Ligand-Induced Tunable Dual-Color Emission Based on Lead Halide

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Ligand-Induced Tunable Dual-Color Emission Based on Lead Halide Perovskites for White Light-Emitting Diodes Yifei Yue, Dongxia Zhu, Ning Zhang, Guangshan Zhu, and Zhong-Min Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01059 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Ligand-Induced Tunable Dual-Color Emission Based on Lead Halide Perovskites for White Light-Emitting Diodes Yifei Yue†, Dongxia Zhu*,†, Ning Zhang*,†, Guangshan Zhu,† Zhongmin Su†,‡

Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Department of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, Jilin Province 130024, P. R. China. †

‡School

of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, 130022, P. R. China. ABSTRACT: Cesium lead halide perovskites (CsPbX3, X=Cl, Br and I) have emerged as an important class of color-tunable light-emitting materials in the past four years. However, single CsPbX3 nanostructures with dual-color emission remain scarce. Here, we demonstrate dual-color emission from lead halide perovskite nanowires induced by the surface ligands, i.e., oligomeric methoxypolyethylene glycol (MEOPEG). In addition to the characteristic emission from the host lattice, an unprecedented emission from the expanded band gap caused by MEOPEG is observed. The ratio of the two emission intensities can be easily adjusted by changing the concentration of the surface ligands. Moreover, the band gaps of CsPbX3-MEOPEG can be further fine-tuned by a simple postsynthetic anion exchange process. As a result, white light-emitting diodes (WLEDs) with high-quality CIE coordinates of (0.33, 0.29) and a high CRI value (84) are realized. These CsPbX3-MEOPEG materials, with tunable dual-color emission, may serve as ideal model systems for WLEDs, which will undoubtedly expand the applications of cesium lead halide perovskites.

INTRODUCTION As a new family of photoelectric materials, perovskites have been widely investigated as the major components in solar cells,1,2 photodetectors,3,4 lasers5,6 and light-emitting diodes (LEDs)7,8 due to their high photoelectric conversion efficiency, strong light absorption and narrow emission line widths. Among these perovskites, CsPbX3 is a promising light-emitting material because of its high photoluminescence quantum efficiency, good charge mobility and excellent color purity. Its color-tunable emission properties further make CsPbX3 a potential candidate for application in optical devices.9 It is known that the emission wavelength is determined by the band gap of a lead halide perovskite, i.e., the distance between the conduction band minimum (CBM) and the valence band maximum (VBM).10 According to previous reports, the CBM of CsPbX3 is formed by Pb 6p orbitals, while the VBM is composed of Pb 6s orbitals and X np orbitals through orbital hybridization.10,11 Hence, changing the host lattice by anion or cation exchange is a straightforward approach for tuning the band gap. Anion exchange is an effective approach in tuning the emission of CsPbX3 in the visible light range, as elucidated by Kovalenko et al.12-14 It has been reported that postsynthetic anion exchange can provide a simple method to prepare lead halide perovskites that are not easily realized by direct synthesis.14 Recently, it has been shown that cation exchange can enable a blueshift in the emission of CsPb1−xMxBr3 nanocrystals (M= Sn2+, Cd2+, and Zn2+; 0 < x ≤ 0.1;).15 In addition to altering the band gap by changing ions, choosing appropriate surface ligands is also a reliable approach. The use of mercapto-β-cyclodextrin as a surface ligand enabled a blueshift in the emission of CsPbBr3.16 Despite successfully achieving tunable single-color

