Efficient Light-Emitting Diodes Based on in Situ Fabricated FAPbBr3

Aug 6, 2018 - ... 5 Zhongguancun South Street, Haidian District, Beijing 100081 , China ... methodology for the improvement of the performance of PeLE...
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Efficient Light-Emitting Diodes Based on In Situ Fabricated FAPbBr3 Nanocrystals: The Enhancing Role of Ligand-Assisted Reprecipitation Process Dengbao Han, Muhammad Imran, Mengjiao Zhang, Shuai Chang, Xiangang Wu, Xin Zhang, Jialun Tang, Mingshan Wang, Shmshad Ali, Xinguo Li, Gang Yu, Junbo Han, Lingxue Wang, Bingsuo Zou, and Hai-Zheng Zhong ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05172 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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Efficient Light-Emitting Diodes Based on In Situ Fabricated FAPbBr3 Nanocrystals: The Enhancing Role of Ligand-Assisted Reprecipitation Process Dengbao Han,† Muhammad Imran,†,║ Mengjiao Zhang,§ Shuai Chang,† Xian-gang Wu,† Xin Zhang,† Jialun Tang,† Mingshan Wang,⊥ Shmshad Ali,‡ Xinguo Li,# Gang Yu,# Junbo Han,⊥ Lingxue Wang,§,* Bingsuo Zou,‡,* Haizheng Zhong,†,* †

Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems,

School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing, 100081, China ‡

School of Physics, Beijing Institute of Technology, 5 Zhongguancun South Street,

Haidian District, Beijing, 100081, China §

School of Optics and Photonics, Beijing Institute of Technology, 5 Zhongguancun

South Street, Haidian District, Beijing, 100081, China ║

NUST Institute of Civil Engineering (NICE), National University of Sciences and

Technology (NUST), NUST campus, H-12, Islamabad, 44000, Pakistan ⊥

Wuhan National High Magnetic Field Center and School of Physics, Huazhong

University of Science and Technology, Wuhan, 430074, China 1 ACS Paragon Plus Environment

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BOE TECHNOLOGY GROUP CO., LTD. No.9 Dize Road, BDA, Beijing, 100176,

China *e-mail: [email protected]; [email protected]; [email protected]

ABSTRACT In this paper, we reported the in situ fabrication of highly luminescent formamidinium lead bromide (FAPbBr3) nanocrystals thin films by dropping toluene as anti-solvent during

spin-coating

a

perovskite

precursor

solution

with

using

3,3-diphenylpropylamine bromide (DPPA-Br) as ligands. The resulting films are uniform and composed by 5-20 nm FAPbBr3 perovskite nanocrystals. By monitoring the solvent mixing of anti-solvent and precursor solution on the substrates, we illustrated the difference between ligand-assisted reprecipitation (LARP) process and nanocrystal pinning (NCP) process. This understanding provides a guideline for film optimization and the optimized films obtained through in situ LARP process exhibit strong photoluminescence emission at 528 nm with quantum yields up to 78% and average photoluminescence lifetime of 12.7 ns. In addition, an exciton binding energy of 57.5 meV was derived from the temperature dependent photoluminescence measurement. More importantly, we achieved highly efficient pure green perovskite based light-emitting diodes (PeLEDs) devices with average external quantum efficiency (EQE) of 7.3% (maximum EQE is 16.3%) and average current efficiency (CE) of 29.5 cd A-1 (maximum CE is 66.3 cd A-1) by adapting a conventional device structure of ITO/PEDOT:PSS/TFB/Perovskite-film/TPBi/LiF/Al. It is expected that 2 ACS Paragon Plus Environment

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the in situ LARP process provides an effective methodology to improve the performance of PeLEDs. KEYWORDS: perovskites nanocrystals, quantum dots, in situ fabrication, photoluminescence, light-emitting diode

