Impurity Ions Codoped Cesium Lead Halide Perovskite Nanocrystals

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Functional Inorganic Materials and Devices

Impurity Ions Codoped Cesium Lead Halide Perovskite Nanocrystals with Bright White Light Emission towards UV-WLED Gencai Pan, Xue Bai, Wen Xu, Xu Chen, Donglei Zhou, Jinyang Zhu, He Shao, Yue Zhai, Biao Dong, Lin Xu, and Hongwei Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14275 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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ACS Applied Materials & Interfaces

Impurity Ions Codoped Cesium Lead Halide Perovskite Nanocrystals with Bright White Light Emission towards UV-WLED Gencai Pan, Xue Bai,* Wen Xu, Xu Chen, Donglei Zhou, Jinyang Zhu, He Shao,Yue Zhai, Biao Dong, Lin Xu, Hongwei Song*

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China.

*E-mail:

[email protected], [email protected].

ACS Paragon Plus Environment 1

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ABSTRACT White light emitting diodes (WLEDs) based on all-inorganic perovskite CsPbX3 (X=Cl, Br, I) nanocrystals (NCs) has attracted extensive interests. However, the native ion exchange among halides makes them extremely difficult to realize the white emission. Herein, we demonstrate a novel strategy to obtain WLED phosphors based on the co-doping of different metal ions pairs, such as Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+ into the stable CsPbCl3 as well as CsPbClxBr3-x NCs. Notably, by typical anion exchange reaction, the highly efficient white emission of Ce3+/Mn2+ co-doped all-inorganic CsPbCl1.8Br1.2 perovskite NCs was achieved, with optimal photoluminescence quantum yield (PLQY) of 75 %, which is much higher than the present record of 49 % for single perovskite phosphors. Moreover, the WLED with luminous efficiency of 51 lm/W, based on 365-nm ultraviolet (UV) chip and CsPbCl1.8Br1.2: Ce3+/Mn2+ nanophosphor was achieved. This work represents a novel device for perovskite-based phosphor-converted WLED. KEYWORDS:

perovskite nanocrystals, co-doping, energy transfer, anion exchange ， white

light-emitting diode.

INTRODUCTION Phosphor-converted WLEDs, the next-generation lighting sources, have attracted much more attention owing to their superior properties such as cost-effective, simple preparation, high efficiency and energy-saving etc.1-9 Currently, the commercial white LEDs are still based on high-efficient phosphor YAG:Ce3+ with blue chip excitation.10,11 However, its color-rendering index (CRI) is poor because of the lack of red light component.9 Moreover, the high correlated color temperature (CCT) is also a negative factor for indoor lighting.6 Fortunately, WLEDs


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based on ultraviolet-pumped tricolor phosphors may be able to avoid these problems.12-14 However, the low luminous efficiency of LEDs and instability of phosphors at all layers are still urgent problems.15, 16 The ultraviolet-pumped WLEDs with a single layer phosphor would be an excellent choice. Lanthanide and transition metal ions co-doping is considered as a technological strategy, which can make single layer phosphor possessing multicolor emissions.17,

18

Many

researchers developed the WLEDs based on single-emitting nanocrystals (NCs) by doping ions into traditional quantum dots (QDs). However, their PLQYs were all too low to achieve real application. For instance, the white light emission based on Cu and Mn co-doped ZnSe QDs was reported with PLQY of 17%.5 Represented by Shao et al, the PLQY for Mn doped ZnSe QDs white phosphors was only 10.2%.19 Wang et al. reported Mn:ZnSe/Cu:ZnS white emission QDs with the maximal PLQY of around 17%.20 All-inorganic perovskite CsPbX3 (X=Cl, Br, I) NCs are very interesting materials owing to their excellent optical properties such as high extinction coefficient, narrow full width at half maximum, wide color gamut, high PLQYs, and the promising optoelectronics applications.21-29 However, the instability of red emission based on CsPbI3 and the anion-exchange reaction among different perovskite NCs severely limit their practical applications on white lighting. Similarly, the doping approach could be able to solve these problems.18,

30

For example, Mn2+ doped

CsPbCl3 NCs show the stable red emission around 600 nm, contributed to 4T1 – 6A1 d–d transitions of the Mn2+ ions emission.31-38 Various lanthanide ions doped perovskite NCs possess the overall visible emission and the NIR emission.18,

