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Perovskite Cesium Lead Halides (CsPbX 3 )

Jizhong Song , Jianhai Li , Xiaoming Li , Leimeng Xu , Yuhui Dong , and Haibo Zeng *

Dr. J. Song, J. Li, X. Li, L. Xu, Y. Dong, Prof. H. Zeng Institute of Optoelectronics and Nanomaterials Herbert Gleiter Institute of Nanoscience School of Materials Science and Engineering Nanjing University of Science and Technology Nanjing 210094 , China E-mail: zeng.haibo@njust.edu.cn

DOI: 10.1002/adma.201502567

optoelectronic devices. The typical QLED device composed of ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al exhibits blue, green, and yellow electroluminescence (EL), which demonstrate that all-inorganic perovskite QDs have potential to become a new class of candidates for low-cost display, lighting, and optical communication applications.

These inorganic perovskite QDs were prepared via a typical synthesis method by hot-injecting cesium stearate into PbBr 2 solution at 170 °C. The involved ionic metathesis reaction takes only several seconds because the nucleation and growth kinetics are very fast. Electronic microscopy images of the perovskite QDs are presented in Figure 1 . The monodisperse CsPbBr 3 QDs have an average diameter of 8 nm, with a cubic shape determined by the perovskite crystal structure (Figure 1 a), which was clearly revealed by X-ray diffraction (XRD) patterns presented in Figure S1 (Supporting Information), and further confi rmed by the lattice fringes of 0.58 nm as shown in high-resolution transmission electron microscopy (HRTEM) images (Figure 1 b). The fast Fourier transformation pattern as inserted in Figure 1 a and the HRTEM image in Figure 1 b confi rm the CsPbBr 3 QDs have a very good crystallinity, which is preferred for improving the luminescence effi ciency and the LED device performance.

The enlarged HRTEM image (Figure 1 c) and the high angle annular dark fi eld scanning transmission electron microscopy (HAADF-STEM) image (Figure S2, Supporting Information) exhibit the single atom arrangement and the cubic crystal lat-tice of CsPbBr 3 QDs, which further confi rm the high crystal quality. The atom confi guration is in good accordance with the cubic CsPbBr 3 crystal structure. It is clearly seen in Figure 1 d that the CsPbBr 3 QDs have the cubic shape, and the QDs are uniformly arranged on the carbon-coated Cu grid, exhibiting a strong tendency to form a fi lm. The elemental mapping result (Figure 1 e) exhibits that the Cs, Pb, and Br atoms are effectively and uniformly incorporated into the QDs.

These as-obtained CsPbBr 3 perovskite QDs exhibit the intrinsical quantum size effects. Their size can be controlled by the reaction temperature evidenced by TEM images (Figure S3, Supporting Information). The size of the QDs increases as the reaction temperature is increased. The typical PL peak of the CsPbBr 3 QDs prepared at 170 °C was located at 510 nm ( Figure 2 a), which shifted to longer or shorter wavelength sides when the size of QDs was increased or decreased, respectively.

Meanwhile, the PL properties can also be tuned by varying the anion element composition. When Cl anions were introduced into the reaction system, the CsPb(Cl/Br) 3 nanocrystals were prepared. The PL peak shifts to the higher energy direction as the Cl anion content is increased (Figure 2 b). It is worth pointing out that the Cl anion addition will not infl uence the crystallinity,

Semiconductor quantum dots (QDs) have attracted intensive studies in recent years, due to their unique optical properties, such as size-dependent emission wavelength, narrow emis-sion spectrum, and high luminescent effi ciency. [ 1–3 ] All of these attractive characteristics make QDs excellent candidates for the next-generation lighting and display, as well as optical communication technologies. Ever since the fi rst CdSe QD-based light-emitting devices (QLEDs) were reported in 1994, [ 4 ] various QD candidates, including CdS, CdTe, InP@ZnSeS, and Cu-doped ZnInS, have been reported as potential emitting materials for QLEDs. [ 5–11 ] Obviously, the dominant materials for QLEDs investigated in the past 20 years have been limited to wurtzite or zinc blende Cd-based QDs.

