Structural Investigation of Cesium Lead Halide Perovskites for High

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Structural Investigation of Cesium Lead Halide Perovskites for High-Efficiency Quantum Dot Light Emitting Diodes Quyet Van Le, Jong Beom Kim, Soo Young Kim, Byeongdu Lee, and Dong Ryeol Lee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01709 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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The Journal of Physical Chemistry Letters

Structural Investigation of Cesium Lead Halide Perovskites for High-Efficiency Quantum Dot Light Emitting Diodes Quyet Van Le‡†, Jong Beom Kim‡††, Soo Young Kim†,*, Byeongdu Lee†††,*, and Dong Ryeol Lee††,* †

School of Chemical Engineering and Materials Science, Integrative research center for two-

dimensional functional materials, Institute of Interdisciplinary Convergence Research, ChungAng University, Seoul 06974, Republic of Korea ††

Department of Physics, Soongsil University, Seoul 06978, Republic of Korea

†††

X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont,

Illinois 60439, United States

Corresponding Authors *Soo Young Kim (Email: [email protected]) *Byeongdu Lee (Email: [email protected]) *Dong Ryeol Lee (Email: [email protected])

ACS Paragon Plus Environment

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ABSTRACT

We have investigated the effect of reaction temperature of hot-injection method on the structural properties of CsPbX3 (X: Br, I, Cl) perovskite nanocrystals (NCs) using the small- and wideangle X-ray scattering. It is confirmed that the size of the NCs decreased as the reaction temperature decreased, resulting stronger quantum confinement. The cubic-phase perovskite NCs were formed despite the reaction temperatures increased from 140 to 180 °C. However, monodispersive NC cubes which are required for densely packing self-assembly film were only formed at lower temperatures. From the X-ray scattering measurements, the spin-coated film from more monodispersive perovskite nanocubes synthesized at lower temperatures resulted in more preferred orientation. This dense-packing perovskite film with preferred orientation yielded efficient light-emitting diode (LED) performance. Thus, the dense-packing structure of NC assemblies formed after spin-coating should be considered for high-efficient LEDs based on perovskite quantum dots in addition to quantum confinement effect of the quantum dots.

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In recent years, the unprecedented development of a new class of electronic materials, called organo/inorgano lead halide perovskites, has occurred. These have the chemical formula ABX3 (A = methyl ammonium (MA), formamidium (FA), or cesium (Cs); B = Pb or Sn; X = halide: I, Br, Cl, or a mixture such as BrxI3−x);1-5 materials based on them exhibit excellent optical, electrical, and chemical properties such as a high absorption coefficient, low exciton binding energy, tunable band gap, high charge-carrier mobility, long charge-carrier diffusion length, and low-temperature solution processability.2 Solar cells with perovskite as the active material have exhibited a power conversion efficiency (PCE) exceeding 20%, making them a very promising candidate to replace expensive silicon solar cells in the near future.1 Moreover, perovskite materials have also showed their potential in a variety of electronic applications such as memristors, photodetectors, field effect transistors, and light emitting diodes (LEDs).6-10 Perovskite LEDs have attracted considerable interest because of their extremely high performance in terms of maximum luminance, current efficiency, and internal quantum efficiency, which are comparable to those of organic light emitting diodes.8,11-15 It is well-known that the performance of perovskite LEDs strongly depends on the crystal structure.12 A perovskite film with a large crystal size exhibits very low quantum yield, leading to poor device performance.12,16 In contrast, perovskite nanocrystals (NCs) show a considerably higher quantum yield, over 90 %, which is suitable for the active layer in LEDs.8,17 Perovskite NCs can be synthesized by various methods, such as recrystallization in an anti-solvent, or a hot injection method.17,18 Recrystallization in an anti-solvent permits fabrication of perovskite NCs at room temperature;18 however, the crystal size of the NCs cannot be precisely controlled. In contrast, the hot injection method enables modification of the crystal size by adjusting the synthesis temperature.17 Nevertheless, the dependence on the synthesis temperature of the film structures


