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Synthesis of CsPbBr and Transformation into CsPbBr Crystals for White Light Emission with High CRI and Tunable CCT Gopi Chandra Adhikari, Saroj Thapa, Hongyang Zhu, Alexei Grigoriev, and Peifen Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03369 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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The Journal of Physical Chemistry

Synthesis of CsPbBr3 and Transformation into Cs4PbBr6 Crystals for White Light Emission with High CRI and Tunable CCT Gopi C. Adhikari, Saroj Thapa, Hongyang Zhu, Alexei Grigoriev, and Peifen Zhu* Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, Oklahoma, 74104, United States

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ABSTRACT: The saponification approach was employed to synthesize all-inorganic halide perovskites at an ambient atmosphere. We report the conversion from CsPbBr3 to lead-depleted Cs4PbBr6 crystals by varying the amount of Cs-oleate precursor at room temperature synthesis. This transformation drastically changes the morphology as well as the structure and hence the optical properties of the perovskites. The cubic CsPbBr3 nanoplatelets with a strong blue emission (462 nm) attenuate a green (529 nm) emission when the material crystallizes in the rhombohedral phase (Cs4PbBr6), which demonstrates the intrinsic luminescence nature of the Cs4PbBr6 crystals. In addition, we have proposed to combine these two compounds with the yellow and red emitting perovskites to generate the white light emission. The correlated color temperature is tuned from 2480 K to 9134 K and the color rendering index (CRI) is maximized up to 96, the highest CRI yet known for this type of materials. These characteristics demonstrate the high potential to replace conventional phosphors in lighting devices.


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INTRODUCTION The advances in the III-nitride light emitting diodes (LEDs) has led to the development of white LEDs.1-8 The commercialized white LEDs are typically obtained by depositing a layer of YAG: Ce3+ yellow phosphors on the blue LED chips. However, those white LEDs have a low color rendering index (CRI) and a high correlated color temperature (CCT) due to a lack of the red component.9 CRI is a quantitative measurement of ability of a light source to reveal the colors of objects realistically or naturally compared to the reference source or daylight (maximum CRI ~ 100). The high CCT shows cool color. Thus, the light sources with low CRI and high CCT are undesirable for indoor lighting applications. In addition, the use of rare-earth metals causes the initial cost of these LEDs to increase substantially.4, 9-10 Therefore, the choice of naturally abundant materials serving as a photon-down converter to get white light emission with high CRI and tunable CCT is important in designing low-cost and efficient white LEDs. In this sense, the metal halide perovskites are promising candidates to replace ordinary phosphors for future solid-state lighting technology. Metal halide perovskites have drawn tremendous attention in the field of modern optoelectronic devices because of their superior photophysical properties.11-30 The general formula of the perovskites is AnBXn+2, where the value of ‘n’ determines the dimension of perovskite in accordance with the connectivity of [BX6]4- octahedra. At n = 1, ABX3 perovskites with a threedimensional network of corner-shared [BX6]4- octahedra are formed and for n = 4, the zerodimensional A4BX6 perovskites are obtained in which the isolated octahedra are bridged by A+ cations. Recently, due to the high efficiency and simple synthesis process, CsPbBr3 perovskites have attracted tremendous attention in the research community; however, the challenges remain regarding the stability and toxicity of lead in these compounds. Specifically, Cs4PbBr6 perovskite



