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Functional Inorganic Materials and Devices

One-step preparation of long-term stable and flexible CsPbBr perovskite quantum dots/ethylene vinyl acetate copolymer composite films for white LEDs 3

Yang Li, Ying Lv, Ziquan Guo, Liubing Dong, Jianghui Zheng, Chufen Chai, Nan Chen, Yijun Lu, and Chao Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02857 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

One-step preparation of long-term stable and flexible CsPbBr3 perovskite quantum dots/ethylene vinyl acetate copolymer composite films for white LEDs Yang Li a, Ying Lv b, Ziquan Guo c, Liubing Dong d, Jianghui Zheng e, Chufen Chai f, Nan Chen c, Yijun Lu c, Chao Chen a*1 a

b

c

College of Materials, Xiamen University, Xiamen 361005, China

Department of Electronic Science, Xiamen University, Xiamen 361005, China

d

e

College of Energy, Xiamen University, Xiamen 361005, China

Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia f

Hualian Electronics Corp., LTD, Xiamen 361008, China

Abstract CsPbBr3 perovskite quantum dots (PQDs)/ethylene vinyl acetate (EVA) composite films were prepared via a one-step method, based on that both supersaturated recrystallization of CsPbBr3 PQDs and dissolution of EVA were realized in toluene. The prepared films display outstanding green emitting performance with high color purity of 92% and photoluminescence quantum yield of 40.5% at appropriate CsPbBr3 PQD loading. They possess long-term stable luminescent properties in the air and in water, benefiting from the effective protection of CsPbBr3 PQDs by EVA matrix. Besides, the prepared CsPbBr3 PQDs/EVA films are flexible enough to be repeatedly

* Corresponding author. E-mail address: [email protected] (C. Chen).

1

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bent for 1000 cycles while keeping unchanged photoluminescence intensity. Optical properties of the CsPbBr3 PQDs/EVA films in white LEDs were also studied by experiments and theoretical simulation. Overall, facile preparation process, good long-term stability and high flexibility allow our green-emitting CsPbBr3 PQDs/EVA films to be applied in lighting applications and flexible displays. Keywords: CsPbBr3 perovskite quantum dots; long-term stable; flexible film; green emission; white LEDs

1. Introduction All inorganic CsPbX3 (X = Cl, Br, I) quantum dots, with perovskite structure and novel electrical and optical properties,1-5 have drawn considerable attention in recent years. They are applied in various fields, such as LEDs,4-8 displays,9,10 lasers,11 and emitters.12 Particularly, CsPbX3 perovskite quantum dots (PQDs) possess unique advantages when used for white LEDs and displays, because their photoluminescence (PL) can cover the whole visible spectra (from 400 to 800 nm) and their band gap energy and emission color are tunable with changing halide;3,4 moreover, CsPbX3 PQDs own narrow full width at half maximum (FWHM) of the emission band, high PL quantum yield (PLQY) and low temperature synthesis process compared with rare earth ion-doped phosphors that are usually prepared via a high-temperature sintering method.13 Despite of above advantages of CsPbX3 PQDs, poor stability always restricts their practical application, i.e., luminescent properties of CsPbX3 PQDs deteriorate quickly when they are exposed to water vapour in atmosphere.14-18 To overcome the 2


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problem, researches have been performed to encapsulate CsPbX3 PQDs within matrices (e.g., glass block, polymer film and silica particle).3,13,16-18 The matrices effectively isolate CsPbX3 PQDs from water and prevent PQDs’ aggregation, leading to a high stability of CsPbX3 PQDs/matrix composite materials. However, high-temperature heat treatment is always required to disperse PQDs in glass matrix,3,18 while for silica and polymer matrices based composite materials, their preparation

generally

goes

through

tedious

procedures,9,13,16,17

involving

supersaturated recrystallization or hot injection in organic solution, repeated centrifugation, re-dispersion of CsPbX3 PQDs in toluene or hexane, PQDs-matrix compositing process, and so on. Therefore, from the perspective of practical application, it is important to develop facile methods to fabricate CsPbX3 PQDs (or CsPbX3

PQDs/matrix composite

materials) with good long-term

stability.