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in perovskite NCs for LED devices has not been reported previously. Additionally, when LED devices are fabricated with colloidal NCs, a self-assembled structure of NCs is formed by the reconstruction of ligand-tethered nanoparticles during spin coating. These structures can be important for device performance because charge/energy transfer occurs at the interface between the emissive NC layer and neighboring electron/hole transport layers in perovskite quantum dot (QD) LEDs. Here, we present a study of the synthesis-temperature dependence of CsPbX3 perovskite for NCs, using X-ray scattering. To this end, we synthesized CsPbX3 perovskite NCs at different reaction temperatures in the form of colloids and spin-coated films, and performed small-angle and wide-angle X-ray scattering measurements to characterize the sizes and crystal structures of the NCs and their assembled structures.19 The synthesis temperature is that of the reaction between Cs-oleate and PbX2 (X: I, Br, or Cl), and it is the reaction temperature rather than the reaction duration that determines the size of the CsPbX3 NCs.

17,20

We also investigated the

dependence on the synthesis temperature of the performance of QD LED devices fabricated with these NCs. In this study, we found that synthesis temperature could significantly affect the width of the size distribution of perovskite NCs as well as their average size. This size variance was found to be critical for the packing density, ordering structure, and crystal orientation of the NC selfassemblies formed after spin coating. In particular, we found that as the reaction temperature decreases, the perovskite NCs became more monodisperse and thus their assembly has a more dense-packed face-to-face simple cubic (SC) structure. We will show that the NC self-assembly structure and its crystal orientation as well as the NC size or crystallinity, which are related to the quantum yield, should be considered to enhance the performance of perovskite QD LEDs.


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Figure 1. Photoluminescence spectra of (a) CsPbBr3, (b) CsPbBrxI3-x, and (c) CsPbBrxCl3-x synthesized at different temperature, respectively. Figure 1 shows the photoluminescence (PL) spectra of colloidal of CsPbr3, CsPbBrxI3-x, and CsPbBrxCl3-x NCs synthesized with different reaction temperatures, accordingly. The NCs synthesized at 140 °C show a distinct blue shift compared to the other high temperatures. This indicates that the NC size for 140 °C is sufficiently small compared to the exciton Bohr diameter, and increases the energy of the emitted photons by quantum confinement.21 We also compared the LED characteristics for devices based on NCs with different synthesis temperatures as the emissive layer. Figure 2 show the performance characteristics of green-emitting CsPbBr3-based LEDs: current density vs. voltage; luminance vs. voltage; luminance efficiency vs current density; and power efficiency vs. current density. It is observed that the operating voltage of perovskite LEDs increases as the synthesis temperature is increased from 140 to 180 °C. Similarly, the maximum luminance of perovskite QD LEDs decreases from 23 to 6 cd/m2 as the temperature increases (140 to 180 °C). These temperature characteristics are similar for CsPbBrxI3-x and CsPbBrxCl3-x, which exhibit red and blue emission, respectively (Figs. 1(b) and (c)). We will show using scanning electron microscopy (SEM) and X-ray scattering that this


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effect of synthesis temperature on the LED performance results from structural properties of the perovskite NCs.

Figure 2. (a) Schematic figure of perovskite crystal, perovskite LED structures, and images of green perovskite LED. (b) Current density vs. voltage, (c) luminance vs. voltage, (d) luminance efficiency vs. current density, and (e) power efficiency vs. current density characteristics of LEDs based on CsPbBr3 nanocrystals synthesized at different reaction temperatures.


