Cesium lead halide perovskite quantum dot-based

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cesium lead halide (CsPb(X)3) perovskite quantum dots ... Recently, all-inorganic perovskite QDs have attracted ... didate for photodetectors (Ramasamy et al.
J Nanopart Res (2017) 19:174 DOI 10.1007/s11051-017-3862-2

RESEARCH PAPER

Cesium lead halide perovskite quantum dot-based warm white light-emitting diodes with high color rendering index Ke Bi & Dan Wang & Peng Wang & Bin Duan & Tieqiang Zhang & Yinghui Wang & Hanzhuang Zhang & Yu Zhang

Received: 23 January 2017 / Accepted: 17 April 2017 # Springer Science+Business Media Dordrecht 2017

Abstract White light-emitting diodes (WLEDs) were fabricated by employing a combination of a commercial yellow emission Ce3+-doped Y3Al5O12 (YAG:Ce)based phosphor and all-inorganic perovskite quantum dots pumped with blue LED chip. Perovskite quantum dot solution was used as the color conversion layer with liquid-type structure. Red-emitting materials based on cesium lead halide (CsPb(X)3) perovskite quantum dots were introduced to generate WLEDs with high efficacy and high color rendering index through compensating the red emission of the YAG:Ce phosphor-based commercialized WLEDs. The experimental results suggested that the luminous efficiency and color rendering index of the as-prepared WLED device could reach up to 84.7 lm/W and 89, respectively. The characteristics of those devices including correlated color temperature (CCT), color rendering index (CRI), and color coordinates were observed under different forward currents. The as-fabricated warm WLEDs showed excellent color stability against the increasing current, while the color K. Bi : D. Wang (*) : P. Wang : B. Duan : T. Zhang : Y. Wang : H. Zhang Femtosecond Laser Laboratory, Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, People’s Republic of China e-mail: [email protected] Y. Zhang (*) State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun 130012, People’s Republic of China e-mail: [email protected]

coordinates shifted slightly from (0.3837, 0.3635) at 20 mA to (0.3772, 0.3592) at 120 mA and color temperature tuned from 3803 to 3953 K. Keywords Perovskite . Quantum dots . White lightemitting diodes

Introduction White light-emitting diodes (WLEDs) have played an important role in lighting source as a promising replacement for traditional lighting such as incandescent bulbs due to its advantages of low cost, high luminous efficacy, long lifetime, and low power consumption (Bai et al. 2014; Park et al. 2005; Sun et al. 2015; Nizamoglu et al. 2008; Sun et al. 2016). Currently, the most commonly used WLEDs are fabricated by combining blue-emitting LED chips with commercial yellow light-emitting Ce3+doped Y3Al5O12 (YAG:Ce) phosphors (Tsukamoto and Isobe 2009; Bachmann et al. 2009). Although this commercial WLEDs have the advantages of relatively high luminous efficacy and excellent color stability, the primary disadvantage is a relatively low color rendering index (CRI) owning to the insufficient red emission in the visible spectral region (Wang et al. 2012; Zhang et al. 2011; Sheu et al. 2003). Moreover, red-emitting component is of great importance for a fabrication of indoor warm white lighting source (Lin and Liu 2011; Xie and Hirosaki 2007). Therefore, the research efforts have been made to enhance red emission by incorporating red-emitting phosphor (Jang et al. 2008; Park et al.

