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Rubidium Doping for Enhanced Performance of Highly Efficient Formamidinium-based Perovskite light emitting diodes Yifei Shi, Jun Xi, Ting Lei, Fang Yuan, Jinfei Dai, Chenxin Ran, Hua Dong, Bo Jiao, Xun Hou, and Zhaoxin Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Applied Materials & Interfaces

Rubidium Doping for Enhanced Performance of Highly Efficient Formamidinium-based Perovskite light emitting diodes Yifei Shi†, Jun Xi†,‡, Ting Lei†, Fang Yuan†, Jinfei Dai†, Chenxin Ran†, Hua Dong†, Bo Jiao† and Xun Hou† and Zhaoxin Wu†§*

†Key Laboratory of Photonics Technology for Information, Key Laboratory for Physical

Electronics and Devices of the Ministry of Education, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China ‡Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 0882

6, Korea §Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006,

China *Corresponding author. E-mail: [email protected] Keywords: Doping engineering; Perovskite; Light emitting diodes; Rubidium; Formamidinium

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Organometal halide perovskites (OHPs) have become the most promising optoelectronic material in past few years with a myriad of applications in photovoltaic, light-emitting and laser fields. However, for light-emitting application, the low photoluminescence quantum yield (PLQY) of OHPs film is critical to hinder the efficiency improvement of OHP film based light emitting diodes (PeLEDs). Herein, we study the effects of rubidium incorporation on the crystal growth, structure, photoelectric and optical properties of formamidinium lead bromide (FAPbBr3)-based perovskite films and light emission performance of PeLEDs. It is found that rubidium incorporation can significantly enhance PLQY of FAPbBr3 film by suppressing the trap density and thus improve withstand voltage as well as the performance of PeLEDs. When FAPbBr3 film with optimal Rb doping ratio is employed as the light emitter of PeLEDs, the maximum luminance and current efficiency is enhanced by ~10 fold and ~5 fold to 66353cd/m2 and 24.22cd/A compared to the controlled device, respectively, the record performance based on FAPbBr3 PeLEDs so far. The enhanced performance can be chiefly attributed to the increase of PLQY and decrease of trap defect density of perovskite film with rubidium incorporation. Our research is expected to stimulate the development of OHPs for next-generation lighting and display field.


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1. INTRODUCTION Organometal halide perovskites (OHPs) have been greatly investigated for the area of photovoltaic fields, whose power conversion efficiencies (PCEs) are dramatically enhanced from 3.8% to 22.1% in a few years1-8. These impressive achievements can be attributed to OHPs’ intriguing semiconductor properties, such as high absorption coefficients, high and balanced carriers mobilities, long exciton diffusion lengths, small exciton binding energies, and so on9-12. Recently, more and more reports have demonstrated OHPs’ applications in light emitting diodes (LEDs)13-18, whose achievements are increasingly attractive. Particularly, OHPs LEDs show high color purity with narrow full width at half maximum (FWHM) (about 20nm)19, such advantage making it available for display, lighting and lasing field20. The general structural formula of OHPs is ABX3, where A is methylammonium (CH3NH3+) (MA+), formamidinium [HC(NH2)2+](FA+), or metal cesium (Cs+) cation, B is lead (Pb2+) or tin (Sn2+) cation, and X is a halide (Cl−, Br−, or I−) anion4, 21-25, In most reports, MAPbBr3 is employed as the emitter in perovskite-based LEDs (PeLEDs). Impressively, Cho et al. reported a highly bright and efficient green PeLEDs with a current efficiency of 42.9 cdA−1 and an external quantum efficiency of 8.53% by adopting a nanocrystal pining method26. Afterwards, Yuan et al. and Wang et al. reported near-infrared EQEs of 8.8% and 11.7% by confining carriers into two-dimensional (2D) perovskites through energy funneling transfer, respectively27, 28. Recently, Xiao et al. also achieved green-emission PeLEDs with a current efficiency of 17.1 cdA−1 and EQE close to 10%, revealing the wide concern on quasi-2D structures to improve the EQE on account of the intrinsic efficient electron transfer via energy funnels29.