emission from lead halide perovskites in the past four years, cesium lead halide perovskites with simultaneous dual-color emission are still scarce. The phenomenon of dual-color emission was first discovered by Kovalenko et al. during the fast anion exchange reaction of CsPbBr3 and CsPbI3.14 However, the dual-color emission was converted into single-color emission within 2 min because of the unstable intermediate state of CsPb(Br/I)3. Very recently, CsPbBrxI3−x nanowires (NWs) prepared by vapor-phase epitaxial growth exhibited stable dual-color lasing.17 They achieved dual-color emission by changing the ratio of bromine and iodine along the length of the NWs. Metal doping is another strategy that has been used to realize dual-color emission, as illustrated by Klimov et al.18 Mn2+-doping generated a new emission at 586 nm in addition to the characteristic emission at 402 nm from CsPbCl3. This new emission was ascribed to the d-d transition of Mn2+.18 Although pioneering work has achieved much success in the synthesis and utilization of perovskites for applications in light-emitting fields, facile preparation strategies, especially those that can achieve perovskites with dual-color emission and their further use as white light-emitting diodes (WLEDs), are still highly demanded. Herein, we present the first example of dual-color emission from CsPbX3-MEOPEG nanowires induced by ligand coordination using a solution-phase synthesis. An easily obtained oligomer with low toxicity,19-21 methoxypolyethylene glycol (MEOPEG, Figure S1) with an average molecular weight of 200 g/mol, was employed for the synthesis of CsPbX3-MEOPEG. In this way, CsPbX3-MEOPEG nanowires were obtained. The nanowires exhibit dual-color emission corresponding to the characteristic emission of the host lattice and a newly generated emission peak caused by the

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interactions between the host lattice and the surface ligands (MEOPEG). Furthermore, the intensity ratio of the dual-color emission peaks can be tuned by adjusting the amount of surface ligands, and a wide emission range can be achieved by postsynthetic anion exchange. As a result, high-quality WLEDs were fabricated from CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG. RESULTS and DISCUSSION CsPbX3-MEOPEG was prepared according to a previously reported hot-injection method with minor modifications.22 Briefly, taking CsPbBr3-MEOPEG as an example, to a mixture of PbBr2, oleic acid (OA), oleylamine (OLA) and MEOPEG in 1-octadecene (ODE) and keep the temperature at 100 °C for 40min, Cs-oleate solution was added. The solution was kept at an elevated temperature (100 °C) for 20 min (see details in the Supporting Information). Afterwards, the product was repeatedly washed with ethyl acetate (EA) and centrifuged to remove excess organic molecules and inorganic salt. CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG were prepared by postsynthetic anion exchange of CsPbBr3-MEOPEG (see details in the Supporting Information). The chemical structures of the reagents and the synthetic route for CsPbBr3-MEOPEG are schematically outlined in Figure 1.

Figure 1. Schematic overview of the synthetic reaction for CsPbBr3-MEOPEG. Solid-state X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to examine the inorganic structure of CsPbBr3-MEOPEG. The peaks at approximately 15, 20 and 30 degrees in the XRD pattern of CsPbBr3-MEOPEG indicate that CsPbBr3-MEOPEG is more similar to the cubic phase according to the standard cubic CsPbBr3 XRD pattern (Figure 2a, PDF#54-0752). Notably, the MEOPEG in CsPbBr3-MEOPEG can be removed by ultrasonication in an organic solvent, e.g., EA. After washing, the formed host lattice, labelled CsPbBr3-Washed, was intact, which was confirmed by FTIR, 1H NMR and XRD analysis. As shown in Figure 3, the typical signals of MEOPEG can be observed in the FTIR and 1H NMR spectra of CsPbBr3-MEOPEG. These signals disappear completely in the FTIR and 1H NMR spectra of CsPbBr3-Washed. The XRD pattern of CsPbBr3-Washed changes to that of the orthorhombic phase (Figure 2a, PDF#18-0364) because the peak at approximately 15 degrees is a doublet, as seen in Figure S2a.23 Notably, the peak at approximately 15 degrees in the XRD pattern of CsPbBr3-MEOPEG shifts to lower values after coordination with the surface ligand (Figures 2a and S2a). According to the Bragg equation (2dsinθ = nλ, where d is the distance between crystal planes, θ is the angle between the incident X-ray and the corresponding crystal plane, λ is the

wavelength of the X-ray, and n is the diffraction order), this change indicates that the distance between crystal planes in CsPbBr3-MEOPEG is larger than that of standard cubic CsPbBr3.23,24 According to a previous report, oleic acid (OA) and oleylamine (OLA) play a key role in controlling the growth direction of perovskite nanoparticles, whereas they have no effect on the host lattice.25-28 CsPbBr3-Washed has the same host lattice as CsPbBr3-OA/OLA prepared with OA and OLA (Figure S2b). Figure S2c suggests that ultrasonication in EA has no effect on the host lattice. Thus, the significant difference in the host lattices between CsPbBr3-MEOPEG and CsPbBr3-Washed is ascribed to the influence of MEOPEG. The peaks at approximately 12.5 degrees in the XRD patterns of CsPbBr3-Washed and CsPbBr3-MEOPEG are from a small amount of Cs4PbBr6, which has no emission in the visible light range.29

