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Green anti-solvent assisted crystallization strategy for air-stable uniform Cs3Cu2I5 perovskite films with highly efficient blue photoluminescence
Journal of Luminescence 223 (2020) 117178
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Green anti-solvent assisted crystallization strategy for air-stable uniform Cs3Cu2I5 perovskite films with highly efficient blue photoluminescence Fanju Zeng a, b, Yuanyang Guo a, Wei Hu a, **, Yongqian Tan b, Xiaomei Zhang b, Jie Yang a, Qiqi Lin a, Yao Peng a, Xiaosheng Tang a, *, Zhengzheng Liu c, d, Zhiqiang Yao e, Juan Du c, d, *** a
Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education), College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044, China School of Big Data Engineering, Kaili University, Guizhou, Kaili, 556011, China c State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China d Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China e State Center for International Cooperation on Designer Low-Carbon and Environmental Materials, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450001, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Green anti-solvent Cs3Cu2I5 perovskite films Long-term air-stability High stable photoluminescence quantum efficiency Blue photoluminescence
Metal halide perovskites have aroused extensive attention due to their excellent photoelectric properties. But the toxicity and instability of lead-based perovskite restrict their development severely. Recently, lead-free Cu-based perovskites have emerged as promising environmental friendly photoelectric materials on account of their low toxicity and equally excellent properties. Here, we report the highly uniform lead-free Cs3Cu2I5 perovskite films with the long-term air-stability prepared by using methyl acetate as anti-solvent during the fabrication. The crystal structure, surface morphology, bandgap, and optical property of the Cs3Cu2I5 perovskite films are investigated. Even though the Cs3Cu2I5 perovskite films prepared with or without anti-solvent demonstrate the same crystal structure of the orthorhombic phase, by the anti-solvent approach, Cs3Cu2I5 perovskite films exhibit an enhanced photoluminescence quantum efficiency (PLQY) from ~62% to ~76.0% and have a relatively smaller surface roughness ~17.5 nm. Most importantly, Cs3Cu2I5 perovskite films exhibited long-term air-sta bility, and the PLQY could still reach 76.3% after storing for several months under an ambient atmosphere. All the results demonstrate the quality and the PLQY of Cs3Cu2I5 perovskite films could be effectively improved by the anti-solvent assisted crystallization strategy, showing potential applications in the fabrication of photo electric devices based on lead-free perovskites.
1. Introduction Metal halide perovskites have aroused wide interests in solar cells, light-emitting diodes, lasers, and photoelectric detectors owing to their high photoelectric conversion efficiency, high photoluminescence quantum efficiency (PLQY), excellent color purity, and tunable bandgap, etc. [1–6] However, the intrinsic toxicity of lead and instability of the lead-based perovskites severely restrict their practical applica tions in view of recent environmental regulations [11]. During the past few years, Sn-based, Sb-based and Bi-based perovskites such as CsSnX3 [7], Cs2SnX6 [8], Cs3Sb2Br9 [9], and Cs3Bi2Br9[10] have been studied as
the substitutes of the lead element in perovskites. Unfortunately, CsSnX3 nanomaterials sensitive be oxidized to Cs2SnX6, and the PLQY of Sn-based perovskite is lower than 0.5% [7,8]. Moreover, Sb and Bi ions are heavy metals that have negative influences on human bodies and the environment [11]. Therefore, how to obtain perovskite materials with non-toxic elements and excellent photoelectric properties is urgent for the development of next-generation lead-free halide perovskites. To date, copper has been recognized as a promising candidate to replace lead to from copper(Cu)-based perovskites materials due to its €tzel et. al [13] reported the synthesis low toxicity [12]. For example, Gra method of MA2CuClxBr4–x hybrid perovskites, and Wang’s groups [14]
* Corresponding author. ** Corresponding author. *** Corresponding author. State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China. E-mail addresses: [email protected] (W. Hu), [email protected] (X. Tang), [email protected] (J. Du). https://doi.org/10.1016/j.jlumin.2020.117178 Received 24 December 2019; Received in revised form 23 February 2020; Accepted 29 February 2020 Available online 5 March 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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employed the high stable C6H4NH2CuBr2I compound in the photovoltaic application. Besides, Cu-based perovskite Cs2CuX4 (X ¼ Cl, Br, and Br/I) quantum dots with tunable photoluminescence spectra were synthesized by Lou’s group by ligand assisted precipitation technique at room tem perature [15]. Furthermore, lead-free Cs3Cu2I5 perovskite films were synthesized by the solution-processed method with a PLQY of 60% [16]. These studies reveal that the low toxic Cu-based perovskite could be one potential substitution for promising optoelectronic devices. However, the correlative explorations about the surface morphology and the crystallization characteristics of the Cu-based perovskite films are still absent. Therefore, it is necessary to illustrate the crystal formation mechanism of Cu-based perovskite films for expanding their application scope. In general, various approaches such as spin-coating, sequential deposition, and thermal evaporation have been employed for synthe sizing perovskite films [17–21]. Among them, the perovskite films ob tained by spin-coating usually possess poor morphologies with pinholes and inhomogeneous grains caused by disordered nucleation and growth. During the past few years, the anti-solvent assisted crystallization strategy has been introduced to prepare uniform organic-inorganic lead halide perovskite films [22–26]. For example, chlorobenzene, toluene, isopropanol, ethyl acetate, or methyl acetate as anti-solvents have been adopted for the fabrication of uniform CH3NH3PbI3 thin films using the spin-coating methods [21,27–30]. Herein, we report a long-term air-stable and highly uniform lead-free Cs3Cu2I5 perovskite films synthesized by spin-coating and by using low toxic methyl acetate (MA) as an anti-solvent. The Cs3Cu2I5 precursor solution is first spun coated on the indium tin oxide (ITO) substrates. In order to induce crystallization immediately, the MA as an anti-solvent is dropped near the end of the spun process. After optimization of spincoating method, the obtained Cs3Cu2I5 perovskite films have a rela tively smaller surface roughness of ~17.5 nm, enhanced (PLQY) from ~62% to ~76.0%, a long-term stable PLQY, the PLQY still reach 76.3% after storing for two months under ambient atmosphere, and can keep the initial PLQY of 88.6% at 373K temperature. In conclusion, the uni form smooth Cs3Cu2I5 perovskite films with low roughness and longterm stable blue luminescence provide new insight for designing po tential blue light-emitting diode and laser devices.