Halide perovskites with a general formula of ABX3 (A = CH3NH3+, MA or NH2CH=NH2+, FA or Cs+; B = Pb2+, Sn2+, and X = Br-, I-, Cl-) are now emerging as new generation functional semiconductors for photonics and optoelectronics due to their excellent device performance and low-cost solution processability.1-4 Considering the light-emitting applications, halide perovskites are desired light emitters with characteristics of brightly, color-tunable and narrow-band emissions, which make them attractive materials for fabricating light-emitting diodes (LEDs) and laser.5-8 Although perovskite based LEDs (PeLEDs) were reported since 1994,9 efficient devices were achieved only after perovskite nanocrystals (PNCs) were introduced.10-18 During the past three years, the external quantum efficiency (EQE) of PeLEDs significantly increased from 0.01~0.1% up to 14.36%.10-25 It has been realized that the dimensionality of perovskite materials (including size and structures) plays an important role in determining the carrier transport mobility, exciton recombination dynamics as well as the device performance.26-31 By introducing organic ligands, both of the molecular and size dimensionality of perovskite materials can be well-controlled through either in situ fabrication on substrates via spin-coating a precursor solution or ex situ fabrication via precipitation technique from colloidal 3 ACS Paragon Plus Environment

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solution.5,6,15-17,32-34 To further improve the device performance, it is much desired to combine the ligand engineering and processing technique to control the dimensionality and morphology of PNCs films.23-25,35 In situ fabrication strategy provides an efficient and convenient way to control the dimensionality of perovskite materials toward efficient and bright devices.36 For example, Cho et al reported the fabrication MAPbBr3 PNCs with average grain size of ~100 nm and achieved recorded device with a current efficiency (CE) of 42.9 cd A-1.17 By introducing long chain alkyl amines as ligands, Rand’s group further reduced the size of MAPbX3 PNCs down to ~10 nm and achieved improved device performance with maximum EQE up to 10.4% for the MAPbI3 system and 9.3% for the MAPbBr3 system.23 Meanwhile, Wang et al. and Yuan et al. demonstrated the dimensionality control of quasi-two dimensional (quasi-2D) perovskites toward efficient devices.20,37 The strategy of quasi-2D perovskites has been successfully applied for other counterparts of FAPbX3 and CsPbX3 PeLEDs and the maximum EQE approaches to 14.36% and 10.4% respectively.24,25 In comparison to the great success of device optimization, the understanding into the in situ fabrication process falls behind. Ligand-assisted reprecipitation (LARP) has been demonstrated to be a versatile ex situ method for fabricating PNCs toward efficient PeLEDs solution by mixing the precursors in polar solvents with nonpolar solvents.6,38,39 During the precipitation process, the solubility change induced the nucleation process and subsequently varied the crystallization into different sizes and shapes. Typically, the in situ fabrication on substrate for PeLEDs involves two steps: spin coating a precursor solution in polar 4 ACS Paragon Plus Environment

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solvents (usually dimethylformamide, DMF; dimethyl sulfoxide, DMSO) and then drop casting anti-solvent at a fixed time.17,23,40-42 Unlike the precipitation process in large volume flask, the in situ fabricated PNCs on substrates experienced different nucleation and growth process. The mixing of anti-solvent with precursor solution on the substrate before nucleation induced a fast precipitation process and subsequently induced the in situ fabrication of PNCs films with improved PL emission.17,23-25 In this work, we investigate the crystallization of in situ fabricated FAPbBr3 PNCs and illustrate the formation process. Highly luminescent and uniform FAPbBr3 PNCs thin films were obtained, resulting in highly efficient PeLEDs with peak EQE up to 16.3%. RESULTS AND DISCUSSIONS Figure 1a schematically illustrates the in situ fabrication process for FAPbBr3 PNCs

films.

A

typical

device

structure

of

ITO/PEDOT:PSS/TFB/Perovskite-film/TPBi/LiF/Al was adapted.43,44 To obtain efficient devices, highly luminescent and uniform FAPbBr3 PNCs films are vitally necessary. According to the literature reports, the incorporation of long chain alkylamine resulted in thin films of PNCs, while the use of aromatic amine induced the formation of quasi-2D perovskites.17,23-25,45-47 In this work, we first introduce an aromatic amine (3,3-diphenylpropylamine bromide, DPPA-Br) as ligands for the fabrication of PNCs films. Similar with the previous works,23,41 DMF was selected as good solvents and toluene was employed as anti-solvents. Except for ligands and anti-solvents, spinning speed and dropping time are also essential to control the final 5 ACS Paragon Plus Environment