Figure 2. a) Solid-state XRD patterns of CsPbBr3-Washed and CsPbBr3-MEOPEG and b) TEM image of CsPbBr3-MEOPEG (scale bar is 100 nm). The inset in b shows a HRTEM image of a CsPbBr3-MEOPEG NW. CsPbBr3-MEOPEG nanowires can be observed in the TEM image shown in Figure 2b. The HRTEM image shows that the distance between crystal planes in the CsPbBr3-MEOPEG nanowires is 0.596 nm (Figure 2b), which is slightly greater than the standard distance between the (100) crystal planes of cubic phase CsPbBr3 (0.580 nm, PDF-#54-0752).25 This result is in agreement with the XRD analysis. Numerous black dots are visible in the TEM image (Figure 2b). As shown in Figure S3a, the lattice spacing of the crystal planes in these black dots is 0.302 nm. This value corresponds to the standard distance between (100) crystal planes in Pb (0.302 nm, PDF-#44-0872). Pb peaks are not present in the XRD pattern of CsPbBr3-MEOPEG (Figure 2a). According to a previous report, the presence of Pb in nanowires is caused by electron irradiation during TEM imaging.15 This result further indicates the high purity of the CsPbBr3-MEOPEG NWs. In Figure S3b, CsPbBr3-Washed is seen to form NWs, which means that the nanostructure of the perovskite does not change during ultrasonication. The high purities of the CsPbBr3-MEOPEG NWs and CsPbBr3-Washed NWs limit the influence of nanoparticle impurities. The chemical structures of CsPbBr3-MEOPEG and CsPbBr3-Washed were further analyzed by FTIR spectroscopy (Figures 3a, b). The characteristic peaks of –OH groups at 3500 cm-1 and C-O-C bonds at 1100 cm-1 in the spectrum of MEOPEG indicate the host lattice is covered by MEOPEG (Figures 3a, b). The characteristic peaks of these two groups disappeared after the ultrasonication treatment (Figures 3a, b). The characteristic peaks at 1640 cm-1, from the N-H bending vibration, 3086 cm-1, from the N-H stretching vibration, and 1550 cm-1, from the carboxylate asymmetric stretching vibration, can be observed in the FTIR spectrum of CsPbBr3-MEOPEG (Figure 3a), which indicates the existence


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ACS Applied Materials & Interfaces of OA and OLA in CsPbBr3-MEOPEG.30,31 After the removal of MEOPEG by ultrasonication, the characteristic peaks at 1581 cm-1, from the N-H bending vibration, 3122 cm-1, from the N-H stretching vibration, and 1540 cm-1, from the carboxylate asymmetric stretching vibration, are preserved in the FTIR spectrum of CsPbBr3-Washed (Figure 3a), indicating that ultrasonication treatment has no effect on the OA and OLA.