39

However, to date, a very few papers

reported the white emission based on single component CsPbX3 (X=Cl, Br, I) provskite NCs with the relatively low PLQYs and poor color-rendering index.30, 40, 41 For example, the white emission based on Mn2+ ions doped CsPb2Cl5 nanoplatelets has been only reported.30 This is the



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most efficient material reported so far, in which its PLQY is only up to 49%, and the chromaticity coordinate (0.35,0.32) is not standard. In the face of low quantum efficiency and poor rendering index for single ion doped perovskite NCs. The impurity ions pairs co-doped CsPbX3 provskite NCs may be a better choice, because ions pairs co-doping can introduce more emission components.42-44 Therefore, it is essential for achieving a high-efficiency white color emission based on impurity ions pairs co-doped CsPbX3 provskite NCs. In this work, a series of metal ions pairs, such as Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+ co-doped CsPbCl3 perovskite NCs were prepared by the modified hot-injection method.18 The stable and efficient white emission for a single composition based on CsPbCl3 perovskite NCs was successfully obtained. It should be highlighted that, the introduction of Ce3+ ions in CsPbCl3 NCs not only complements the blue and green components, but also sensitizes the emission of the other ions, such as Mn2+, Eu3+ and Sm3+ with red emission. Moreover, by typical anion exchange reaction,45 the highest efficient white emission of Ce3+/Mn2+ co-doped all-inorganic CsPbCl1.8Br1.2 perovskite NCs are achieved, with PL QY of 72 ± 3%. Finally, the WLED based on a 365-nm chip and CsPbCl1.8Br1.2 : Ce3+/Mn2+ nanophosphor with the luminous efficiency of 51 lm/W and the CRI of 89 were realized. EXPERIMENTAL SECTION Materials. Cs2CO3 (99.9%), octadecene (ODE, 90%), oleic acid (OA, 90%) oleylamine (OLA, 70%), PbCl2 (99.99%), CeCl3•6H2O (99.99%), SmCl3•6H2O (99.99%), EuCl3•6H2O (99.99%), MnCl2 (90%) ， BiCl3 (90%) , bromotrimethylsilane (TMS-Br, 98%), toluene (ACS grade, Fischer), ODE were purchased Macklin, others were purchased from Sigma-Aldrich and all were used directly.


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Synthesis of Ce3+ and Mn2+ co-doped CsPbCl3 NCs. Cerium chloride (0.188mmol) was completely dissolved into 1.5 ml OA and 10 ml ODE in 50 mL three-necked flask and heated at 160 °C and stirred for 1 h under purging N2 flow. And then PbCl2 (0.376mmol), MnCl2 and 1ml OLA were added and continued heating at 160 °C and stirring for 1 h. After the adequate dissolution of PbCl2 and MnCl2, The temperature was raised to 210 °C under purging N2 gas. The as-prepared Cs-oleate was injected promptly, and after 1min, the three-necked flask was immediately placed in an ice-water bath and cooled down to room temperature. The Ce and Mn co-doped perovskite NCs in the ODE was purified by centrifuge. A series of molar doping concentrations (2.3%, 4.1%, 9.1% and 11.6% ) of Mn2+ ions were got by the corresponding molar feed amount of MnCl2 (0.376mmol, 0.564mmol, 0.795mmol and 1.589mmol). Synthesis of Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+ co-doped CsPbCl3 NCs. Similar to the synthesis of Ce and Mn co-doped CsPbCl3 NCs, cerium chloride (bismuth chloride) and europium chloride (samarium chloride) were completely dissolved into 1.5ml OA and 10 ml ODE in 50 mL three-necked flask and heated at 160 °C and stirred for 1 h under purging N2 flow. And then, PbCl2 and 1ml OLA were added and continued heating at 160 °C for 1 h. After the adequate dissolution of PbCl2, the temperature was raised to 210 °C under purging N2 gas. The as-prepared Cs-oleate was injected promptly, and after 1min, the three-necked flask was immediately placed in an ice-water bath and cooled down to room temperature. Anion exchange reaction of Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+ co-doped CsPbCl3 NCs. The anion exchange reaction was performed at room temperature under ambient conditions. To get impurity ions pairs (Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+) co-doped CsPbClxBr3-x NCs, the as-synthesized impurity ions pairs co-doped CsPbCl3 NCs were dispersed in toluene. A TMS-Br stock solution was slowly added to a



