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Abstract
The spectral optimization for maximizing limited luminous efficacy (LLE) of correlated color temperature (CCT) tunable white light-emitting diodes (LEDs) with two, three and four color perovskite quantum dots (QDs) excited by a blue chip was investigated under the constraint of designated ultrahigh color rendering index (CRI) and color quality scale (CQS). The results show that high quality white lights with CRIs of 96-97, CQSs of 95-96, and LLEs of 243-225 lm/W at CCTs of 2700 K to 6500 K could be realized by the QDs-converted white LEDs (QD-WLEDs) using a blue chip and different combinations of perovskite quantum dots. The luminous efficacies of QD-WLEDs are expected to reach 114 to 122 lm/W at CCTs of 2700 K to 6500 K, if the radiant efficiency of a blue chip is 60% and the real PLQY values of perovskite QDs are 75%. These results demonstrate that perovskite QD-based LEDs are expected to be promising in next generation display and lighting technologies.
© 2017 Optical Society of America
1. Introduction
In the past decade, the metal halide perovskite nanocrystals have attracted great attention from scientists in various areas due to its remarkable optoelectronic properties, such as the high absorption coefficient, high photoluminescence quantum yield (PL QY), narrow emission, and convenient bandgap tunability [1–6]. Specifically, the perovskite quantum dots (QDs) have been successfully applied in the fields of solar energy [7], lasers [8-9], and light-emitting diodes (LEDs) [7, 10-11]. As illumination becomes the second largest energy consumption factor taking up approximately 20% of global electrical energy production [12], it imposes increasing demands of both energy-efficiency and light quality on light sources. QDs-converted white LED (QD-WLED) is considered as one of the most promising lighting techniques in addressing such challenges of reducing the energy consumption and achieving qualified performance. Currently, the most popular commercial WLEDs are mainly based on the integration of traditional yellow phosphors (YAG:Ce) which perform weak emission in the red spectra region with blue InGaN/GaN chips [13]. This trend presents challenges for this type of LEDs to achieve a high luminous efficacy (LE) as well as a high color rendering.
As new-generation phosphor materials, semiconductor nanocrystals or colloidal QDs have tunable and relatively narrow emission across the visible spectral range and small overlap between their emission and absorption spectra in comparison with traditional phosphors. It was reported that many QD-WLEDs using II-VI [14–19], III-V [20–22], and I-III-VI [23–30] type QDs can attain good color quality white lights with CRI over 90. However, they are not preferable in terms of efficiency — most commonly, with LEs less than 80 lm/W. Compared with II−VI group semiconductors, lead halide perovskite nanocrystals have higher absorption coefficient and have narrow emission of relatively longer excited-state lifetime. Additionally, the energy loss introduced by interference of surface or trap states is less in these devices than in traditional QDs [3]. In this circumstance, inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite QDs, as a promising alternative approach, began to attract a great deal of scientific attention.