2.3. Characterization X-ray diffraction (XRD) with Cu Kα radiation (XRD-6100, Shimadzu, Japan) is used to characterize the crystal structures. The surface, crosssectional scanning electron microscope (SEM) images, energy disperse spectroscopy (EDS), and elemental mapping are observed by JSM-7800F (Japan). The transmittance spectra are investigated by UV–Vis spec trophotometer (UV-1800, Shimadzu, Japan). The X-ray photoelectron spectroscopy (XPS) and energy level are evaluated by ultraviolet photoelectron spectroscopy (UPS) (PREVAC XPS, UPS System, R3000 VUV5K MX-650). The atomic force microscope (AFM) images are characterized by an Atomic Force Microscope (Dimension ICON, Bruker, Germany). The steady-state photoluminescence (PL) spectra are ob tained by a fluorescence spectrophotometer (Cary Eclipse, Agilent). The photoluminescence quantum efficiency (PLQY) of films is obtained by the absolute quantum efficiency measuring instrument (C9920-02G, Hamamatsu, Japan). Through a fluorescence lifetime measurement system (QM TM NIR, PTI, America), the time-resolved PL measurements are collected. 3. Results and discussion Fig. 1a illustrates the spin-coating process with/without anti-solvent of MA to fabricate Cs3Cu2I5 perovskite films. We firstly drop the Cs3Cu2I5 precursor solution on the ITO glass substrate and spurt MA as anti-solvent near the end of the spin-coating process to induce crystal lization immediately. Subsequently, the as-prepared films are trans ferred on a hotplate and annealed at 100 � C for 60 min in the glove box to evaporate the residual solvent. It is obvious that the as-fabricated Cs3Cu2I5 perovskite films without anti-solvent show a rough surface with noticeable holes (3D AFM image in Fig. 1c). Nevertheless, the morphology of the compact films with uniform grains is remarkably improved by the anti-solvent strategy (3D AFM in Fig. 1b). Besides, it can be clearly observed the transmittance of the films prepared by the two methods is different. The perovskite films with anti-solvent are more transparent (photograph in Fig. 1b) but the films without anti-solvent are foggy (photograph in Fig. 1c). Fig. 2a shows the simulated crystal structure of as-prepared Cs3Cu2I5 perovskite films. Such a structure could be attributed to the ortho rhombic space group of Pnma. The Cs3Cu2I5 crystal includes binuclear anion Cu2I35 . Specifically, the I atoms are organized to trigonal bi pyramids, one Cu atom fills up one of the two I tetrahedral holes, the other a trigonal site of the neighboring tetrahedron, each Cu2I35 is isolated by Csþ ions [31,32]. The crystal structure of Cs3Cu2I5 perovskite films are further investigated by XRD measurements, as shown in Fig. 2b. The Cs3Cu2I5 perovskite films prepared with/without MA as an anti-solvent show the same crystal structure. The strong diffraction peaks located at 15.8� , 25.8� , 27.0� , 29.5� , 35.8� , and 48.4� for 2θ scan are distinguished, matching with the planes of (002), (122), (222), (313), (040), and (152) of the films, respectively, which are agreeing with orthorhombic phase (PDF card of No. 45–0077). No diffraction peaks for the secondary phases have appeared, which demonstrates the purity of the prepared Cs3Cu2I5 materials. Moreover, the intensity of the diffraction peaks of the Cs3Cu2I5 perovskite films with MA exhibit a significant enhancement in contrast to the films without MA, further implying that the treatment by MA as an anti-solvent should be one efficient strategy to improve the crystallinity of Cu-based perovskite. In order to further identify the detailed composition of the as-prepared Cs3Cu2I5 perovskite films, the XPS and Energy-disperse EDS are car ried out for element analysis. The surface XPS survey spectra of Cs3Cu2I5 perovskite films are shown in Fig. 2c and the high-resolution XPS spectra of Cs 3d, I 3d, and Cu 2p are shown in Fig. S1. It could be found that the Cs 3d with two distinct peaks at 724 eV and 738 eV prove existence of Csþ in the films surface. And the peaks of I 3d located at 618.8 eV and 630.4 eV convince the existence of I . Finally, the two strong peaks for Cu 2p at 932 eV and 951.8 eV could be assigned to Cuþ [15]. The XPS
2. Experimental section 2.1. Materials Copper iodide (CuI, 99.95%), Cesium iodide (CsI, 99.9%), N, NDimethylformamide (DMF, 99.8%), Dimethyl sulfoxide (DMSO, � 99.5%), and Methyl acetate (98%) were purchased from Aladdin, Xi’an Baolaite Company, and Sigma Aldrich, respectively. 2.2. Fabrication of Cs3Cu2I5 perovskite films The Cs3Cu2I5 precursor solution was prepared using 1.8 mmol CsI and 1.2 mmol CuI dissolved in 1 mL mixed solvent of DMF and DMSO (v: v ¼ 4: 1) stirred at 60 � C for 1 h in the glove box. The indium tin oxide (ITO) substrates were sequentially washed by deionized water, acetone, isopropanol, and ethyl alcohol for 30 min and treated by UV ozone for 30 min before the deposition of thin films. Subsequently, the 60 μL filtered precursor solution was deposited onto ITO substrates by using a two-step spin-coating method, first 1000 rpm for 15 s and then 4000 rpm for 30 s, 100 μL Methyl acetate was added on the spinning substrates near the end of the second spin-coating step. The films used for contrast do not have the anti-solvent dropping procedure described above. Finally, the Cs3Cu2I5 precursor films were annealed at 100 � C for 60 min in the glove box in order to evaporate the residual solvent and promote crystallization.
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Fig. 1. (a) The schematic illustration of the spin-coating procedure with/without MA in the fabrication of Cs3Cu2I5 perovskite films. And the photograph and 3D AFM image of as-prepared films on the ITO substrate. (b) with MA. (c) without MA.