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size and morphology of resulting perovskite films. The concept of nanocrystal pinning (NCP), proposed by Cho et al, has been successfully applied for the fabricating efficient MAPbBr3 based PeLEDs.17 They proposed that the NCP process includes nucleation at first and subsequent growth into fixed size with a final immediate crystallization after dropping anti-solvents. In our work, we discovered the transition from NCP process to LARP process by controlling the dropping time of anti-solvents. The biggest difference between NCP process and LARP process is effects of dropping anti-solvents. In LARP process, the anti-solvent was dropped before nucleation and the dropped anti-solvents induced fast nucleation on substrates and then precipitated into thin films with solvent evaporation. Therefore, the key point is dropping before and after nucleation, which varied the crystallization process between NCP and LARP. To clarify the nucleation point, we first monitored the photoluminescence (PL) evolution of substrate under ultra-violet (UV) lights during spin coating of a typical precursor solution using digital camera. As described in the experimental section, a precursor solution (0.2 M) was obtained by dissolving a mixture of FABr, PbBr2 and DPPA-Br with molar ratio of 1:1:0.4 into DMF for fabricating FAPbBr3 PNCs films. As shown in video S1 in the supporting information, the substrate experienced a fast color change from colorless to light green during the spin coating process. An obvious color change at about 7th second from the beginning can be attributed to the initialization of nucleation process. Therefore, the dropping anti-solvents was set at a fixed time of 7 seconds or longer from the beginning for NCP process, but 3~7 6 ACS Paragon Plus Environment

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seconds for LARP process. Figure 1b described the profiles of NCP process and LARP process for the in situ fabrication of FAPbBr3 PNCs films. To illustrate the difference of NCP and LARP processes, a series of samples were fabricated by varying the dropping time from 4th, 5th, 6th to 8th second from the beginning for the samples of S1, S2, S3 and S4 respectively. A typical photo of these samples under UV lamp is shown in the inset of figure 1b and their UV-vis absorption and PL spectra are presented in figure 1c and 1d. The absorption and PL spectra of these samples were only slightly varied by the dropping time. However, the samples (S1, S2, S3) obtained from LARP process show very strong PL emission with absolute PL quantun yields (PLQYs) of 70~80% as shown in figure 1e. In contrast, the PLQY of the sample (S4) obtained from NCP process is about 20%, which is comparable with the previous report.17 Except for the high PLQY, smooth surface is another prerequisite for fabricating high efficency PeLEDs devices.24 As discussed above, the formation of PNCs film was accomplished within one second after dropping anti-solvents. This means that the in situ LARP process on substrate is very fast and complicated than that in the flask. We then investigated the morphology of resulting in situ fabricated FAPbBr3 PNCs films from LARP process by applying optical microscopy. As shown in the figure S1 in the supporting information, the morphologies of as fabricated FAPbBr3 PNCs films can be significantly influenced by the dropping time. Sample S3 possesses very smooth and uniform film, while the surface of sample S1 and S2 are very rough with some ring defects in the center. Although the formation of PNCs during the in situ 7 ACS Paragon Plus Environment

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LARP process is not well understood, it has been well recongized that ligands play an important role in determining the PLQYs and surface morphologies of resulting in situ fabricated PNCs films.17,20-24,41 The amount of DPPA-Br was varied to optimize the surface morphology and PL properties for in situ FAPbBr3 PNCs films. As depicted in scanning electron microscope (SEM) images, absorption and PL spectra of FAPbBr3 PNCs films with different molar ratios of DPPA-Br:FAPbBr3 in the precursor solution were shown as figure S2 and S3 respectively. The in situ FAPbBr3 PNCs film that obtained using a precursor solution of FABr, PbBr2 and DPPA-Br with molar ratios of 1:1:0.4 has less pin holes and highest PL intensity with an optmized PLQY up to 78%. In addtion, DPPA-Br also provides highly hydrophobic proeprties. As shown in figure S4, the resulting in situ fabricated FAPbBr3 PNCs films using DPPA-Br have a larger contact angle in comparasion with the films using n-butylammonium bromide (BABr) or phenylethylammonium (PEA). This feature may accounts for the improved stability against moisters in ambient air.