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vigorously stirring NC solution. Progress in the halide exchange reaction was monitored by performing PL measurements taken periodically during the reaction. With the addition of the Brprecursor, a monotonic red shift of the band-edge PL was observed. Characterizations. The TEM images were acquired on a Titan transmission electron microscope (FEI Company) with an operated voltage of 300 kV. The high resolution TEM (HRTEM) images were acquired in EFTEM mode with a 20 KeV energy slit inserted around the zero energy loss of electrons. The energy dispersive X-ray spectroscopy (EDS) elemental mapping were got on a Talos F200X transmission electron microscope with an operated voltage of 200 kV equipped with an energy dispersive detector. Powder XRD patterns were recorded by using a Rigaku D/Max-Ra X-ray diffractometer with a monochrom at Cu Kα radiation (λ = 1.54178 Å). Inductively coupled plasma optical emission spectrometry (ICP-OES) were performed with a Varian 720-ES ICP-optical emission spectrometer. X-ray photoelectron spectroscopy (XPS) characterizations were conducted on a Kratos Axis Ultra DLD spectrometer. The ultraviolet-visible (UV-vis) absorption spectra were collected by using a Shimadzu UV-3101PC scanning spectrophotometer. The photoluminescence (PL) spectra were pumped using a laser-system consisting of a Nd: YAG pumping laser (1064 nm), a third-order Harmonic-Generator (355 nm) and a tunable optical parameter oscillator (OPO, Continuum Precision II 8000) with a pulse duration of 10 ns, a repetition frequency of 10 Hz and a line width of 4-7 cm-1. A visible photomultiplier (350-850nm) combined with a double-grating monochromator were used for spectral collection. The lifetime was got by spectroﬂuorometer (FLS980, Edinburgh Instruments). The PLQYs were acquired using an integrating sphere incorporated into a spectroﬂuorometer (FLS980, Edinburgh Instruments). Quantum yield was


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then calculated by using the Edinburgh L980 software package. The luminous efficiency of the white LEDs was determined by an Ocean Optics spectrometer equipped with an integral sphere.

RESULTS AND DISCUSSION The impurity ions pairs (Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+ , Bi3+/Eu3+ and Bi3+/Sm3+) co-doped CsPbCl3 perovskite NCs were prepared by the modified hot-injection method, respectively.18 Considering the higher temperature resistance of synthesizing CsPbCl3 NC than CsPbBr3 NC, CsPbCl3 NC was selected as the host materials, owing to the high synthesizing temperature is favorable for impurity ions doping into the lattice of NC host.18, 35 To achieve the co-doping of impurity ions pairs, the rare earth chloride (CeCl3, EuCl3 and SmCl3) , BiCl3 and oleic acid (OA) were firstly completely dissolved in octadecene (ODE). Then, the MnCl2, PbCl2 and oleylamine (OLA) were added and continued heating at 160 °C for 1 h. Afterwards, the temperature was raised to 210 °C under purging N2 gas. Finally, the as-prepared Cs-oleate was injected into the above contents, and after 1min, the flask was immediately placed in an ice-water bath. After purification by centrifugation, the products were dispersed in toluene. The details of the synthesis are in Experimental Section. Transmission electron microscope (TEM) images in Figure 1a demonstrate that both the undoped and doped NCs possess uniform size and similar tetragonal shape, but the size of doped NCs decreases slightly from 7.7±0.5 to 7.1±0.5 nm, 7.2±0.5 nm, 7.3±0.5 nm, 7.3±0.5 nm and 7.4±0.5 nm for the co-doping with, Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+, respectively (Figure S1). As revealed by X-ray diffraction (XRD) patterns (Figure 1b), the cubic crystalline structure is formed for both the undoped and doped CsPbCl3 NCs (PDF#18-0366). The doping of impurity ions pairs leads to the diffraction peaks shift toward higher diffraction



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angles (Table S1), which is due to the lattice contraction induced by partly replacing Pb2+ ions with the smaller impurity ions in host lattice.18,

35

The legible crystal lattice fringes can be

observed by high resolution transmission electron microscopy (HRTEM) images of undoped and

Figure 1. (a) TEM images, (b) XRD patterns of impurity ion pairs (Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+) co-doped CsPbCl3 NCs. (c) and (d) HR-TEM images of undoped

and

2.7%Ce3+/9.1%Mn2+

2.7%Ce3+/9.1%Mn2+

co-doped

2.7%Ce3+/9.1%Mn2+

co-doped

co-doped

CsPbCl3 CsPbCl3

CsPbCl3

NCs. NCs.

