Study on the Color-Compensation Effect of Hybrid Orange-Red Quantum Dots in WLED Application

Quantum dots (QDs) as emerging light-converting materials show the advantage of enhancing color quality of white light-emitting diode (WLED). However, WLEDs employing narrow-emitting monochromic QDs usually present unsatisfactory color rendering in the orange region. Herein, orange-red emitting polychromic hybrid QDs (hybrid-QDs) are developed through mixing CdSe/ZnS based orange QDs (O-QDs) and red QDs (R-QDs) to compensate the orange-red light for WLEDs. We investigated the effect of self-absorption and fluorescence resonance energy transfer (FRET) process in hybrid-QDs on the spectral controllability and fluorescent quenching in WLEDs. The concentration and donor/acceptor ratios were also taken into account to analyze the FRET efficiency and help identify suitable hybrid-QDs for color compensation in the orange-red light region. As the result, the optimized hybrid-QDs effectively improve the color rendering index of the WLED compared with monochromatic QDs at the same color coordinates. QDs to enhance the spectral continuity and color of the orange-red light-emitting region QWLEDs. (FWHM) as the component of the hybrid orange-red QDs (hybrid-QDs). The FRET in the hybrid-QDs was studied by considering the effects of concentration and proportion of hybrid-QDs. The results were used to optimize the quantum efficiency (QE) and spectral controllability of the hybrid-QDs. Moreover, the hybrid orange-red QDs were used with LuAG:Ce green phosphor in blue LEDs to form QWLEDs. The as-prepared QWLEDs exhibit enhanced color quality with a more balanced full spectrum in the orange-red region.


Introduction
Light-emitting diodes (LEDs) have attracted significant research interests in solid-state lighting applications due to their high efficiency, long lifetime, low-power consumption, fast response time and high reliability [1,2,3,4,5,6]. WLEDs are usually fabricated by packaging yellow, green and red-emitting phosphors with blue-LED chips [7,8,9]. The full-spectrum WLEDs employ hybrid phosphors with a high proportion of red phosphor [10].
However, classical red phosphors have a broad emission that causes lumen loss in the redlight emitting region because the human eye is insensitive to the wavelength longer than 650 nm [11].
Recently, quantum dots (QDs) have been employed to fabricate high-quality WLEDs.
Compared to classical phosphors, QDs have unique optical properties, such as size-dependent wavelength tunability, high photoluminescence quantum yield and strong absorption [12,13,14,15,16,17]. Because of the narrow emitting characteristics at redlight region, the red-emitting QDs are particularly useful for inhibiting the abovementioned lumen loss and improving color rendering index (CRI) of WLEDs [18,19]. Therefore, using QDs to compensate for the orange-red region has become an effective measure for enhancing the color quality of WLEDs [20]. Generally, QD-based WLEDs (QWLEDs) can be divided into two categories by mixing monochromic or polychromatic QDs in the LEDs [20,21,22,23]. For example, Xie et al. used red-emitting CdSe/CdS/ZnS QDs to replace classical red phosphor with LuAG:Ce green phosphor to fabricate high performance WLED [24]. Li et al. fabricated QWLEDs by integrating a mixture of red, yellow and green light-emissive CdZnS/ZnSe QDs on the blue-emitting GaN LED chip, which exhibited a CRI of 85.2 and correlated color temperature (CCT) of 4072 K [25].
To date, the full spectrum QWLEDs for lighting application are commonly developed by incorporating broad emitting green-yellow phosphors and narrow emitting monochromatic red QDs [24]. These QWLEDs present superb spectral continuity in the green-yellow region but a clear valley in the orange-red region. Theoretically, a hybrid QDs made of several monochromic QDs in the orange-red region are capable to fill the valley and further improve the spectral continuity of QWLEDs. However, it is difficult to regulate the spectra of the hybrid QDs because of the self-absorption and fluorescence resonance energy transfer (FRET) process among polychromatic QDs [26]. Therefore, although the color property of the QWLEDs have been investigated by manipulating the peak position and broadness of the monochromatic red QDs, the hybrid orange-red QDs have not been studied in WLEDs on account of the self-absorption and FRET process.
Herein, hybrid orange-red QDs were studied to enhance the spectral continuity and color quality of the orange-red light-emitting region for QWLEDs. We prepared CdSe/ZnS based 4 orange QDs (O-QDs) and red QDs (R-QDs) with different full width at half maximum (FWHM) as the component of the hybrid orange-red QDs (hybrid-QDs). The FRET in the hybrid-QDs was studied by considering the effects of concentration and proportion of hybrid-QDs. The results were used to optimize the quantum efficiency (QE) and spectral controllability of the hybrid-QDs. Moreover, the hybrid orange-red QDs were used with LuAG:Ce green phosphor in blue LEDs to form QWLEDs. The as-prepared QWLEDs exhibit enhanced color quality with a more balanced full spectrum in the orange-red region.