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Wide-angle X-ray scattering (WAXS) from NCs in solution (Fig. 3(a)) clearly shows a cubicphase of CsPbBr3 perovskite crystals formed at reaction temperatures of 140-180 °C; this result is consistent with previous studies.17,20 For temperatures of 140 °C and above, there is almost no difference in crystalline structure. In contrast, an orthorhombic polymorph of CsPbBr3 perovskite crystals is seen for all temperatures, including 120 °C, as indicated by asterisks in Fig. 3 (a). The orthorhombic, tetragonal and cubic polymorphs of the CsPbX3 perovskite structure are known, with the cubic phase formed mainly at high temperatures.17,22,23 In addition to these polymorphs, red-emitting CsPbBrxI3-x and blue-emitting CsPbBrxCl3-x NCs also show a lamellar structure at 180 °C (Figs. 3 (d) and (g)). It should further be noted that there are diffraction peaks appearing only at 120 °C, which result from PbBr2 orthorhombic crystals. This is because PbBr2, which is one of the precursors in the CsPbBr3 synthesis, does not dissolve and a considerable amount remains suspended in the solution. As the hot-injection method is a rapid process that synthesizes CsPbX3 NCs within a few seconds after the injection of a Cs precursor (such as Cs-oleate), it appears that the PbX2 does not dissolve at a low temperature of 120 oC. However, as the WAXS curves from the polymorph mixtures (cubic-phase-dominant) synthesized at temperatures of 140180 °C are almost identical, the differences seen in the LED characteristics for these temperatures (Fig. 2) are not readily explained. On the other hand, small-angle X-ray scattering (SAXS) on the colloidal NCs (Fig. 3 (b)) shows that the size of the CsPbBr3 cubic-phase perovskite NCs formed at 140 °C and above depends on the reaction temperature. Since the knee-point position in the SAXS curve is inversely proportional to the particle size, Fig. 3(b) indicates that the size of the NCs increases with reaction temperature. This difference in the NC sizes is consistent with the PL characteristics, which result from quantum confinement. As seen from the PL characteristics in


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Fig. 1, the NCs synthesized at 140 °C show a distinct blue shift relative to the higher-temperature QDs. Quantum confinement occurs when the particle size is much smaller than the exciton Bohr diameter, and a blue shift appears because the quantized energy adds to the bulk-semiconductor bandgap energy. The theoretically predicted exciton Bohr diameter for CsPbX3 is 7 nm,17 which exceeds the average size of the 140 oC NCs as determined from the SAXS data (Fig. 3(c)), and a blue-shift is observed. The dependence of the NC size and PL shift on synthesis temperature is also observed in red-emitting CsPbBrxI3-x and blue-emitting CsPbBrxCl3-x (Figs. 1(b) and (c)). Similar phenomena are found in CsPbBrxI3-x and CsPbBrxCl3-x (Figs. 3(d)-3(i)). We note a significant broadening of the distribution of NC radii (the SAXS analysis in Fig. 3(c)) with increasing synthesis temperature. In the hot-injection method, where NCs are formed within a few seconds after the Cs precursor is injected, it is inevitable that the variance in size as well as the average size of the NCs increases with the reaction temperature. The increase in the size variance, or polydispersity, of these NCs is relatively insignificant for the structural properties of the NCs in solution, but has a significant effect on the NC self-assembly structure that occurs when the material is spin-coated into LED thin films. This is a critical structural parameter for LED performance, as we will show later.


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Figure 3. X-ray scattering from colloidal CsPbX3 nanocrystal synthesized at different temperature. (a),(d),(g) Wide-angle X-ray scattering intensities of CsPbBr3, CsPbBrxI3-x and CsPbBrxCl3-x, respectively. The black dotted lines indicate the positions of peaks for cubic-phase perovskite crystals with a lattice constant of 0.586 nm, 0.612 nm, and 0.574 nm accordingly. (b), (e), (h) small-angle X-ray scattering intensity. (c), (f), (i) distribution function for NC volume to


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the radius shown, determined from the best fit (solid line) in (b), (e), (h). The excition Bohr diameter (predicted) is 7 nm.