In the last two years, the halide perovskite materials were demonstrated to be amazing semiconductors with high perfor-mance for not only solar cells but also for LEDs and lasers. [ 12–16 ] However, the stability issue has become the crucial problem of hybrid organic–inorganic halide perovskites (CH 3 NH 3 PbX 3 , X = I, Br, Cl). [ 12–16 ] Compared to hybrid organic–inorganic hal-ides, all-inorganic perovskites exhibit higher stability and have huge potential in various optoelectronics. [ 14 ] To integrate high stability with quantum confi nement effects, Kovalenko and co-workers fabricated the cesium lead halides (CsPbX 3 , X = Cl, Br, and I) QDs, which exhibited outstanding optical proper-ties, especially their tunable and high-quantum-yield photo-luminescence (PL). [ 17 ] Inspired by recent lasing and LEDs of CH 3 NH 3 PbX 3 , [ 18,19 ] these all-inorganic perovskite QDs have huge potential to become a new class of emitting materials in future practical QLEDs. Up to now, QLEDs based on all-inor-ganic perovskite cesium lead halide nanocrystals have not been reported.

Here, for the fi rst time, we report the QLEDs based on all-inorganic perovskite cesium lead halide (CsPbX 3 , X = Cl, Br, I) nanocrystals. The high-quality CsPbX 3 QDs were synthesized through hot-injecting cesium stearate (CsSt) to PbBr 2 solution. The luminescence wavelength can be tuned by both the size of the QDs and varying the halide composite (Cl, Br, and I). The QDs can be easily dispersed in various nonpolar solvents (e.g., toluene, octane, and hexane) serving as ink for solution-based

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monodispersity, and morphology of the QDs as proved in Figure S4 (Supporting Information). On the other hand, the I anions can shift the PL peak of CsPbBr 3 to the lower energy direc-tion. From the PL spectra shown in Figure 2 b, it can be clearly seen that the luminescence can be effectively tuned in the range

from 420 to 585 nm by introducing the Cl and I anions. The dependence of PL spectra on the halide molar ratio is summa-rized in Table S1 (Supporting Information). The alloying QDs pre-serving the basic cubic CsPbBr 3 structures were revealed by XRD patterns (Figure S5, Supporting Information). The XRD pattern

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Figure 1. a) TEM images and b) HRTEM images of monodisperse perovskite QDs. Inset of (a): selected area electron diffraction (SAED) pattern of CsPbBr 3 QDs. c) Enlarged HRTEM images of perovskite CsPbBr 3 QDs and the corresponding cubic crystal structure. d) STEM images and e) elemental mappings of CsPbBr 3 QDs.

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features have no obvious change, with slightly changed peak posi-tions (the smaller Cl − ions lead to a shift of the diffraction peaks toward the large angle direction, while the larger I − ions result in a shift toward the small angle direction). The corresponding QDs dispersed in toluene can emit light in the range from violet to orange under 365 nm light excitation as shown in Figure 2 c. Besides the tunable emission color, their quantum yields are very high in the range of 60%–90%, especially the CsPbBr 3 QDs-assembled thin fi lms with a PL quantum yield of >85%.

The synthesis of perovskite QDs was conducted with oleylamine (OAm) and oleic acid (OA) as surfactants. Therefore, the carboxy and amine groups in OA and OAm were adsorbed or grafted onto the surface of the QDs during the reaction. These surface alkyl groups facilitate the QDs to disperse in

various organic media as inks (Figure 2 c). The as-synthesized all-inorganic QDs can be stored for more than two months, exhibiting higher stability in contrast to organic–inorganic hybrid CH 3 NH 3 PbBr 3 perovskite nanocrystals (Figure S6, Sup-porting Information). In a word, these all-inorganic perovskite QDs, exhibiting a single-crystal structure, good dispersity, and high ink stability, are suitable for low-cost, solution-processed, and fl exible optoelectronics. [ 20–22 ] Associated with the high PL effi ciency and tunable light emission that cover the whole vis-ible wavelength range, the cesium lead halide perovskite QDs have huge potential in light-emitting diodes.

The typical as-prepared QLED device ( Figure 3 a,b) consists of multiple layers in the following order: indium tin oxide (ITO), poly(ethylenedioxythiophene):polystyrene sulfonate

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Figure 2. a) Size-dependent PL spectra of monodisperse perovskite CsPbBr 3 QDs. The controlled sizes were realized by changing the reaction tempera-ture. The reaction temperature was 140, 155, 170, and 185 °C, respectively. b) Composition-tunable PL spectra of perovskite CsPbX 3 QDs by adding the different halides. c) The corresponding sample of perovskite CsPbX 3 QDs shown in (b).

Figure 3. Illustration of multilayer perovskite QLED device. a) The device structure. b) Cross-sectional TEM image showing the multiple layers of mate-rial with distinct contrast. Scale bar, 50 nm. c) Flat-band energy level diagram.