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2012; Su et al. 2016; Chen et al. 2013). In the past few years, rare earth ions and red phosphor have been exploited and applied in WLED lighting filed. However, these (oxy)nitride compounds red phosphors with relatively low luminous efficiency and high production cost limiting its further development. For red-emitting quantum dots (QDs), the broader emission band (CuInS2 or ZnCuInS) of ∼110 nm can improve the CRI value of WLED, but decrease the luminous efficiency of device to the lower sight function. Hence, it is necessary to develop an efficient red-emitting QDs with high quantum efficiency, appropriate emission band, and low production cost for warm WLED (Beltran-Huarac et al. 2013; Zhong et al. 2015). Recently, all-inorganic perovskite QDs have attracted considerable attention because of high photoluminescence (PL) quantum yield (QY), narrow emission full width at half maximum (FWHM), and low production cost (Protesescu et al. 2015; Akkerman et al. 2015; Yakunin et al. 2015). All of those advantages make it become a new class of promising lighting materials and a potential candidate for photodetectors (Ramasamy et al. 2016), solar cells (Kulbak et al. 2016), and light-emitting diodes (Zhang et al. 2016; Song et al. 2015). In particular, this type of nanomaterial with high quantum yield (up to 50– 90%) makes it become a tremendous potential for the application of WLED lighting. Thus, red-emitting phosphor based on perovskite QDs was expected to generate warm WLEDs with high luminous efficacy and improve color rendering index through compensating the red emission of YAG:Ce-phosphor-based WLEDs. However, the PL performance of solid-state perovskite QDs is instability, mainly because of its photoluminescence quenching with increasing surface temperature which may be attributed to the aggregation effect and the fundamental properties of QD (Jang et al. 2010; Jang et al. 2013). Another considerable issue is the so-called coffee ring suffering from QD uniform distribution in the solidification process (Kshirsagar et al. 2011). To avoid the above issues, liquidtype QD was employed as an efficient color conversion layer in sealing packages. Kuo’s group (Sher et al. 2016) demonstrates that the liquid-type QD-WLEDs are highly efficient and reliable owning to the improved quantum efficiency and the lower surface temperature (below 50 °C). Therefore, WLEDs were fabricated by employing the red-emitting perovskite QD solution as color conversion layer integrated with as-prepared YAG:Ce-based LEDs. Liquid-type QD layers were used to maintain initial

optical properties of QDs and to enhance quantum efficiency of devices. The experimental results exhibited that this type of device has excellent color stability toward sustainable high currents and high luminous efficacy of above 84.7 lm/W and high CRI of 89 at correlated color temperature (CCT) of 3717 K. Moreover, warm, natural, and cool white light devices can be achieved simultaneously by increasing the QD concentration, which demonstrated that perovskite QDs are promising red phosphors for efficient LED lighting.

Experimental section Material Oleic acid (OA, 90%) and 1-octadecene (ODE, 90%) were purchased from Alfa Aesar. Oleylamine (OLA, across 80–90%), toluene, and chloroform were purchased from Aladdin. Cs2CO3, PbBr2, PbI2, UV glue, and polymethylmethacrylate (PMMA) were purchased from Sigma-Aldrich. All chemicals were used directly without further purification. Synthesis of CsPb(Br/I)3 quantum dots These all-inorganic perovskite quantum dots were synthesized by following the previous report (Chen et al. 2013). Firstly, the Cs-oleate precursor was prepared by adding Cs2CO3 (0.8 g) and OA (2.5 mL) into a 100-mL three-neck flask along with ODE (30 mL), and then, the mixture was dried for 2 h at 120 °C and heated to 150 °C under N2 until all Cs2CO3 was dissolved. In a typical synthesis of red-emitting CsPb(Br1.44I1.56) perovskite quantum dots, ODE (10 mL) with PbBr2 (0.066 g) and PbI2 (0.088 g) were loaded into a 50-mL three-neck flash, exhausted and aerated with few times, and then dried under vacuum for 2 h at 120 °C. Dried OLA (1 mL) and dried OA (1 mL) were injected under N2 at this temperature. After the mixtures were dissolved and became a clear solution, the reaction temperature was raised to 180 °C. And then, the as-prepared Csoleate solution (1 mL, heated to 100 °C before injection) was quickly injected. Five seconds later, the reaction mixture cooled down to room temperature by the icewater bath. After cooled down, the mixture was separated by a centrifugation for 10 min at 5000 rpm. And then, the precipitate was redispersed in toluene (3 mL) and kept centrifuging for 10 min at 12000 rpm. Then,