Furthermore, with rapid development of wearable electronics recently,19 flexible LEDs and flexible displays begin to receive growing concern,20,21 whereas flexible CsPbX3 PQDs/matrix composites have rarely been researched. Herein, we proposed a novel strategy for the facile synthesis of long-term stable and flexible CsPbBr3 PQDs/ethylene vinyl acetate copolymer (EVA) films with outstanding luminescent properties. Specifically, EVA was dissolved in toluene, and then the precursor solution that composed of PbBr2, CsBr, ligands and dimethyl formamide (DMF) solvent was quickly added into above EVA-toluene system to fabricate CsPbBr3 PQDs/EVA films after natural drying. Obviously, the key to the fabrication of the CsPbBr3 PQDs/EVA films was that both supersaturated 3



recrystallization of CsPbBr3 PQDs and dissolution of EVA could be realized in toluene, i.e., toluene was used not only as the solvent of EVA, but also as poor solvent during the process of supersaturated recrystallization to synthesize CsPbBr3 PQDs. The CsPbBr3 PQDs/EVA films with different loading of CsPbBr3 PQDs were prepared, and their component, micro-morphology, luminescence performance and mechanical property were comprehensively studied. The films display outstanding green emitting performance and high flexibility and are very stable when exposed to water vapour in atmosphere or even directly soaked in water. Potential mechanisms for such good performance and possible applications in white LEDs of the films were also investigated. The facile and ingenious strategy proposed in this work for fabricating long-term stable and flexible CsPbBr3 PQDs/EVA films will promote the practical use of CsPbX3 PQDs in lighting applications and flexible displays.

2. Experimental 2.1 Materials PbBr2 (99.99%) and CsBr (99.9%) were obtained from Xi'an Polymer Light Technology Co., Ltd, and were used as received without further purification. DMF and toluene purchased from Sinopharm Chemical Reagent Co., Ltd. Oleic acid (OA) and oleyl amine (OAm) were produced by Aladdin Reagent Co., Ltd. EVA containing ~32% vinyl acetate was supplied by DuPont Company, USA. The commercial green phosphor of (Sr,Ba)2SiO4:Eu2+ and red phosphor of (Sr,Ca)AlSiN3:Eu2+ were supplied by Shenzhen Looking Long Technology Co., Ltd and Xiamen Hualian Electronics Co., Ltd, respectively. 4


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2.2 Preparation of CsPbBr3 PQDs/EVA film CsPbBr3 PQDs/EVA films in the present work were synthesized at room temperature. In a typical synthesis, PbBr2 (0.4 mmol) and CsBr (0.4 mmol) were dissolved in DMF (10 mL), and then OA (1 mL) and OAm (0.5 mL) were added to stabilize the DMF-PbBr2-CsBr precursor solution. EVA (0.5 g) was dissolved in toluene (5 mL). Subsequently, a certain volume (10~300 µL) of the above precursor solution was added to EVA-toluene system under vigorous stirring. Finally, toluene solvent naturally volatilized at room temperature to form CsPbBr3 PQDs/EVA films. In the prepared films, loading of CsPbBr3 PQDs was controlled by changing the volume of the precursor solution that added to EVA-toluene system, and when the volume of added precursor solution is x µL, the synthesized CsPbBr3 PQDs/EVA film is noted as EVA-x (mass ratio of CsPbBr3 PQDs to EVA in EVA-10, EVA-40, EVA-100, EVA-160, EVA-200 and EVA-300 films is 0.04%, 0.2%, 0.4%, 0.6%, 0.8% and 1.2% respectively). Pure EVA film was also prepared if the precursor solution was not added into EVA-toluene system. 2.3 Characterization X-ray diffraction (XRD) analyzer (model: Ultima-IV) with a Cu Kα (λ = 1.5418 Å) radiation was used to analyze crystal structure of the synthesized CsPbBr3 PQDs/EVA films. Valence states were studied by X-ray photoelectron spectroscopy (XPS) measurements on a PHI Quantum-2000 instrument. Fourier transform infrared (FTIR) spectra were tested on a spectrograph (model: Nicolet iS5). Scanning electron microscopy (SEM; model: SUPRA 55) and high-resolution transmission electron 5



