Ultrastable inorganic perovskite nanocrystals coated with thick long

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Ultrastable inorganic perovskite nanocrystals coated with thick long-chain polymer for efficient white light-emitting diodes Honge Wu, Sheng Wang, Fan Cao, Jipeng Zhou, Qianqian Wu, Haoran Wang, Xiaomin Li, Luqiao Yin, and Xuyong Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04634 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Chemistry of Materials

Ultrastable inorganic perovskite nanocrystals coated with thick long-chain polymer for efficient white light-emitting diodes

Honge Wu,1, 2, 3, # Sheng Wang,1, # Fan Cao,1 Jipeng Zhou,1 Qianqian Wu,1 Haoran Wang,1 Xiaomin Li,1 Luqiao Yin,1 and Xuyong Yang1,*

1 Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai 200072, China 2 College of Biological and Chemical Engineering, Anhui Polytechnic University, Wuhu 241000, China 3 Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China

E-mail: [email protected]

# These

authors equally contributed to this work.

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ABSTRACT:

The unstable nature of perovskites has severely limited their practical applications. Here, we report on ultrastable CsPbBr3 nanocrystals (NCs) with thick (~25 nm) polymer coating prepared via an effective postsynthetic strategy. The thick poly (maleicanhydride-alt-1-octadecene) (PMAO) with long hydrophobic alkyl chains bounded with the surface ligands of perovskite NCs acts as a protection layer to effectively prevent perovskite degradation from external environment. The photoluminescence (PL) for the thick PMAO coated CsPbBr3 NCs maintains over 90% of its initial emission intensity under continuous ultraviolet (UV) illumination of 144 h, while that of the pristine NCs is decreased to ~ 6%. After exposure in air for 40 days, only a very little PL degradation appears for the thick polymer coated NCs as compared to the dramatic decrease in PL emission for the pristine NCs. Upon immersion into water for 24 h, the perovskite NCs maintain 60% of its initial PL intensity, whereas the PL emission for the pristine NCs is completely quenched within only a few minutes. Moreover, there is no any side effect on the luminescent properties of perovskite NCs by the transparent polymer coating, and the PL quantum yields (PLQYs) are obviously improved due to the surface defect passivation of NCs. The resulting thick PMAO coated CsPbBr3 NCs are combined with commercially available red-emitting phosphor on blue InGaN chip to fabricate a high-performance warm white light-emitting diode (WLED) with a high power efficiency of 56.6 lm/W.

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All-inorganic metal halide CsPbX3 (X= Cl, Br, or I) perovskite nanocrystals (NCs) have been regarded as most promising materials for lighting and display applications thanks to their superior optical properties such as high photoluminescence quantum yields (PLQYs),1-5 narrow emission linewidth,6-9 wide emission spectra tunability,10-13 as well as low material cost.14-20 However, the intrinsic chemical instability of perovskites with ionic nature is a bottle-neck for their commercialization application. The external environment such as moisture, oxygen, and light can easily induce the degradation of perovskite materials because of their low crystal formation energy, and thus leads to PL attenuation.1, 21-24 Furthermore, the short-chain ligands such as oleic acid (OA) and oleylamine (OAm) on the surface of perovskite NCs cannot also efficiently suppress the destruction of material structure. An efficient strategy for achieving stable perovskite NCs is to overcoat polymer materials with excellent stability and transparency.25-28 For example, Sun et al. recently introduced crosslinkable and polymerizable ligands to prepare polymer-NCs composites via in-situ thermally induced crosslinking approach, which significantly enhanced moisture stability and suppressed the photodegradation for perovskite NCs.25

Meyns

et

al.

also

demonstrated

that

long-chain

poly

(maleicanhydride-alt-1-octadecene) (PMAO) with good barrier characteristic can be incorporated with perovskite NCs by adding PMAO into the perovskite NCs synthesis reaction, and form PMAO-NCs composites to improve the chemical and optical stability of material.28 Despite their ongoing research efforts to enhance the stability of perovskite NCs by polymer coating, the progress in this direction has still been limited so far. The in-situ synthesis methods encounter an issue that the amount of the coated polymer is little and therefore the formed thin polymer “shell” cannot effectively prevent the damage of NCs from external environment. The formation of 3 ACS Paragon Plus Environment

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the thin polymer is mainly ascribed to two points: One is the loss of the polymer during the purification process for the perovskite NCs-polymer composites. The other is that the used solvents such as octadecene and some excess surface ligands of NCs prevent polymer from being adsorbed on the surface of NCs. Herein, we report on efficient and stable 25 nm-thick PMAO polymer coated all-inorganic CsPbBr3 NCs prepared through a facile postsynthetic strategy. The thick polymer coating layer can be served as a steric barrier to significantly improve the stability of perovskite NCs. Meanwhile, the PL quantum yields (PLQYs) of CsPbBr3 NCs are also enhanced after polymer coating due to the surface defect passivation of NCs. By combining the green perovskite NCs improved stability with the commercially available red-emitting nitride phosphor cured on the blue-emitting InGaN chip, we further demonstrate a warm white light emitting diode (WLED), with the Commission Internationale de L’Eclairage (CIE) colour coordinates at (0.390, 0.332) and a low colour correlated temperature (CCT) at 3320 K. The highest power efficiency for the WLED reaches 56.6 lm/W, three times more than that (17 lm/W) of incandescent lamp.29 These results not only provide us with an effective approach for the preparation of thick polymer coated perovskite NCs but also shed some light on the light-emitting applications of perovskite NCs.

