by the convert type and using surface-mounted device (SMD) typed InGaN-based blue emitting LEDs (λem = 450 nm, 2.8 V, and 17 mW) with 13 mil, as shown in Fig. 1. The bi-color CdSe QDs were blended with different weight ratios of UV resin (0, 15, 30, 45 and 60 wt. %) to prepare the QDs-mixtures which were dropped into blue emitting LED chip and cured to form the bi-color GRx QDs-based WLEDs. The schematic diagram of CdSe QDs-based WLED is depicted in Fig. 2. Commercial blue, green, and red color filters were used to identify the purity of GRx and calculate the color gamut for display application.
Fig. 1 Property of blue-emitting LED (SMD 3020). (a) I-V curve and (b) EL spectrum. Insert photograph shows the B-LED under 20 mA.
2.3 Characterization of bi-color CdSe QDs and WLED
The optical properties and morphologies have been acquired by ultraviolet-visible (UV-vis, Jasco V-670 spectrometer) absorption/photoluminescence (PL, Hitachi F-7000) emission spectroscopy/time-resolved photoluminescence (TRPL, Olympus FV-300) and high resolution transmission electron microscopy (HRTEM, JEOL JEM-ARM300F). Besides, the PL lifetime spectra of GRx QDs were excited using a diode laser wavelength of 470 nm at room temperature (RT), and the average lifetime (τavg) was calculated by below equations [33–35].
On the other hand, the Commission Internationale de l'éclairage (CIE), CCT, CRI and luminous efficacy of the bi-color CdSe QDs-based WLEDs have been also measured by integrating sphere system (Isuzu Optics ISM-360).
3. Results and discussion
GRx bi-color CdSe QDs can be prepared by multiple injections of anions precursor solution into cationic/capping reagents co-existing solution in desired reaction temperatures and times. In order to obtain two individual sizes of QDs, we use different capping reagents to limit the growth rate of QDs: HDA for larger sized QDs and HDA/TOPO for the smaller one, because the steric hindrance of TOPO retards the growth rate of smaller QDs. Based on the PL temporal evolution of GR1.5 (shown in Fig. 3) QDs, we can find that the emission peak of red QD is almost fixed at 600 nm under the period of green QDs growth due to the well passivation by HDA, while the emission wavelength after secondary injection is red-shifted from 490 to 530 nm after 30 s. Figure 4 shows the emission spectra of GRx QDs. It can be observed that the emission spectra of GRx QDs are composed of two observable peaks in which the shorter wavelength is green (G) and the longer wavelength is red (R) color emissions. The emission area of green light (AG) is 6, 13, and 22 times of red one (AR) for GR1.5, GR1.1 and GR0.75, respectively, meaning that the initial TOP-Se precursor of Rx solution affects the growth rate and number of nuclei to control the relative emission intensity of two colors [32].
Figure 5 shows the room temperature PL lifetime, and the τavg of the GRx QDs samples. The τavg for 537 nm (G) and 602 nm (R) is ~72 ns and ~40 ns, and thatof GR1.5, GR1.1 and GR0.75 is 44, 48 and 54 ns, respectively. The emission wavelength of QDs with larger particle size is longer than that with smaller particle size, suggesting that the emission energy of smaller QDs can be reabsorbed by larger QDs [33, 36]. Moreover, the τavg of GRx QD is close to R samples, and the τavg becomes longer as the emission area ratio (AG/AR) increases, suggesting that the τavg not only depends on particle size but also reabsorption effect. Table 1 summarizes the relationship between reabsorption and τavg of GRx QDs samples. It can be found that the τavg dramatically decreases with increasing the diameter of QDs due to the size effect, suggesting that the surface-to-volume ratio plays an important role in determining the surface localization of the hold states. As a result, more trap sites on the surface can be found when the surface-to-volume ratio increases and the diameter decreases for QDs.
Table 1. Correlation between reabsorption and PL lifetime of GRx QDs.
Besides, the HRTEM reveals that the GRx QDs with two emission wavelengths mainly come from the contribution of two different particle sizes, as shown in Fig. 6. The smaller and lager size is 2.6 (zinc blende structure) and 4.0 nm (wurtzite structure), respectively, corresponding to the emission wavelengths of 537 and 602 nm in FL spectra.