microscopy (HRTEM; model: JEM2100) were used to observe micro-morphologies. Atomic force microscope (AFM; model: Asylum Research Cypher) was applied to characterize the surface roughness of the film samples. Thermogravimetric (TG) measurement was performed on a simultaneous thermal analyzer (model: Netzsch STA 449 F5 Jupiter) in nitrogen atmosphere with a heating rate of 5 oC/min. Photoluminescence (PL) emission spectra and PLQY were measured using a fluorescence spectrophotometer (model: FLS980, Edinburgh Instruments) equipped with visible and near infrared photomultiplier tube detectors. All the above measurements were performed at room temperature. Water contact angle measurement and tensile behavior test were carried on a contact angle measuring device and universal testing machine, respectively. To discuss possible applications of the CsPbBr3 PQDs/EVA films in white LEDs, a white LED was constructed by placing the film and commercial red phosphor of (Sr,Ca)AlSiN3:Eu2+ on an InGaN blue LED chip. Optical properties of the LED, such as luminous efficacy (LE), color rendering index (CRI) and correlated color temperature (CCT) were measured using an integrating sphere (LHS-1000, Everfine, China) with a forward current of 20 mA.

3. Results and Discussion 3.1 Morphology and component of CsPbBr3 PQDs/EVA films Figure 1 shows the digital photographs of the CsPbBr3 PQDs-EVA-toluene colloid and CsPbBr3 PQDs/EVA films. Original EVA-toluene solution was colorless and transparent, while when the colorless precursor solution that composed of PbBr2, 6


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CsBr, ligands and DMF was added, yellowish colloid formed (Figure 1a). When the colloid is irradiated using a UV (365 nm) source, intense green emission can be observed (Figure 1b). The phenomenon indirectly demonstrates the formation of CsPbBr3 PQDs.3,4 According to previous researches,4,22-24 it is reasonable to believe that the CsPbBr3 PQDs form through a process of supersaturated recrystallization, and toluene serves as poor solvent during the process. After complete evaporation of toluene solvent, the colloid spontaneously formed CsPbBr3 PQDs/EVA films. As exhibited in Figure 1c, pure EVA film and CsPbBr3 PQDs/EVA composite films with thickness of 200~250 µm are transparent, although introduction of CsPbBr3 PQDs makes the films become yellow. Unsurprisingly, the composite films also show intense green emission under UV (365 nm) light (Figure 1d), which is associated with the presence of CsPbBr3 PQDs inside the EVA based films. Besides, TG analysis in Figure S1 reveals that DMF in the colloid completely evaporated during the formation of PQDs/EVA films, while the ligands of OA and OAm with relatively high boiling points (350~360 oC) are more likely to preserve in the obtained composite films. The ligands are generally considered to be beneficial for stabilizing the PQDs and helping to control the PQDs’ size.4,14,25 XRD patterns of the pure EVA film and typical CsPbBr3 PQDs/EVA composite film of EVA-160 sample are given in Figure 2a. The broad diffraction peak appeared at around 21o is assigned to EVA.26 Besides, a series of diffraction peaks, such as the peaks at 15o, 22o, 31o, 38o and 44o are observed in the XRD pattern of EVA-160 film, which well math characteristic peaks of monoclinic CsPbBr3 (PDF#18-0364). PbBr2 7