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Although OHPs exhibit high photoluminescence quantum yield (PLQY>90%) in solution for nanocrystals or quantum dots23, 30, PLQY of OHPs, the intrinsic photo-emission dependent parameter, in film state is rather low20. To improve the PLQY of perovskite film, low dimensional perovskites were proposed to achieve high PLQY perovskite film31. Unfortunately, the ligands are indispensable to form quasi-2D structure, in which the insulative ligands will restrict the conduction of perovskite films and hinder the movement of carriers. Additionally, the stability of both perovskite film and PeLEDs device is crucial to be addressed due to the potential weak chemical interaction thermally unstable nature of organic components together with the on inorganic octahedral [PbX6]4-. Many perovskite solar cells (PSCs) reports proposed that partial substitution of A-site cations is an effective method to stabilize the crystal lattice32, 33

. For example, FA and the alkali metals (cesium, rubidium, potassium, sodium) are employed

as the A-site substitutive ion to improve efficiency and thermal stability of PSCs. Particularly, for alkali metal doped OHPs film, tuned tolerance factor, reduced trap density, increased crystallinity and enhanced PLQY could be achieved. Also, the alkali metal halides could passivate the grain boundaries and interface states, fill dangling bond, thus avoiding fluorescence quenching in perovskite34. Additionally, the alkali metals are oxidation-stable A-site cations that can avoid perovskite electronic properties distortion due to oxidation-prone Pb/Sn mixtures35. Recently, Sun et al.36 reported a FA/Cs mixed nanocrystal perovskite LED with an impressive luminance of 55005cd m−2, current efficiency of 10.09cd A−1, EQE of 2.8%. Crucially, PLQY of the Cs incorporated film is significantly enhanced compared to the pristine film, which inspires us to further explore potential progress of other alkali metal ion substitutions. Conventionally, appropriate Goldschmidt tolerance factor =


√( )

(in

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which RA, RB, RX is ionic radius of A+,B2+, X- in ABX3 structure) is essential to form an “established perovskite” structure37, 38, as shown in Figure S1, in which the ionic radius of Cs and Rb is 167 pm and 152 pm, respectively. However, the small difference probably has a great influence on property of the corresponding film. The tolerance factor of CsPbBr3 is about 0.81, just locating on the minimum edge of "established perovskites"，other alkali metals（Li, Na, K） is obviously too small to form stable perovskite structure. Notably, RbPbBr3 only misses this range a small margin. Up to date, unfortunately, there is no reports on the systemically investigation of optoelectronic property and related LEDs by Rb substitution in perovskite films. In this work, we study the effect of alkali metal Rb ion incorporation on the crystallization kinetics, morphology, crystal structure, optical properties of FAPbBr3 perovskite and the light emitting performance of PeLEDs. RbBr is introduced as a dopant to substitute the FA+ partially. We observe that Rb ion incorporation can increase the PLQY by around 10 folds on account of the substantially suppressed trap density. Besides, it is demonstrated that Rb incorporation can stabilize the perovskite film crystal phase and improve the crystallinity of polycrystalline perovskite films and enhance the withstand voltage and performance of device. The optimized PeLEDs with 7% Rb content exhibit the maximum bright luminance of 66353cd m−2 and highest current efficiency of 24.22cd A−1, representing the most bright and efficient green emission PeLEDs based on FAPbBr3. Rb ion incorporation is expected as a useful approach to accelerate the development of high performance PeLEDs in the future.



2. RESULTS AND DISCUSSION

Figure1. (a) Schematic diagram of spin-coating perovskite films by the means of anti-solvent dropping. (b-e) show the top-viewed SEM images of pure FAPbBr3 film (b) and FAPbBr3 films doped different Rb content.