Figure 3. a) Full and b) enlarged (from 1000 to 1180 cm-1) FTIR spectra of CsPbBr3-MEOPEG, CsPbBr3-Washed and MEOPEG. c) Full and d) enlarged (from 3.0 to 3.8 ppm) 1H NMR spectra of MEOPEG, CsPbBr3-Washed and CsPbBr3-MEOPEG. To verify the manner of binding of MEOPEG onto the host lattice, proton nuclear magnetic resonance (1H NMR) analysis was conducted. The proton signal for the OH group from MEOPEG in the 1H NMR spectrum of CsPbBr3-MEOPEG disappears, which indicates that MEOPEG binds to the surface of the host lattice via its –OH groups (Figure S4). 32 In addition, the proton signals of OA and OLA can be found in the 1H NMR spectra of CsPbBr3-MEOPEG and CsPbBr3-Washed (Figures S5a, b).31 After removing MEOPEG, the peak at approximately 3.3 ppm, which is the proton signal of OLA, in the 1H NMR spectrum of CsPbBr3-Washed became obvious (Figures S5b, c).33 X-ray photoelectron spectroscopy (XPS) measurements were performed to further investigate the elemental composition and intrinsic interactions. The presence of Cs, Pb, Br, C, O and N are seen in the XPS survey scans of CsPbBr3-MEOPEG and CsPbBr3-Washed (Figure S6). This result further confirms that OA and OLA are present on the surface of the host lattice in CsPbBr3-MEOPEG and CsPbBr3-Washed, which is in agreement with the FTIR and 1H NMR results. Deconvolution XPS analysis of Cs and Pb are both unchanged before and after the removal of MEOPEG (Figures S7a, b). However, the binding energy of Br noticeably changes (Figure 4a). In CsPbBr3-MEOPEG, MEOPEG coordination results in a lower binding energy for Br (67.45 eV) compared to that in CsPbBr3-Washed (67.6 eV). Simultaneously, the O 1s peak in the XPS core level spectrum of CsPbBr3-MEOPEG shifts by ~0.85 eV to a high binding energy compared to that of MEOPEG (Figure 4b). The O 1s peak of OA in CsPbBr3-Washed, shown in Figure 4b, shifts to a lower binding energy and becomes a single peak. Thus, most of the O atoms in CsPbBr3-MEOPEG are provided by

MEOPEG. Therefore, we attribute the interaction between MEOPEG and the host lattice to electron transfer that occurs between O and Br. This charge transfer is characteristic of Br · · ·O halogen bonding complexes, in which Br atoms show Lewis acidity and O atoms acts as charge donors.34 This interaction weakens the chemical bond between Pb and Br, thus enlarging the characteristic band gap of the host lattice to form a new wider band gap.10 After removing MEOPEG, the effect of MEOPEG on the Pb-Br bonds disappears and the widened band gap is reduced to the characteristic band gap. Ultraviolet photoelectron spectroscopy (UPS) spectra and absorption spectra of CsPbBr3-MEOPEG and CsPbBr3-Washed were measured to confirm these inferences. The UPS spectrum of CsPbBr3-MEOPEG (Figure S8a) shows the two VBMs (-5.63 eV and -5.11 eV) in CsPbBr3-MEOPEG while only one VBM (-5.11 eV) was observed in CsPbBr3-Washed in Figure S8b. As shown in Figure S8c, there are two band gaps (2.95 eV and 2.43 eV) in CsPbBr3-MEOPEG and only one band gap (2.43 eV) in CsPbBr3-Washed. Therefore the CBMs (-2.68 eV) in CsPbBr3-MEOPEG and CsPbBr3-Washed are the same. Based on the above discussion, the influence of MEOPEG on the host lattices expands the partial characteristic band gap by lowering the VBM, thereby enabling the new-type perovskite to show dual-color emission.

Figure 4. XPS core level spectra of a) Br 3d and b) O 1s in CsPbBr3-MEOPEG, CsPbBr3-Washed and MEOPEG. One of the intriguing applications of lead halide perovskites is as light-emitting diodes due to their excellent photoluminescent properties. Therefore, the photoluminescence of CsPbBr3-MEOPEG was investigated. Figure 5a shows the photoluminescence (PL) spectrum of CsPbBr3-MEOPEG. There are two significant emission peaks centered at 470 nm and 515 nm. The corresponding absorption spectrum also indicates two bands (Figure S8). After the enlarged band gap was reduced to the characteristic band gap after removing MEOPEG, there was only one peak centered at 515 nm in the PL spectrum of CsPbBr3-Washed (Figure 5a). The green emission at 515 nm is the characteristic emission of the host lattice, orthogonal phase CsPbBr3.15,23 Accordingly, there is only one peak in the absorption spectrum of CsPbBr3-Washed (Figure S8). The blue emission from CsPbBr3-MEOPEG is not caused by MEOPEG, according to the absorption and emission spectra of MEOPEG (Figure S9). It can be determined that the dual-color emission of CsPbBr3-MEOPEG is caused by the interaction between MEOPEG and the host lattice based on the above discussion. In all the above investigations, the average molecular weight of MEOPEG was maintained at 200 g/mol. To clarify the effect of MEOPEG length on the emission properties, we prepared CsPbBr3-MEOPEG using short (2-methoxyethanol, Figure S10), medium (MEOPEG 200, Figure S1) and long (MEOPEG 2000, Figure S11) ligands. The characteristic