manifests promising optoelectronic properties such as strong and narrow photoluminescence (PL), enhanced thermal stability, and large exciton binding energy (favorable for LEDs applications) that are distinct from the CsPbBr3 perovskite.31-33 These remarkable properties of Cs4PbBr6 perovskites will lead to potential applications in various optoelectronic devices, preferably white LEDs. Despite the superior properties of Cs4PbBr6 reported recently,31 the origin of the photophysical properties yet to be fully studied and understood. In addition, this perovskite is less explored than other similar materials due to the complexity of the preparation process.34-35 It was reported that the defect states within the large bandgap of Cs4PbBr6 (Eg > 3.6 eV) resulted in green light emission.36 31-32, 37-38 Zhai et al. and Akkerman et al. have attributed the green emission to the embedded CsPbBr3 in Cs4PbBr6 matrix.31, 35-36 However, Zhong et al. have reported that the green emission was coming from Cs4PbBr6 crystals itself.39 In this work, we report on the controllable synthesis of CsPbBr3 nanoplatelets and leaddepleted hexagonal shaped Cs4PbBr6 crystals by employing a saponification technique at room temperature.40 By increasing the amount of Cs-oleate, cubic CsPbBr3 nanocrystals gradually transform to rhombohedral Cs4PbBr6 crystals, which leads to tunable blue-green light emission. The CsPbBr3 nanoplatelets obtained at room temperature emit blue light. This indicates that the green emission can be attributed to the Cs4PbBr6 crystals. Our finding provides strong evidence that Cs4PbBr6 is a superior material for optoelectronic device applications. Moreover, the ratio of blue to green emission can be tuned by adjusting the amount of Cs-oleate used in the synthesis process, which provides a cost-effective approach for tuning CCT of white emission while retaining a good CRI. We have demonstrated the tuning of CCT of white light emission by combining the appropriate fractions of these two crystals with yellow [MAPb(Br0.7I0.3)3, MACH3NH3] and red [MAPb(Br0.5I0.5)3] emitting nanocrystals. The wide-spanning range of tuning


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from warm to cool white emission (2480 K - 9134 K) with CRI up to 96 is demonstrated in this work. To the best of our knowledge, this is the highest value of CRI yet reported for this type of materials.5, 41-42

EXPERIMENTAL METHODS Using the saponification approach reported earlier by our group,40 we synthesized CsPbBr3 nanocrystals at room temperature and then transformed the material into hexagonal Cs4PbBr6 crystals by increasing the amount of the Cs-oleate precursor. Specifically, the amount of the Csoleate precursor was increased gradually (from 0.04 mL) until Cs4PbBr6 crystals were obtained at 0.4 mL precursor concentration. All experiments were performed in an ambient atmosphere. We have synthesized [MAPb(Br0.7I0.3)3] and [MAPb(Br0.5I0.5)3] by using a synthesis technique developed and described by our group earlier.43 X-ray diffraction (XRD) measurements were carried out by Rigaku Smart Lab system with Cu Kα1 radiation (λ =1.54 Aͦ) coming from a fixed anode operated at 40 kV and 44 mA. Transmission electron microscopy (TEM) studies were performed using a Hitachi H-7000 TEM at 75 kV. The absorption and PL spectra were measured using A VARIAN Carry 50 Scan UVSpectrophotometer, and A Spectro- Fluorophotometer (Shimadzu, RF-6000) with a xenon lamp as an excitation source, respectively. The photoluminescence quantum yield (PLQY) of as grown samples were measured using a QE-pro spectrometer (QEP02037, Ocean Optics) with an integrated sphere (819C-SF-6, Newport) excited at a wavelength of 405 nm using a laser source. The approximate error of this PLQY measurement is within the limit of ± 2 %. Four different emission spectra (blue, green, yellow, and red) were combined and the spectral data were analyzed to yield the CRI, CCT, and CIE coordinates.



RESULTS AND DISCUSSION Here, we used the saponification process for the structural and compositional control of the cesium lead bromide nanocrystals at room temperature. This method enables the successful transformation

Figure 1. (a) The XRD scans by varying the amount of Cs-oleate precursor, exhibits the conversion from cubic CsPbBr3 to rhombohedral Cs4PbBr6. (b) and (c) are primitive cells of cubic structure of CsPbBr3 and rhombohedral structure of Cs4PbBr6 respectively. of CsPbBr3 nanocrystals to Cs4PbBr6 crystals via increasing the amount of Cs-oleate precursor at an ambient atmosphere. The saponification process of CsPbBr3 nanoplatelets growth is under a Pb2+ rich environment with a low amount of Cs+. By gradually increasing the amount of Cs-oleate precursor (from 0.04 to 0.4 mL), the product changes from cubic CsPbBr3 to rhombohedral Cs4PbBr6, which is confirmed by XRD analysis shown in Figure 1a. At 0.04 mL of Cs-oleate precursor, the cubic structure of CsPbBr3 was produced with diffraction peaks at about 14.9°, 21.3°, 26.4°, 30°, 33.8°, 37.5°, and 44.2°, which correspond to the crystal planes in Pm3̅m (ICSD 29073) space group of (100), (110), (111), (200), (210), (211), and (300), respectively. However, 6 ACS Paragon Plus Environment