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the precipitate was discarded and the supernatant was reserved, because the latter accounts for higher PL QY. The perovskite QDs CsPb(Br1.74I1.26) of 583 nm and CsPb(Br1.26I1.74) of 630 nm were synthesized according to the above process, expecting the amount of original materials with PbBr2 (0.08 g) and PbI2 (0.0722 g), PbBr2 (0.058 g) and PbI2 (0.0998 g), respectively. By the way, PMMA matrix was prepared by a mixture of PMMA powder and chloroform (15% by weight), mixing with perovskite quantum dot solution to form films as red-emitting layers. Device fabrication Two types of samples were prepared at the same time: one is the sample of QDs phosphor as color-conversion layer, and the other is the sample of QDs solution as color-conversion layer. Figure 1a illustrates the process flow of the sample of QDs phosphor as color-conversion layer. We use YAG and PMMA to drop into the blue chip to fabricate white LED. After the mixtures are cured, anther mixtures such as red emission layer which contains QD and PMMA were dropped on its surface. And then, the QDs phosphor-based warm white LED was already prepared. Figure 1a shows the fabricated structure and process flow of QDs solution-based warm white LED. The process is as follows: First, the glass substrate was divided into four thin stripes, including the size of 1.3 cm × 0.3 cm for two, the size of 1.6 cm × 0.3 cm for one, and the size of 1 cm × 0.3 cm for one. Second, take these thin stripe combinations with the right size proportion by using AB-based glue. Third, the middle gap with the size of 1 cm × 1 cm was left to inject QD solution and sealed with the AB-based glue to complete the process of Fig. 1 The structure and process flow of QDs phosphor-based warm white LED (a) and QDs solution-based warm white LED (b)

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the glass box. A small amount of YAG:Ce phosphor was added into UV glue and shocked for a few minutes. Then, the mixture was coated on blue LED chip quickly. After ultraviolet baked in the oven for 1 min, YAG:Cebased LED was obtained. Finally, the device fabrication consisting of the as-prepared YAG:Ce-based LED and encapsulation box was finished. Characterization Absorption spectra were measured on a Shimadzu 3600 UV-Vis spectrophotometer. The photoluminescence (PL) spectra were taken on a Cary Eclipse Spectrofluorimeter. Transmission electron microscopy (TEM) images were achieved on a FEI Tecnai F20 microscope. The PL QYs were calculated by an integrating sphere and its inner face incorporate with BENFLEC. The electroluminescence (EL) spectra of the WLEDs device were measured by a Zolix Omniλ300 monochromator/ spectrograph.

Results and discussion The red luminescent all-inorganic perovskite QDs were synthesized through a hot injection method as previously reported. Figure 1a shows the UV visible absorption spectra of red emission QD-based all-inorganic CsPb(X)3 perovskites. The size of red emission perovskite QDs was also estimated from illustration of the TEM images in the inset of Fig. 2a. Impressively, with the decrease of the ratio of bromine ion and iodine ion in the perovskite chemical composition, the lattice parameters and of perovskite become larger, and the optical

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Fig. 2 a The UV absorption and TEM image (inset) of red emission CsPb(Br1.44I1.56) perovskite-based QDs. The inset of TEM image is the red-emitting quantum dot solution. b Emission intensity of blue LED chip (blue solid line) and red emission

wavelength perovskite QDs based on three different halide ratios (from left to right: CsPb(Br1.74I1.26) of 583 nm, CsPb(Br1.44I1.56) of 607 nm, CsPb(Br1.26I1.74) of 630 nm) excited at 450 nm

band gap of perovskite become shorter, which further leads to the red shift of the emission wavelength of the perovskite quantum dots (Cui et al. 2015; Wang et al. 2016; Gil-Escrig et al. 2015). In order to obtain suitable red emission wavelength for fabricating high-quality warm WLED, three different red emission wavelengths were measured as shown in Fig. 2b. Their corresponding emission peak wavelengths were approximately centered at 583 nm (PL QY ∼69%), 607 nm (PL QY ∼63%), and 630 nm (PL QY ∼58%), with narrow FWHM of 27, 30, and 35 nm, respectively. The QY of the phosphors are almost 73% based on these perovskite quantum dots. Impressively, a narrow band emission is

more beneficial for fabrication of the high efficacy warm WLED. Besides, the EL emission spectra of commercial blue LED chip were also measured as shown in Fig. 2b. Their emission spectra cover the whole visible spectral region, which makes it possible to achieve warm WLED with high color rendering properties. Thus, the red emissive perovskite QDs can be a red-emitting material to compensate the lack of red color regions. It is generally known that red-component is of great importance to generate warm WLED device with high color rendering properties though compensating red color region. As shown in Fig. 3a, the EL emission spectra of WLED devices without red-component (S1) and with red-

Fig. 3 a The EL emission spectra of YAG:Ce-based WLED (S1); the EL emission spectra of three warm WLED devices combined with YAG:Ce based WLED and three different red emission wavelength perovskite QDs of 582 nm (S2), 607 nm (S3), and