To obtain a well-designed QD-WLED suitable for lighting applications, some evaluation metrics must be carefully considered and optimized at many levels of spectrum design to satisfy certain criteria. Examples of these metrics are color rendering index (CRI) or color quality scale (CQS), LE, and correlated color temperature (CCT) [31, 32]. A good white light source must render the real colors of the objects that it illuminates. This feature of light sources has great importance for indoor lighting applications. The CIE CRI is widely used and the only internationally accepted metric for assessing the color-rendering performance of light sources. Recent momentum to commercialize lamps using LEDs for general illumination is exposing some short comings of the CRI. Considering these problems with the CRI, a new metric, CQS, for color rendering evaluation of light sources is developed, which is especially useful for narrow-band emitters such as QDs. In our previous work, optimization of QD-WLEDs was studied under the constraint of both a designated CCT and CRI for high radiation LE [33]. The challenge in the design of QD-WLEDs with a tunable CCT consists of achieving excellent colorimetric properties over a reasonable range of CCTs, while at the same time maximizing LE. According to the results presented in [33], QD-WLEDs using the blue chip and RGB CdTe QDs could realize CCT tunable white lights with CRIs of 95–96, CQSs of 93–94, and limited luminous efficacy (LLEs) of 256–272 lm/W, which increase by 12–21%, compared with those of optimal phosphor-coated white LEDs (pc-WLEDs) [34]. Nevertheless, the photometric optimization of CCT tunable perovskite QD-WLEDs with excellent color rendition has not been explored until now. In this work, the photometric optimization for maximizing LLE of CCT tunable perovskite QD-WLEDs, including down-conversion energy loss, was developed under the constraint of the designated color rendering properties. The CCT was tuned by changing power density ratios of light components while keeping chromaticity coordinates on the Planckian locus for CCT below 5000 K, or the daylight locus for CCT above 5000 K. That is, the chromaticity difference from the Planckian or daylight locus on the CIE 1960 uv chromaticity diagram (Duv) is equal to zero. The reason for this constraint (Duv = 0) is to avoid being out of the range of the chromaticity tolerance quadrangles of white-light sources [35, 36] due to the deviations of peak wavelengths (WLs) of LEDs and QDs. In our experiments, all-inorganic perovskite QDs, which roughly meet the requirement of the optimal peak WLs, have been prepared. Besides, the simulation shows that all-inorganic perovskite QD-WLED could achieve the expected colorimetric properties.
2. Experimental
The all-inorganic perovskite QDs explored in this work were synthesized by typical hot-injection methods with a minor modification based on the previous studies [37, 38].
2.1. Chemicals
Lead chloride (PbCl2, 99.999% trace metals basis), lead bromide (PbBr2, 99.999% trace metals basis), lead iodide (PbI2, 99.999% trace metals basis), and cesium carbonate (Cs2CO3, Reagent Plus, 99%) were purchased from Aladdin. Hexane (97%, Analytical grade) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Octadecene (ODE, technical grade, 90%), oleylamine (OLA, 90%), and oleic acid (OA, 90%) were purchased from Aladdin. All reagents were used as received without further purification.
2.2. Preparation of Cs-Oleate
Cs2CO3 (0.080 g) was loaded into a 100 ml, 3-neck flask along with ODE (30 ml) and oleic acid (2.5 ml), and the mixture was dried for 1 h at 120oC under vacuum. After degassing, the temperature of reaction solution was then raised to 150oC and kept under N2 atmosphere until it became clear (i.e., until all Cs2CO3 was dissolved).
2.3. Synthesis of CsPbX3 QDs
In a typical synthesis of CsPbX3 QDs, CsPb(Br1.4/l1.6) QDs, 0.066 g PbBr2 and 0.088 g PbI2 with 10 ml ODE were loaded into a 50 ml 3-neck flask and dried under vacuum for 1 h at 120°C. Then 1 ml oleylamine (OLA) and 1 ml OA were injected into the flask. After the mixture was completely dissolved, the reaction temperature was raised to 150°C under N2 and 1ml, the as-prepared Cs-oleate solution was swiftly injected into the flask. Five seconds later, the reaction mixture was cooled down to room temperature by an ice-water bath. The as-synthesized perovskite QDs could be purified via high-speed centrifugation (at 15 000 rpm for 15 min). After centrifugation, the supernatant was discarded and the particles were re-dissolved in hexane. The similar synthetic processes were adopted to prepare violet, blue, cyan, green, yellow, and red emitting QDs. The optical properties of perovskite QDs were monitored through the temporal evolution of UV-Visible and PL spectra. The absorption and PL spectra of QDs were collected at room temperature by ultraviolet spectrophotometer (Shimadzu, UV-3600) and fluorescence spectrophotometer (Shimadzu RF-5301PC), respectively. The PL QY of perovskite QDs was calculated by using the relative quantum efficiency of the QDs compared to the reference of Rhodamine 6G (RD6, QY = 95% dissolved in ethanol [38]- [39], using the following equation.