peaks of the –OH groups at 3500 cm-1 disappear, and the characteristic peak of the C-O-C bonds at 1100 cm-1 can still be seen in the FTIR spectra of CsPbBr3-2-methoxyethanol and CsPbBr3-MEOPEG 2000, which means the ligands 2-methoxyethanol and MEOPEG 2000 bind to the host lattice though their –OH groups (Figures S12a, b). However, compared with CsPbBr3-MEOPEG 200 and CsPbBr3-MEOPEG 2000, there are only a few C-O-C bonds in CsPbBr3-2-methoxyethanol, and the proton signals of 2-methoxyethanol are not visible in the 1H NMR spectrum of CsPbBr3-2-methoxyethanol (Figure S12c), which proves that only a small amount of 2-methoxyethanol is bound to the host lattice. Therefore, the XRD pattern of CsPbBr3-2-methoxyethanol is similar to those of CsPbBr3-Washed and CsPbBr3-OA/OLA (Figure S13a). CsPbBr3-2-methoxyethanol shows a hexagon nanoplatelet morphology in its TEM image (Figure S13b). The characteristic peak of the host lattice is inconspicuous in the XRD pattern of CsPbBr3-MEOPEG 2000, which can be attributed to the extended backbone of MEOPEG 2000 (Figure S13c). Correspondingly, there are irregularity globules in the TEM image of CsPbBr3-MEOPEG 2000 (Figure S13d). Only one peak in the PL spectrum of CsPbBr3-2-methoxyethanol (Figure S14a) can be seen, while CsPbBr3-MEOPEG 2000 shows dual-color emission (Figure S14b). Both of their PL spectra are different from those of 2-methoxyethanol and MEOPEG 2000 (Figure S15). In the CsPbBr3-2-methoxyethanol system, the single-color emission can be attributed to the fact that the C-O-C bonds are too few to affect the characteristic band gap of the host lattice. MEOPEG 200 is the best surface ligand candidate for perovskites with dual-color emission and high-quality nanotopography among these three surface ligands. Subsequently, we explored the effect of the surface ligand concentration on the photoluminescent properties. The concentrations of MEOPEG, OA and OLA were simultaneously reduced to avoid additional variables (Table S1). As the dosage of the surface ligand decreases from 2.0 mL to 0.5 mL (Table S1), the mass percent (wt %) of MEOPEG decreases, while the wt % of OA and OLA both increase (Figure S16a). The characteristic emission intensity of the host lattice dramatically increases and that of the expanded band gap decreases synchronously (Figure 5b). These changes mean that the intensity ratio of the blue emission and green emission can be easily tuned, which endows CsPbBr3-MEOPEG with substantial flexibility for application as a phosphor in WLEDs. Upon closer inspection of the photoluminescence, a slight blueshift of both emission peaks occurs as the surface ligand concentrations decrease (Figure 5b, Table S2). According to previous reports, the perovskite nanoplatelets have a larger blueshift in their emission compared with that of other morphologies.25,26,31,35-37 As the surface ligand concentrations decrease, CsPbBr3-MEOPEG tends to form wider NWs and even nanoplatelets (Figure S17). The ratio of OA to OLA noticeably increases when the surface ligands are reduced to a certain extent (Figure S16b), which leads to the perovskites growing into nanoplatelets (Figure S17).25,26 Thus, the results suggest that changing the surface ligand concentration enables obtaining dual-color emitting perovskites with various morphologies.