630 nm (S4), respectively. b The luminous efficiency (red line) and CRI (black line) value as a function of emission wavelength for YAG:Ce-based WLED and three warm WLED devices

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component (S2, S3, S4) were discussed. And inset of Fig. 3a is the schematic diagram of WLED device. From Fig. 3b, suitable red emission wavelength can make a contribution to the improved CRI value and the relatively high luminous efficiency. Furthermore, the luminous efficiency and CRI value of warm WLED device had a large shift by changing the red emission peak. The EL emission spectra of three warm WLED devices based on three different red emission peaks are depicted in Fig. 3a of S2, S3, and S4. And the characteristics of the color temperature of these warm WLED devices are all around 3837 K as a basic condition for comparison. From Fig. 3b, with increasing the red emission wavelength, the luminous efficiency decreases while the CRI value rises. It is theoretically reasonable that the improved CRI value makes up red or red-deep color areas. And the luminous efficiency decrease is probably attributed to the relatively lower quantum efficiency and broader FWMH within reddeep emission region. Obviously, we can obtain a warm WLED with a relatively high efficacy and good CRI value according to the above experimental results, and finally, the warm WLED based on red emission CsPb(Br1.44I1.56) perovskite QDs was selected. Red emission perovskite QDs were introduced in order to not only improve color rendering index but also obtain various correlated color temperatures. The EL emission spectra of these WLED devices were observed as depicted in Fig. 4a. With increasing QD concentration in solutions, we can observe tunable color temperature along with the Planckian locus from 2854 to 11,068 K and their corresponding color coordinates on CIE1931 chromaticity diagram as shown in Fig. 4b. From Fig. 4a,

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with increasing QD concentration (from 0.5 to 2.7 g/mL) in solution, the blue region decreases while the red region increases obviously, which exhibit the absorbed effectively blue light emitted from blue LED chip. As shown in Fig. 4b, the color coordinates of all WLED with various CCT are very close to the Planckian locus, which means that we can obtain a standard and comfortable color temperature for those types of warm WLED to protect human eyesight. Furthermore, the photographs of three kinds of WLED devices emitting cool white, natural white, and warm white light were primarily demonstrated as illustration in the inset of Fig. 4b. Their corresponding color temperatures are 3817 K (warm white light), 5227 K (natural white light), and 7851 K (cool white light), respectively. And the Commission Internationale de L'Eclairage (CIE) coordinates of natural white LED device of (0.3349, 0.3284) is obviously close to a standard pure white light color coordinates of (0.3333, 0.3333). Table 1 summarizes that the parameters of these devices make it possible to fabricate high-quality warm WLED combined with red emission perovskite QDs and a commercialized WLED though compensating its red color region to meet the demands of high-quality warm WLED for indoor lighting. Moreover, the results exhibit that the obtained warm WLED device has a relatively great CRI value (89) and a relatively high luminous efficiency (84.7 lm/W). And it is worth to be noted that its advantages of low cost, easy fabrication, and better optical properties make it a promising candidate for indoor light application. Optical stability is always a huge issue in terms of currently perovskite QD-LED-based lighting application, including color rendering index (CRI), correlated

Fig. 4 EL emission spectra (a) and CIE color coordinates of these WLED devices; insets of b are three photographs as illustrations with different color temperature WLED devices

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Table 1 The CIE coordinates (x, y), correlated color temperature (Tc), and color rendering index (Ra) of as-fabricated WLED device samples operated at 20 mA; the ppm (g/mL) is the red emission QD concentration in each sample devices

Device

LE (lm/W)

x

y

Tc (K)

Ra

ppm (g/mL)

S1

84.4

0.2829

0.2695

11,068

76

0.5

S2

84.6

0.3018

0.2893

7851

87

0.9

S3

84.5

0.3200

0.3102

6209

87

1.1

S4

84.8

0.3349

0.3284

5227

88

1.4

S5

84.5

0.3618

0.3512

4398

89

1.6

S6

84.7

0.3835

0.3726

3717

89

1.9

S7

84.6

0.4203

0.3874

3159

87

2.3

S8

84.1

0.4459

0.4041

2854

86

2.7

color temperature (CCT), and CIE color coordinates, which were regarded as the crucial factor for evaluating the performance of those devices. Thus, to illustrate the superior characterizations of liquid-type QD-LED,

phosphor-converter QD-LED mixed with red emission perovskite QDs and PMMA matrix was fabricated for a comparison. Figure 5a shows that the red emission region of phosphor-converter QD-LED has obviously