To study the effects of Rb ion incorporation on the morphology of perovskite film, the one step spin-coating with anti-solvent drop method was adopted to form compact and uniform perovskite films, as shown in Figure1(a). To exclude the interference of Br ion on the effect of Rb ion, FA(1-x)Rb(x)PbBr3 precursor solution was prepared by mixing FABr, RbBr and PbBr2 (molar ratio(1.7-x):x:1) in N,N-Dimethyl formamide (DMF) solvent. Chlorobenzene (CBZ) as the anti-solvent was employed to drive the precursor fast crystallization during spin-coating. Afterwards, the perovskite film was followed by annealing at 80°C for 15min to remove


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residual solvent. A group of top-view SEM images of different Rb content in FAPbBr3 film is shown in Figure1 (b-e). When Rb content was low (Rb3%), there was no obvious morphology difference compared to pure FAPbBr3 film due to the low doping level. However, with Rb content increasing (Rb7%), some white flakes crystals were protruded from perovskite film, whose size were smaller than the mean crystal size of the controlled film. When the Rb content was increased to 14%, plenty of white flakes emerged on the surface of perovskite film. To fight out what the white flake area is, energy dispersive x-ray analysis (EDX) image of the corresponding films is demonstrated in Figure S2. We observed that spot1 is in the white flake area had higher bromide and rubidium atomic ratios compared to spot2 in the uniform area. Presumably, RbBr were precipitated during anti-solvent over fast crystallization. The elemental mapping analysis was carried out to identify doping positions of Rb ions in the perovskite film, as shown in Figure S3. For the Rb element signal, there was no distinct difference between grain boundaries and the grains, suggesting homogeneous Rb ion distribution in the perovskite film.

Figure2. (a)PL spectra of pure FAPbBr3 and different Rb incorporated FAPbBr3 films, (b) 7 ACS Paragon Plus Environment


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PLQY value and XRD peak position (~14ᴼ) as a function of Rb content, the inset shows the photo of different Rb content incorporated FAPbBr3 films under 405nm wavelength UV lamp, (c) XRD patterns of different Rb content incorporated FAPbBr3 films, (d) XRD patterns of FAPbBr3 films with different Rb incorporation proportion zoomed around 14ᴼ, (e) schematic diagram of Rb incorporated into FAPbBr3 lattice and substituted FA+ partially.

Furthermore, to study the dependence of the optical characteristics of perovskite film on Rb incorporation, photoluminescence spectra (PL) and photoluminescence quantum yield (PLQY) were measured, as shown in Figure2(a). From the PL spectra we can find that, with Rb content increasing, the intensity of individual PL spectrum was improved successively. Compared with pure perovskite film, the PL intensity of Rb incorporation (Rb3%, Rb7%, Rb14%) film showed 3-fold, 4.7-fold and 5.5-fold enhancement, respectively. Interestingly, the PL peak position of pure film, Rb3%, Rb7%, Rb14% was located at 540nm, 537nm, 533nm, 531nm, indicating the Rb ion incorporation can slightly tune the luminescence spectra. To further identify whether the nonradiative recombination of perovskite film was suppressed as a result of Rb incorporation, PLQY of the perovskite was measured under ambient conditions (60% relative humidity, room temperature). When the pump energy was fixed, PLQY can be described by the following equation(1)39 and the generated carrier dynamics can be described by equation(2)40, 41 :

PLQY =

=

!"#$ !%"#& !"#$ !'()"

=

*++ *,*+ + *- -

= . − 01 2 − 0 2 − 03 23

(1) (2)

Where n is photo generated carrier density, G is the generation ratio of carrier density, k1 is


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coefficient of trap-assisted monomolecular recombination, k2 is coefficient of bimolecular recombination (radiative recombination), k3 is coefficient of Auger recombination. As shown in Figure 2(b), pure FAPbBr3 film exhibited a low QY(~2%). Encouragingly, the PLQY was increased with sequence 6.85%, 15.41%, 21.42% with Rb content of the corresponding sequence 3%, 7%, 14%. The high PL quantum yield suggests promising potential for high EQE light emitting application. In addition, FAPbBr3 films within different Rb contents in daylight and ultraviolet lamp are shown in Figure S4, where the film color changed from yellow to green yellow in daylight implied the improved PLQY after Rb incorporation. Above all, all the data, PL and PLQY, addressed the reduced non-radiative recombination inside the perovskite layer incorporated by Rb ion. To determine the composition and crystallinity of the films, X-ray diffractions (XRD) of the perovskite films with different Rb contents were conducted as shown in Figure2(c), diffraction peaks at 14.707°, 20.865°, 29.669°, 33.253° of the pure FAPbBr3 film were assigned to the —