In PL spectra, the AG is 6, 13, and 22 times of AR in GR1.5, GR1.1, and GR0.75 samples, respectively. However, the CIE and AG/AR of bi-color CdSe QDs-based WLED in electroluminescence (EL) are (0.40, 0.24), 0.28/1, (0.40, 0.31), 0.52/1, and (0.40, 0.38), 1.02/1 for GR1.5, GR1.1, and GR0.75, respectively [32] and only GR0.75 QDs-based white LED locates in white region. In bi-color CdSe QD-based device, the AG decreases dramatically, meaning that a strong self-absorption occurred and about 95% of green light absorbs by red QDs. Therefore, the AG/AR ratio is a key factor for fabrication of QDs-based WLEDs [26]. On the other hand, the optical properties of QDs are dispersion-dependent, meaning that their optical properties can be controlled by the polymer [25, 37]. Therefore, we choose the GR0.75 QDs to blend with UV resin under different weight ratios, and the result is shown in Fig. 7(a). The EL emission intensity of GR0.75 QD decreases and color of GR0.75 QDs-based WLEDs changes with increasing UV resin content which may result in the increase of CCT and the blue-shift of CIE. The devices properties of GR0.75 QDs-based WLEDs with different UV resin contents including CIE, CRI, CCT and luminous efficacy under a drive current of 20 mA are summarized in Figs. 7(b) and (c). As the UV resin content increases, the CIE and color of devices are dramatically changed from warm white to cold white side, the CCT is increased from 3478 to 8042 K, and luminous efficacy is enhanced from 1.8 to 3.1 lm/W, but the CRIs are almost unchanged around 70.
The long-term stability test of GR0.75 QD-based WLED is conducted under 20 mA, and the EL and CIE results are shown in Fig. 8 and summarized in Table 2. The stability of GR0.75 QD-based WLED is improved from 40 to 1080 min after encapsulation with 45 wt% of UV gel.
Table 2. Long-term stability and CIE of GR0.75 QD-based WLED after 20 mA operating.
Besides, the mixture of GR0.75 QDs and 30 wt. % UV resin is selected for backlight application because the position of white light point is close to the sunlight. The transmittance spectra of R/G/B CF are obtained in Fig. 9 (a). The three primary colors (RGB) of QDs-based WLED are perfectly transmitted over the color filter (CF) due to the narrow band emission. Compared with CCFL, 1p-LED and 2p-LED [9, 10], Fig. 9 (b) shows that the bi-color GR0.75 QDs-based WLED can clearly reduce the overlapped regions in CF with high color purity owing to the G and R emissions with narrow FWHM. Furthermore, the inset in Fig. 9 (b) demonstrates the three primary colors of bi-color GR0.75 QDs-based WLED after the CF transmission. The performance of QDs-based WLED backlight indicates that the B, G and R colors with corresponding EL emission wavelength, FWHM, CIE are 454 nm, 26 nm, (0.15, 0.03), 548 nm, 28 nm, (0.33, 0.66), and 616 nm, 30 nm, (0.68, 0.32), respectively. The white point of bi-color GR0.75 QDs-WLED is located at (0.35, 0.34), the coverage of color gamut is NTSC of 89% and sRGB standards of 126% in CIE 1931 color space as shown in Fig. 10. The results show that the bi-color GR0.75 QDs-based WLED is better than the typical YAG-based WLED backlights. The well-separated emission can provide the wider color gamut because of using the combination of shorter-wavelength G- and longer-wavelength R-QDs.
Fig. 9 (a) The EL spectra of bi-color GR0.75 QDs-based WLED (30 wt. % UV resin content) and transmission spectra of R/G/B color filters. (b) The EL spectra and the color images of bi-color GR0.75 QDs-based WLED (30 wt. % UV resin content) after color filters.
Fig. 10 Color gamut of bi-color GR0.75 QDs-WLED (30 wt. % UV resin content) compared to NTSC and sRGB color standard in CIE 1931 color space.
4. Conclusion
In this study, the direct bi-color CdSe GRx QDs have been successfully synthesized and used as luminescent materials to fabricate the on-chip type GRx QDs-based WLEDs. The AG/AR in PL spectra should be larger than 22, so that the CIE of devices can be located in white region. Among them, GR0.75 can successfully emit white light which compensates the reabsorption effect. By varying amounts of UV resin, the color of GR0.75 QDs-based WLED can be adjusted from warm to cold white light, as well as the luminous efficacy can be improved from 1.8 to 3.1 lm/W. The stability of GR0.75 QD-based WLED is enhanced 40 to 1080 min after encapsulation with 45% of UV gel. Moreover, the WLED with white point of (0.35, 0.34) is applied as backlight source and provides color gamut of 89 and 126% in NTSC and sRGB standards, respectively. Therefore, the GR0.75 QDs have the potential as novel bi-color nanophosphor that can be directly encapsulated with a blue emitting LED chip to form the WLED. The work presented here shows that the direct bi-color CdSe QDs-based WLED is an outstanding candidate for full color displays.
Funding
Ministry of Science and Technology (MOST) of Taiwan (104-2628-E-008-005-MY3, 105-2221-E-150-058 and 106-2221-E-150-049).
Acknowledgment
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan under Contract no. 104-2628-E-008-005-MY3, 105-2221-E-150-058 and 106-2221-E-150-049.
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