Exchanging the halogen atoms in CsPbX3 via postsynthetic anion exchange is a popular approach to tune the band gap of CsPbX3.12-14,36,38 The influence of anion exchange on the dual-color emission of CsPbX3-MEOPEG was also investigated in this work. To avoid the influence of the cations, a PbX2 precursor stock solution was used for the postsynthetic anion exchange of CsPbX3-MEOPEG and CsPbX3-Washed (see details in the Supporting Information). The XRD patterns of CsPb(Br/Cl)3-Washed and CsPb(Br/I)3-Washed are very close to that of the standard pattern (Figure S18a). As demonstrated in the above investigation, the interactions between MEOPEG and the host lattice weaken the Br and Pb bonds. Thus, a portion of the Br atoms are more easily replaced by Cl or I atoms. Therefore, the XRD patterns of CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG become more complicated (Figure S18b). These new peaks can represent the emergence of new crystal planes during the anion exchange process. The peaks in the XRD pattern of CsPb(Br/I)3-MEOPEG are shifted to lower angles compared to those of CsPb(Br/Cl)3-MEOPEG. The smaller Cl atoms cause lattice contraction, while the larger I atoms cause lattice expansion.24 As shown in Figure S19, the CsPb(Br/Cl)3-MEOPEG NWs are thinner than the CsPb(Br/I)3-MEOPEG NWs. The characteristic peaks of MEOPEG, OA and OLA can be clearly observed in the FTIR and 1H NMR spectra of CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG (Figure S20). In the normalized 1H NMR spectra, the intensities of the MEOPEG proton signals in the 1H NMR spectrum of CsPbBr3-MEOPEG are stronger than those of CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG (Figures S20a, c). Additionally, the proton signal at approximately 3.3 ppm, which comes from OLA, becomes more obvious after the anion exchange (Figures S20c, d). It can be deduced that the concentration of MEOPEG after anion exchange is decreased. Although there is more OA and OLA in the PbCl2 precursor stock solution than in the PbI2 precursor stock solution, the amount of MEOPEG in CsPb(Br/I)3-MEOPEG is the lowest of the samples according to the 1H NMR spectra, which indicates that OA and OLA have negligible effects on the concentration of MEOPEG during the postsynthetic anion exchange process (Figures S20c, d). The different amounts of MEOPEG in CsPbBr3-MEOPEG, CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG can be explained by the different bond energies between the O-X bonds and Pb-X bonds. The bond energy of O-Br (235 kJ/mol) is only slightly lower (by 5.6%) than that of Pb-Br (249 kJ/mol) in CsPbBr3-MEOPEG. For CsPb(Br/Cl)3-MEOPEG, the bond energy of O-Cl (269 kJ/mol) is weaker than that of Pb-Cl (301 kJ/mol) by 11%. The bond energy of O-I (130 kJ/mol) in CsPb(Br/I)3-MEOPEG is much weaker than that of Pb-I (194 kJ/mol) by 33%. The smaller bond energy difference between the O-X bonds and Pb-X bonds means MEOPEG is more likely to bind to the host lattice.18 Thus, a small amount of MEOPEG is removed during the anion exchange process because of the increase in the bond energy difference between the O-X and Pb-X bonds. The Cl 2p peak shifts to higher binding energies in the XPS core level spectrum of CsPb(Br/Cl)3-MEOPEG (179.6 eV) compared with that of CsPb(Br/Cl)3 (197.5 eV) (Figure S21b). The I 3d peak in the XPS core level spectrum of CsPb(Br/I)3-MEOPEG (518.24 eV) is shifted by ~0.08 eV to a lower binding energy compared to that of CsPb(Br/I)3 (Figure