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after increasing the Cs-oleate precursor concentration to 0.4 mL, the obtained product shows diffraction peaks at about 12.6°, 12.9°, 20.1°, 22.4°, 25.6° and 30.5° corresponding to diffraction from (110), (113), (300), (024), and (223) planes of a rhombohedral phase (R3̅c (ICSD 73-2478) space group) of Cs4PbBr6 . The structural models of cubic CsPbBr3 and rhombohedral Cs4PbBr6 are shown in Figure 1b, c. At 0.04 mL of Cs-oleate precursor, the networks of thin nanoplatelets of CsPbBr3 with the narrow particle shape and size (of dimensions 39.1 ± 6.8 nm × 1.9 ± 0.3 nm) have been synthesized (Figure 2a). When the amount of the Cs-oleate precursor is increased gradually, the nanoplatelets networks become less organized (Figure 2b-d), thereby altering the size of the nanoplatelets from

Figure 2. The TEM images of as-grown samples with the variation of amount of Cs-oleate precursor as: (a) 0.04 mL, (b) 0.06 mL, (c) 0.1 mL, (d) 0.2 mL, (e) 0.3 mL and (f) 0.4 mL. 35.0 ± 5.3 nm × 2.1 ± 0.2 nm to 31.4 ± 5.7 nm × 4.2 ± 0.4 nm and then to 28.43 ± 1.3 nm × 4.66 ± 0.4 nm respectively. Upon increasing the precursor concentration further, some hexagonal crystals begin to show up (Figure 2e) with the crystal size of 190.1 ± 17.7 nm × 47.1 ± 2.1 nm. Eventually, a uniform network of hexagonal Cs4PbBr6 crystals with a size of 110.5 ± 15.1 nm × 7 ACS Paragon Plus Environment


110.5 ± 15.1 nm has been obtained (Figure 2f) when the concentration of precursor reaches 0.4 mL. The optical properties of as-grown samples were investigated by photoluminescence (Figure 3). All the emission spectra are collected at a fixed excitation wavelength of 365 nm. A single intense blue emission peak (~ 462 nm) is observed from the nanocrystals synthesized at the lowest

Figure 3. The plot of PL emission of as-produced samples with different amount of Cs-oleate precursor. For low amount of Cs-oleate (0.04 mL) blue peak appears due to CsPbBr3, whereas, for 0.4 mL, peak is shifted to the green region due to Cs4PbBr6. concentration of the Cs-oleate precursor (0.04 mL) with a PLQY of 88 %. This peak is due to the cubic CsPbBr3 nanocrystals alone, which is confirmed by the XRD scan (Figure 1a). When we gradually increased the amount of the precursor between 0.04 mL and 0.2 mL, both blue (~ 462 nm) and green (~ 529 nm) emission peaks were observed. The integrated intensity of the green peak gradually increases with the concentration of the Cs-oleate precursor between 0.06 mL and


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Table 1. The percentage (%) of integrated intensity of blue and green color emitted by the samples with the variation of Cs-oleate precursor. Cs-oleate

Blue Color

Green Color

(mL)

(%)

(%)