and CsBr are not detected in the CsPbBr3 PQDs/EVA films. The diffraction peaks of CsPbBr3 possess relatively weak intensities, mainly resulting from the following two reasons: (1) CsPbBr3 PQDs have very low contents in the EVA-based composite films and (2) CsPbBr3 PQDs are wrapped by EVA matrix (Figure S2 and Figure S3). Despite of this, XRD analysis reconfirms the existence of CsPbBr3 PQDs in the as-prepared composite films. XPS characterization was performed to study the component and valence state of the CsPbBr3 PQDs/EVA films (Figure 2b-d). The elements of Cs, Pb and Br can be detected, and their main peaks, including Cs 3d, Pb 4f and Br 3d, have similar binding energies to those of CsPbBr3 PQDs that reported in literature.18,25,27 For instance, Pb 4f XPS spectrum presents two spectral peaks with binding energies of 138 and 142.1 eV, which correspond to Pb 4f7/2 and Pb 4f5/2 levels respectively, and the peaks of Br 3d5/2 and Br 3d3/2 are centered at binding energies of 68.2 and 69.3 eV respectively. Further, HRTEM was used to characterize micro-morphologies of CsPbBr3 PQDs in the EVA-based composite films. HRTEM images in Figure 2e-g show that with increasing precursor volume that added to EVA-toluene solution, the size of CsPbBr3 PQDs in the obtained composite films (especially in EVA-300 film) becomes larger and not uniform. It should be emphasized that EVA is chosen as the matrix to protect CsPbBr3 PQDs herein, because EVA film is waterproof, transparent, highly flexible than many previously reported polymer matrices and easy to prepare (EVA is generally synthesized by polymerization of ethylene and vinyl acetate, as illustrated in Figure S4; its flexibility and hydrophobicity will be exhibited in the following content); 8


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besides, considering that in solar cells, EVA film has been widely applied as packaging material and CsPbX3 (X=Br, Cl, I) PQDs can be used as downconverter,28,29 our CsPbBr3 PQDs/EVA composite films will presumably be utilized to enhance efficiency of solar cells (this is not the research issue of this work). Incorporation of CsPbBr3 PQDs has no obvious effect on intermolecular interactions of EVA matrix itself, according to FTIR spectroscopy analysis in Figure 3a and Table S1. Furthermore, FTIR spectrum also proves that DMF in the precursor solution does not preserve in the as-prepared composite films (the ligands of OA and OAm are also not detected by FTIR spectrum, possibly because their content is very low and they are enveloped within EVA matrix). The EVA-based films with CsPbBr3 PQDs show smaller surface roughness compared with pure EVA film (Figure S5), possibly because the existence of PQDs suppresses the free shrinkage of EVA polymer chains during the evaporation of toluene to form solid-state films. From another perspective, during

the

supersaturated

recrystallization

process

of

CsPbBr3 PQDs

in

precursor-EVA-toluene colloid, the movement and regrowth of CsPbBr3 PQDs will be impeded by EVA polymer chains, which is favorable to obtain small-sized and uniform CsPbBr3 PQDs.30,31 3.2 Luminescence properties of CsPbBr3 PQDs/EVA films Luminescence properties of CsPbBr3 PQDs/EVA films are comprehensively studied. Figure 4 shows the PL emission spectra of the prepared CsPbBr3 PQDs/EVA films under an excitation wavelength of 460 nm. For comparison, PL emission spectrum of commercial green phosphor of (Sr,Ba)2SiO4:Eu2+ that synthesized via high 9