Fig. 2. (a) Cs3Cu2I5 crystal structure, the green balls, blue balls, and purple balls are representing Cs atoms, Cu atoms, and I atoms, respectively. (b) XRD patterns. (c) Survey XPS spectra.
results indicate that the as-prepared films are composed of the three elements of Csþ, Cuþ, and I . As shown in Fig. S2 and Table S1 and S2, the atomic ratio of the Cs, Cu, I of the films is 29.02%: 17.11%: 53.87% without anti-solvent, and 28.59%: 18.01%: 53.04% for the films with anti-solvent respectively. These ratios are approximate 3: 2: 5, which is consistent with the chemical formula of the Cs3Cu2I5 crystal structure. Moreover, the EDS mapping results of Cs3Cu2I5 perovskite films
prepared by using MA are shown in Fig. S3, and it exhibits the uniform distribution of Cs, Cu, and I element. Based on the analysis of XRD, XPS and EDS results, it could clearly demonstrate the Cs3Cu2I5 perovskite films are successfully prepared. We further investigate the role of the addition of anti-solvent on the surface morphology and crystal grains of Cs3Cu2I5 perovskite films. Fig. 3a and d shows the top-view SEM images of Cs3Cu2I5 perovskite 3
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films prepared with/without MA as an anti-solvent. It can be clearly observed that there are many non-negligible holes in the Cs3Cu2I5 perovskite films prepared without anti-solvent (Fig. 3a), and the grain size distribution is in a wide-range from 50 nm to 900 nm (Fig. 3c). Nevertheless, the Cs3Cu2I5 perovskite films prepared using anti-solvent show smaller and uniform grain size range from 80 nm to 320 nm (Fig. 3f), especially dense surface without any holes (Fig. 3d). It is obvious that dropping MA during the spin-coating process could effi ciently reduce the size and improve the uniformity of Cs3Cu2I5 grains. The 2D AFM images show that the films without anti-solvent demon strate a rough surface with root-mean-square (RMS) roughness of 64.4 nm (Fig. 3b), comparatively, the films with anti-solvent have a smooth surface with RMS roughness of 17.5 nm without any holes (Fig. 3e). To determine the thickness of the Cs3Cu2I5 perovskite films with antisolvent, the cross-section of the films is investigated by cross-sectional SEM. The cross-sectional SEM images and elemental mapping are shown in Fig. S4. The thickness of the dense films is about 340 nm without any hole. The above XRD, SEM, and AFM results reveal the evident improve ment on the morphology of Cs3Cu2I5 perovskite films while using MA as an anti-solvent. We further propose the procedure for nucleation and grain growth of Cs3Cu2I5 perovskite films with/without MA as an antisolvent are shown schematically in Fig. 4. In the beginning, the liquid films become thinning mainly because the excess solution is spun off by the centrifugal forces. After a transition point, supersaturation, nucle ation, and growth occurred, the vapor pressure of the solvent dominated films thinning. Finally, the number of materials deposited at the end of the spinning process in the dry film (T) can be expressed as an equation (1) [33,34]. T ¼ c0
� �13 E ω 3v
2 3
Cs3Cu2I5 become slow in the solution deposition process. As a result, the number of nuclei on the substrate is few. Based on the Volmert Weber growth [35], the few crystal nucleation results in the formation of “islands” with many uncovered holes areas (see Figs. 3a and 4a). The key point to prepare uniform Cs3Cu2I5 perovskite films is to control the morphology of Cs3Cu2I5 perovskites. When it comes to two mixture solvents, the whole vapor pressure of the mixed solution is not far from the sum of the two solvents vapor pressures, and the boiling point of the mixture should not be higher than each solvent [36]. Therefore, by introducing high vapor pressure and low boiling point anti-solvent in the perovskite films preparation process, it could promote the vapor pres sure and reduce the boiling point of the solution. Among the generally used anti-solvent, the MA with low toxicity, low boiling point (56.9 � C) and high vapor pressure (28.8 kPa) is employed to prepare Cs3Cu2I5 perovskite films. A transparent Cs3Cu2I5 perovskite films could be immediately formed after using MA as an anti-solvent during spin-coating process (Fig. 4b), which is probably caused by the increased vapor pressure and decreased boiling point, and finally led to fast crystallization of the Cs3Cu2I5 perovskite. Eventually, the uniform morphology Cs3Cu2I5 perovskite films obtained by using MA as anti-solvent. Fig. 5 shows the optical properties of both Cs3Cu2I5 perovskite films with/without anti-solvent characterized by using UV–Vis, PL (excited by 290 nm) spectrometers and PLE (excited by 445 nm). Fig. 5a shows the UV–Vis transmittance spectra of both Cs3Cu2I5 perovskite films. Obvi ously, the transmittance in the visible band of Cs3Cu2I5 perovskite films with MA as an anti-solvent is higher than that without using MA films. This result is in accord with the phenomenon in Fig. 1. The reason could be ascribed to the films becomes thinner and the surface becomes uni form with less scattering caused by using the MA as anti-solvent (Figs. S4a and S5). According to the Tauc plot [37] in the model of films, the bandgap of the films is 3.86eV (Fig. S6). In order to study the band structure of the Cs3Cu2I5 perovskite films, the He I (21.2 eV) UPS data for films is investigated, as shown in Fig. 6. The UPS results indicate that the valence band and the Fermi level of the Cs3Cu2I5 perovskite films with MA as an anti-solvent are shift down slightly, compared to the Cs3Cu2I5 perovskite films without MA. This shift can be attributed to the different crystallinities of the MA treated perovskite. Fig. 5b shows the PL and PLE spectras of the films. A blue emission peak can be found at about 445 nm and the PLE located at about 290 nm. There is a large
(1)
where c0 , E, v, and ω represent the concentration of the solution, the evaporation rate, the viscosity of the solution, and the spin-coating speed, respectively. In our work, the concentration of Cs3Cu2I5 precur sor solution is same and the spin-coating parameters is same. Therefore c0 , v and ω are fixed values. However, the DMF with high boiling point (153 � C) and low vapor pressure (0.52 kPa) would have effect on the evaporation speed of DMF, and hence the crystal nucleation rate of the
Fig. 3. (a) Top-view SEM image. (b) AFM image, and (c) Size distribution of the Cs3Cu2I5 perovskite films without MA. (d) Top-view SEM image. (e) AFM image, and (f) Size distribution of the Cs3Cu2I5 perovskite films with MA. 4
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Fig. 4. The proposed procedure for nucleation and grain growth of Cs3Cu2I5 perovskite films with/without MA. (a) without MA. (b) with MA.
Fig. 5. (a) UV–vis transmittance spectra. (b) The PLE and PL of Cs3Cu2I5 perovskite films. (c) The PLQY of Cs3Cu2I5 perovskite films without dropping MA, the photograph is shown inside. (d)The PLQY of Cs3Cu2I5 perovskite films dropping MA, the photograph is shown inside.