0.04

100

0

0.06

98

2

0.1

86

14

0.2

19

81

0.3

0

100

0.4

0

100

0.2 mL. The increase in the intensity of the green emission is followed by the concomitant decrease in the intensity of the blue emission. For instance, at 0.2 mL precursor concentration, the integrated intensity of the green emission exceeds the integrated intensity of the blue emission by more than 50 %. The relative integrated intensities are summarized in Table 1. The different color emission peaks are due to the mixture of CsPbBr3 and Cs4PbBr6 crystal phases which is consistent with XRD patterns. The blue peak completely disappears from emission spectra at the precursor concentration exceeding 0.3 mL and the obtained crystals show a strong green emission with PLQY of 49 %. This can be attributed to the formation of pure rhombohedral and hexagonal Cs4PbBr6 crystals (Figure 1a, 2f). These characteristics affirm the intrinsic luminescence nature of the Cs4PbBr6 crystals. Thus, in our experiment, the blue emission from pure CsPbBr3 strongly suggests that the emission in the green region is due to Cs4PbBr6 particles alone, unlike the emission reported due to the embedded CsPbBr3 impurities in Cs4PbBr6.31, 35-36 If the traces of CsPbBr3 nanoparticles were incorporated (not observed in XRD) in the matrix of as-synthesized 9 ACS Paragon Plus Environment


Cs4PbBr6 in our study, we expect the emission to be different from the green light emission observed for Cs4PbBr6. This is because even few traces of CsPbBr3 embedded in Cs4PbBr6 matrix (having a high value of PLQY)44 is sufficient to produce the intense emission at its desired wavelength (blue). Thus, we should conclude that the emission from the transformed product in the green region is due to Cs4PbBr6 instead of minor CsPbBr3 impurities. The obtained luminescence from Cs4PbBr6 may be due to the halogen-related defect states within the bandgap, which act as radiative recombination centers for the excitons trap.45 Optical behaviors of the synthesized materials were further investigated by UV-visible absorption and excitation spectra. These studies indicate that as-grown CsPbBr3 and Cs4PbBr6

Figure 4. The absorption and excitation spectra along with corresponding emission spectrum of: (a) CsPbBr3 and (b) Cs4PbBr6. Insets show digital images of thin films under 400 nm UV lamp illumination. crystals exhibit different absorption peaks with different shapes of excitation spectra shown in Figure 4. The UV-visible absorption peak for CsPbBr3 is at ~ 457 nm and it is shifted to the blue region to ~ 328 nm in Cs4PbBr6. This implies that the transformed crystals have a wider bandgap. We approximated the bandgap using the corresponding peak of the absorption spectra where the estimated value corresponds to ~ 2.71 eV and ~ 3.78 eV for CsPbBr3 and Cs4PbBr6, respectively. 10 ACS Paragon Plus Environment

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These absorbance characteristics are consistent with previous experimental and theoretical studies.35, 45-51 CsPbBr3 nanocrystal emission has a full width at half maximum (FWHM) of ~ 18 nm and that of Cs4PbBr6 crystals is ~ 16 nm. This narrow FWHM indicates that the produced crystals have narrow size distributions and generates a consistent emission with high color purity.43, 52 The insets of the thin films were used to demonstrate the different luminescence nature of CsPbBr3 and Cs4PbBr6 and for making a comparative analysis of stability between these two different crystals. We observed that the Cs4PbBr6 thin film exhibits its luminescent behavior even after 30 days (without much loss in intensity) when placed in an ambient atmosphere, however, the CsPbBr3 thin films significantly loses its emitting nature, provided the same reaction conditions. Moreover, the observed normalized excitation spectra (Figure 4) are broadband spectra starting

Figure 5. Emissions from the combination of four different colors emitting nanocrystals (having different intensity) with tunable CCT. from UV region and continuing into the visible range, which underscores the potential practical use of these compounds in a wide range of wavelengths. The quantization of energy levels at the 11 ACS Paragon Plus Environment


bottom of the conduction band leads to a broadening of the bandgap. The extension of excitation from UV to visible and the emission in the significant portion of the visible region suggests that these compounds can be used as photon down converters in conjunction with III-nitride based UV/blue LEDs in order to achieve white light emission. Thus, these materials have a high potential to be used as versatile light sources for white LED applications. In addition, we have estimated the optical properties of white light emission by the combination of these two blue and green light emitting crystals with yellow [MAPb(Br0.7I0.3)3, λpeak ~ 562 nm, Table 2. The CCT, CRI and CIE coordinates (x, y) of as fabricated samples. CCT (K)