(100), (110), (200) and (210) planes of a Pm3m cubic perovskite structure, agreeing well with previously reports42, 43. For Rb incorporated perovskite film, there was a shift to wider angles for the XRD peak position, as shown in Figure 2(d). The low-angle XRD peaks for Rb14% perovskite film and pure FAPbBr3 film occurred at 14.739° and 14.707°, respectively, revealing that Rb indeed substituted the FA+ partially and modified the FAPbBr3 crystal lattice. The values of XRD peak position around 14° and PLQY are listed in Table S1. Considering the difference between ionic radius of FA+(253pm ) and Rb+(152pm), as shown in Figure 2(e), Rb substituting induced crystal distortion and contracted perovskite cubic volume, which decreased the electron overlap caused by relatively reductive metal-halide-metal bond, so that



the valence band maximum would be down-shifted. As a result, the band gap would be broadened, proved by the blue-shifted PL spectra peak. Generally, the contraction of crystal lattice may contribute to stabilize the perovskite phase. Phase stability of perovskites is determined by the volumetric ratio between [PbX6]4- (Cl, I, Br) octahedra and A-site cations. To further verify the contraction of cubic volume due to Rb incorporation, and identify the chemical state of pure and Rb incorporated FAPbBr3 films, pure FAPbBr3 film and 7% Rb incorporated FAPbBr3 film were selected to analyze the X-ray photoelectron spectroscopy (XPS). XPS spectra of Pb4f, Br3d and Rb3d are shown in FigureS5, calibrated based on C 1s (284.8eV). Photoemission of Rb 3d could not be resolved in pure FAPbBr3, while the 7% Rb incorporated FAPbBr3 film showed a pronounced peak at 109.7eV assigned to Rb 3d. Notably, there was a shift of the Pb4f and Br3d peak position to higher binding energy for 7% Rb incorporated FAPbBr3 film compared to pure FAPbBr3 film. For Pb 4f, the peak position shifted to higher binding energy from 138.25 to 138.4eV. For Br 3d, the peak position shifted to higher binding energy from 68.1eV to 68.35eV, respectively. The shifts undoubtedly stemmed from the Rb cations incorporated into the perovskite lattice, a direct evidence for the changes in chemical state between lead cations and halides. The shift of Pb 4f peaks toward higher binding energy indicated tightening halides and shrinking the PbBr6 octahedral volume. Then, the A-site cations strongly reinforced the corner-shared [PbBr6]4- octahedra, leading to the enhancement of the phase stability.


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Figure3. (a) Cross sectional SEM image of device, (b) schematic illustration of device structure. (c) Energy level diagram of different functional layer in PeLEDs. (d) EL spectra of PeLEDs based on different Rb incorporated FAPbBr3 films. The inset shows a bright electroluminescence photography of LED based on 7% Rb incorporated FAPbBr3 films, driven at 5 V, shot under daylight.

From above all, the excellent optoelectronic properties of the perovskite films with Rb incorporation are promising to enhance the performance of PeLEDs. Here, we adopted a conventional device configuration: ITO/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)(~40nm)/Perovskite film (~280nm)/ 1,3,5- Tris(1-phenyl -1H -benzimidazol-2-yl)benzene(TPBi)(60nm)/LiF(1nm)/Al(100nm), as shown in cross sectional



SEM image in Figure 3(a). Figure 3(b) is the corresponding schematic diagram of device configuration. The flat-band energy levels of the different functional layer in the PeLEDs are demonstrated in Figure 3(c). Electrons are injected from the lowest unoccupied molecular orbital (LUMO) level of TPBi into the conduction band (CB) of perovskite films, while holes are transferred from the highest occupied molecular orbital (HOMO) level of PEDOT:PSS into the valance band (VB) of perovskite films in the ongoing devices, and then the electrons and holes are recombined radiatively inside the perovskite emission layer. This process can be described by differential equation (2). Figure 3(d) presents the electroluminescence (EL) curves of different Rb content device and inset is a very bright photo shot for Rb 7% incorporated device in daylight, a blue shift was observed for the Rb incorporated device, which was in accordance with PL spectra in Figure 2(a).


