Defect Passivation for Red Perovskite Light-Emitting Diodes with

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Defect Passivation for Red Perovskite Light-Emitting Diodes with Improved Brightness and Stability You Ke, Nana Wang, Decheng Kong, Yu Cao, Yarong He, Lin Zhu, Yuming Wang, Chen Xue, Qiming Peng, Feng Gao, Wei Huang, and Jianpu Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03664 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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The Journal of Physical Chemistry Letters

Defect Passivation for Red Perovskite LightEmitting Diodes with Improved Brightness and Stability You Ke a, Nana Wang a*, Decheng Kong a, Yu Cao a, Yarong He a, Lin Zhu a, Yuming Wang b, Chen Xue c, Qiming Peng a, Feng Gao b, Wei Huang a,c, Jianpu Wang a* a

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China. b Department

of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, SE-

58183, Sweden. c Shaanxi

Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU),

127 West Youyi Road, Xi'an 710072, Shaanxi, China. Corresponding Author [email protected] (N. W.); [email protected] (J. W.)

ACS Paragon Plus Environment

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ABSTRACT: Efficient and stable red perovskite light-emitting diode (PeLED) is important for realizing fullcolor display and lighting. Red PeLEDs can be achieved either by mixed-halide or lowdimensional perovskites. However, the device performance, especially the brightness, is still low owing to the phase separation or poor charge transport issues. Here, we demonstrate red PeLEDs based on three-dimensional (3D) mixed-halide perovskites where the defects are passivated by using 5-aminovaleric acid. The red PeLEDs with an emission peak at 690 nm exhibit an external quantum efficiency of 8.7% and a luminance of 1,408 cd m-2. A maximum luminance of 8,547 cd m-2 can be further achieved as tuning the emission peak to 662 nm, representing the highest brightness of red PeLEDs. Moreover, those LEDs exhibit a half-life of up to 8 h under a high constant current density of 100 mA cm-2, which is over 10 times improvement compared to literature results.

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Solution-processed perovskite light-emitting diodes (PeLEDs) have received considerable attention due to their potential for flexible, low-cost and large-scale display and lighting applications.1–3 The near-infrared and green PeLEDs have achieved remarkable progress, with external quantum efficiency (EQE) of >20%,4–6 which is approaching the best performing organic LEDs.7,8 Moreover, the near-infrared and green PeLEDs demonstrated remarkable brightness, with a radiance of 390 W sr-1 m-2 and a luminance of 179,000 cd m-2, respectively.5,9 However, it is still a challenge to achieve high brightness in red and blue PeLEDs. The main reason is that the red and blue emissions are either from low-dimensional perovskites which show inferior charge transport properties due to the existence of large organic spacer, or mixedhalide perovskites which usually suffer from halide segregation under electrical stress.10–14 Cs based single-halide nanocrystal and multiple quantum well (MQW) perovskites have been demonstrated to achieve red emission with a peak EQE of 7.3%.14–18 And recently it has been demonstrated that CsPb(Br/I)3 quantum dot based red PeLED shows a high peak EQE of >20% at a current density of ~0.01 mA cm-2.19 However, the maximum luminance of those red PeLEDs with quantum confinement effect is low (100 mW cm-2 when the trap induced non-radiative recombination centers are filled. It has been reported that well-passivated perovskites possess significantly enhanced PLQE.22 After adding 5AVA with a ratio of 0.5, the PLQE increases to ~30% at the low excitation of