temperature sintering method is also presented. In each PL emission spectrum of the CsPbBr3 PQDs/EVA films with various CsPbBr3 loading, a highly symmetric band centered at around 520 nm is always observed, indicating the outstanding green emission performance of our CsPbBr3 PQDs/EVA films. Meanwhile, with the increase of CsPbBr3 PQD loading in films, corresponding emission peak intensity increases at first and then descends after reaching a maximum value at EVA-160. In fact, EVA-160 film not only possesses the highest PL intensity, but also has a narrow FWHM of only 18.6 nm (Figure S6). FWHM of our CsPbBr3 PQDs/EVA composite films fluctuates around an average value of 19.7 nm, smaller than many previously-reported FWHM values,14,25,30,32 whereas larger than Li et al reported FWHM in 2018.31 Even though the difference in FWHM between the composite films may be caused the temperature fluctuation when we prepared the films or measured their PL spectra, the fact that CsPbBr3 PQDs aggregate to form large particles inside EVA-300 film (Figure 2g) will inevitably result in a larger FWHM.31 Furthermore, PLQY and internal quantum yield of the EVA-160 film are measured to be 40.5% and 33.2%, respectively (Figure S7), notably higher than the figure for previously reported CsPbBr3 PQDs/ethyl cellulose composite film.13 High PLQY value of EVA-160 can be explained as follows. For our prepared EVA-x composite films, increasing x value leads to higher CsPbBr3 PQD content in the films. However, when the content of CsPbBr3 PQDs exceeds a certain threshold, EVA polymer chains cannot effectively prevent the direct contact of PQDs, thus undesirable aggregation of PQDs will happen. This point is supported by HRTEM observations that in EVA-300 film, CsPbBr3 10


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PQDs show much larger particles than those in EVA-40 and EVA-160 (Figure 2e-f), and consequently, luminescence performance of EVA-300 is even worse.17,31,32 In addition, it is worth mentioning that the CsPbBr3 PQDs/EVA films with appropriate CsPbBr3 PQD loading, such as EVA-160 and EVA-200, perform much better than commercial green phosphor of (Sr,Ba)2SiO4:Eu2+ in PL intensity (Figure 4), even though our CsPbBr3 PQDs/EVA films are very easy to prepare. The Commission International De L'Eclairage (CIE) chromaticity coordinate and the dominant wavelength point of EVA-160 film and (Sr,Ba)2SiO4:Eu2+ phosphor powder were also calculated using the colorimetry calculator software developed by Lu group,33 and the results are summarized in Table 1. It can be seen that our EVA-160 is located in the green region corresponding to the dominant wavelength of 527 nm. Moreover, its color purity of 92.0% is significantly higher than that of commercial green (Sr,Ba)2SiO4:Eu2+ phosphor with the color purity of 70.0%. Poor stability in the air is an important limiting factor for the practical application of pure CsPbBr3 PQDs, which is associated with the degradation of CsPbBr3 PQDs when exposed to water or water vapour.13,17,18 In order to evaluate the availability of CsPbBr3 PQDs/EVA films in the fields of white LEDs and display devices, long-term stability of the CsPbBr3 PQDs/EVA films in the air and in water was studied. The EVA-160 film was kept in air for 0~8 days or directly soaked in water for 0~30 days, and then its PL characters were measured every 24 h. As shown in Figure 5a, the PL intensity retention of EVA-160 film always keeps around 100% after staying in the air for 0~192 h. Directly soaking in water for 0~240 h also does 11