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Figure4. (a)Current density –Voltage (J–V), (b) Luminance–Voltage (L-V), (c) Current efficiency-Voltage (CE–V) and d) EQE–V characteristics curves of perovskite LEDs based on FAPbBr3 film incorporated different Rb content.

The device characteristic curves of PeLEDs are shown in Figure4. The current density–voltage (J–V) curve is displayed in Figure 4(a). We observed that a slight decrease of the current density when Rb ions were incorporated into the perovskite emission layer, which might be attributed to the much more non-conductive RbBr precipitation when Rb content was increasing. In addition, the withstand voltage was improved significantly. Generally, the onset of current density decreasing indicated that the device was broken-down in this nodal voltage. For pure FAPbBr3 PeLEDs, current density started to decrease at 7.8V, PeLEDs with 3%, 7%, 14% Rb content showed higher breakdown voltage at 9.6V, 10.2V, 11V, respectively, revealing the durability of device was enhanced by Rb doping. Figure 4(b), (c) and (d) show the luminance-voltage

(L-V),

current

efficiency-voltage(CE-V),

and

external

quantum

efficiency-voltage (EQE-V) characteristics curves of four types of device. Apparently, the maximum luminance and current efficiency were significantly enhanced as Rb content was increased from 0 to 7%. Particularly, the pure FAPbBr3 LEDs demonstrated the highest luminance, CE and EQE of 6421cd/m2, 4.98cd/A, 1.47%, respectively. Excitedly, when only 3% Rb content was incorporated into FAPbBr3 film, the corresponding LEDs showed an obviously enhanced luminance, CE, EQE of 33732cd/m2, 16.73cd/A, 4.95%, respectively, whose current efficiency represented an enhancement of 3.3-fold compared to the LEDs based on pure FAPbBr3. Furthermore, the best performance of LEDs was based on FAPbBr3 containing 7%



Rb, exhibiting the maximum luminance, CE, EQE of 65353cd/m2, 24.22cd/A, 7.17%, respectively. Significantly, the CE was boosted by about 5 times improvement compared to pure FAPbBr3 LEDs, which was the champion result in this work. So far as we know, the highest values of luminance, CE and EQE that we achieved are the best reported values so far for FAPbBr3 based LEDs. Moreover, Figure S6 (a) displays histograms of peak CEs measured from 40 devices with the devices based on FAPbBr3 film incorporated 7% Rb, indicating a benign reproducibility. Nevertheless, there was no further increment for the luminance and efficiency of PeLEDs when Rb content reached excess 7%. The typical excessive Rb content 14% was also doping into the FAPbBr3 film as the photoactive layer of PeLEDs. Compared with the optimal LEDs, the luminance CE, EQE with Rb content 14%-based LED dramatically dropped down to 29532cd/m2, 11.48cd/A, 3.39%, respectively. Table1 summarizes the detailed data of four types of device. The decreased reason when excessive Rb ions were doped could be attributed to that the RbBr precipitation destroyed the uniform of perovskite film, which impeded the conduction of perovskite film and quenches the electrons and holes or the combination of both of them. Additionally, the increment of RbBr content in the precursor may induce the halogen vacancies, which can play a role of quenching centers. Device incorporated with Rb ion also exhibited a better stability than pure FAPbBr3 based LEDs, the device lifetime decay curve compared device with or without Rb ion are demonstrated in FigureS6(b). The device lifetime measurement was operated under 5 mA/cm2 constant current density, and the initial luminance was about 600cd/m2. To avoid the water and oxygen decreasing device performance, all the device fabrication and measurement processes were done in glovebox in this work, which is equivalent to encapsulation in terms of the effect of oxygen and moisture.