where QYQDs and QYRD6 are the PL QY for the QDs and the standard Rhodamine 6G, respectively; IQDs and IRD6 represent the integral PL intensity of QDs and the standard Rhodamine 6G at a specified wavelength; AQDs and ARD6 respectively show the absorption intensities at the same excitation wavelength; nQDs and nRD6 are the refractive indices of the solvents for dissolution. To reduce the re-absorption impact, the absorbance of the sample solutions was maintained less than 0.1 by dilution. The PLQY data are respectively shown in the PL spectra of the corresponding QDs.The as-obtained perovskite QDs are obviously composition-dependent and the PL properties can also be tuned by varying the anion element composition. When chloride ions were introduced into the reaction system, the CsPb(Cl1-xBrx)3 nanocrystals were prepared. The PL peak shifts to the higher energy direction as the chloride ion content is increased. In contrast, the iodine ions can shift the PL peak to the lower energy direction. According to the PL spectra shown in Figure. 1(a) (Fig. 1(a)), it can be clearly seen that the luminescence can be effectively tuned from 402 to 704 nm by introducing the chloride ions and iodine ions. Figure 1(a) also presents typical normalized PL spectra of perovskite QDs under 360 nm excitation wavelength, from which narrow full-widths at half-maximum (FWHMs) of perovskite QDs can be seen, indicating that all the as-obtained QDs possess a reasonably narrow size distribution. Seven typical color photographs of samples in solution under UV exposure are shown in Fig. 1(b). The TEM images of the yellow emitting perovskite QDs were shown in Fig. 2(c), which suggested that the perovskite QDs perform high monodispersity which agrees well with the narrow FWHM of PL spectra in Fig. 1(a). Figure 1(d) demonstrates that the size distribution of aforementioned QDs have an average size of 9.28 nm.
Fig. 1 Composition-dependent PL spectra of CsPbX3 QDs (a) and seven typical color photographs of samples in solution under UV exposure (violet, blue, cyan, green, yellow, orange, and red from left to right) (b), the TEM images and size distribution of the yellow emitting perovskite QDs are shown in (c) and (d) respectively.
3. Optimization of CCT Tunable QD-WLEDs
To analyze the possible properties of QD-WLED with blue chip and CsPbX3 QDs, the simulation program is developed according to the principle of additive color mixture and CIE method of measuring and specifying color rendering properties of light sources [40]. Considering a spectral power distribution (SPD) that contains emission spectrum from a blue chip, four color CsPbX3 QDs in the QD-WLED, the relative SPD of QD-WLED, SQD-WLED(λ), is given by,
where Sb, qb, λ0b and ∆λb refer to the relative SPD, proportion of the relative SPD, WL, FWHM, respectively, for a blue chip. The subscripts i = 1, 2, 3, and 4 refer to cyan, green, yellow and red CsPbX3 QDs, respectively. Si, qi, and λ0i refer to the relative SPD, proportion of the relative SPD, peak WL, respectively, for each perovskite QD. Twenty four color perovskite QDs (486 nm-704 nm) are used in optimization. The real emission spectra of these CsPbX3 QDs are shown in Fig. 2.For the blue chip, the chosen WL is varied between 450 nm and 470 nm, while the FWHM is varied between 25 nm and 35 nm. We employ Ohon’s model [41] of SPD for the blue chip. Because of three mixed constraints, 11-dimensional space will be reduced to 8-dimensional space for a particular CCT (Duv = 0) [42]. Unity quantum efficiency is adopted for the ideal case in this model. Thus, LLE, including down-conversion energy loss, can be calculated by [34],