not change the PL emission spectrum of the EVA-160 film (Figure 5b), and in fact, even after soaking in water for 720 h (i.e., 30 days), appearance and luminescence property of our EVA-160 film are remarkably unchanged. Above results favorably manifest the extraordinary long-term stability of the CsPbBr3 PQDs in EVA-160 film, which is attributed to the protection of EVA matrix. In order to confirm this, hydrophobic property of the film was quantitatively characterized by water contact angle measurement. As displayed in Figure 6, the EVA-160 film indeed possesses hydrophobic nature, with a water contact angle of 96o. This means that water is not easy to infiltrate the whole film, i.e., CsPbBr3 PQDs can be effectively prevented contact with water, thus leading to the high stability of CsPbBr3 PQDs/EVA films when exposed to water or water vapour. Photo-stability of our prepared composite films were also evaluated, as exhibited in Figure S8. During the irradiation of 460 nm blue-light, EVA-160 film shows decreasing PL intensity. After the initial 18 h irradiation, the PL intensity reduces by ~22%, while during the subsequent irradiation, the PL intensity declines slowly. Photo-induced regrowth of PQDs is generally considered as an important reason for the deterioration in PL intensity during blue-light (or UV) irradiation.30 In addition, the EVA-160 film was heated to 25~75 oC for 10 min and then cooled down to room temperature for PL spectrum measurement (Figure S9). The film exhibits relatively stable luminescence property when the temperature is low than 45 oC, whereas heating at higher temperature will lead to a notable deterioration of luminescence performance. Photo-stability and thermal-stability of the CsPbBr3 PQDs/EVA films 12


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will be further optimized in our future researches. 3.3 Mechanical flexibility of CsPbBr3 PQDs/EVA films To evaluate the feasibility of the prepared CsPbBr3 PQDs/EVA films in flexible LEDs and flexible displays, mechanical flexibility and luminescence performance under deformation states of our CsPbBr3 PQDs/EVA films were studied. As exhibited in Figure 7a, the EVA-160 film is flexible enough to be bent from 0 to 180o, and even when at 180o, the film still shows intense green emission under UV (365 nm) light. More importantly, after bending for 0~1000 cycles, no attenuation of the PL intensity of the film is observed. This is strongly dependent on the good flexibility of EVA matrix itself.34-36 Moreover, tensile test results in Figure 7b also prove that CsPbBr3 PQDs/EVA films possess similar mechanical behavior to pure EVA film. When the CsPbBr3 PQDs/EVA film is stretched to 350% of its original length, it is capable of glowing green under UV (365 nm) light. All in all, the EVA-160 film is very stable in luminescence performance when it suffers from bending deformation or tensile deformation, thus is promising to be used for flexible LEDs and flexible displays. 3.4 Optical properties of CsPbBr3 PQDs/EVA films in white LEDs A white LED device was constructed by combining the green-emitting EVA-160 film with (Sr,Ca)AlSiN3:Eu2+ red phosphor on an InGaN blue LED chip. Figure 8 presents the electroluminescence (EL) spectrum of the assembled white LED (inset in Figure 8) at a driving current of 20 mA for bright white generation. We can observe an emission band at 460 nm which is attributed to the InGaN blue chip, a narrow green emission band that originated from the CsPbBr3 PQDs/EVA film, and a broad yellow-red 13



emission band that attributed to the (Sr,Ca)AlSiN3:Eu2+. The assembled white LED shows a CRI of 74.7, a low CCT of 2347 K and a LE of 37.7 lm/W (experimental results in Table 2). The optical parameters such as LE, CCT and CRI can be easily tuned by modifying the ratio of the green CsPbBr3 PQDs/EVA film to the red phosphor in theory. To confirm this, we performed the investigation on the trade-off relationship between the CRI and LE of radiation (LER) for blue-green-red tricolor system.37 The LER is defined as the ratio of the optical power perceived by the human eyes to the total optical power. The parameters of CRI and LER are only determined by the spectral shape of the white light. Therefore, we only change the relative intensity of the blue LED, green-emitting film and the (Sr,Ca)AlSiN3:Eu2+ red phosphor. Their relative intensities are all changed from 0.1 to 5, with an interval of 0.1. Table 2 lists five optimal results (marked as Opt-1 to Opt-5). In particular, the Opt-1 result shows the close CCT and CRI values with the experiment data, but has the LER of 259.6 lm/W, which represents the theoretical maximum LE value of the tricolor system. For Opt-4 and Opt-5 at higher CCT, the CRI exceeds 80 and LER is larger than 300 lm/W, indicating that these corresponding tricolor systems with our green CsPbBr3 PQDs/EVA films are potential candidates for the general lighting.