Fig. 5 The EL emission spectra of the warm WLED device based on red-emitting perovskite QDs in phosphor (a) and in solution (b) as color conversion layer. The inset: the PL emission spectra toward different temperatures for QD solution. c The EL spectra

of the warm WLED device under increasing forward current bias. d The CIE color coordinates of the warm WLED device under each current bias from 20 to 120 mA, respectively (Tc = CCT)

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decreased while that of liquid-type QD-LED (Fig. 5b) has changed slightly with the time increased under a high current bias of 120 mA, which is mainly ascribed to the optical gap decreases with increasing surface temperature (Yu et al. 2011; Huang and Lambrecht 2013). And the inset of Fig. 5a is the function of PL emission intensity and the increasing measurement temperature. As illustrated in Fig. 5c, the EL emission spectra of the as-prepared warm WLED device were measured under a forward bias current from 20 to 120 mA, respectively. From Fig. 5d, the corresponding emission peak positions are shifted slightly from low- to high-current bias, indicating that this type of warm WLED device exhibited excellent optical stability under increasing high current conditions. As Table 2 summarizes, when the forward bias currents were injected from 20 to 120 mA, the characteristics of the warm WLED device that has no obvious change are as follows: the CIE color coordinates shifted slightly from (0.3837, 0.3635) at 20 mA to (0.3772, 0.3592) at 120 mA and color temperature tuned from 3803 to 3953 K (Fig. 5d) as well as reserved a high CRI of 89, which demonstrates that it has excellent color stability, especially the high CRI value which is comparable to those obtained with other double or tricolor phosphor-based WLEDs. The color coordinates of warm WLED device have a slight shift which can be attributed to the LED device surface temperature increases in emission energy owing to that the device was heated during prolonged operation (Yu et al. 2011). The excellent optical performance perhaps attributes to the liquid-type QD structure, which exhibits that the relatively low surface temperature can reduce thermal quenching and the liquid-type form also can preserve the PL quantum efficiency of the raw QD to improve the luminous efficiency of WLED device (Sher et al. 2016). The improved optical properties make it a Table 2 The CIE coordinates (x, y), correlated color temperature (Tc), and color rendering index (Ra) of the warm WLED device sample at a forward bias current from 20 to 120 mA Current (mA)

x

y

Tc (K)

Ra

20

0.3837

0.3635

3803

89

40

0.3823

0.3630

3837

89

60

0.3819

0.3624

3843

89

80

0.3799

0.3613

3893

89

100

0.3783

0.3603

3930

89

120

0.3772

0.3592

3953

89

potential competitor as a red emission material for warm WLED application with high luminous efficiency.

Conclusion In summary, we report the potential of all-inorganic perovskite-based QDs as a novel red-emitting material to fabricate warm WLED for indoor lighting applications. The WLED device was generated by a combination of yellow emission YAG:Ce phosphor and red emission perovskite QDs pumped with commercial blue LED chip. The red-emitting perovskite QDs were utilized to integrate with the red-deficient commercialized WLED in order to obtain warm WLED with high luminous efficiency and improved color rendering properties though compensating its red emission in the entire visible spectral region. The liquid-type QD as color conversion layer not only maintained the initial quantum efficiency but also provided a comfortable color temperature for human eyesight protection. Moreover, the adjustable color temperature from 2853 to 11,068 K of very close to the Planckian locus can be easily achieved though increasing the QD concentration in solution. It is worth to be noted the generated warm WLED with a high CRI of 89 and high luminous efficiency of above 84.7 lm/W under high currents bias, which make them suitable candidates for indoor light technology. Furthermore, we believe that red-emitting all-inorganic CsPb(X)3perovskite QDs as a novel emission material will have a bright future for lighting application. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51502109, 21573094, 11274142, and 11474131), the Open Project of State Key Laboratory of Superhard Materials (Jilin University), the National Found for Fostering Talents of Basic Science (No. J1103202), and the Chinese Scholarship Council for providing financial support during visiting University of California at Irvine. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.

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