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ACS Applied Materials & Interfaces S22b). The changes for the three kinds of halogens are shown in Table S3. Because the Lewis acidity of the Cl atoms is much weaker than that of the Br atoms and I atoms, it is harder for Cl atoms to obtain the lone pair of electrons from the O atoms. However, the O atoms can easily obtain electrons from the Cl atoms due to the higher electronegativity of the O atoms. Thus, the electron cloud density of the Cl atoms decreases under the influence of the O atoms.34 These results indicate that the chemical environment of the Cl and I atoms change in the presence of MEOPEG to form a new expanded band gap, which is the same as the case for CsPbBr3-MEOPEG. After adding the PbCl2 precursor stock solution, both emission peaks show a blueshift (Figures 5c and S23a). Specifically, the emission peak from the expanded band gap of CsPb(Br/Cl)3-MEOPEG has a greater change than that of the characteristic band gap (Figures 5c and S23a). The different degrees of exchange of the two kinds of Br atoms result in the blue emission peak of CsPbBr3-MEOPEG significantly blueshifting, while the green emission peak begins to blueshift. This phenomenon appears during the Br-to-I anion exchange (Figures 5d and S23b). The decrease in the amount of MEOPEG causes the characteristic emission peak of the host lattice in the spectra to become stronger after the postsynthetic anion exchange process (Figures 5c, d and S23). After anion exchange from CsPbBr3-Washed to CsPb(Br/Cl)3-Washed and CsPb(Br/I)3-Washed, both maintain single-color emission, as shown in Figure S24. These results indicate that MEOPEG still has a huge influence on the dual-color emission of CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG. The dual-color emission of CsPbX3-MEOPEG can be tuned to cover the entire visible spectral range through postsynthetic anion exchange (Figure S23).

prepare WLEDs via the “remote phosphor” approach.8,39 In this case, CsPbBr3-MEOPEG was dispersed in a PbCl2 or PbI2 precursor stock solution to prepare a CsPb(Br/Cl)3-MEOPEG or CsPb(Br/I)3-MEOPEG solution. These solutions were separately dropped onto two pieces of quartz glass to form uniform films. Then, a UV LED (395 nm, 3 W) was applied to pump the CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG films to obtain white light emission. White emission covering the entire visible light range can be observed in Figure 6a. The CIE coordinates are (0.33, 0.29) with a high CRI value (84) (Figure 6b). The highest luminance was 298 cd/m2. These WLEDs, with all-perovskite emission, successfully simplified high-quality WLEDs.

Figure 6. a) Emission spectrum and (inset) image of the WLED as a down converter for a UV LED (395 nm). b) The CIE coordinates and CRI of the WLED. CONCLUSIONS In summary, we reported a series of CsPbX3-MEOPEG perovskites with unique dual-color emission. The influence of MEOPEG on the host lattice via the halogens was verified by XRD, HRTEM, FTIR, 1H NMR and XPS analysis. This influence widened the characteristic band gap of the host lattice to form a new expanded band gap. The intensity ratio of the dual-color emission can be easily fine-tuned by varying the amount of MEOPEG. Both of the band gaps for the dual-color emission of CsPbX3-MEOPEG can be feasibly tuned via the exchange of different halogen atoms through a postsynthetic anion exchange process. Moreover, WLEDs prepared from CsPb(Br/Cl)3-MEOPEG and CsPb(Br/I)3-MEOPEG realized high-quality white emission with CIE coordinates of (0.33, 0.29) and a high CRI value (84). Our work will provide novel insights into the design and synthesis of perovskites with various optical properties.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details on the materials, methods, experimental procedures, and characterization data. Figure 5. Photoluminescence spectra (390 nm excitation) of a) CsPbBr3-MEOPEG and CsPbBr3-Washed, b) CsPbBr3-MEOPEG synthesized with various amounts of the surface ligand, c) CsPbBr3-MEOPEG and CsPbBr1.47Cl1.53-MEOPEG and d) CsPbBr3-MEOPEG and CsPbBr2.07I0.93-MEOPEG. Because of the simple synthesis, low cost, high color purity, tunable ratio of the dual-color emission intensity and wide emission range, CsPbX3-MEOPEG meets the requirements for application in WLEDs. These novel perovskites were used to

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENT This work was funded by NSFC (No. 51473028), the key scientific and technological project of Jilin province (20160307016GX, 20190701010GH), the development and



reform commission of Jilin province (20160058). This project was supported by the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

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Ligand-Induced Tunable Dual-Color Emission Based on Lead Halide

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