4. Conclusions CsPbBr3 PQDs/EVA composite films with various CsPbBr3 PQD loading were prepared via a one-step method. The key to the fabrication of the CsPbBr3 PQDs/EVA films was that both supersaturated recrystallization of CsPbBr3 PQDs and dissolution of EVA were realized in toluene. The prepared films display outstanding green 14


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emitting performance with high color purity of 92% and PLQY of 40.5% at appropriate CsPbBr3 PQD content. Benefiting from the effective protection of CsPbBr3 PQDs by EVA matrix, the films possess long-term stable luminescence properties in the air and in water. Besides, the prepared CsPbBr3 PQDs/EVA films are flexible enough to be repeatedly bent for 1000 cycles while keeping almost unchanged PL intensity. Facile preparation process, good long-term stability and high flexibility allow our green-emitting CsPbBr3 PQDs/EVA films to be applied in lighting applications and flexible displays.

Associated content Supporting Information. The supporting information is available free of charge on the ACS Publications website. TG curve, SEM and AFM images, polymerization illustration of ethylene and vinyl acetate, quantum yield measurements, FTIR analysis, FWHM, photo-stability and thermal-stability.

Acknowledgements The authors would like to thank Meng Li (College of Energy, Xiamen University) for technical supports.

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[15] Chen, W.; Hao, J.; Hu, W.; Zang, Z.; Tang, X.; Fang, L.; Niu, T.; Zhou, M. Enhanced Stability and Tunable Photoluminescence in Perovskite CsPbX3/ZnS Quantum Dot Heterostructure. Small 2017, 13, 1604085-1604093. [16] Park, D. H.; Han, J. S.; Kim, W.; Jang, H. S. Facile Synthesis of Thermally Stable CsPbBr3 Perovskite Quantum Dot-Inorganic SiO2 Composites and Their Application to White Light-Emitting Diodes with Wide Color Gamut. Dyes Pigments 2018,149, 246-252. [17] Wang, Y.; Zhu, Y.; Huang, J.; Cai, J.; Zhu, J.; Yang, X.; Shen, J.; Jiang, H.; Li, C. CsPbBr3 Perovskite Quantum Dots-Based Monolithic Electrospun Fiber Membrane as an Ultrastable and Ultrasensitive Fluorescent Sensor in Aqueous Medium. J. Phys. Chem. Lett. 2016, 7, 4253-4258. [18] Di, X.; Hu, Z.; Jiang, J.; He, M.; Zhou, L.; Xiang, W.; Liang, X. Use of Long-Term Stable CsPbBr3 Perovskite Quantum Dots in Phosphor-Silicate Glass for Highly Efficient White LEDs. Chem. Commun. 2017, 53, 11068-11071. [19] Dong, L.; Xu, C.; Li, Y.; Huang, Z. H.; Kang, F.; Yang, Q. H.; Zhao, X. Flexible Electrodes and Supercapacitors for Wearable Energy Storage: A Review by Category. J. Mater. Chem. A 2016, 4, 4659-4685. [20] Li, X.; Liang, R.; Tao, J.; Xu, Q.; Peng, Z.; Han, X.; Wang, X.; Wang, C.; Zhu, J.; Pan, C.; Wang, Z. L. Flexible Light Emission Diode Arrays Made of Transferred Si-microwires ZnO-Nanofilm with Piezo-Phototronic Effect Enhanced Lighting. ACS Nano 2017, 11, 3883-3889. [21] Lee, S. Y.; Parka, K.; Huh, C.; Koo, M.; Yoo, H. G.; Kim, S.; Ah, C.S.; Sung, G. 18