Stokes shift at 155 nm between PLE and PL. This result is in accord with previous reports and they mainly explained the PL mechanism of Cs3Cu2I5 with an excited-state structural reorganization [16]. Using Jahn–Teller distortion, the excited-state structural reorganization can be explicated [38]. Ordinarily, copper (I) halide with the d10 closed-shell prefers a tetrahedral geometry [39]. When the bandgap photon energy has absorbed photons, the electronic configuration of copper (I) 3 d10 changes to copper (II) 3 d9, inducing Jah–Teller distortion and
subsequent reorganization of the excited-state structure. According to this mechanism, the Stokes shift is ruled largely by the energy variation between copper (I) 3 d10 and copper (II) 3 d9. Thus, the situations of the first excitation peak and the luminescence peak are relatively dissimilar. Besides, the PL and PLE peaks of perovskite films with MA are higher than the films without anti-solvent. To determine the reason for this difference, The PLQY of the films excited by 290 nm is investigated. As shown in Fig. 5, the Cs3Cu2I5 perovskite films using MA have higher 5
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Journal of Luminescence 223 (2020) 117178
Fig. 6. (a) UPS spectrum of the Cs3Cu2I5 perovskite films without MA. (b) The determined energy level of the Cs3Cu2I5 perovskite films without MA. (c) UPS spectrum of the Cs3Cu2I5 perovskite films using MA. (d) The determined energy level of the Cs3Cu2I5 perovskite films with MA.
PLQY of 76.0% than that without MA (62.0%). These results indicate the films with a full-covered and smooth surface could improve their luminescence because the defect of the smooth films is lower than rough films. The time-resolved photoluminescence (TRPL) spectra obtained from the Cs3Cu2I5 perovskite films under excitation at 290 nm are shown in Fig. 7. The TRPL decay time and amplitude curves are fitted by below single-exponential equation (2) [40]. f ðtÞ ¼ expð t = τÞ
(2)
Here, τ represents decay time. For the Cs3Cu2I5 perovskite films without anti-solvent,τ is 969.4 ns, and for films with anti-solvent τ is 994.6 ns. It can be known that the average decay time (τavg) of films is equal to τ because of the TRPL decay time are fitted by singleexponential. Therefore, the τavg of Cs3Cu2I5 perovskite films (994.6 ns) with MA is longer than one (969.6 ns) without MA. The longer τavg could be resulted from the higher crystal quality of the uniform perovskite grains due to the anti-solvent which induced fast crystallization form uniform films with low defects density. Furthermore, to study the stability of Cs3Cu2I5 perovskite films with anti-solvent, we store the films in an air ambient for 60 days. The XRD measurements show that the crystal structure of Cs3Cu2I5 perovskite films with anti-solvent is still in agreement with the PDF card of No. 45–0077 and displays no obvious change compared with the initial XRD results, as shown in Fig. S7. It proves that Cs3Cu2I5 perovskite films with anti-solvent have long-term stable crystal structures in atmospheric environment. Finally, we investigated the photoluminescence stability
Fig. 7. The time-resolved photoluminescence of Cs3Cu2I5 perovskite films.
of the Cs3Cu2I5 perovskite films with MA as shown in Fig. 8. We pre liminarily characterized the PL and PLQY of films stored under the ambient atmosphere (humidity about 65.0% in Chongqing, China) from 1-60 days. The PLQY of films could still reach 76.3% after 60 days which is almost equal to the first result characterized initially. This result 6
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Fig. 8. The PL and PLQY of Cs3Cu2I5 perovskite films with anti-solvent from 1 day to 60 days in the ambient atmosphere. (a) PL. (b) PLQY. The PLQY of Cs3Cu2I5 perovskite films with MA variation with temperature. (c) PL. (d) PLQY.
indicates that the films have long-term photoluminescence stability in the ambient atmosphere. Sequentially, we heated the films from 300 K to 373 K and characterized the PL and PLQY of films in the ambient atmosphere are shown in Fig. 8c and d, respectively. The PLQY slightly reduced with the increasing of the temperature but still maintain higher than 67.3% at 373 K that keep the initial PLQY of 88.6%, implying the high thermal stability of luminescence. We continue to store the films in air ambient for 150 days. The crystal structure and PLQY of films are investigated, as shown in Figs. S7 and S8, the XRD result does not show obvious change and the PLQY of films is 76.4%. Accordingly, the Cs3Cu2I5 perovskite films obtained by anti-solvent assisted crystalliza tion strategy demonstrate long-term air-stability of crystal structure and high PLQY, which show potential applications in photoelectric devices.
manuscript. Declaration of competing interest The authors declare no competing financial interest. Acknowledgment The authors gratefully acknowledge financial support from the Youth Science and technology talent growth program of Guizhou edu cation department (Qian Jiao He KY Zi [2019] No. 188), the Funda mental Research Funds for the National Key Research and Development Program of China (No. 2018YFB2200500), the Natural Science Foun dation of China (No. 51602033, No. 61975023, No. 61520106012, No. 61875211), the Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2017jcyjB0127, No. cstc2018jcyjAX0633, No. cstc2019jcyj-msxmX0040, No. cstc2017jcyjAX0197), the Funda mental Research Funds for the Central Universities (No. 2018CDQYGD0008), the Strategic Priority Research Program of CAS (No. XDB16030400), the International S&T Cooperation Program of China (No. 2016YFE0119300); the Open Fund of the State Key Labo ratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics), the Youth Science and technology talent growth program of Guizhou education department (Qian Jiao He KY Zi [2017] No. 335), the Major Research Projects of Innovative Groups in Education Department of Guizhou Province of China (Qian Jiao He KY Zi [2018] No. 035).
4. Conclusions In summary, we successfully obtained the long-term air-stable and highly uniform lead-free Cs3Cu2I5 perovskite films by using the green solvent of methyl acetate as an anti-solvent near the end of the spincoating process. In Comparison with the films without anti-solvent, the crystal structure of the films showed no changes, but the surface of films became much more smoothly with the RMS decreasing from 64.4 nm to 17.5 nm. Moreover, the films with anti-solvent became more transparency and the PLQY increases from 62.0% to 76.0% with longterm stable blue luminescence and crystal structural in the ambient at mosphere. Demonstrating that the anti-solvent assisted crystallization strategy could effectively improve the quality of Cs3Cu2I5 perovskite films and the photoluminescence quantum efficiency, showing the po tential applications in the fabrication of the photoelectric devices based on lead-free perovskites.
Appendix A. Supplementary data Supplementary data related to this article can be found at https ://doi.org/10.1016/j.jlumin.2020.117178.