(f-k) (L)

NCs. EDX

XPS

(e)

SAED

mapping

spectra

of

pattern

of

images

of

undoped

and

2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3 NCs.


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2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3 NCs (Figure 1c and 1d). Taking Ce/Mn co-doped NCs as an example, the lattice constants of undoped and 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3 NCs for the (101) diffraction plane are 3.96Å and 3.93Å, respectively. The lattice constant of the 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3 NCs decreases slightly comparing to the undoped NCs, which is consistent with the result of XRD patterns. In addition, Figure 1e shows the selected area electron diffraction (SAED) pattern of the 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3 NCs, in which (101) and（200）planes of cubic phase were identified.

Combined with the HR-TEM

images (Figure 1 c-d), it can be concluded that the doped NCs maintains the same perovskite structure with the bare one. The energy-dispersive X-ray (EDX) mapping images further evidence that Ce and Mn elements were involved in the CsPbCl3 perovskite NCs (Figure 1f-k). To further verify the doping sites of Ce3+/Mn2+ ions, the X-ray photoelectron spectroscopy (XPS) comparative analysis of undoped and 2.7%Ce3+/9.1%Mn2+ ions co-doped CsPbCl3 NCs were also performed. The result indicates that the undoped and 2.7%Ce3+/9.1%Mn2+ ions co-doped CsPbCl3 NCs both comprise Cs, Pb, Cl, C, and O elements (Figure 1L). However, two additional signals in 2.7%Ce3+/9.1%Mn2+ ions co-doped CsPbCl3 NCs appear, which can be ascribed to the Ce3+ 3d and Mn2+ 2p signals, respectively. The high-resolution XPS spectra of Ce3+ 3d and Mn2+ 2p further affirm this result (Figure S2a and S2b). Moreover, from high resolution XPS of Mn, the Mn peak at 641.1eV with binding energy hints at the presence of Mn2+, which suggests the valence state of the manganese ion is still +2 in CsPbCl3 NCs.46 Furthermore, it is observed that the binding energy of Pb2+ 4f5/2 and Pb2+ 4f7/2 decreases obviously as introducing Ce3+/Mn2+ ions (Figure S2c), while the binding energy of Cs+ 3d and Cl- 2p exhibits little variations (Figure S2d, S2e). These results indicate that Ce3+/Mn2+ ions have incorporated into the lattice of CsPbCl3



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perovskite NCs and replace the Pb2+ sites.18, 39 The doping concentrations of impurity ions were identified by the inductively coupled plasma optical emission spectrometry (ICP-OES) measurements, and the doping concentrations of impurity ions were marked in Figure 1a. The absorption spectra of impurity ions pairs co-doped CsPbCl3 NCs show that the excitonic absorption peaks of the host NCs blue shift with various impurity ions pairs doping and the increase of doping concentration of impurity ions (Figure 2a-2e). It suggests that the band gap of host NCs after impurity ions doing gradually becomes larger, which is contributed to the lattice contraction of doped NCs.18,

47

Consistently, under the excitation of 365 nm light, the

emission spectra for the impurity ions pairs co-doped CsPbCl3 NCs exhibit a narrow band-edge emission, which shifts to blue comparing with that of CsPbCl3 NCs. Interestingly, the other two

Figure 2. (a-e) Absorption and emission spectra of undoped and impurity ions pairs (Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+) co-doped CsPbCl3 perovskite NCs. (f) Optimal PLQY of impurity ions pairs (Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+) co-doped CsPbCl3 perovskite NCs.


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emission components appear, corresponding to blue and green emission originated from the intrinsic transitions (4f-5d) of Ce3+ or the intrinsic transitions (3P1–1S0) of Bi3+ ions and red emission originated from the intrinsic transitions (4T1 – 6A1) of Mn2+, (5D0-7FJ (J=1,2,3,4)) of Eu3+ and

(4G5/2-6HJ (J = 5/2, 7/2, 9/2, 11/2)) of Sm3+ ions. These results indicate that an

efficient energy transfer may occur from CsPbCl3 NC host to the energy levels of impurity ions.39 Moreover, for Ce3+/Sm3+ co-doped CsPbCl3 perovskite NCs, the characteristic peak (4G5/2-6H5/2) of Sm3+ was not observed, it may be overlaid by the stronger emission of Ce3+ ions.