CRI

x

y

2480

96

0.4642

0.3901

3047

91

0.4302

0.3956

3363

95

0.4017

0.3660

3972

90

0.3834

0.3837

4964

92

0.3462

0.3534

5372

93

0.3353

0.3385

7396

92

0.2958

0.3338

9134

90

0.2801

0.3080

side length of 9.8 ± 0.6 nm] and red [MAPb(Br0.5I0.5)3, λpeak ~ 630 nm, side length of 13.9 ± 0.7 nm] emitting rectangular nanocrystals. We have used the emission spectra of these different colors to determine CCT and CRI. Then, we tuned the CCT (2480 K to 9134 K) of white light by choosing the appropriate fraction of these perovskites from warm to cool, shown in Figure 5. These spectra indicate that the relatively low intensity of blue and green color compared to that of yellow and 12 ACS Paragon Plus Environment

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red color is suitable for a warm color. The intensity of blue and green should be stronger in order to achieve a cool color. The calculations of CCT and CRI along with corresponding CIE coordinates (x, y) are shown in Table 2. We found the CRI as high as 96. To the best of our knowledge, this value of CRI is the highest value yet known for this type of compounds.5, 41-42 The selection of four different colors instead of the usual three colors provide the continuous broadband emission spectrum, which is a possible reason for having high CRI and desirable CCT values. The

Figure 6. CIE Chromaticity showing tuning CCT from warm to cool white color. The black curve passing through the center of the CIE diagram is the Planckian locus, which is a path followed by an incandescent blackbody radiator in a color space with a change in its temperature. CIE color coordinates diagram is shown in Figure 6. The black curve passing through the center of the CIE diagram is a Planckian locus, which is a path followed by an incandescent blackbody radiator in a color space with a change in its temperature. The color coordinates defined for any light source when lies in the proximity of this curve or on the curve itself, are considered to be 13 ACS Paragon Plus Environment


capable of emitting the light comfortable for the human eyesight. The tunable color temperature of white light emission we obtained with CIE coordinates very close the Planckian locus is desirable for general illuminations. Furthermore, the CIE coordinate of the neutral white light is (0.3353, 0.3385), which is very close to the standard neutral white light (0.3333, 0.3333). Therefore, low cost, stability, and scalability of fabrication, and the excellent optical properties make these materials promising for LED applications as replacement materials of conventional phosphors.

CONCLUSIONS Using an inexpensive and simple saponification synthesis method, we showed that cubic CsPbBr3 nanocrystals (PLQY ~ 88 %) change their crystal structure to a more stable rhombohedral Cs4PbBr6 structure (PLQY ~ 49 %) at the room temperature. An added benefit of this phase transition is a reduction in overall lead content in the material. Specifically, the complete phase transformation was done using a novel method that involved the control of the amount of Cs-oleate precursor at an ambient atmosphere. The PL spectra confirm the intrinsic blue and green luminescence of CsPbBr3 and Cs4PbBr6 respectively. The luminescence observed for Cs4PbBr6 is likely due to the radiative recombination of excitons in the presence of halogen-related defect states within the bandgap. Finally, we demonstrated the tunability of CCT of white light from 2480 K to 9134 K by combining these two compounds with the yellow and red perovskite nanocrystals, which led to the CRI as high as 96, the highest CRI yet reported for this type of materials. These properties indicate good prospects for these materials to be used as a replacement of conventional phosphors in the lighting devices.

AUTHOR INFORMATION 14 ACS Paragon Plus Environment

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Corresponding Author *Email:

[email protected]. Phone: +1 (918) 631-5125

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge for the financial support by The University of Tulsa through a Startup Fund. The authors also would like to acknowledge Dr. Parameswar Harikumar’s help with absorption measurements.

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