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Figure 1. (a) Schematic illustration of the in situ LARP process for fabricating FAPbBr3 PNCs films. (b) Spin-coating speed-time profile. The photo (inset) of PNCs film under UV-light obtained by dropping anti-solvents at different time. (c, d) Absorption and PL spectra of FAPbBr3 PNCs films obtained with different dropping time of anti-solvents. (e) Statistical PLQYs of FAPbBr3 PNCs films with different dropping time of anti-solvents.

The resulting in situ FAPbBr3 PNCs films were characterized by applying different characterization techniques. Figure 2a shows a typical X-ray diffraction (XRD) pattern of resulting in situ FAPbBr3 PNCs film (black line). The diffraction peaks at 15°, 21.2°, 30°, 33.6° can be attributed to the (100) (110) (200) and (210) —

planes of cubic FAPbBr3 crystal (Pm3 m), which is consistent with the previous reports.48,49 In addition, the observed broadness in these peaks confirms the formation 9 ACS Paragon Plus Environment

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of small-sized FAPbBr3 PNCs. Furthermore, we also determined the composition characteristics of the in situ fabricated FAPbBr3 PNCs film. As shown in figure S5, the X-ray photoelectron spectroscopy (XPS) spectra of Pb4f7/2, Pb4f5/2, Br3d5/2, Br3d3/2 and N1s at 138.0 eV, 142.9 eV, 68.0 eV, 69.0 eV and ~400.0 eV indicated the presence of Pb, Br, N elements.17,31 Transmisison electron microscopy (TEM) was then utilized to characterize the morphology of resulting in situ FAPbBr3 PNCs films. Figure 2b and 2c show the cross-sectional images of a typical PeLEDs device based on FAPbBr3 PNCs film. It is clearly seen that the in situ fabricated FAPbBr3 PNCs film is composed by condensed nanoparticles with broad size distribution ranging from 5-20 nm. The observed broad size distribution is correlated with the fast precipitation process. Moreover, the resulting film has a clear and smooth interface between FAPbBr3 PNCs and TPBi layer without any obvious pin-hole existing. The in situ fabricated FAPbBr3 PNCs can be removed from the substrates and dissolved into chlorobenzene. The formation of stable colloidal solution in chlorobenzene (figure S6), suggest that these in situ fabricated PNCs are well-capped by DPPA-Br as ligands on the surface. The obtained colloidal solution was dropped on to copper mesh for TEM observation. As shown in figure 2d and 2e, the in situ fabricated FAPbBr3 PNCs are irregular shaped nanoparticles. From the high-resolution TEM (HRTEM) image and corresponding FFT pattern (Figure 2f), two crossed lattice planes of 0.42 nm and 0.59 nm corresponding to (011) and (001) can be identified. Based on the above results, we can conclude that the in situ fabricated FAPbBr3 PNCs

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through LARP process have much smaller average sizes than that fabricated through NCP process.

Figure 2. (a) XRD patterns of FAPbBr3 PNCs film (black line) and FAPbBr3 powder (red line). The corresponding Miller indexes are labeled at the top of the diffraction peaks. (b, c) Cross-sectional TEM images of the typically multilayered device. (d) Typical TEM image of the FAPbBr3 PNCs transferred onto a copper grid. (e) Typical HRTEM image of a PNC and its corresponding FFT pattern (f).

The achieved high PLQY of PNCs film has been attributed to the efficient exciton recombination due to the increased exciton binding energy (Eb) and well-surface capping.6 In our case, DPPA-Br act as surface capping ligands to improve radiative recombination of charge carriers by effective exciton confinement. The increased exciton binding energy of PNCs has been demonstrated by the temperature-dependent PL measurement.6,50,51 As shown in figure 3a, the pseudocolor map of the 11 ACS Paragon Plus Environment

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temperature-dependent PL spectra presents the evolution of PL emission with temperature from 5 to 300 K for FAPbBr3 PNCs film. Figure S7a shows the temperature-dependent fluorescence spectra, the fluctuation of PL peak position blue shift and the full width at half maximum (FWHM) gradually widens with the increase of temperature were demonstrated in figure S7b. Figure S8 plots the relationship between PL intensity with temperature and the exciton binding energy was calculated by fitting the plot using equation 1.52 I (T ) =