Figure 3. (a) Absorption spectra and (b) Emission spectra of undoped and 2.7%Ce3+ and Mn2+ ions with different concentrations doped CsPbCl3 perovskite NCs. (c) Excitation spectra of 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3 perovskite NCs. (d) Decay time of exciton and Ce3+ ions , and total PL QY versus different Mn2+ ions doping concentrations. (e) Energy transfer efficiency versus different Mn2+ ions doping concentrations. (f) Energy level diagram of Ce3+/Mn2+ co-doped CsPbCl3 perovskite NCs.



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After optimizing the doping concentration, the optimal PLQYs of impurity ions pairs (Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+) co-doped CsPbCl3 perovskite NCs were obtained, as shown in Figure 2f. The PLQY of as-prepared CsPbCl3 NCs is 5%, and it increases to 13%, 20%, 22%, 34%, and 58% for Bi3+/Sm3+, Bi3+/Eu3+, Ce3+/Sm3+, Ce3+/Eu3+, and Ce3+/Mn2+ co-doped CsPbCl3 NCs. It should be highlighted that the PLQY of 2.7%Ce/9.1% Mn co-doped CsPbCl3 NCs improves more than 10 times compared to the bare CsPbCl3 NCs. And the CRI of all the doped CsPbCl3 NCs reaches around 70. To understand the luminescent mechanism for impurity ions co-doped CsPbCl3NCs, taking Ce/Mn co-doped CsPbCl3 NCs as an example, the dependence of photoluminescence on the Mn doping concentration was investigated, as displayed in Figure 3. As mentioned in Figure 2a, the absorption spectra of Ce/Mn co-doped CsPbCl3 NCs show the blue-shift of the excitonic absorption peaks compared to CsPbCl3 NCs with the increase of doping concentration of Mn2+ ions (Figure 3a),due to lattice contraction of the host NC. Similarly, the excitonic emission peaks for the Ce3+/Mn2+ ions co-doped CsPbCl3 NCs exhibit a narrow band-edge emission, which shifts to blue comparing with that of CsPbCl3 NCs (Figure 3b). Remarkably, the exciton emission intensity of 2.7%Ce3+ doped CsPbCl3 NCs becomes stronger than that of undoped perovskite NCs, attributed to that some nonradiative recombination pathways (such as Cl vacancy) may be removed through the introducing of metal ions.18, 39 As expected, the exciton emission position shifts continuously to blue with the increase of the doping concentration of Mn2+ ions. Meanwhile, the other two band emission components appear, corresponding to 4f-5d transition for Ce3+ ions and 4T1-6A1 transition for Mn2+ ions, which center at 434 nm and 592 nm, respectively. These results indicate that an efficient energy transfer occurs from CsPbCl3 NC host to the energy levels of Ce3+/Mn2+ ions. To deeply explore the process of energy transfer, the


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excitation spectra selectively monitoring at the emission peak of excitonic emission of CsPbCl3 NCs (394 nm), Ce3+ (434 nm) and Mn2+ (592 nm) ions were recorded (Figure 3c). The excitation peak of Ce3+ ions matches closely with the excitonic excitation peak of CsPbCl3 NC host, which confirms that the emissions of Ce3+ ions are sensitized by the CsPbCl3 NC host. By the Gaussian decomposition, the excitation spectrum monitoring at the of emission peak of Mn2+ ions contains three components, in which two broadbands matches closely with the excitonic absorption of CsPbCl3 NC host, while the third broadband should be corresponding to the absorption of Ce3+ ions. Previous reports demonstrated the excitation spectrum of Mn doped CsPbCl3 NC monitoring the Mn2+ ions emission doesn’t contain the third broadband.35 Therefore, it can be drawn that the emissions of Mn2+ ions are sensitized by the Ce3+ ions and CsPbCl3 NC host. Similarly, the excitation spectra of Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+ ions pairs co-doped CsPbCl3 NCs were also measured (Figure S3). The results demonstrate that the emissions of Eu3+ ions or Sm3+ ions in Ce3+/Eu3+ or Ce3+/Sm3+ co-doped CsPbCl3 NCs originates from the sensitization of both CsPbCl3 NC host and Ce3+ ions, similarly with the Ce3+/Mn2+ co-doped NCs (Figure S3a and S3b). Differently, the emissions of Eu3+ ions or Sm3+ ions in Bi3+/Eu3+ or Bi3+/Sm3+ co-doped CsPbCl3 NCs mainly result from CsPbCl3 NC host itself (Figure S3c and S3d). In addition, the emission decays of exciton, Ce3+ ions, and Mn2+ ions were obtained, as shown in Figure 3d and Figure S4. The emission decays of Mn2+ ions present a single exponential behavior, which is consistent with the previous reports for the intrinsic transitions of Mn2+ ions.35 The excitonic emission of the CsPbCl3 NCs displays a bi-exponential process, formulated by 𝐼(t) = 𝐼1exp ( ― 𝑡/𝜏1) + 𝐼2exp ( ― 𝑡/𝜏2)