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Although the Rb incorporation can enhance the stability of LEDs to a certain extent, there is still a large room to improve the stability of LEDs before it can be industrialized in practice. In fact, aside from the ambient atmosphere such as water and oxygen, the intrinsic instability of perovskite is another critical factor, such as ion migration, halide separation, thermal instability, the chemical reactions between the acid with cathodes and so on. The strategies to improve the stability of perovskite materials and PeLEDs have been widely studied, such as A-site cation engineering(Cs cations doping)36, Ruddlesden–Popper phase (phenylethylamine(PEA), butylamine (BA) halide doping )27, 29, suppressing ion migration with additives and blocking layers (an amine-based small molecule (ethylenediamine (EDA)) and an amine-based polymer (polyethylenimine (PEI)) have been adopted as passivating agents for perovskite44). Last, fabricating stable perovskite nanoparticles is also an effective method to achieve stable PeLEDs45. Nevertheless, the instability of perovskite LEDs is still an open challenge to be settled in a long run. Table1. A summary performance of device incorporated different Rb content. Rb content Max.Luminance (cd/m2) No Rb 6421 Rb3% 33732 Rb7% 66353 Rb14% 29532

Max.CE (cd/A) 4.98 16.73 24.22 11.48

Max. EQE (%) 1.47 4.95 7.17 3.39

PL peak position(nm) 540 537 533 531


EL peak position(nm) 540 536 532 532


Figure5. (a)Time-resolved PL spectra of perovskite thin films incorporated with different Rb content. (b) PL intensity as a function of pump fluence. The intersection point indicates the trap state saturation threshold fluence. The blue and red lines represent the linear fits to experimental data, respectively.

Table2. The summary of fast PL lifetime (τ1), slow PL lifetime (τ2) and average lifetime (τave) depending on different Rb incorporation content. Rb content τ1(ns) f1(%) τ2(ns) f2(%) χ2 τave (ns) No Rb 1.72 28.68 40.28 71.32 1.116 29.22 Rb3% 0.71 2.76 75.43 97.24 1.051 73.37 Rb7% 2.19 2.46 151.29 97.54 1.012 147.63 Rb14% 2.77 0.37 180.98 99.63 1.056 180.32 τ1 and τ2 are the lifetimes of fast decay and slow decay, respectively. f1 and f2 are the fractions of the fast decay and slow decay parts, respectively. χ2 close to 1,indicating the fitted results are in accordance with the experimental data.

To investigate the kinetics of excitons and the formation and transport of photo excited charge carriers in the perovskite film incorporated with Rb, time resolved photoluminescence (TRPL) decay curves fitted from a bi-exponential function were also carried out in Figure5(a). PL decay curves can be fitted with following equation (3):


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4 = 51 6

78%

,

+ 5 6

78%

+

(3)

Where A is normalized PL intensity, τ1 and τ2 are the lifetime of two decay components, f1 and f2 are the fractions of two decay components. As previously reported, the PL decay lifetime was considered as the summation of fast-decay related to trap-assisted recombination at grain boundaries as an indicative of a short lifetime (τ1) and slow-decay related to radiative recombination inside the grains as an indicative of a long lifetime (τ2). The average recombination lifetime (τave) was calculated with the fi and τi (i=1,2) values from the fitted curve data according to the following equation (4):

:; , the P=> of Rb 7% incorporated FAPbBr3 film were 8.6µJ/cm2, corresponding to a trap state density of about 6.1×1017 cm-3. While, the pure FAPbBr3 film possessed a higher threshold about 11.7µJ/cm2, indicating a higher trap state density, about 8.4×1017 cm−3. The decreased trap density was consistent with the PLQY measurement. Owing to the trap can also be divided