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0.1 mW cm-2, indicating that trap-assisted non-radiative recombination is effectively suppressed. More importantly, the PLQE can maintain ~40% at light intensities between 5 mW cm-2 and 400 mW cm-2. We attribute the reduced trap density to the defect passivation of 5AVA, in which the protonated NH3+ group can bind to FA/Cs vacancy and the –COO- group can coordinate to the lead ion in perovskite surface.23–25 To further verify the passivation effect, the perovskite films were characterized by time-resolved PL measurement. After the incorporation of 5AVA, the PL lifetime of the perovskite films are significantly increased (Figure 3d) and the trap-assisted non-radiative monomolecular recombination rate is reduced (Table S2). The time-resolved PL curves at different excitation intensities are fit by using stretched-exponential decay function (Figure S5 and Table S3),26 which show that for the FA0.47Cs0.53Pb(I0.87Br0.13)3 film without 5AVA, the average lifetime () is ~62 ns and increases with increased excitation intensities, indicating high rate of trap-assisted non-radiative recombination within the perovskite films. For the perovskite film with 5AVA (Figure S5b and Table S3),  is ~157 ns and decreases as the excitation intensities increase. The transient PL measurement results are consistent with the excitation-intensity-dependent PLQE (Figure 3c), suggesting that 5AVA can passivate the defects of perovskites, leading to reduced non-radiative recombination. Moreover, the PL transients at different emission wavelengths also show that the perovskite film with 5AVA has less wavelength-dependent PL lifetime (Figure S5c,d), indicating decreased energetic disorder in FA0.47Cs0.53Pb(I0.87Br0.13)3 film.27 To quantify the electronic disorder of perovskite film, urbach energy (Eu) was measured by using Fouriertransform photocurrent spectroscopy (FTPS). Figure 3e shows that the Eu for 0, 0.3, 0.5 and 0.7 films are 15.00.4, 14.50.2, 14.30.3 and 13.70.4 meV, respectively.


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The above results suggest that 5AVA can aid the grain growth and passivate defects of perovskite crystallites, leading to suppressed nonradiative recombination and phase segregation. To further increase the luminance of red PeLEDs, we next change the ratio of iodide and bromide to shift the emission spectrum to short-wavelength. The EL emission peaks of PeLEDs based on 5AVA0.3-FA0.47Cs0.53Pb(I1-yBry)3 with different Br ratios (y=0.23, 0.33, 0.43, 0.53, 0.64) locate at 675, 662, 650, 630 and 605 nm, respectively (Figure 4). Note that the additive of 5AVA has similar effect on the suppression of phase separation in different I-Br perovskites, resulting in stable EL spectra of these PeLEDs under various applied voltages (Figure S6). The 5AVA0.3FA0.47Cs0.53Pb(I0.67Br0.33)3 device shows a EL peak of 662 nm and maximum brightness of 8,547 cd m-2 at a low voltage of 3.3 V (Figure 4c), which is the record luminance of red PeLEDs (Table S1). The high luminance of red PeLEDs can be mainly owing to (i) high quality perovskite crystallites with low defect density, (ii) good charge transport property of 3D perovskite and (iii) relative short emission spectrum with more sensitive to the human eye. Meanwhile, the device exhibits very pure red emission with Commission Internationale de l’Eclairage (CIE) color coordinates of (0.72, 0.28) (Figure 4e). The peak EQE, current efficiency, luminous efficacy are 4.9%, 1.32 cd A-1, 1.50 lm W-1 respectively. The lifetime of device under a constant current density of 100 mA cm-2 for an initial luminance of ~700 cd m-2 is 3 h, representing over 10 times improvement compared to previous reported red PeLEDs under the similar condition (Table S1).


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Figure 4. Characterizations of red PeLEDs based on 5AVA0.3-FA0.47Cs0.53Pb(I1-yBry)3 with different Br ratios. (a) Normalized EL spectra. (b) Dependence of current density on the driving voltage. (c) Dependence of luminance on the driving voltage. (d) Dependence of EQE on the current density. (e) Corresponding CIE coordinate of device with Br ratio of 0.33. (f) Stability of device with Br ratio of 0.33 measured at a constant current density of 100 mA cm-2. We have demonstrated that the 5AVA additive can stable the 3D mixed-halide perovskites by enhancing the crystallinity and reducing defect densities, and retain the good charge transport of the 3D perovskites. This enables us to achieve high efficient and stable red PeLEDs with a record luminance of 8,547 cd m−2. We believe that our finding is also useful for achieving high performance blue perovskite LEDs, and can be further extended to other mixed-halide perovskite optoelectronic devices.