Synthesis of O-QDs.
The synthetic procedure was based on the report in the literature [27]. Cd(St) 2 (2 mmol) and stearic acid (0.2 mmol) were loaded into a 50 mL three-neck flask with 10 mL of ODE.
After stirring with nitrogen bubbling, the solution was heated to 270 °C. Then 0.5 mL of TOP-Se (2 mmol of Se powder dissolved in 1 mL of TOP) was rapidly injected into the flask and maintained at 270 °C for 2 min. Afterward, 0.5 mL of TOP-S (4 mmol of S powder dissolved in 2 mL of TOP, stirred well) was rapidly injected into the flask and maintained 5 at 270 °C for 40 min, and then the flask was cooled to 30 °C. Cd(St) 2 (0.75 mmol), Zn(Ac) 2 (2.25 mmol), and 5 mL of ODE were added into the above solution. After stirring with nitrogen bubbling, the flask was heated to 160 °C. 1.5 mL of TOP-S was slowly injected into the flask and maintained at 160 °C for 4 h, and then the flask was cooled to room temperature. After a centrifuged purification procedure with ethanol, the as-prepared CdSe/ZnS QDs were dispersed in 10 mL of dimethylbenzene for further use.
Synthesis of R-QDs.
The synthetic procedure was similar to that of O-QDs expect for the following two points. Then, the different concentrations of hybrid-QDs gels were added into the same type of 6 mold and removed the bubbles. Finally, the hybrid-QDs silicone gel thin films were built by curing at 150 °C for 1 h.

Preparation of hybrid-QDs silicone gel thin films
With the same weight ratio of O-QDs:R-QDs (10:1), the hybrid-QDs were mixed into the different volumes of silicone gels to form hybrid-QDs gels with different concentrations (0.35, 0.5, 0.75, 1, 1.5 and 3 mg/mL). Then, the as-prepared hybrid-QDs gels were added into the same type of molds and removed the bubbles. Finally, the hybrid-QDs silicone gel thin films with different hybrid-QDs concentrations were built by curing at 150 °C for 1 h.

Fabrication of WLEDs
The LED chips (typical 2835 lead frame package) with the emission peak at 450 nm were

Measurement and Characterization
Photoluminescence (PL) was recorded on an Ideaoptics FX2000-EX PL spectrometer.
Transmission electron spectroscopy (TEM) was performed on a FEI Tecnai G2 Spirit TWIN transmission electron microscope operating at 100 kV. The quantum efficiency (QE) measurements were carried out on an OceanOptics QEpro QY test system under 365 nm blue laser irradiation. The luminous efficiency and optical power were recorded on an EVERFINE ATA-1000 LED automatic temperature control photoelectric analysis and measurement system. UV-Vis absorption was measured by using a Persee T6 UV-Vis spectrometer. The excitation spectra and time-resolved PL spectroscopy (TRPL) have been measured by an Edinburgh FLS920 fluorescence spectrometer.