I0 1 + Ae − Eb / kBT

(1)

in which I0 is the PL intensity at 0 K, Eb is the exciton binding energy, and kB is the Boltzmann constant. The exction binding energy of ~57.5 meV was extracted from the fitting line from figure 3b. According to the literature reports, the exciton binding energy of bulk FAPbBr3 film is less than 25 meV,53 while it can increase up to 140 meV with the size reduction ( 10%) are maintained in the range of current density from 3.2 to 33.9 mA cm-2, illustrating that the injection of electrons and holes is always balanced even though at high current density or high operating voltage in PeLEDs. The bright images of operative device is present in figure 4f. In addition, it is noticed that the as-fabricated FAPbBr3 NCs based PeLEDs are not stable in the ambient air. As described in figure S10, the naked devices without encapsulation experienced very fast degradation within 1 minutes. The illuminated device at 60 mA cm-2 only last for 1 hour at a low temperature of 78 K in the vacuum environment, suggesting a complicated degradation mechanism beyond heat accumulation and ion movements.59

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Figure 4. (a) Device structure of FAPbBr3 based PeLEDs. (b) AFM image of the surface of FAPbBr3 PNCs film. (c) EL spectra of FAPbBr3 based PeLEDs under varying voltage bias (insert: CIE coordinate of typical PeLEDs). (d) Current density and luminance of the devices as a function of applied bias. (e, f) Current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of the PeLEDs as a function of current density. The optical pictures of operative device were inserted in figure 4f. According to the above description, the in situ LARP fabrication can provide high quality PNCs films for achieving recorded efficiency of PeLEDs devices. While, reproducibility is regarded as an important factor of fabrication methodology. The fast fabrication process makes it quite a challengeable for device optimization. To illustrate the reproducibility of in situ LARP fabrication, the device performance of 70 PeLEDs devices was recorded and analyzed. A diagram is shown in figure 5 to summarize the results of maximum luminance and current efficiencies (Histogram statistical distribution see figure S11). The statistic results of device parameters give an average CE of 29.5 cd A-1 and average luminance of 9 453 cd m-2. It is noted that the optimized luminance of these devices exceeded 20 000 cd m-2 with a moderate CE of about 20 cd A-1. While, the optimized CE of these devices approached to 66.3 cd A-1. Because the nucleation and growth in in situ LARP process are accomplished within one second (see video S1), the in situ LARP fabrication is sensitive to the dropping time and speed of anti-solvents. Therefore, this fast LARP process accounts

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for the low reproducibility of high performance PeLEDs based on PNCs films and will be further optimized by developing more advanced processing techniques.

Figure 5. The statistical distribution map of the maximum current efficiencies and luminance for 70 FAPbBr3 PNCs films based PeLEDs devices.

CONCLUSIONS In this work, we firstly introduced the DPPA-Br as ligands for the in situ fabrication of PNCs films by dropping anti-solvents during spin-coating a perovskite precursor solution in DMF. To gain deep insights into the formation process, we monitored the formation process and illustrated the important role of LARP process. By optimizing the film fabrication, high quality films consisted of 5-20 nm FAPbBr3 PNCs are obtained and the resulting films are very uniform with surface roughness of 1.5 nm. PL measurements show that the resulting FAPbBr3 PNCs films give out strong PL emission at 528 nm with an optimized PLQY up to 78% and average PL lifetime of 12.7 ns. These features suggest the PL decay mainly took place through exciton radiative recombination in these FAPbBr3 PNCs films, correlating with the 17 ACS Paragon Plus Environment

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exciton binding energy of 57.5 meV and very good surface capping by DPPA-Br. By adapting

a

conventional

device

structure

of

ITO/PEDOT:PSS/TFB/Perovskite-film/TPBi/LiF/Al, we fabricated best efficient pure green PeLEDs devices with a maximum EQE of 16.3% and CE of 66.3 cd A-1. In all, it is concluded that LARP process also plays an important role in the in situ fabrication of high quality PNCs films and provides an effective methodology to improve the performance of PeLEDs. It is noted that the in situ LARP process is particularly sensitive to the dropping time of anti-solvents and the reproducibility need to be improved in the further works. Advanced design and intelligent auto-control equipment for such films are highly desired.