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(PEDOT:PSS, 40 nm), poly(9-vinlycarbazole) (PVK, 10 nm), per-ovskite QDs (10 nm), TPBi (40 nm), and LiF/Al (1/100 nm). Figure 3 c shows a schematic illustration of the fl at-band energy level diagram of the layers. Thin fi lms of TPBi are employed as electron-transporting layers. The PVK is used as hole-transporting and electron blocking layers. The PVK can reduce the hole injection barrier, block the electron in the active layer, and hence allow the holes and electrons to effectively recom-bine in the QD emitting layers.

The current density–luminance–voltage ( J – L – V ) characteristic of the QLEDs based on the typical green CsPbBr 3 QDs is presented in Figure 4 a. The turn-on voltages (calculated with a luminance of 1 cd m −2 ) required for the QLED device are about 4.2 V. The lumi-nance increases as the voltage is increased, which achieves the maximum value of 946 cd m −2 under the applied voltage of 8.8 V. The normalized EL spectrum of the QLEDs is shown in Figure 4 b. The device gives a very narrow EL, which is solely attributed to the band-edge emission of QDs with a slightly redshifted emission from the PL spectrum taken in the QD colloid solution. Excitingly,

no any notable parasitic emissions originated from the charge transport layer (i.e., TPBi or PVK) can be observed in the entire EL spectrum under varied voltages (Figure S7, Supporting Informa-tion). The devices emit bright and uniform green light from the whole pixel under a bias of 5 V as shown in the inset in Figure 4 b.

The current effi ciency and external quantum effi ciency (EQE) as a function of the luminance for the typical green QLEDs are shown in Figure 4 c. A maximum current effi ciency of 0.43 cd A −1 and the EQE of 0.12% are achieved at a luminance of 100 cd m −2 . The power effi ciency curve is also shown in Figure 4 d with a value of 0.14 lm W −1 . The blue and orange QLED devices can also be obtained by using the above structure, and the device performances are summarized in Table 1 . In our blue, green, and orange LEDs using the structure of ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al, the luminances of 742 cd m −2 , 946 cd m −2 , and 528 cd m −2 , with external quantum effi ciencies of 0.07%, 0.12%, and 0.09%, respectively, have been reached. The per-formance of these LEDs based on the all-inorganic perovskite QDs can be further optimized in the future by balancing the

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Figure 4. Device performances. a) Current density and luminance versus driving voltage characteristics for the QLED device. b) EL spectrum at an applied voltage of 5.5 V and, inset, a photograph of a device with an active area of 2 × 2 mm 2 . c) Current density and external quantum effi ciency as a function of luminance for the QLEDs. d) Power effi ciency as a function of luminance for the QLEDs.

Table 1. Characteristics of QDs and QLEDs. PL (photoluminescence), EL (electroluminescence), V on (turn-on voltage), L (Luminance), EQE (external quantum effi ciency), CE (current effi ciency), and PE (power effi ciency).

QD PL λ max. (FWHM) EL λ max. (FWHM) V on Max. EQE Max. CE Max. PE Max. L

Blue 452 nm (23 nm) 455 nm (20 nm) 5.1 V 0.07% 0.14 cd A −1 0.07 lm W −1 742

Green 514 nm (29 nm) 516 nm (23 nm) 4.2 V 0.12% 0.43 cd A −1 0.18 lm W −1 946

Orange 583 nm (21 nm) 586 nm (23 nm) 4.6 V 0.09% 0.08 cd A −1 0.06 lm W −1 528

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charge transporting, improving the PL effi ciency, as well as by removing the surfactants. High crystal quality and high PL quantum yields guarantee that all-inorganic perovskite QDs are promising semiconductors, and have huge potential in other optoelectronic devices, such as mono chromatic narrowband photodetectors, solar cells, and lasers.

The EL spectra can be readily tuned by substitution of Br ions with I ions or Cl ions (CsPb(Cl/Br) 3 , CsPb(Br/I) 3 ) as refl ected in the PL spectra (Figure 2 ). Figure 5 a–c shows the photographs of the QLED devices fabricated on a 4 × 4 cm 2 substrate under operation. These devices can emit uniform and large-area blue, green, and orange light. The normalized EL spectra of blue, green, and orange QLEDs with emission wavelength peaks at 455, 516, and 586 nm are shown in Figure 5 d, with the full width at half maximum (FWHM) smaller than 30 nm for all colors (Table 1 ). All devices exhibit very saturated and pure colors, as demonstrated by the Commission Internationale de l’Eclairage (CIE) chromaticity diagram shown in Figure 5 e. Compared to the National Television System Committee (NTSC) standard color (stars), the blue, green, and orange QLEDs can cover the upper left part of the CIE diagram. Although the all-inorganic perovskite CsPbX 3 nanocrystals can show the bright emission with a wide color gamut, the pure CsPbI 3 or CsPb(Br/I) 3 with a high molar ratio of I ions is not stable for preparing QLED due to its metastable state in the cubic phase. [ 23,24 ]