Author contributions
References
The manuscript was written through the contributions of all of the authors. All of the authors have given approval to the final version of the
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Journal of Luminescence 223 (2020) 117178
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Green anti-solvent assisted crystallization strategy for air-stable uniform Cs3Cu2I5 perovskite films with highly efficient blue photoluminescence Fanju Zeng a, b, Yuanyang Guo a, Wei Hu a, **, Yongqian Tan b, Xiaomei Zhang b, Jie Yang a, Qiqi Lin a, Yao Peng a, Xiaosheng Tang a, *, Zhengzheng Liu c, d, Zhiqiang Yao e, Juan Du c, d, *** a
Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education), College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044, China School of Big Data Engineering, Kaili University, Guizhou, Kaili, 556011, China c State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China d Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China e State Center for International Cooperation on Designer Low-Carbon and Environmental Materials, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450001, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Green anti-solvent Cs3Cu2I5 perovskite films Long-term air-stability High stable photoluminescence quantum efficiency Blue photoluminescence
Metal halide perovskites have aroused extensive attention due to their excellent photoelectric properties. But the toxicity and instability of lead-based perovskite restrict their development severely. Recently, lead-free Cu-based perovskites have emerged as promising environmental friendly photoelectric materials on account of their low toxicity and equally excellent properties. Here, we report the highly uniform lead-free Cs3Cu2I5 perovskite films with the long-term air-stability prepared by using methyl acetate as anti-solvent during the fabrication. The crystal structure, surface morphology, bandgap, and optical property of the Cs3Cu2I5 perovskite films are investigated. Even though the Cs3Cu2I5 perovskite films prepared with or without anti-solvent demonstrate the same crystal structure of the orthorhombic phase, by the anti-solvent approach, Cs3Cu2I5 perovskite films exhibit an enhanced photoluminescence quantum efficiency (PLQY) from ~62% to ~76.0% and have a relatively smaller surface roughness ~17.5 nm. Most importantly, Cs3Cu2I5 perovskite films exhibited long-term air-sta bility, and the PLQY could still reach 76.3% after storing for several months under an ambient atmosphere. All the results demonstrate the quality and the PLQY of Cs3Cu2I5 perovskite films could be effectively improved by the anti-solvent assisted crystallization strategy, showing potential applications in the fabrication of photo electric devices based on lead-free perovskites.
1. Introduction Metal halide perovskites have aroused wide interests in solar cells, light-emitting diodes, lasers, and photoelectric detectors owing to their high photoelectric conversion efficiency, high photoluminescence quantum efficiency (PLQY), excellent color purity, and tunable bandgap, etc. [1–6] However, the intrinsic toxicity of lead and instability of the lead-based perovskites severely restrict their practical applica tions in view of recent environmental regulations [11]. During the past few years, Sn-based, Sb-based and Bi-based perovskites such as CsSnX3 [7], Cs2SnX6 [8], Cs3Sb2Br9 [9], and Cs3Bi2Br9[10] have been studied as
the substitutes of the lead element in perovskites. Unfortunately, CsSnX3 nanomaterials sensitive be oxidized to Cs2SnX6, and the PLQY of Sn-based perovskite is lower than 0.5% [7,8]. Moreover, Sb and Bi ions are heavy metals that have negative influences on human bodies and the environment [11]. Therefore, how to obtain perovskite materials with non-toxic elements and excellent photoelectric properties is urgent for the development of next-generation lead-free halide perovskites. To date, copper has been recognized as a promising candidate to replace lead to from copper(Cu)-based perovskites materials due to its €tzel et. al [13] reported the synthesis low toxicity [12]. For example, Gra method of MA2CuClxBr4–x hybrid perovskites, and Wang’s groups [14]
* Corresponding author. ** Corresponding author. *** Corresponding author. State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China. E-mail addresses: [email protected] (W. Hu), [email protected] (X. Tang), [email protected] (J. Du). https://doi.org/10.1016/j.jlumin.2020.117178 Received 24 December 2019; Received in revised form 23 February 2020; Accepted 29 February 2020 Available online 5 March 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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employed the high stable C6H4NH2CuBr2I compound in the photovoltaic application. Besides, Cu-based perovskite Cs2CuX4 (X ¼ Cl, Br, and Br/I) quantum dots with tunable photoluminescence spectra were synthesized by Lou’s group by ligand assisted precipitation technique at room tem perature [15]. Furthermore, lead-free Cs3Cu2I5 perovskite films were synthesized by the solution-processed method with a PLQY of 60% [16]. These studies reveal that the low toxic Cu-based perovskite could be one potential substitution for promising optoelectronic devices. However, the correlative explorations about the surface morphology and the crystallization characteristics of the Cu-based perovskite films are still absent. Therefore, it is necessary to illustrate the crystal formation mechanism of Cu-based perovskite films for expanding their application scope. In general, various approaches such as spin-coating, sequential deposition, and thermal evaporation have been employed for synthe sizing perovskite films [17–21]. Among them, the perovskite films ob tained by spin-coating usually possess poor morphologies with pinholes and inhomogeneous grains caused by disordered nucleation and growth. During the past few years, the anti-solvent assisted crystallization strategy has been introduced to prepare uniform organic-inorganic lead halide perovskite films [22–26]. For example, chlorobenzene, toluene, isopropanol, ethyl acetate, or methyl acetate as anti-solvents have been adopted for the fabrication of uniform CH3NH3PbI3 thin films using the spin-coating methods [21,27–30]. Herein, we report a long-term air-stable and highly uniform lead-free Cs3Cu2I5 perovskite films synthesized by spin-coating and by using low toxic methyl acetate (MA) as an anti-solvent. The Cs3Cu2I5 precursor solution is first spun coated on the indium tin oxide (ITO) substrates. In order to induce crystallization immediately, the MA as an anti-solvent is dropped near the end of the spun process. After optimization of spincoating method, the obtained Cs3Cu2I5 perovskite films have a rela tively smaller surface roughness of ~17.5 nm, enhanced (PLQY) from ~62% to ~76.0%, a long-term stable PLQY, the PLQY still reach 76.3% after storing for two months under ambient atmosphere, and can keep the initial PLQY of 88.6% at 373K temperature. In conclusion, the uni form smooth Cs3Cu2I5 perovskite films with low roughness and longterm stable blue luminescence provide new insight for designing po tential blue light-emitting diode and laser devices.