(1)



where 𝜏1 and direct

𝜏2 represent

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the fast and slow decay process, associated to the quenching and

radiative recombination of free charge carriers in CsPbCl3 NCs, respectively.18 The

average lifetimes of exciton and Ce3+ ions depended on different Mn doping concentrations were calculated (Figure 3d and Table S2). The results indicate the average lifetimes of the excitonic and Ce3+ ions emission of the Ce/Mn co-doped CsPbCl3 NC both decrease with the increase of Mn doping amount. It indicates efficient energy transfer occur from NCs and Ce3+ to Mn2+ ions, according to the observed result in Figure 3c. The energy transfer efficiency ηT from NCs host or Ce3+ ions to Mn2+ ions were determined by the following formula, respectively (Figure 3e):

𝜂𝑇 = 1 ―

𝜏𝑆

𝜏 𝑆0

(2)

where 𝜏𝑆0 and τS stand for the lifetimes of NCs host or Ce3+ ions in the absence and the presence of Mn2+, respectively. The energy transfer efficiency from host NC to Mn2+ ions is only up to about 26%, while the energy transfer efficiency from Ce3+ ions to Mn2+ ions is up to around 60%. The results suggest that Ce3+ doping is beneficial to the red emission of Mn2+ ions, which is consistent with the results of the excitation spectra. And more, the total PLQYs for undoped and 2.7%Ce3+ and Mn2+ ions with different concentrations doped CsPbCl3 perovskite NCs were measured with YAG: Ce3+ (BM302D, Jiangsu Bree Optronics Co., Ltd., peaking at 551 nm) as a reference (Figure 3d). The maximal PLQY of 58% was achieved for 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3 perovskite NCs. However, for 2.7%Ce3+/11.6%Mn2+ co-doped CsPbCl3 perovskite NCs, the PLQY reduced to 55%, which is due to concentration quenching. According to the above results, the energy level diagram of Ce3+/Mn2+ co-doped CsPbCl3 perovskite NCs is schemed in Figure 3f. Under the excitation of 365 nm light, the CsPbCl3 perovskite NCs absorb


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the light energy and emit the 394 nm light via the radiative recombination of excitons between the valance band and the conduction band of CsPbCl3 NC. Besides the excitons recombination, a part of energy would directly transfer to Ce and Mn, leading to the emission of Ce and Mn. More importantly, a part of the energy on Ce3+ ions would further transfer to Mn2+, resulting in the enhanced emission of Mn2+. Because the energy level between Ce and Mn is much closer that of

Figure 4. (a-d) TEM images and size distributions of 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3, CsPbCl2.57Br0.43(Cl:Br=6:1), CsPbCl2.25Br0.75(Cl:Br=3:1), and CsPbCl1.8Br1.2(Cl:Br=1.5:1) NCs. (e) XRD patterns of 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl3, CsPbCl2.57Br0.43, CsPbCl2.25Br0.75, and CsPbCl1.8Br1.2 NCs. (f) Survey XPS of 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl1.8Br1.2 NCs.