In summary, we fabricated blue, green, and yellow QLEDs with sharp emissions (FWHM < 30 nm) based on all-inorganic perovskite CsPbX 3 QDs for the fi rst time. The color of QLEDs can be tuned from blue to orange by changing the content and the category of anions. In the blue, green, and orange LEDs with a stacking layered structure of ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al, we achieved luminances of 742 cd m −2 , 946 cd m −2 , and 528 cd m −2 , with external quantum effi ciencies of 0.07%, 0.12%, and 0.09%, respectively. The resultant QLEDs demonstrate that the all-inorganic perovskite CsPbX 3 QDs can grow as a new family of photoelectric material with great potential in a wide variety of optoelectronics, such as lighting,

displays, monochromatic narrowband photodetectors, solar cells, and lasers.

Experimental Section Synthesis of CsPbBr 3 QDs : 15 mL of octadecene (ODE), 3 mL of OAm,

1.5 mL of OA, and PbBr 2 (0.2 g) were loaded into a 100 mL four-neck fl ask, degassed at 100 °C for 10 min, mixed at 100 °C for 30 min, and heated to 170 °C in 10 min under Ar fl ow. 0.55 mL of Cesium Stearate (CsSt) solution (0.15 M in ODE) was quickly injected. After 5 s, the reaction mixture was cooled by the ice-water bath. The resultant QDs were precipitated by 20 mL of acetone and separated via centrifugation. The separated QDs were redispersed in 8 mL of 1-octane serving as ink for solution process QLEDs active layer.

Device Fabrication : PEDOT:PSS solutions (Baytron P VPAl 4083, fi ltered through a 0.22 µm fi lter) were spin-coated onto the ITO-coated glass substrates at 4000 rpm for 60 s and baked at 140 °C for 15 min. The hole transporting and electron blocking layer were prepared by spin-coating PVK chlorobenzene solution (concentration: 6 mg mL −1 ) at 4000 rpm for 1 min. Perovskite QDs were deposited by spin-coating at 2000 rpm for 60 s. TPBi (40 nm) and LiF/Al electrodes (1 nm/100 nm) were deposited using a thermal evaporation system through a shadow mask under a high vacuum of 2 × 10 −4 Pa. The device active area was 4 mm 2 as defi ned by the overlapping area of the ITO and Al electrodes. The devices were encapsulated in the glove-box by a cover glass slide using curable resin.

Characterization and Device Measurements : One drop of as-synthesized perovskite QDs dispersion was diluted and dispersed onto a carbon-coated Cu grid, and hexane was allowed to evaporate at room temperature. TEM and HRTEM images and energy-dispersive X-ray spectroscopy were taken on a Tecnai G2 F30 S-TWIN and Titan TM G2 60-300 TEM instrument. The PL spectra of the QDs were obtained by using a Varian Cary Eclipse spectrometer. The EL spectra and luminance ( L )–current density ( J )–voltages ( V ) characteristics were collected by using a Keithley 2400 source, a calibrated luminance meter, and a PR-705 SpectraScan spectrophotometer (Photo Research) in air and at room temperature.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Figure 5. a–c) Photographs of QLED devices with the Nanjing University of Science and Technology (NUST) logo. d) The EL spectra (straight line) of sample shown in (a–c) under applied voltage of 5.5 V, and the PL spectra (dashed line) of QDs dispersed in hexane. e) CIE coordinates of the three-color QLEDs (circular) compared to the NTSC color standards (stars).

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Acknowledgements J.S., J.L., and X.L. contributed equally to this work. This work was fi nancially supported by the National Basic Research Program of China (2014CB931700/2014CB931702), NSFC (61222403), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Received: May 29, 2015 Revised: August 9, 2015

Published online: October 7, 2015

[1] H. B. Shen , W. R. Cao , N. T. Shewmon , C. C. Yang , L. S. Li , J. G. Xue , Nano Lett. 2015 , 15 , 1211 .

[2] Y. X. Yang , Y. Zheng , W. R. Cao , A. Titov , J. Hyvonen , J. R. Manders , J. G. Xue , P. H. Holloway , L. Qian , Nat. Photonics 2015 , 9 , 259 .