Figure 4. (a), (b), (c) FE-SEM images of CsPbBr3 at different temperature. (d), (e), (f) FE-SEM images of CsPbBrxI3-x at different temperature. (g), (h), (i) FE-SEM images of CsPbBrxCl3-x at different temperature. Spin-coated NCs are distinguished from colloidal ones by having a dense self-assembled structure, as shown in the SEM image of Figs. 4(a)-4(i). The CsPbX3 cubic-phase perovskite NCs will have this dense structure because the NC shape is close to being a cube. While facecentered-cubic packing is the most common result with nanosized spheres, the self-assembled structure of ligand-tethered nanocubes is known to favor face-to-face SC structures.24 The van


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der Waals interaction is then strongest because the surface faces of the nanocubes are aligned with the gap determined by the ligand-to-ligand distance. With this geometry, the nearestneighbor contact area is maximized in LEDs for the interfaces of NC to NC and NC to chargetransfer layer, where the charge and energy transfer occurs, and thus the device performance is significantly improved. However, in our synthesis, the perovskite NC self-assembled structure seemed to depend strongly on the reaction temperature, ranging from 140 to 180 oC. As the reaction temperature increases, the assembly of a dense face-to-face SC structure is hindered because of the increased polydispersity of the NCs. In the grazing-incidence small-angle X-ray scattering (GISAXS) curves for the spin-coated NC assembly layers (shown in Fig. 5(a)), the 140 oC sample clearly shows the high order peaks resulting from the structure factor of the SC lattice. Further, the 2D GISAXS for the red-emitting CsPbBrxI3-x NC layer shows a self-assembly lattice peak in the outof-plane as well as in-plane directions at 140 °C (Fig. 5(b)). On the other hand, in the 160 and 180 °C cases, these lattice peaks are not evident and the SAXS curves are similar to those for dispersive NCs in solution (Fig. 3 (b)). In the SEM images, the ordered NC domains for 160-180 °C are significantly smaller than those for 140 °C; the ordered domains are scarcely visible at 180 °C.


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Figure 5. GISAXS of (a) CsPbBr3, (b) CsPbBrxI3-x, and (c) CsPbBrxCl3-x at various temperature obtained from the line profile cuts horizontally at Qz = 0.22 nm-1 from 2D GISAXS images shown in Figs. S(3)-S(5), respectively. The ordering in the self-assembled structure can be also confirmed by comparing the gap distance between nanocubes, which can be determined from the GISAXS analysis in Fig. 5(a). The lattice parameter deduced from the structure factor, and the particle edge length from the form factor oscillation, are 17.3 and 13.9 nm, respectively, for the 140 oC sample, giving a gap distance of 3.4 nm. This length is close to double the ligand length and indicates a face-to-face SC structure. On the other hand, for the 180 oC case, the lattice parameter and the particle edge length are 28.7 nm and 19.0 nm, respectively, and the gap distance is 9.7 nm, so that the ligandtethered cube faces are not in contact with each other. A main cause of these results is that the polydispersity of perovskite nanocubes for synthesis at 180 oC is significantly larger than that for synthesis at 140 oC, so it is more difficult to obtain an ordered SC arrangement, where the faces of the cubes are parallel to each other.25 If the spin-coated perovskite nanocubes have a face-to-face SC assembly structure, as in the 140 oC case, the crystal planes of the NC cubes should be aligned along the out-of-plane direction perpendicular to the surface. Since the nanocubes may have randomly-oriented assembly domains in the in-plane direction, NC assemblies should have a preferred orientation along one of the faces of the cube in a direction perpendicular to the surface. To analyze these crystalline orientations of perovskite nanocube assemblies, we measured 2D GIWAXS for the spin-coated NC layers. Figure 6(a) shows a clearly-preferred orientation along the (001) direction of cubic-phase perovskite crystals, for the 140 °C synthesis sample. This preferred orientation means that the (001) crystal planes of all NC cubes are well aligned along the out-of-