2.3. Characterization X-ray diffraction (XRD) with Cu Kα radiation (XRD-6100, Shimadzu, Japan) is used to characterize the crystal structures. The surface, crosssectional scanning electron microscope (SEM) images, energy disperse spectroscopy (EDS), and elemental mapping are observed by JSM-7800F (Japan). The transmittance spectra are investigated by UV–Vis spec trophotometer (UV-1800, Shimadzu, Japan). The X-ray photoelectron spectroscopy (XPS) and energy level are evaluated by ultraviolet photoelectron spectroscopy (UPS) (PREVAC XPS, UPS System, R3000 VUV5K MX-650). The atomic force microscope (AFM) images are characterized by an Atomic Force Microscope (Dimension ICON, Bruker, Germany). The steady-state photoluminescence (PL) spectra are ob tained by a fluorescence spectrophotometer (Cary Eclipse, Agilent). The photoluminescence quantum efficiency (PLQY) of films is obtained by the absolute quantum efficiency measuring instrument (C9920-02G, Hamamatsu, Japan). Through a fluorescence lifetime measurement system (QM TM NIR, PTI, America), the time-resolved PL measurements are collected. 3. Results and discussion Fig. 1a illustrates the spin-coating process with/without anti-solvent of MA to fabricate Cs3Cu2I5 perovskite films. We firstly drop the Cs3Cu2I5 precursor solution on the ITO glass substrate and spurt MA as anti-solvent near the end of the spin-coating process to induce crystal lization immediately. Subsequently, the as-prepared films are trans ferred on a hotplate and annealed at 100 � C for 60 min in the glove box to evaporate the residual solvent. It is obvious that the as-fabricated Cs3Cu2I5 perovskite films without anti-solvent show a rough surface with noticeable holes (3D AFM image in Fig. 1c). Nevertheless, the morphology of the compact films with uniform grains is remarkably improved by the anti-solvent strategy (3D AFM in Fig. 1b). Besides, it can be clearly observed the transmittance of the films prepared by the two methods is different. The perovskite films with anti-solvent are more transparent (photograph in Fig. 1b) but the films without anti-solvent are foggy (photograph in Fig. 1c). Fig. 2a shows the simulated crystal structure of as-prepared Cs3Cu2I5 perovskite films. Such a structure could be attributed to the ortho rhombic space group of Pnma. The Cs3Cu2I5 crystal includes binuclear anion Cu2I35 . Specifically, the I atoms are organized to trigonal bi pyramids, one Cu atom fills up one of the two I tetrahedral holes, the other a trigonal site of the neighboring tetrahedron, each Cu2I35 is isolated by Csþ ions [31,32]. The crystal structure of Cs3Cu2I5 perovskite films are further investigated by XRD measurements, as shown in Fig. 2b. The Cs3Cu2I5 perovskite films prepared with/without MA as an anti-solvent show the same crystal structure. The strong diffraction peaks located at 15.8� , 25.8� , 27.0� , 29.5� , 35.8� , and 48.4� for 2θ scan are distinguished, matching with the planes of (002), (122), (222), (313), (040), and (152) of the films, respectively, which are agreeing with orthorhombic phase (PDF card of No. 45–0077). No diffraction peaks for the secondary phases have appeared, which demonstrates the purity of the prepared Cs3Cu2I5 materials. Moreover, the intensity of the diffraction peaks of the Cs3Cu2I5 perovskite films with MA exhibit a significant enhancement in contrast to the films without MA, further implying that the treatment by MA as an anti-solvent should be one efficient strategy to improve the crystallinity of Cu-based perovskite. In order to further identify the detailed composition of the as-prepared Cs3Cu2I5 perovskite films, the XPS and Energy-disperse EDS are car ried out for element analysis. The surface XPS survey spectra of Cs3Cu2I5 perovskite films are shown in Fig. 2c and the high-resolution XPS spectra of Cs 3d, I 3d, and Cu 2p are shown in Fig. S1. It could be found that the Cs 3d with two distinct peaks at 724 eV and 738 eV prove existence of Csþ in the films surface. And the peaks of I 3d located at 618.8 eV and 630.4 eV convince the existence of I . Finally, the two strong peaks for Cu 2p at 932 eV and 951.8 eV could be assigned to Cuþ [15]. The XPS
2. Experimental section 2.1. Materials Copper iodide (CuI, 99.95%), Cesium iodide (CsI, 99.9%), N, NDimethylformamide (DMF, 99.8%), Dimethyl sulfoxide (DMSO, � 99.5%), and Methyl acetate (98%) were purchased from Aladdin, Xi’an Baolaite Company, and Sigma Aldrich, respectively. 2.2. Fabrication of Cs3Cu2I5 perovskite films The Cs3Cu2I5 precursor solution was prepared using 1.8 mmol CsI and 1.2 mmol CuI dissolved in 1 mL mixed solvent of DMF and DMSO (v: v ¼ 4: 1) stirred at 60 � C for 1 h in the glove box. The indium tin oxide (ITO) substrates were sequentially washed by deionized water, acetone, isopropanol, and ethyl alcohol for 30 min and treated by UV ozone for 30 min before the deposition of thin films. Subsequently, the 60 μL filtered precursor solution was deposited onto ITO substrates by using a two-step spin-coating method, first 1000 rpm for 15 s and then 4000 rpm for 30 s, 100 μL Methyl acetate was added on the spinning substrates near the end of the second spin-coating step. The films used for contrast do not have the anti-solvent dropping procedure described above. Finally, the Cs3Cu2I5 precursor films were annealed at 100 � C for 60 min in the glove box in order to evaporate the residual solvent and promote crystallization.
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Fig. 1. (a) The schematic illustration of the spin-coating procedure with/without MA in the fabrication of Cs3Cu2I5 perovskite films. And the photograph and 3D AFM image of as-prepared films on the ITO substrate. (b) with MA. (c) without MA.