[3] Y. Zhang , C. A. Xie , H. P. Su , J. Liu , S. Pickering , Y. Q. Wang , W. W. Yu , J. K. Wang , Y. D. Wang , J. I. Hahm , N. Dellas , S. E. Mohney , J. A. Xu , Nano Lett. 2011 , 11 , 329 .

[4] V. L. Colvin , M. C. Schlamp , A. P. Alivisatos , Nature 1994 , 370 , 354 . [5] W. K. Bae , J. Kwak , J. Lim , D. Lee , M. K. Nam , K. Char , C. Lee ,

S. Lee , Nano Lett. 2010 , 10 , 2368 . [6] X. L. Dai , Z. X. Zhang , Y. Z. Jin , Y. Niu , H. J. Cao , X. Y. Liang ,

L. W. Chen , J. P. Wang , X. G. Peng , Nature 2014 , 515 , 96 . [7] H. C. Yoon , J. H. Oh , M. Ko , H. Yoo , Y. R. Do , ACS Appl. Mater.

Interfaces 2015 , 7 , 7342 . [8] K. H. Lee , J. H. Lee , H. D. Kang , B. Park , Y. Kwon , H. Ko , C. Lee ,

J. Lee , H. Yang , ACS Nano 2014 , 8 , 4893 . [9] J. Lim , M. Park , W. K. Bae , D. Lee , S. Lee , C. Lee , K. Char , ACS Nano

2013 , 7 , 9019 .

[10] W. J. Zhang , Q. Lou , W. Y. Ji , J. L. Zhao , X. H. Zhong , Chem. Mater. 2014 , 26 , 1204 .

[11] W. K. Bae , J. Lim , D. Lee , M. Park , H. Lee , J. Kwak , K. Char , C. Lee , S. Lee , Adv. Mater. 2014 , 26 , 6387 .

[12] F. Zhang , H. Zhong , C. Chen , X.-g. Wu , X. Hu , H. Huang , J. Han , B. Zou , Y. Dong , ACS Nano 2015 , 9 , 4533 .

[13] G. Li , Z.-K. Tan , D. Di , M. L. Lai , L. Jiang , J. H.-W. Lim , R. H. Friend , N. C. Greenham , Nano Lett. 2015 , 15 , 2640 .

[14] M. Kulbak , D. Cahen , G. Hodes , J. Phys. Chem. Lett. 2015 , 6 , 2452 . [15] F. Hao , C. C. Stoumpos , D. H. Cao , R. P. H. Chang ,

M. G. Kanatzidis , Nat. Photonics 2014 , 8 , 489 . [16] Y. H. Kim , H. Cho , J. H. Heo , T. S. Kim , N. Myoung , C. L. Lee ,

S. H. Im , T. W. Lee , Adv. Mater. 2015 , 27 , 1248 . [17] L. Protesescu , S. Yakunin , M. I. Bodnarchuk , F. Krieg , R. Caputo ,

C. H. Hendon , R. X. Yang , A. Walsh , M. V. Kovalenko , Nano Lett. 2015 , 15 , 3692 .

[18] Z. K. Tan , R. S. Moghaddam , M. L. Lai , P. Docampo , R. Higler , F. Deschler , M. Price , A. Sadhanala , L. M. Pazos , D. Credgington , F. Hanusch , T. Bein , H. J. Snaith , R. H. Friend , Nat. Nanotechnol. 2014 , 9 , 687 .

[19] H. Zhu , Y. Fu , F. Meng , X. Wu , Z. Gong , Q. Ding , M. V. Gustafsson , M. T. Trinh , S. Jin , X. Y. Zhu , Nat. Mater. 2015 , 14 , 636 .

[20] J. Song , S. A. Kulinich , J. Li , Y. Liu , H. Zeng , Angew. Chem. Int. Ed. 2015 , 54 , 462 ; Angew. Chem. 127 , 472 .

[21] J. Z. Song , J. H. Li , J. Y. Xu , H. B. Zeng , Nano Lett. 2014 , 14 , 6298 .

[22] J. Z. Song , H. B. Zeng , Angew. Chem. Int. Ed. 2015 , 54 , 9760 ; Angew. Chem. 127 , 9896 .

[23] C. C. Stoumpos , C. D. Malliakas , M. G. Kanatzidis , Inorg. Chem. 2013 , 52 , 9019 .

[24] D. M. Trots , S. V. Myagkota , J. Phys. Chem. Solids 2008 , 69 , 2520 .