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High-Performance

Perovskite

Photodetectors

Based

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on

Solution-Processed

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21



Figure captions Figure 1. Photos of (a)(b) CsPbBr3 PQDs-EVA-toluene colloid and (c)(d) CsPbBr3 PQDs/EVA films. (a)(c) are under room light and (b)(d) are under UV (365 nm) light. Figure 2. (a) XRD patterns of pure EVA film and EVA-160 composite film; XPS spectra of (b) Cs 3d, (c) Pb 4f and (d) Br 3d for EVA-160 film. HRTEM images of CsPbBr3 PQDs in (e) EVA-40, (f) EVA-160 and (g) EVA-300 films. Figure 3. FTIR spectra of pure EVA film and EVA-160 composite film. Figure 4. PL emission spectra of CsPbBr3 PQDs/EVA films and commercial (Sr,Ba)2SiO4:Eu2+ green phosphor powder. Figure 5. Long-term stability tests of EVA-160 film: (a) PL intensity retention and photographs under UV (365 nm) light after keeping in the air for 0~192 h; (b) PL emission spectra and photographs under UV (365 nm) light after soaking in water (the film is pointed out by dotted line circles) for 0~240 h. Figure 6. Water contact angle measurement of CsPbBr3 PQDs/EVA films. Figure 7. (a) PL intensity retention of EVA-160 film after repeatedly bending for 0~1000 cycles (inset: photographs of EVA-160 film at original and bending states). (b) Tensile curves of pure EVA film and EVA-160 film (inset: photograph of EVA-160 film at stretching state under UV light). Figure 8. EL spectrum of the constructed white LED.

22


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Figure 1. Photos of (a)(b) CsPbBr3 PQDs-EVA-toluene colloid and (c)(d) CsPbBr3 PQDs/EVA films. (a)(c) are under room light and (b)(d) are under UV (365 nm) light.

23



Figure 2. (a) XRD patterns of pure EVA film and EVA-160 composite film. XPS spectra of (b) Cs 3d, (c) Pb 4f and (d) Br 3d for EVA-160 film. HRTEM images of CsPbBr3 PQDs in (e) EVA-40, (f) EVA-160 and (g) EVA-300 films.

24


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Figure 3. FTIR spectra of pure EVA film and EVA-160 composite film.

Figure 4. PL emission spectra of CsPbBr3 PQDs/EVA films and commercial (Sr,Ba)2SiO4:Eu2+ green phosphor powder.

25



Figure 5. Long-term stability tests of EVA-160 film: (a) PL intensity retention and photographs under UV (365 nm) light after keeping in the air for 0~192 h; (b) PL emission spectra and photographs under UV (365 nm) light after soaking in water (the film is pointed out by dotted line circles) for 0~240 h.

Figure 6. Water contact angle measurement of CsPbBr3 PQDs/EVA film.

26


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Figure 7. (a) PL intensity retention of EVA-160 film after repeatedly bending for 0~1000 cycles (inset: photographs of EVA-160 film at original and bending states). (b) Tensile curves of pure EVA film and EVA-160 film (inset: photograph of EVA-160 film at stretching state under UV light).

Figure 8. EL spectrum of the constructed white LED.

27



Page 28 of 29

Table 1 CIE chromaticity coordinate, dominant wavelength and color purity of EVA-160 and (Sr,Ba)2SiO4:Eu2+ Sample

Chromaticity coordinate

Dominant wavelength

Color purity

EVA-160 (Sr,Ba)2SiO4:Eu2+

(0.122,0.789) (0.195,0.673)

527 nm 528 nm

92.0% 70.0%

Table 2 Optical parameters of the as-assembled white LED and simulation results by the color calculator software CCT (K)

CRI

LE (lm/W)

LER (lm/W)

Experiment

2347

74.7

37.7

-

Opt-1

2498

73.1

-

259.6

Opt-2

3685

74.7

-

292.9

Opt-3

4169

76.6

-

306.3

Opt-4

5775

82.2

-

316.3

Opt-5

6162

81.3

-

333.0

28


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Graphic Abstract

29


ethylene vinyl acet - ACS Publications

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