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plane direction and the other crystal planes are also well aligned within each nanocube assembly domain. The crystalline direction of assembly domains is randomly oriented in the in-plane direction and shows in-plane powder rings in the diffraction reciprocal space, exhibiting spatially-discrete diffraction peaks with a 2D detector. Therefore, the preferred orientation observed confirms that spin-coated NC cubes synthesized at 140 oC have a dense face-to-face SC assembly structure. In contrast, as shown in Fig. 6(b) and (c), the GIWAXS patterns for the nanocrystalline assemblies synthesized at 160-180 oC show a typical powder ring pattern resulting from three-dimensionally random orientations of the nanocube crystalline planes. As described above, as the reaction temperature increases, the polydispersity of the NC increases, preventing the nanocubes from forming a face-to-face SC structure. The polydispersity of an object is negative for efficient packing not just considering geometry.25 The strength of van der Waals interaction that increases with increasing particle size would irregularly distributed around a particle, which would lead the packing structure of an assembly to more amorphous. (shown schematically in Fig. 6). This observation of a preferred orientation only in the 140 oC sample has been also confirmed in the 2D GIWAXS results of red-emitting CsPbBrxI3-x and blueemitting CsPbBrxCl3-x (Figs. 6 (d)-6(i)).


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Figure 6. Grazing-incidence wide-angle X-ray scattering (GIWAXS) images measured from self-assemblies of spin-coated (a-c) CsPbBr3, (d-f) CsPbBrxI3-x, and (g-i) CsPbBrxCl3-x nanocrystals synthesized at different reaction temperatures. Insets: Schematics of self-assemblies of cubic-shaped perovskite nanocrystals. The circle dots indicate the calculated positions of the peaks diffracted from the cubic-phase crystals whose (100) planes are parallel to the substrate surface Performance can be significantly improved for LEDs based on perovskite nanocube SC assemblies if the packing density of the emissive source can be maximized. In face-to-face SC packing, the charge and energy transfer between the charge-transport layer (CTL) and the quantum-dot layer (QDL) can also be enhanced by maximizing the nearest-neighbor contact area


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at the QDL/CTL and QDL/QDL interfaces. In addition, by reducing the void ratio of the QD layer, direct contact between the charge transport layers adjacent to the QD layer can be avoided, thereby reducing exciton annihilation and current leakage at the unwanted p-n junction. Therefore, the main identified factors in high-efficiency perovskite QD LEDs are the structural advantage of a dense face-to-face SC packing of nanocube assemblies, and the increase in quantum yield from quantum confinement in the QDs. Further studies on the effect of the tethering ligand length on the nanocube self-assembly are required to separate these QD size and assembly structure effects. In summary, we have investigated the effect of reaction temperature in the hot-injection method on the structural properties of CsPbBr3 perovskite NCs and their spin-coated selfassemblies, using small- and wide-angle X-ray scattering and comparisons of high-efficiency LED performance and PL characteristics. First, we confirmed that as the reaction temperature decreases the size of the NCs decreases, and quantum confinement thereby becomes stronger. Next, we found that cubic-phase perovskite NCs are formed for reaction temperatures between 140 and 180 °C; for the lower temperature synthesis, the size distribution of NCs is less dispersive, and nanocubes spin-coated into thin films have a more densely packed self-assembly structure. From the 2D GIWAXS measurements, we found for the first time that spin-coated assemblies of the more monodispersive nanocubes synthesized at lower temperatures show a distinct preferred orientation, which confirms a face-to-face SC packing of cubic-phase perovskite nanocubes. This dense-packing structure yielded higher efficiency LED performance. Therefore, our study suggests that in fabricating high-efficiency LEDs based on perovskite quantum dots, achieving a densely-packed structure of nanocrystal assemblies by spin-coating


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should be considered, in addition to exploiting the effect of quantum confinement in quantum dots.