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Experimental Methods Synthesis and materials preparation. Colloidal ZnO nanocrystals were synthesized by a solutionprecipitation process with some modifications.28 The precursor solutions of perovskite films with various 5AVA ratios were prepared by dissolving 5AVA, FAI, FABr, CsI and PbI2 with a molar ratio of x:0.4:0.5:1:1 in a mixed solvent of N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) with 7 wt.% concentration. The precursor solutions of 5AVA0.3FA0.47Cs0.53Pb(I1-yBry)3 films are from 5AVA, FAI, FABr, CsI, PbI2 and PbBr2 with a molar ratio of 0.3:0.4:0.5:1:(1.25-1.95y):(1.95y-0.25). All the precursor solutions were stirred at 60 °C for 2 h in a glovebox. Device fabrication. The devices were fabricated with an architecture of indium tin oxide (ITO)/polyethylenimine ethoxylated modified zinc oxide (ZnO-PEIE)/ perovskite/ poly(9,9dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB)/molybdenum oxide (MoOx,)/gold (Au). The ZnO films were prepared by spin coating solutions of ZnO nanocrystals onto the ITOcoated glass substrates at 4,000 rpm for 45 s and annealed in air at 150 °C for 30 min. Then an untrathin PEIE layer was prepared by spin coating a solution of PEIE in 2-methoxyethanol (0.4 wt.%) onto the ZnO films and rinsing twice with DMF. The precursor solutions of perovskites were spin-coated onto the PEIE treated ZnO films, then annealed at 100 °C for 30 min to form perovskite films. The TFB layers were deposited from an m-xylene solution (8 mg mL-1). Finally, MoOx/Au electrodes were deposited using a thermal evaporation system. The device area was 3 mm2. Characterization. The perovskite LED devices were measured at room temperature in a nitrogen-filled glovebox. A fiber integration sphere (FOIS-1) coupled with a QE65 Pro spectrometer and a Keithley 2400 source meter were used for the measurements.29 The devices


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were swept from zero bias to forward bias with a scanning rate of 0.05 V s-1. The time evolution of EQEs and voltages were measured in air with simple glass-epoxy encapsulation of devices. The film morphology was measured by a JEOL5 JSM-7800F SEM. The X-ray diffraction patterns were taken on a Bruker D8 Advance. UV-vis absorbance and PL spectra were carried out by a UV-vis spectrophotometer with an integrating sphere (PerkinElmer, Lambda 950) and a fluorescent spectrophotometer (F-4600, HITACHI) with a 200 W Xe lamp as the excitation source, respectively. The PLQE measurement was taken by combination of a 445 nm continuous wave laser, optical fiber, spectrometer, and integrating sphere.30 The sample size is 2 cm2 cm and the light spot is a circle with a diameter of 5 mm. The time-resolved PL transients were measured by using an Edinburgh Instruments spectrometer (FLS920). Perovskite films were excited by a 445 nm pulsed laser with different intensities. The time-resolved PL curves are fit to extract the PL decay rate by

dn  k1n  k2 n 2  k3 n3 dt

(1),

where k1 is trap-assisted and geminate monomolecular recombination constant, k2 is bimolecular recombination constant, and k3 is Auger recombination constant. To obtain the average PL lifetime, the PL decay curves were fit by using the stretched-exponential decay function,26

I (t )  I 0 e  (t / c )



(2),


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where I(t) is PL intensity, c is the time taken after excitation for the PL intensity to drop to 1/e of the initial intensity (I0), and β is distribution coefficient. The average lifetime () is calculated by



c 1 ( )  

(3),

1 where ( ) is defined as 



1 ( )   x (1  )/  e  x dx



(4).

0

FTPS was measured by combination of a Fourier transform infrared spectroscopy (FTIR, Vertex 70, Bruker Optics) and a low-noise current amplifier (SR570, Stanford Research Systems). Acknowledgments This work is financially supported by the Major Research Plan of the National Natural Science Foundation of China (91733302), the Joint Research Program between China and European Union (2016YFE0112000), the National Basic Research Program of China-Fundamental Studies of Perovskite Solar Cells (2015CB932200), National Key Research and Development Program of China (2017YFB0404500), the Natural Science Foundation of Jiangsu Province, China (BK20150043, BK20150064, BK20180085), the National Natural Science Foundation of China (11474164, 61875084, 61634001, 21601085, 11804156), the National Science Fund for


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Distinguished Young Scholars (61725502), the Synergetic Innovation Center for Organic Electronics and Information Displays. Supporting Information. The following files are available free of charge. Absorbance spectra, PL spectra, XRD, time-resolved PL decay transients of perovskite films; EL spectra, luminance, efficiency, lifetime of PeLEDs (PDF)

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Defect Passivation for Red Perovskite Light-Emitting Diodes with

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