Results And Discussion
The optical properties of the two monochromic QDs were firstly studied. Figure 1a  Moreover, the 2 mg/mL R-QDs and 10 mg/mL O-QDs based silicone gel films also exhibit the highest PL intensity under different exciting power, as shown in Fig. S1 a and b, respectively. At the above two concentrations, the optical properties of the monochromic QDs are effectively preserved, which weakens the PL quenching caused by the host matrix effect [28,29]. The study helps to find the suitable concentration of the monochromic QDs in silicone film.
To further investigate the concentration influence of QDs in the monochromic QDs based silicone thin films, the time-resolved PL (TRPL) spectra of the thin films with different concentrations are measured and the decay curves are depicted in Fig. 3. It is known that PL decay curves can be expressed with a multi-exponential function, as illustrated by Eq. 1 [ 30], where I(t) is the PL intensity at time t. Ai and τi represent the relative amplitude and the excited state lifetime of each exponential component of PL decay, n is the number of decay times. These decay curves, as shown in Fig. 3a and b, can be well fitted by a double exponential function according to Eq. 1.
The fitting parameters Ai and τi are listed in Table S1 and S2. The amplitude-weighted lifetimes of the R-QDs and O-QDs are selected as their lifetimes (τ ave ) for further investigation. The lifetime can be calculated from the following Eq. 2 [ 31] and is listed in Table S1 and S2.  Table S3. The lifetime of the O-QDs sample was found to be 30.25 ns. When the acceptor R-QDs are introduced, the lifetime of the donor O-QDs decreases (Table S3) owing to the intervene of the energy-transferring channel. The lifetime of the donor becomes shorter with the rise of the acceptor concentration. On the contrary, the lifetime of the R-QDs sample was found to be 13.08 ns. When the donor O-QDs are introduced, the acceptor R-QDs present an increase in lifetime as a result of energy feeding (Table S3) [34]. The calculated results are shown in Fig. 5c, which clearly demonstrates the phenomena.
The FRET process is also investigated by energy transfer efficiency. The efficiency of FRET can be calculated according to lifetime as illustrated by Eq. 3.
Where τ DA is the donor fluorescence lifetime in the presence of the acceptor, τ D is the donor fluorescence lifetime in the absence of acceptor [26]. It shows that τ DA is inversely proportional to the energy transfer efficiency. Therefore, as the acceptor-donor ratio increases, the τ DA becomes shorter and the energy transfer efficiency increases. Larger energy transfer efficiency reflects higher impact on fluorescence. We further analyze the FRET efficiency of hybrid-QDs in Fig. 5a. The calculated results are listed in Table 1   As the best continuity spectrum for LED lighting in orange-red light, the hybrid-QDs with the 1:10 weight ratio of R-QDs:O-QDs is selected for further study.  Table S4 lists the amplitudes, lifetime components, and amplitude-weighted lifetimes for the hybrid-QDs in Fig. S2. Their FRET efficiencies are calculated and shown in Table S5. Furthermore, the changes in lifetimes and FRET efficiency with concentration are clearly shown in Fig.6d. In detail, the FRET efficiency exhibits a decrement trend from 22% to 9% with the rise of concentration. Meanwhile, the lifetime recorded at the emission wavelength of the donor O-QDs increases with increasing concentration (orange dots in

Conclusion
In summary, we prepared the hybrid orange-red QDs and studied their optical properties and the energy transfer dynamic in the hybrid-QDs for LED applications. Our study reveals that the concentration of the hybrid-QDs and the proportion of donor-QDs and receptor-QDs play an important role in the energy transfer efficiency and spectrum stability.
Meanwhile, the self-absorption has a significant influence on the FRET between different monochromic QDs in the hybrid-QDs. The relative stable and high QE can be achieved by adjusting the donor-to-receptor ratio in the hybrid-QDs, which is meaningful to enhance the color quality of WLED by compensation of the light gap in the orange-red region. As a result, the WLED fabricated based on the hybrid-QDs exhibits a highly improved color quality and more natural light spectrum compared to the spectra of the monochromic-QDs based WLEDs.

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