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between NCs host and Mn, thus the Ce3+ doping is beneficial to the red emission of Mn2+ ions, leading to the high PLQY (55±3%). From the above results, we obtained a series of white light emission single component materials, in which Ce3+/Mn2+ co-doped CsPbCl3 perovskite NCs is with the PLQY of 55 ± 3% and CRI of 70. To further improve the PLQY and CRI , the Br- ions replacing Cl- ions partly by anion-exchange reaction was conducted in 2.7% Ce3+/9.1% Mn2+ co-doped CsPbCl3 NCs.45 After anion-exchange reaction, the samples with the different ratios of Br- and Cl- ions were characterized by TEM images. The samples exhibit similar tetragonal morphology and slight size variation (Figure 4a-d), and the size increases from 7.7 ± 0.5 to 8.2 ± 0.5 nm with replacing Clwith Br-. The powder X-ray diffraction (XRD) patterns further show that the anion-exchange reaction does not affect cubic crystalline structure of CsPbCl3 NCs. Moreover, with increasing of bromotrimethylsilane (TMS-Br) ratio in reaction systems, the peak position of (101) plane slightly shifts toward smaller angle (Figure 4e), which indicates that the interplanar distance becomes larger with the increasing of Br- ions. To further verify the influence of anion-exchange reaction, X-ray photoelectron spectroscopy (XPS, Figure 4f) of the as-prepared NCs was studied. The results show that the 2.7%Ce/9.1%Mn co-doped CsPbClxBr3-x NCs comprise Cs, Pb, Ce, Mn, Cl, Br, C and O elements after the anion-exchange reaction, which reveals that the NCs still contain Ce and Mn elements after anion-exchange reaction. Moreover, The ICP results of 2.7%Ce/9.1%Mn co-doped CsPbCl3 NCs and 2.7%Ce/9.1%Mn co-doped CsPbCl1.8Br1.2 indicate the doping concentration of Ce and Mn hardly change, which further proves anion-exchange reaction hardly affect Ce3+ and Mn2+ ions doping. Furthermore, the steady-state UV-vis absorption spectra of Ce3+/Mn2+ co-doped CsPbClxBr3-x NCs show that the first excitonic peak of NCs shifts to the lower energy side with the increasing


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of Br- ions (Figure 5a), which indicates the bandgaps of NCs become narrower, which is consistent with previous report.45 The PL spectra also show that a red-shift from 394 to 429 nm

Figure 5. (a) Absorption and (b) PL spectra of 2.7%Ce/9.1%Mn co-doped CsPbClxBr3-x NCs. (c) CIE chromaticity coordinate of the LED from 2.7%Ce/9.1%Mn co-doped CsPbClxBr3-x NCs(A (0.42,0.33),B(0.39,0.32),C(0.37,0.30), D(0.33,0.29)). The inset is PL images of 2.7%Ce/9.1%Mn co-doped CsPbClxBr3-x NCs under 365 nm UV lamp. (d) PL spectra of the WLED. The inset 1 is white phosphor powder of 2.7%Ce/9.1%Mn co-doped CsPbCl1.8Br1.2 NCs with PS. The inset 2 is the photograph of the device operated at 3.0V (The WLED is fabricated by coating Ce3+/Mn2+ co-doped CsPbCl1.8Br1.2 NCs mixed PS composites on 365-nm chip). (e) PL spectra of WLED as a function of time. (f) Normalized integrals of the emission peaks at 429 nm, 460 nm and 592 nm for Ce3+/Mn2+ co-doped CsPbCl1.8Br1.2 NCs mixed PS composites.



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with increasing the concentration of Br- ions (Figure 5b). These are ascribed to the typical halide component dependent luminescent behavior in peroskite NCs. Moreover, the intensity of PL peak of NCs host enhances obviously with increasing the concentration of Br- ions, meanwhile, the emission intensity of Mn2+ ions does not reduce. The excitation spectra after anion-exchange reaction

of

Ce3+/Mn2+

co-doped

CsPbCl3

perovskite

NCs,

CsPbCl2.57Br0.43

(Cl:Br=6:1),CsPbCl2.25Br0.75 (Cl:Br=3:1), and CsPbCl1.8Br1.2 (Cl:Br=1.5:1) NCs further displays that the emission behavior of Ce3+ and Mn2+ does not change, coming from the energy transfer from NCs host to Ce3+, and NCs host and Ce3+ to Mn2+ respectively. In view of this, the ratio of Cl and Br is only adjusted to 1.8:1.2 and the Br- ion doesn’t increase more. Because the ratio of Br- ions increases too high, the bandgap of perovskite NC host becomes narrower. It will be not benefit for transferring energy from perovskite NC host to Ce3+ ions, and the sensitization of Ce3+ ions would significantly be reduced. More interestingly, the chromaticity coordinate has also been adjusted by anion-exchange reaction (Figure 5c), and the CRI reach to 89 (Table S3). And the overall PLQY of white light has also been greatly improved, and the maximal PLQY is up to 75% (Table S3), which is a new record for single component NCs with white color emission. The fluorescent images of corresponding samples are shown in the inset in Figure 5c. Moreover, the PLQY and CRI for Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+ co-doped CsPbCl3 NCs via the anion-exchange reaction were also recorded in Table S3. The optimized PLQYs of Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+ and Bi3+/Sm3+ co-doped CsPbCl1.8Br1.2 NCs are 53%, 41%, 43% and 29%, respectively. Moreover, the CRI of Ce3+/Eu3+ co-doped CsPbCl1.8Br1.2 NCs is up to 92. It can be concluded that the PLQYs and CRI have been highly improved by anion-exchange reaction.