where , and V(λ) is 1988 CIE photopic luminous efficiency function. In order to optimize spectra of the CCT tunable QD-WLEDs with excellent color rendering, we introduce an objective functionwhere (m, n) = (90, 82), (95, 93) and (96, 95), the subscripts j = 1, 2, 3, 4, 5, 6, 7 and 8 refer to 2700 K, 3000 K, 3500 K, 4000 K, 4500 K, 5000 K, 5700 K and 6500 K of CCTs (Duv ≤ 0.0054), respectively. Hence, the optimization problem is simplified to find the maximum of the objective functions (F). To solve the above optimization problem, we apply a fast Pareto genetic algorithm [43] due to its capability of exploring a vast set of solutions independently from the starting solution, its efficiency and scalability in addressing complex optimization problems, and its customizability to different kinds of optimization criteria4. Results and discussion
4.1 Simulation of CCT Tunable QD-WLEDs with two color perovskite QDs
The optimal peak WLs and FWHMs, as well as their photometric and colorimetric properties of CCT tunable QD-WLEDs with CRI ≥ 90 and CQS ≥ 82 at CCTs of 2700 K to 6500 K (Duv = 0) have been obtained by nonlinear program for maximizing F. The simulation results show that CCT tunable QD-WLEDs with CRI ≥ 90 and CQS ≥ 82 at CCTs of 2700 K to 6500 K (Duv = 0) consist of the blue chip (465 nm, 25 nm), green CsPb(Br0.7I0.3)3 (542 nm, 38 nm), and red CsPb(Br0.4I0.6)3 (617 nm, 59 nm) QDs.(As shown in Fig. 3)
Fig. 3 Measured UV-Vis absorption and normalized PL spectra of green CsPb(Br0.7I0.3)3, and red CsPb(Br0.4I0.6)3 QDs under 360 nm excitation wavelength. Inset is fluorescence photograph of corresponding QDs.
The photometric and colorimetric properties of the optimal CCT tunable QD-WLEDs is shown in Table 1. R(9-12) in the table is the average of the special CRI R9 to R12 of the four saturated colors (red, yellow, green and blue). The simulation results show that the optimal QD-WLEDs with two CsPbX3 QDs could realize CCT tunable white-lights with CRIs of 90-92, R(9-12)s of 66-73, CQSs of 82-86 and LLEs of 273-294 lm/W at CCTs of 2700 K to 6500 K (Duv = 0). Furthermore, their special CRIs of R13 and R15 corresponding to the colors of the skin on the face of European and Chinese women are also very high. Both R13 and R15 are especially important for interior lighting. The optimal SPDs of CCT tunable QD-WLEDs with the blue chip (465 nm, 25 nm), green CsPb(Br0.7I0.3)3 (542 nm, 38 nm), and red CsPb(Br0.4I0.6)3 (617 nm, 59 nm) QDs at CCTs of 2700 K to 6500 K are shown in Fig. 4.
Table 1. Simulated relative radiant flux (Φe%) of each color component, photometric and colorimetric properties of CCT tunable QD-WLEDs with the blue chip, green CsPb(Br0.7I0.3)3, and red CsPb(Br0.4I0.6)3 QDs for CRI ≥ 90 and CQS ≥ 82 at CCTs of 2700 K to 6500 K (Duv = 0).
Fig. 4 Optimal SPDs of CCT tunable QD-WLEDs with the blue chip, green CsPb(Br0.7I0.3)3, and red CsPb(Br0.4I0.6)3 QDs at CCTs of 2700 K to 6500 K (Duv = 0).
4.2 Simulation of CCT Tunable QD-WLED with three color perovskite QDs
Under conditions of CRI ≥ 95 and CQS ≥ 93, the simulation results show that CCT tunable QD-WLEDs at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054) consist of the blue chip (457 nm, 25 nm), green CsPbBr3 (516 nm, 21 nm), yellow CsPb(Br0.5I0.5)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs (As shown in Fig. 5).