Results and Discussion A typical postsynthetic procedure to prepare CsPbBr3 with thick PMAO polymer coating layer is shown in Figure 1. The purified CsPbBr3 NCs and PMAO were first dispersed and dissolved in toluene, respectively. The CsPbBr3 NCs was then drop-wise added into PMAO solution under rigorously stirring at room temperature. The protonated groups of polymers such as amine and thiol easily damage the 4 ACS Paragon Plus Environment

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structure of perovskite materials,30 while the weak-polar groups are in favor of polymer coating on the surface of perovskite NCs. The PMAO with long alkyl chain is this kind of polymer with weak-polar groups.31,

32

For our case, the PMAO

molecules can be tightly bounded with the surface ligands (OA and OAm) of CsPbBr3 NCs through Van der Waals interactions, and finally form thick polymer coating on perovskite NCs after 12 h. The resulting CsPbBr3/PMAO NCs can be well dispersed in various solvents such as toluene/chloroform, and easily to form smooth and uniform films.

Figure 1. Schematic illustration of postsynthetic treatment for obtaining perovskite NCs with thick PMAO polymer coating layer.

The morphology and size distribution of pristine CsPbBr3 NCs and thick PMAO coated CsPbBr3 NCs were elucidated by transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis (Figure 2). It can be observed that the morphology of the perovskite NCs remains unchanged after being coated with PMAO polymer (Figures 2a,b). However, we found that the narrow particle size distribution for the polymer coated NCs with an average hydrodynamic diameter of 61.5 nm is 5 ACS Paragon Plus Environment

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much larger than that (10.9 nm) of pristine CsPbBr3 NCs by DLS analysis (Figures 2c,d), and the corresponding coating thickness for the PMAO polymer on NCs is estimated to be 25.3 nm.

Figure 2. TEM images of (a) CsPbBr3 PNCs and (b) CsPbBr3/PMAO NCs, and the corresponding DLS figures (c) and (d), respectively.

X-ray diffraction (XRD) patterns and Fourier transform infrared spectroscopy (FTIR) (Figure 3) were further used to analyze phase structure and composition of the resulting CsPbBr3/PMAO NCs. Figure 3a shows the XRD patterns of the PMAO, pristine NCs, and PMAO coated NCs. Both the pristine and CsPbBr3/PMAO NCs have similar diffraction peaks located at 15.2° and 30.2° assigned to the (100) and (200) crystal planes of CsPbBr3. However, we also observed a broadened peak at around 19° for CsPbBr3/PMAO NCs, suggesting that CsPbBr3 NCs are wrapped by PMAO polymer.33,

34

Note that there are no decomposition and phase transition of 6 ACS Paragon Plus Environment

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CsPbBr3 NCs produced during the PMAO postsynthetic process. The FTIR spectra of the perovskite NCs samples with and without PMAO polymer coating are shown in Figure 3b. Two obvious variations are found in the FTIR spectra after coating PMAO: the strong characteristic signals of stretching vibrations of C=O at ~1859 cm-1 and ~1780 cm-1 and the distinct peaks of C-O stretching vibration at ~1222 cm-1 and ~1080 cm-1 appeared.35 The C=O and C-O stretching vibrations belong to the anhydride group of PMAO polymer, which are in accordance with that of the FTIR spectra for pure PMAO polymer. Meanwhile, we also note that there is a peak at around 3100 cm-1 in the FTIR spectrum for CsPbBr3/PMAO is disappeared. These results provide a clear indication for the formation of PMAO polymer on the surface of perovskite NCs.

Figure 3. (a) XRD patterns and (b) FTIR spectra of the PMAO, pristine CsPbBr3 NCs and CsPbBr3/PMAO NCs.

The optical properties and stability of the resulting CsPbBr3/PMAO NCs were well studied to estimate the suitability of materials in the practical lighting and display applications. Figure 4a shows the comparison of the PL and absorption spectra for pristine and CsPbBr3/PMAO NCs. Both the two samples exhibit a similar narrow 7 ACS Paragon Plus Environment