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As is known, the polystyrene (ps) can dramatically improve the stability of perovskite NCs.48 Therefore, the Ce3+/Mn2+ co-doped CsPbCl1.8Br1.2 NCs are mixed with PS in toluene. After evaporating the mixture for 20 min at 30°C in vacuum drying oven, the sticky mixture was coated on a commercially available 365-nm GaN LED chip, and putted the as-prepared LEDs in fume cupboard for 10 hours. The WLED device exhibits white emission (The inset 2 in Figure 5d). The emission spectrum is shown in Figure 5d and the color coordinate is (0.33, 0.29). The luminous efficiency is 51 lm/W and the Rendering Index is up to 89. The stability of WLED was surveyed (Figure 5e, 5f and Figure S6). As can be seen, by blending Ce3+/Mn2+ co-doped CsPbCl1.8Br1.2 NCs with PS, the as-prepared WLED shows robust stability under UV excitation and ambient environment. Moreover, WLED based on two layers coating Ce3+/Mn2+ co-doped CsPbCl1.8Br1.2 NCs and CsPbBr3 NCs was prepared (Figure S7). Its luminous efficiency is 42 lm/W, and the Rendering Index can exceed 90.

CONCLUSIONS In conclusion, we successfully prepared a series of metal ions pairs (Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+ , Bi3+/Eu3+ and Bi3+/Sm3+) co-doped CsPbCl3 perovskite NCs. The stable white emission for a single composition based on impurity ions pairs co-doped CsPbCl3 perovskite NCs is obtained successfully. Interestingly, the Ce3+ ions introducing not only complements the blue and green components, but also sensitizes the emission of the ions (Mn2+, Eu3+ and Sm3+) with red light emission. The highest effective white light emission is obtained from 2.7%Ce3+/9.1%Mn2+ co-doped CsPbCl1.8Br1.2 NCs, the PLQY is up to 75%. To our knowledge, the value is a new record in perovskite NCs. And the CRI is up to 89. Furthermore, the Ce3+/Mn2+ co-doped CsPbCl1.8Br1.2 NCs mixing with PS was employed to prepare stable WLED.



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The results indicate that the Ce3+/Mn2+ co-doped CsPbCl1.8Br1.2 phosphor exhibits great potential to serve as a white emitting phosphor for UV-WLED.

ASSOCIATED CONTENT Supporting information The Supporting information is available free of charge on the ACS Publications website at DOI: XXX. Figure S1-S7, Table S1, S2 and S3, particle size distribution, high-resolution XPS spectra, excitation spectra, emission decays and average lifetimes calculation. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

*E-mail:

[email protected]

Notes The authors declare no competing financial interests. Author contributions H.W.S., G.C.P., X.B. and W.X. designed the experiments, interpreted the data and co-wrote the paper. G.C.P., X.C., D.L.Z., J.Y.Z., H.S., Y.Z., B.D. and L.X. carried out the synthesis, characterization, optical measurements and data analysis. X.B., G.C.P. and H.W.S. discussed and commented on the manuscript.


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ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant no. 11674127, 11674126, 21403084, 61674067, 61775080), Jilin Province Science Fund for Excellent Young Scholars (20170520129JH, , 20170101170JC , 20170520111JH), National Key Research and Development Program (2016YFC0207101), Major State Basic Research Development Program of China (973 Program) (No. 2014CB643506), the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3) and the Jilin Province Natural Science Foundation of China (No. 20180101210JC, 20170101170JC, 20160418055FG).

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Table of contents

TOC figure


Impurity Ions Codoped Cesium Lead Halide Perovskite Nanocrystals

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