Materials and Methods Materials. All reagents were used as received without further purification: PbBr2 (lead (II) bromide 99%, Aladdin). HBr (hydrobromic acid, 48 wt % in water, Aladdin). DPPA

(3,3-Diphenylpropylamine,

97%,

Energy

Chemical).

DMF

(N,N-dimethylformamide, 99.8%, Super Dry, with molecular sieves, J&K Seal). CB (Chlorobenzene, 99%, J&K Seal). Toluene (analytical grade, Beijing Chemical Reagent Co., Ltd., China). LiF (Lithium fluoride, 99.85%, Alfa Aesar). PEDOT:PSS (poly (ethylenedioxythiophene):polystyrene sulphonate, Heraeus-Clevios P VP AI4083),

TFB

(Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl)),

TPBi

(1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene)

and

FABr 18

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(formamidinium bromide) were purchased from Xi’an Polymer Light Technology Corp. Synthesis of DPPA-Br. DPPA-Br was synthesized by the reaction of the DPPA with HBr. Firstly, DPPA in absolute ethanol was stirred and cooled to 0 ℃ with the addition of HBr in a 1:1 molar ratio. The reaction solution was stirred for 2 h in an ice bath. Then, the excess solvent was removed using rotary evaporation under a pressure of 0.1 MPa at 50 ℃ . After that, the product of DPPA-Br was precipitated from the mixture, purified with diethylether for three times, and dried into white crystals in vacuum oven (60 ℃, 12 h) for future use. Fabrication of hybrid FAPbBr3 PNCs film. A mixture of PbBr2, FABr and DPPA-Br with a molar ratio of 1:1:x (x = 0, 0.2, 0.3, 0.4) were dissolved in DMF to obtain a 0.2 M FAPbBr3 precursor solution. Unless otherwise specified, the characterization data of FAPbBr3 PNCs film and PeLEDs correspond to x = 0.4. The precursor was stirred over 2 h at 60 ℃ and then through 0.22 um organic nylon filter for spin coating. All of the FAPbBr3 PNCs films were fabricated on O2 plasma treated TFB films. The spin coating rate was 4000 rpm and toluene was dropped on the spinning substrate at 4th, 5th, 6th and 8th second from the beginning respectively, then dried at 80 ℃ for 5 min to remove residual solvent and ensure the complete reaction of the precursors. All of the above experiments were finished in a nitrogen-filled glove box. Fabrication of LED devices. The parented ITO-coated glass substrates with working area of 9 mm2 were sequentially cleaned in detergent, deionized water, acetone, 19 ACS Paragon Plus Environment

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ethanol and isopropanol by sonication. The cleaned substrates were treated with O2 plasma for 2 min to make the surface hydrophilic, then PEDOT:PSS was spin-coated at 4000 rpm for 60 s and baked at 150 ℃ for 15 min in ambient air. Thereafter, the substrates were transferred into a nitrogen-filled glove box, and TFB (6 mg mL-1 in chlorobenzene) was spin-coated on the PEDOT:PSS film at 4000 rpm for 40s and baked at 120 ℃ for 30 min. TFB film was then treated with O2 plasma for 10 min to improve wettability, on the other hand, the treated TFB film was still highly soluble in chlorobenzene or m-dichlorobenzene, but resist to toluene washing. FAPbBr3 PNCs films were fabricated as show above details with x = 0.4 and the anti-solvent dropping time was at 6th s from the beginning. TPBi, LiF, and Al layers were thermally evaporated with thicknesses of 30, 1, and 100 nm respectively in a high vacuum thermal evaporator at a pressure below 2 × 10-4 Pa. Characterizations. UV-Vis absorption spectra of FAPbBr3 PNCs films were measured on a UV-6100 UV-Vis spectrophotometer (Shanghai Mapada Instruments Co., Ltd., China) and PL spectra were taken using a F-380 fluoresnece spectrometer (Tianjin Gangdong Sci. & Tech. Development. Co., Ltd., China). The PLQYs of thin films were determined using a fluorescence spectrometer with an integrated sphere (C9920-02, Hamamatsu Photonics, Japan) excited at a wavelength of 405 nm using a purple LED light source. The photo images of FAPbBr3 PNCs films with different dropping time of anti-solvents were observed by 3D digital optical microscopy (VHX 5000, KEYENCE) excited by 405 nm portable laser PGL-RV 405. SEM measurements of the FAPbBr3 PNCs films surface were recorded on Hitachi S-4800 20 ACS Paragon Plus Environment