EXPERIMENTAL SECTION Synthesis of CsPbX3 nanocrystals. First, Cs-oleate was prepared by loading Cs2CO3 (0.814 g) into a three-necked flask with octadecene as the solvent (ODE, 20 mL) and oleic acid (OA, 2.5 mL). The mixture was dried at 120 °C under a N2 atmosphere for 1 h before heating to 150 °C and holding for 2 hrs. The Cs-oleate solution was then kept on the hot plate at 100 oC to prevent recrystallization of the Cs2CO3. Next, PbBr2 or an appropriate PbX2 mixture (0.188 mmol) (e.g., 1:1 PbBr2/PbI2 or 1:1 PbBr2/PbCl2) was loaded into a 25-mL three-necked flask with ODE (5 mL) and dried under vacuum at 120 °C for 1 h. Dried OA (0.5 mL) and dried oleylamine (0.5 mL) were injected at 120 °C for 30 min. To obtain the different sized NCs, the mixtures were heated to 140, 160 or 180 oC. The as-prepared Cs-oleate was swiftly injected into the Pb-halide mixture. The reaction mixture was then cooled with an ice-water bath to terminate the reaction. The perovskite NCs were washed with hexane three times and dispersed in toluene for further characterization and device fabrication. Fabrication of perovskite LEDs. CsPbX3 NC-based LEDs were fabricated on ITO/glass. The ITO/glasses were cleaned with acetone, isopropyl alcohol, and deionized water in sequence. The ITO/glass

substrates

were

then

treated

with

UV/ozone

for

15

min.

Poly(3,4-

ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) were deposited by spin coating at a rate of 4000 rpm. These PEDOT:PSS-coated substrates were then loaded into a thermal evaporator for the deposition of a 40-nm-thick N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,10biphenyl)-4,40-diamine (NPB) layer. The colloidal NCs of CsPbX3 (20 mg/mL) in toluene were


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spin-coated at 600 rpm and heated to 100 °C for 15 min. The thicknesses of CsPbBrxCl3–x, CsPbBrx, and CsPbBrxI3–x nanoparticle films were measured to be 50 nm. These quantum dotcoated substrates were transferred to the thermal evaporator for bathocuproine (BCP, 30 nm), LiF (1 nm), and Al (100 nm) deposition. Characterization. UV-vis absorption spectra (V670 UV-vis spectrophotometer) were used for measuring the absorption characteristics of the CsPbX3 NCs. A xenon arc lamp and a detector (PMT 1527 photomultiplier) were used to detect emissions from the samples for PL measurements. The electrical and light emitting properties of CsPbX3-based LEDs were characterized using a Keithley 2622A source meter and a MINOLTA CS100A luminance meter. The perovskite NCs synthesized at different reaction temperatures were washed with hexane/methyl acetate, and kept in colloid with toluene solvent or spin-coated onto a SiO2 surface to form a thin film. In the case of thin films, X-ray scattering was measured at the grazing-incidence angle to increase the intensities scattered from the NC nanostructures on the substrate surface, and compared to scanning electron microscopy (SEM) measurements. Using the small-angle X-ray scattering (SAXS) method, the size distribution of the NCs formed at different synthesis temperatures, the structure of the NC self-assembly, and the gap distance between the NCs were determined. In addition, wide-angle X-ray scattering (WAXS) was used to investigate the crystalline structures and orientations of the NCs and their self-assemblies. All X-ray scattering experiments were performed at the 12-ID-B beamline at the Advanced Photon Source (APS) at Argonne National Laboratory. At the 12-ID-B beamline, it is very convenient to simultaneously perform several types of SAXS and WAXS experiments on a sample.

ASSOCIATED CONTTENT


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AUTHOR INFORMATION Corresponding Authors *Soo Young Kim (Email: [email protected]) *Byeongdu Lee (Email: [email protected]) *Dong Ryeol Lee (Email: [email protected]) Author contribution ‡

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by NRF grants 2013R1A1A2011326, NRF-2016K1A3A7A09005585, 2015K1A3A1A59073839, and 2017H1D8A1030599. This work was also supported by Korea Agency for Infrastructure Technology Advancement grant funded by Ministry of Land, Infrastructure and Transport (17IFIP-B133622-01). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.


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Supporting Information Statistical analysis of maximum luminance of green perovskite LEDs, characteristics of LEDs based on CsPbBrxCl3-x and CsPbBrxI3-x nanocrystals synthesized at 140 oC, Grazing-incidence small-angle X-ray scattering (GISAXS) images

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Figure 1 366x123mm (96 x 96 DPI)



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Structural Investigation of Cesium Lead Halide Perovskites for High

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