Fig. 2. (a) Cs3Cu2I5 crystal structure, the green balls, blue balls, and purple balls are representing Cs atoms, Cu atoms, and I atoms, respectively. (b) XRD patterns. (c) Survey XPS spectra.
results indicate that the as-prepared films are composed of the three elements of Csþ, Cuþ, and I . As shown in Fig. S2 and Table S1 and S2, the atomic ratio of the Cs, Cu, I of the films is 29.02%: 17.11%: 53.87% without anti-solvent, and 28.59%: 18.01%: 53.04% for the films with anti-solvent respectively. These ratios are approximate 3: 2: 5, which is consistent with the chemical formula of the Cs3Cu2I5 crystal structure. Moreover, the EDS mapping results of Cs3Cu2I5 perovskite films
prepared by using MA are shown in Fig. S3, and it exhibits the uniform distribution of Cs, Cu, and I element. Based on the analysis of XRD, XPS and EDS results, it could clearly demonstrate the Cs3Cu2I5 perovskite films are successfully prepared. We further investigate the role of the addition of anti-solvent on the surface morphology and crystal grains of Cs3Cu2I5 perovskite films. Fig. 3a and d shows the top-view SEM images of Cs3Cu2I5 perovskite 3
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films prepared with/without MA as an anti-solvent. It can be clearly observed that there are many non-negligible holes in the Cs3Cu2I5 perovskite films prepared without anti-solvent (Fig. 3a), and the grain size distribution is in a wide-range from 50 nm to 900 nm (Fig. 3c). Nevertheless, the Cs3Cu2I5 perovskite films prepared using anti-solvent show smaller and uniform grain size range from 80 nm to 320 nm (Fig. 3f), especially dense surface without any holes (Fig. 3d). It is obvious that dropping MA during the spin-coating process could effi ciently reduce the size and improve the uniformity of Cs3Cu2I5 grains. The 2D AFM images show that the films without anti-solvent demon strate a rough surface with root-mean-square (RMS) roughness of 64.4 nm (Fig. 3b), comparatively, the films with anti-solvent have a smooth surface with RMS roughness of 17.5 nm without any holes (Fig. 3e). To determine the thickness of the Cs3Cu2I5 perovskite films with antisolvent, the cross-section of the films is investigated by cross-sectional SEM. The cross-sectional SEM images and elemental mapping are shown in Fig. S4. The thickness of the dense films is about 340 nm without any hole. The above XRD, SEM, and AFM results reveal the evident improve ment on the morphology of Cs3Cu2I5 perovskite films while using MA as an anti-solvent. We further propose the procedure for nucleation and grain growth of Cs3Cu2I5 perovskite films with/without MA as an antisolvent are shown schematically in Fig. 4. In the beginning, the liquid films become thinning mainly because the excess solution is spun off by the centrifugal forces. After a transition point, supersaturation, nucle ation, and growth occurred, the vapor pressure of the solvent dominated films thinning. Finally, the number of materials deposited at the end of the spinning process in the dry film (T) can be expressed as an equation (1) [33,34]. T ¼ c0
� �13 E ω 3v
2 3
Cs3Cu2I5 become slow in the solution deposition process. As a result, the number of nuclei on the substrate is few. Based on the Volmert Weber growth [35], the few crystal nucleation results in the formation of “islands” with many uncovered holes areas (see Figs. 3a and 4a). The key point to prepare uniform Cs3Cu2I5 perovskite films is to control the morphology of Cs3Cu2I5 perovskites. When it comes to two mixture solvents, the whole vapor pressure of the mixed solution is not far from the sum of the two solvents vapor pressures, and the boiling point of the mixture should not be higher than each solvent [36]. Therefore, by introducing high vapor pressure and low boiling point anti-solvent in the perovskite films preparation process, it could promote the vapor pres sure and reduce the boiling point of the solution. Among the generally used anti-solvent, the MA with low toxicity, low boiling point (56.9 � C) and high vapor pressure (28.8 kPa) is employed to prepare Cs3Cu2I5 perovskite films. A transparent Cs3Cu2I5 perovskite films could be immediately formed after using MA as an anti-solvent during spin-coating process (Fig. 4b), which is probably caused by the increased vapor pressure and decreased boiling point, and finally led to fast crystallization of the Cs3Cu2I5 perovskite. Eventually, the uniform morphology Cs3Cu2I5 perovskite films obtained by using MA as anti-solvent. Fig. 5 shows the optical properties of both Cs3Cu2I5 perovskite films with/without anti-solvent characterized by using UV–Vis, PL (excited by 290 nm) spectrometers and PLE (excited by 445 nm). Fig. 5a shows the UV–Vis transmittance spectra of both Cs3Cu2I5 perovskite films. Obvi ously, the transmittance in the visible band of Cs3Cu2I5 perovskite films with MA as an anti-solvent is higher than that without using MA films. This result is in accord with the phenomenon in Fig. 1. The reason could be ascribed to the films becomes thinner and the surface becomes uni form with less scattering caused by using the MA as anti-solvent (Figs. S4a and S5). According to the Tauc plot [37] in the model of films, the bandgap of the films is 3.86eV (Fig. S6). In order to study the band structure of the Cs3Cu2I5 perovskite films, the He I (21.2 eV) UPS data for films is investigated, as shown in Fig. 6. The UPS results indicate that the valence band and the Fermi level of the Cs3Cu2I5 perovskite films with MA as an anti-solvent are shift down slightly, compared to the Cs3Cu2I5 perovskite films without MA. This shift can be attributed to the different crystallinities of the MA treated perovskite. Fig. 5b shows the PL and PLE spectras of the films. A blue emission peak can be found at about 445 nm and the PLE located at about 290 nm. There is a large
(1)
where c0 , E, v, and ω represent the concentration of the solution, the evaporation rate, the viscosity of the solution, and the spin-coating speed, respectively. In our work, the concentration of Cs3Cu2I5 precur sor solution is same and the spin-coating parameters is same. Therefore c0 , v and ω are fixed values. However, the DMF with high boiling point (153 � C) and low vapor pressure (0.52 kPa) would have effect on the evaporation speed of DMF, and hence the crystal nucleation rate of the
Fig. 3. (a) Top-view SEM image. (b) AFM image, and (c) Size distribution of the Cs3Cu2I5 perovskite films without MA. (d) Top-view SEM image. (e) AFM image, and (f) Size distribution of the Cs3Cu2I5 perovskite films with MA. 4
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Fig. 4. The proposed procedure for nucleation and grain growth of Cs3Cu2I5 perovskite films with/without MA. (a) without MA. (b) with MA.
Fig. 5. (a) UV–vis transmittance spectra. (b) The PLE and PL of Cs3Cu2I5 perovskite films. (c) The PLQY of Cs3Cu2I5 perovskite films without dropping MA, the photograph is shown inside. (d)The PLQY of Cs3Cu2I5 perovskite films dropping MA, the photograph is shown inside.