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emission linewidth, and there is an enhancement in PL intensity for the CsPbBr3/PMAO sample. The corresponding PLQY is increased from 75.9% to 88.8%. This can be attributed to that the decrease in the surface defects of CsPbBr3 NCs resulting from the effective polymer coating by Van der Waals interaction.36, 37 The reduced surface defects of NCs are further proved by time-resolved PL spectra, which indicate a longer PL lifetime (42.50 ns) for the CsPbBr3/PMAO NCs compared with that (36.89 ns) of pristine CsPbBr3 NCs (Figure 4b). The stability of CsPbBr3 NCs is significantly improved after coating the thick PMAO polymer due to the protection of the thick polymer. As shown in Figure 4c, the PMAO coated green CsPbBr3 NCs in toluene are still very bright and transparent (observed from Figure 1), accompanied by only slight decrease in PL intensity after exposure in air for 40 days, while that of the pristine CsPbBr3 NCs is decreased to close to half. Figure 4d shows the temporal evolution of the PL intensity for the pristine CsPbBr3 and CsPbBr3/PMAO NCs under continuous illumination by a 365 nm UV lamp with the power of 12 W in ambient condition (please also refer to Figure S1). We can observe that the pristine CsPbBr3 NCs display a very fast PL decay, and after continuous irradiation for 144 h the PL intensity of the pristine CsPbBr3 NCs was dramatically decreased to ~ 6% of its initial intensity. As comparison, the PL intensity of the CsPbBr3/PMAO NCs maintains over 90% of the initial intensity. Due to the hydrophobic nature of polymer, the CsPbBr3/PMAO NCs show much improved water-resistant capability. When totally immersing into water, the pristine CsPbBr3 NCs were immediately quenched (Figure 4e), while the PL emission intensity for CsPbBr3 NCs coated with hydrophobic PMAO polymer maintained ~60% even after 24 h. These results can be ascribed to the fact that the thick PMAO polymer forms a

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steric barrier to suppress the structure damage of inorganic perovskite NCs decomposition induced by the invasion of oxygen, light and water.

Figure 4. (a) Absorption and PL spectra (λex = 400 nm), (b) time-resolved PL decay curves, (c) stability in the air, (d) photostability, and (e) water resistance of pristine CsPbBr3 NCs and CsPbBr3/PMAO NCs.

As a proof-of-concept lighting application, the resulting CsPbBr3 NCs coated with thick polymer were used as the green-emitting component to fabricate WLEDs. The CsPbBr3/PMAO NCs combined with a commercially available red emissive nitride phosphor (ER6436) are encapsulated with ET-821A and ET-821B silicone resin on a blue-emitting InGaN chip. By adjusting R/G/B colour composition ratio, a high-performance LED with excellent white emission is obtained. The emission spectrum of the resulting WLED is shown in Figure 5a, which is composed of three R/G/B emission peaks: blue one is from the blue-emitting InGaN chip centered at ~ 450 nm, green one is from the CsPbBr3 NCs centered at ~ 512 nm, and the red emission is from the nitride phosphor centered at ~ 623 nm. The CIE coordinates of 9 ACS Paragon Plus Environment

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(0.390, 0.332) and a relatively CCT value of 3320 K are obtained for the resulting WLED (Figure 5b), which corresponds to an excellent warm white light emission. Figure 5c shows the luminance and luminous efficiency as a function of forward-bias current of the WLED. The luminance for the resulting WLED is increased with increasing injection current, and the maximum luminance can be over 6,000,000 cd/m2. The maximum power efficiency (PE) reaches 56.6 lm/W, which is three times more than that of a common incandescent lamp (17 lm/W).29 Figure 5d displays a bright and uniform warm white emission from the photograph of the WLED output recorded at the current of 20 mA. In addition, the PMAO coated CsPbBr3 NCs based WLED also show the dramatic enhancement in stability compared with the white device with the pristine CsPbBr3 NCs, as shown in the time-dependent emission spectra in Figure S2.

Figure 5. Performance of the fabricated WLEDs using CsPbBr3/PMAO NCs. (a) White emission spectrum at a forward-bias current of 20 mA. (b) Colour coordinate in 10 ACS Paragon Plus Environment

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CIE1931 diagram. (c) Luminance and luminous efficacy under different forward-bias currents. (d) Photograph of a working WLED at 20 mA current.

In summary, we have developed a postsynthetic strategy to prepare highly luminescent and stable CsPbBr3 NCs coated with 25 nm-thick PMAO polymer. The overcoated thick polymer with long hydrophobic alkyl chain as steric barrier can effectively inhibit the degradation of NCs caused by the erosion of water, oxygen and/or light. In addition, there is no any decrease in PL emission after coating the polymer, and on the contrary the PLQYs of the CsPbBr3/PMAO NCs are slightly increased due to the suppressed surface defects of NCs. Making use of the green-emitting CsPbBr3/PMAO NCs combined with commercially red emitting phosphor, a high-performance warm WLED with the Commission Internationale de L’Eclairage (CIE) colour coordinates at (0.390, 0.332) and a low colour correlated temperature (CCT) at 3320 K is obtained. Our postsynthetic strategy is an efficient approach to prepare thick polymer coated perovskite NCs, and the obtained CsPbBr3/PMAO NCs with improved luminescent properties and stability are promising candidates for lighting and display applications.

ASSOCIATED CONTENT Supporting Information. Experimental details, photostability data of perovskite NCs with different thicknesses of PMAO, and time-dependent EL spectra of the resulting WLEDs are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT The authors would like to thank the financial support from National Natural Science Foundation of China (Nos. 61605109, 51675322 and 61735004), National Key Research and Development Program of China (No. 2016YFB0401702), Shanghai Rising-Star Program (No. 17QA1401600), Shanghai Science and Technology Committee (No. 16JC1400602) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2015037).

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