Fig. 5 Measured UV-Vis absorption and normalized PL spectra of the green CsPbBr3, the yellow CsPb(Br0.5I0.6)3, and red CsPb(Br0.2I0.8)3 QDs under 360 nm excitation wavelength. Inset in each case is the digital photo of the emission under UV light for each sample.
The photometric and colorimetric properties of the optimal CCT tunable QD-WLEDs with three CsPbX3 QDs is shown in Table 2. The optimal QD-WLEDs with three CsPbX3 QDs could realize CCT tunable white-lights with CRIs of 95, R(9-12)s of 85-90, R13s of 91-95, R15s of 90-93,CQSs of 93 and LLEs of 250-260 lm/W at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054). The optimal SPDs of CCT tunable QD-WLEDs with the blue chip (457 nm, 25 nm), the green CsPbBr3 (516 nm, 21 nm), the yellow CsPb(Br0.5I0.5)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs at CCTs of 2700 K to 6500 K are shown in Fig. 6.
Table 2. Simulated relative radiant flux (Φe%) of each color component, photometric and colorimetric properties of CCT tunable QD-WLEDs with the blue chip, green CsPbBr3, the yellow CsPb(Br0.5I0.5)3, and red CsPb(Br0.2I0.8)3 QDs for CRI ≥ 95 and CQS ≥ 93 at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054).
Fig. 6 Optimal SPDs of CCT tunable QD-WLEDs with the blue chip, green CsPbBr3, yellow CsPb(Br0.5I0.6)3, and red CsPb(Br0.2I0.8)3 QDs at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054).
4.3 Simulation of CCT tunable QD-WLED with four color perovskite QDs
Under conditions of CRI ≥ 96 and CQS ≥ 95, the simulation results show that CCT tunable QD-WLEDs at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054) consist of the blue chip (448 nm, 25 nm), cyan CsPb(Cl0.1Br0.9)3 (501 nm, 34 nm), green CsPb(Br0.9I0.1)3 (526 nm, 27 nm), yellow CsPb(Br5I6)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs (As shown in Fig. 7).
Fig. 7 Measured UV-Vis absorption and normalized PL spectra of the cyan CsPb(Cl0.1Br0.9)3, green CsPb(Br0.9I0.1)3, yellow CsPb(Br0.5I0.5)3, and red CsPb(Br0.2I0.8)3 QDs under 360 nm excitation wavelength. Inset in each case is the digital photo of the emission under UV light for each sample.
The photometric and colorimetric properties of the optimal CCT tunable QD-WLEDs with four color CsPbX3 QDs are shown in Table 3. The optimal QD-WLEDs with four color CsPbX3 QDs could realize CCT tunable white-lights with CRIs of 96-97, R(9-12)s of 94-97, R13s of 90-93, R15s of 92-95, CQSs of 95-96 and LLEs of 243-254 lm/W at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054). The optimal SPDs of CCT tunable QD-WLEDs with the blue chip (448 nm, 25 nm), cyan CsPb(Cl0.1Br0.9)3 (501 nm, 34 nm), green CsPb(Br0.9I0.1)3 (526 nm, 27 nm), yellow CsPb(Br0.5I0.5)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs at CCTs of 2700 K to 6500 K are shown in Fig. 8.
Table 3. Simulated relative radiant flux (Φe%) of each color component, photometric and colorimetric properties of CCT tunable QD-WLEDs with the blue chip, cyan CsPb(Cl0.1Br0.9)3, green CsPb(Br0.9I0.1)3, yellow CsPb(Br0.5I0.5)3, and red CsPb(Br0.2I0.8)3 QDs for CRI ≥ 96 and CQS ≥ 95 at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054).
Fig. 8 Optimal SPDs of CCT tunable QD-WLEDs with the blue chip, cyan CsPb(Cl0.1Br0.9)3, green CsPb(Br0.9I0.1)3, yellow CsPb(Br0.5I0.5)3, and red CsPb(Br0.2I0.8)3 QDs at CCTs of 2700 K to 6500 K (Duv ≤ 0.0054).