Stokes shift at 155 nm between PLE and PL. This result is in accord with previous reports and they mainly explained the PL mechanism of Cs3Cu2I5 with an excited-state structural reorganization [16]. Using Jahn–Teller distortion, the excited-state structural reorganization can be explicated [38]. Ordinarily, copper (I) halide with the d10 closed-shell prefers a tetrahedral geometry [39]. When the bandgap photon energy has absorbed photons, the electronic configuration of copper (I) 3 d10 changes to copper (II) 3 d9, inducing Jah–Teller distortion and
subsequent reorganization of the excited-state structure. According to this mechanism, the Stokes shift is ruled largely by the energy variation between copper (I) 3 d10 and copper (II) 3 d9. Thus, the situations of the first excitation peak and the luminescence peak are relatively dissimilar. Besides, the PL and PLE peaks of perovskite films with MA are higher than the films without anti-solvent. To determine the reason for this difference, The PLQY of the films excited by 290 nm is investigated. As shown in Fig. 5, the Cs3Cu2I5 perovskite films using MA have higher 5
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Fig. 6. (a) UPS spectrum of the Cs3Cu2I5 perovskite films without MA. (b) The determined energy level of the Cs3Cu2I5 perovskite films without MA. (c) UPS spectrum of the Cs3Cu2I5 perovskite films using MA. (d) The determined energy level of the Cs3Cu2I5 perovskite films with MA.
PLQY of 76.0% than that without MA (62.0%). These results indicate the films with a full-covered and smooth surface could improve their luminescence because the defect of the smooth films is lower than rough films. The time-resolved photoluminescence (TRPL) spectra obtained from the Cs3Cu2I5 perovskite films under excitation at 290 nm are shown in Fig. 7. The TRPL decay time and amplitude curves are fitted by below single-exponential equation (2) [40]. f ðtÞ ¼ expð t = τÞ
(2)
Here, τ represents decay time. For the Cs3Cu2I5 perovskite films without anti-solvent,τ is 969.4 ns, and for films with anti-solvent τ is 994.6 ns. It can be known that the average decay time (τavg) of films is equal to τ because of the TRPL decay time are fitted by singleexponential. Therefore, the τavg of Cs3Cu2I5 perovskite films (994.6 ns) with MA is longer than one (969.6 ns) without MA. The longer τavg could be resulted from the higher crystal quality of the uniform perovskite grains due to the anti-solvent which induced fast crystallization form uniform films with low defects density. Furthermore, to study the stability of Cs3Cu2I5 perovskite films with anti-solvent, we store the films in an air ambient for 60 days. The XRD measurements show that the crystal structure of Cs3Cu2I5 perovskite films with anti-solvent is still in agreement with the PDF card of No. 45–0077 and displays no obvious change compared with the initial XRD results, as shown in Fig. S7. It proves that Cs3Cu2I5 perovskite films with anti-solvent have long-term stable crystal structures in atmospheric environment. Finally, we investigated the photoluminescence stability
Fig. 7. The time-resolved photoluminescence of Cs3Cu2I5 perovskite films.
of the Cs3Cu2I5 perovskite films with MA as shown in Fig. 8. We pre liminarily characterized the PL and PLQY of films stored under the ambient atmosphere (humidity about 65.0% in Chongqing, China) from 1-60 days. The PLQY of films could still reach 76.3% after 60 days which is almost equal to the first result characterized initially. This result 6
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Fig. 8. The PL and PLQY of Cs3Cu2I5 perovskite films with anti-solvent from 1 day to 60 days in the ambient atmosphere. (a) PL. (b) PLQY. The PLQY of Cs3Cu2I5 perovskite films with MA variation with temperature. (c) PL. (d) PLQY.
indicates that the films have long-term photoluminescence stability in the ambient atmosphere. Sequentially, we heated the films from 300 K to 373 K and characterized the PL and PLQY of films in the ambient atmosphere are shown in Fig. 8c and d, respectively. The PLQY slightly reduced with the increasing of the temperature but still maintain higher than 67.3% at 373 K that keep the initial PLQY of 88.6%, implying the high thermal stability of luminescence. We continue to store the films in air ambient for 150 days. The crystal structure and PLQY of films are investigated, as shown in Figs. S7 and S8, the XRD result does not show obvious change and the PLQY of films is 76.4%. Accordingly, the Cs3Cu2I5 perovskite films obtained by anti-solvent assisted crystalliza tion strategy demonstrate long-term air-stability of crystal structure and high PLQY, which show potential applications in photoelectric devices.
manuscript. Declaration of competing interest The authors declare no competing financial interest. Acknowledgment The authors gratefully acknowledge financial support from the Youth Science and technology talent growth program of Guizhou edu cation department (Qian Jiao He KY Zi [2019] No. 188), the Funda mental Research Funds for the National Key Research and Development Program of China (No. 2018YFB2200500), the Natural Science Foun dation of China (No. 51602033, No. 61975023, No. 61520106012, No. 61875211), the Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2017jcyjB0127, No. cstc2018jcyjAX0633, No. cstc2019jcyj-msxmX0040, No. cstc2017jcyjAX0197), the Funda mental Research Funds for the Central Universities (No. 2018CDQYGD0008), the Strategic Priority Research Program of CAS (No. XDB16030400), the International S&T Cooperation Program of China (No. 2016YFE0119300); the Open Fund of the State Key Labo ratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics), the Youth Science and technology talent growth program of Guizhou education department (Qian Jiao He KY Zi [2017] No. 335), the Major Research Projects of Innovative Groups in Education Department of Guizhou Province of China (Qian Jiao He KY Zi [2018] No. 035).
4. Conclusions In summary, we successfully obtained the long-term air-stable and highly uniform lead-free Cs3Cu2I5 perovskite films by using the green solvent of methyl acetate as an anti-solvent near the end of the spincoating process. In Comparison with the films without anti-solvent, the crystal structure of the films showed no changes, but the surface of films became much more smoothly with the RMS decreasing from 64.4 nm to 17.5 nm. Moreover, the films with anti-solvent became more transparency and the PLQY increases from 62.0% to 76.0% with longterm stable blue luminescence and crystal structural in the ambient at mosphere. Demonstrating that the anti-solvent assisted crystallization strategy could effectively improve the quality of Cs3Cu2I5 perovskite films and the photoluminescence quantum efficiency, showing the po tential applications in the fabrication of the photoelectric devices based on lead-free perovskites.
Appendix A. Supplementary data Supplementary data related to this article can be found at https ://doi.org/10.1016/j.jlumin.2020.117178.
Author contributions
References
The manuscript was written through the contributions of all of the authors. All of the authors have given approval to the final version of the
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