into bulk trap (such as vacancies, interstitials, antisites) and surface trap (like dangling bonds). We fitted these two types of trap in Figure S8 according to previous report. From the fitted data, we can conclude that both of the surface and bulk trap density were decreased when the Rb ion incorporated into FAPbBr3. 3. CONCLUSION In conclusion, we systematically investigate the effects of Rb incorporation on FAPbBr3 perovskite film, including crystal formation, morphology, optical property, crystal structure and corresponding LEDs. Rb ion replaces the site of FA+ partially, and distorts the crystal structure of FAPbBr3, thus affects the optoelectronic properties of FAPbBr3 in turn. The enhanced PLQY of perovskite film makes high performance PeLEDs is more accessible caused by Rb incorporation. The optimal LEDs with 7% Rb incorporation exhibit a maximum reported luminance of 65353cd/m2, EQE of 7.17% and a highest efficiency of 24.22 cd/A, the highest luminance and efficiency based on FAPbBr3 film so far. The Rb incorporation provides a new way to tune optical spectra, increase PLQY of perovskite film, and enhance the performance and stability of device. This work suggests a feasible road to improve the light emission performance of PeLEDs, working as a new milestone for the development of PeLEDs. 4. EXPERIMENTAL SECTION

Materials: RbBr2 (99.999 wt%)and PbBr2 (99.999 wt%) were purchased from Alfa Aesar. TPBi(CC033,99%,made in Taiwan, China) and LiF(CC033,99.99%,made in Taiwan, China) was purchased from Nichem. FABr was synthesized and purified according to a previously reported method47, 48. 24 mL formamidine solution (33 wt% in ethanol, Sigma Aldrich) and


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10mL hydrobromic acid (57 wt% in water, Sigma Aldrich) were diluted by using 100mL ethanol in a 250 mL round bottom flask by constant stirring at 0°C for 2 hours. The precipitate of FABr was obtained by rotary evaporation at 40°C and washed with dry diethyl ether until the solid became white. The final product was dried at 60°C in a vacuum oven for 24 hours. TPBi is purchased from Alfa Aesar. Aqueous dispersions of PEDOT:PSS (CLEVIOS PVP Al 4083) were obtained from Heraeus and used as received.

Film and device Fabrication: ITO glass substrates were cleaned with detergent, acetone and ethyl alcohol mixed solvent and deionized water sequentially and repeatedly. After these procedures, the cleaned ITO glass will be dried under bake lamp. The dried ITO glass substrates were treated with ultraviolet ozone plasma for 5 minutes to improve wettability and work function. Then, diluted PEDOT:PSS(the volume of deionized water and PEDOT:PSS mother solution is 3:1) solution was spin-coated on treated ITO glass at 1000 rpm for 30 seconds and annealed at 120°C for 20 minutes at ambient atmosphere, forming a PEDOT:PSS layer of 40nm. After that, the substrates transferred into a nitrogen-filled glove box, a DMF solution of FAPbBr3 (FABr/ RbBr/PbBr2 in (1.7-x): x: 1 molar ratio) was spin-coated on the substrate at 3000 rpm for 30s and CBZ dropping after 7s from the beginning of spin-coating, then the films were annealed at 80°C for 15 minutes, after that, the perovskite film was transported into thermal evaporation chamber without air exposure for the fabrication of 60nm TPBi, 1nm LiF and 100 nm Al sequentially. The evaporation rates for the organic layer, LiF and Al were 0.3 nm s−1, 0.1 nm s−1 and 1 nm s−1, respectively.

Characterization:



The film thickness was determined by a Dektak profilometer. It was found that all perovskite film thicknesses were around 280 nm. The emission area of the device was about 12 mm2. Luminance–current–voltage (L–I–V) characteristics, as well as the stability performance of the devices, were measured using a computer-controlled source meter (Keithley 2602) and a calibrated silicon photodiode. All measurements were carried out at room temperature in the glove box in a 99.999% N2 atmosphere to exclude the influence of an air atmosphere on the device performance. Electroluminescence spectra were measured using a PR650 spectrometer. The surface morphology was investigated by scanning electron microscopy (Quanta F250). The perovskite film was characterized by X-ray diffraction (XRD) (D/MAX-2400, Rigaku, Japan) with Cu Kα radiation. Photoluminescence spectra of the perovskite films were recorded using a fluorescence spectrophotometer (Fluoromax 4, HORIBA Jobin Yvon). PLQY were measured with a integrating sphere (excited at 405nm), Time-resolved PL spectra were recorded with a 100ps time resolution using a time-correlated single photon counting (TCSPC) system (FLS920 spectrometer) (excited by picosecond pulsed LEDs, pulse duration:

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