For the real blue chip and CsPbX3 QDs, we chose that the radiant efficiency (ηeB) of a blue chip is 60%, the real PLQY values of CsPbX3 QDs are about 75%. The luminous efficacy (LE) can be estimated by
where . The LEs of QD-WLEDs with two, three, and four CsPbX3 QDs by excited blue chip at CCTs of 2700 K to 6500 K. are shown in Table 4. The results show that LEs of CCT tunable QD-WLEDs with the blue chip (465 nm, 25 nm), green CsPb(Br0.7I0.3)3 (542 nm, 38 nm), and red CsPb(Br0.4I0.6)3 QDs for CRI ≥ 90 and CQS ≥ 82 are expected to reach 133 to 137 lm/W, with the blue chip (457 nm, 25 nm), the green CsPbBr3 (516 nm, 21 nm), the yellow CsPb(Br0.5I0.5)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs for CRI ≥ 95 and CQS ≥ 93 are expected to reach 114 to 122 lm/W, and with the blue chip (448 nm, 25 nm), cyan CsPb(Cl0.1Br0.9)3 (501 nm, 34 nm), green CsPb(Br0.9I0.1)3 (526 nm, 27 nm), yellow CsPb(Br0.5I0.5)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs for CRI ≥ 95 and CQS ≥ 93 are expected to reach 114 to 122 lm/W at CCTs of 2700 K to 6500 K, assuming ηeB = 60% and PLQY = 75%.Table 4. Estimated LES (lm/W) of the QD-WLEDs with two, three, and four CsPbX3 QDs (PLQY = 75%) by excited blue chip (ηeB = 60%) at CCTs of 2700 K to 6500 K.
5. Conclusions
In this work, the spectral optimization for maximizing LLE of CCT tunable QD-WLEDs with two, three and four color perovskite QDs excited by the blue chip, including down-conversion energy loss, was investigated under the constraint of the designated ultrahigh color rendition. The results show that white lights with CRIs of 90-92, R(9-12)s of 66-73, CQSs of 82-86 and LLEs of 273-294 lm/W at CCTs of 2700 K to 6500 K could be realized by the QD-WLEDs using the blue chip (465 nm, 25 nm), green CsPb(Br0.7I0.3)3 (542 nm, 38 nm), and red CsPb(Br0.4I0.6)3 (617 nm, 59 nm), with CRIs of 95, R(9-12)s of 85-90, R13s of 91-95, R15s of 90-93, CQSs of 93 and LLEs of 250-260 lm/W by the QD-WLEDs using the blue chip (457 nm, 25 nm), green CsPbBr3 (516 nm, 21 nm), yellow CsPb(Br0.5I0.5)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs, as well as with CRIs of 96-97, R(9-12)s of 94-97, R13s of 90-93, R15s of 92-95, CQSs of 95-96 and LLEs of 243-254 lm/W by the QD-WLEDs using the blue chip (448 nm, 25 nm), cyan CsPb(Cl0.1Br0.9)3 (501 nm, 34 nm), green CsPb(Br0.9I0.1)3 (526 nm, 27 nm), yellow CsPb(Br0.5I0.5)3 (572 nm, 25 nm), and red CsPb(Br0.2I0.8)3 (636 nm, 42 nm) QDs at CCTs of 2700 K to 6500 K. The luminous efficacies of QD-WLEDs are expected to reach 114 to 122 lm/W at CCTs of 2700 K to 6500 K, if the radiant efficiency of a blue chip is 60% and the real PLQY values of perovskite QDs are 75%. These results demonstrate that perovskite QD-based LEDs are expected to be promising in next generation display and lighting technologies, also provide an important basis for the design and manufacture of CCT tunable QD-WLEDs with high properties.
Funding
National Natural Science Foundation of China (61675049, 61377046, 61177021, and 51575099), and Natural Science Foundation of Shanghai (15ZR1401700).
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