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microscope, working at 15 kV. The sample of cross-section TEM observations were prepared with FIB (Focused Ion beam) method utilizing FEI 1td. Helios660 and using a Hitachi 1td. H-9000UHR machine operating at an acceleration voltage of 300 kV by Toray Research Center, Inc. in Japan. The TEM and HR-TEM images of the FAPbBr3 nanocrystals were captured on a JEOL-JEM 2100F TEM machine operating at an acceleration voltage of 200 kV. The FAPbBr3 PNCs were detached from TFB substrate via mechanical stripping by immersing the PNCs film in chlorobenzene, and the mixed chlorobenzene solution via centrifuging resulting supernatant was dipped on to copper mesh for TEM measurements. The XRD sample preparation method is similar, except that the mixed chlorobenzene solution was dropped on the silicon wafer. The XRD measurements were performed on a Bruker/D8 FOCUS X-ray diffractometer, using a Cu Kα radiation source (wavelength at 1.5405 Å). The XPS was collected on an ULVAC-PHI machine (PHIQUANTERA-II SXM) using Al Kα X-rays as the excitation source. TRPL was collected using a fluorescence lifetime measurement system (Deltaflex Horiba Jobinyvon IBH Inc., Japan). As shown in figure S12, no PL emission was observed for the O2 plasma treated TFB layer. Low-temperature-dependent PL spectra measurements were performed on a lab-made setup. The sample was mounted in a helium-follow cryostat (Microstat Mo, Oxford) with temperature range of 5-300 K. A frequency-doubled Ti:sapphire laser (Mira900, Coherent) with the repetition rate of 76 MHZ and plus duration around 130 fs was used as a PL excitation source. The wavelength is 405 nm and the power is set to be ~30 uW and the PL spectra were record by using an EM-CCD (Andor DU970P) 21 ACS Paragon Plus Environment

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through a monochromator (Andor SR500). All of the FAPbBr3 based PeLEDs without encapsulation were measured at room temperature in ambient air. AFM images were got using a Shimadzu SPM-9600 atomic force microscope in noncontact mode. The typical electroluminescence (EL) spectra was estimated on a dual channel fiber optic spectrometer (AvaSpec-ULS2048-2-USB2, Holland). The current density-voltage and luminance-voltage characteristics of PeLEDs devices were measured using connective Keithley 2400 power source analyzer and spectroradiometer (Photo Research Inc. PR-655). The EQE of device was calculated according to Lambertian profile.

ASSOCIATED CONTENT Supporting Information. Photoimages, fluorescence microscope images, SEM images, Absorption and PL spectra, XPS survey spectra of FAPbBr3 PNCs films. The contact angles of water on different perovskite films. The analyses of temperature-dependent PL and transmission spectra. Photographs of FAPbBr3 nanocrystal solution. The PL spectra of TFB film before and after O2 plasma treatment. Band alignment and stability of FAPbBr3 based LEDs. Statistical maximum current efficiency of PeLEDs and video S1 of in situ fabrication of FAPbBr3 PNCs film through LARP process. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

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* E-mail: [email protected] (HZ); [email protected] (BZ); [email protected] (LW) Author Contributions †

D.H. and M.I. contributed equally to this work.

ACKNOWLEDGMENT The authors would like to thank Prof. JW Hong and Mr. JM Deng for AFM measurements, Ms. YN Wen for assistance with the digital optical microscopy measurements, Prof. JB You for the device measurements and Prof. JP Wang for helpful discussions. This study was supported by National Natural Science Foundation of China (Nos. 61722502, 21603012, 61735004) and the project is also supported by BOE Technology Group Co., Ltd., China.

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