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Efficient and Color-Tunable Quasi-2D CsPbBrxCl3x Perovskite Blue Light-Emitting Diodes Kun-Hua Wang, Yande Peng, Jing Ge, Shenlong Jiang, Bai-Sheng Zhu, JiSong Yao, Yi-Chen Yin, Jun-Nan Yang, Qun Zhang, and Hong-Bin Yao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01490 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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ACS Photonics

Efficient and Color-Tunable Quasi-2D CsPbBrxCl3-x Perovskite Blue Light-Emitting Diodes

Kun-Hua Wang1,2, Yande Peng1,2, Jing Ge1,3, Shenlong Jiang 1,3, Bai-Sheng Zhu1,2, Jisong Yao 1,2,

Yi-Chen Yin1,2, Jun-Nan Yang1,2, Qun Zhang1,3, Hong-Bin Yao1,2*

1 Hefei National Laboratory for Physical Sciences at the Microscale, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, China 2 Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China 3 Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

*Corresponding author: [email protected] Group website: http://staff.ustc.edu.cn/~yhb/

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ABSTRACT: Metal halide perovskites are emerging as promising candidate materials for light-emitting diodes (LEDs) due to their high luminescence, color purity, tunable bandgaps, and solution processabilities. In the past several years, the metal halide perovskite based LEDs (PeLEDs) in the green and red light spectral range have been demonstrated in high brightness and good efficiency. However, the performance of blue PeLED is still limited by the low blue light emission efficiency of present metal halide perovskite materials. Here, we report a facile solution method to fabricate a series of quasi-2D CsPbBrxCl3-x perovskite films with high photoluminescent quantum yields (PLQYs) in the blue light spectral range. By compositional engineering via increasing the content of chloride, we achieved the tunable light emission wavelength of quasi-2D CsPbBrxCl3-x perovskite films from green (504 nm) to blue (470 nm) with a high PLQY up to 42% at 486 nm. In addition, we developed a novel NiOx/LiF hole transport layer with high affinity to precursor solution for improving the film quality of quasi2D CsPbBrxCl3-x perovskite while reducing the quenching effect of hole transport layer to asformed perovskite film. As a result, the blue PeLEDs based on quasi-2D CsPbBrxCl3-x perovskite films show color-tunable emissions and the external quantum efficiency of 0.52% as well as high brightness of 1446 cd m-2 are achieved at 490 nm.

KEYWORDS: light-emitting diodes, CsPbBrxCl3-x, perovskite, quasi-2D, high brightness,

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Metal halide perovskites have attracted tremendous research interest on light-emitting diodes (LEDs)1-4 over the past few years due to their intriguing optoelectronics properties such as high photoluminescence quantum yields (PLQYs), narrow emission bandwidths, high chargecarrier mobilities, and tunable bandgaps to cover entire visible light spectrum.5-7 Since the first metal halide perovskite based LED (PeLED) was demonstrated with an external quantum efficiency (EQE) around 0.1% in 2014,8 the EQEs of PeLEDs have been unprecedentedly improved within the green and red PeLEDs to exceed 10%.9-12 However, the device performance of blue PeLEDs still lags significantly behind that of their green and red counterparts, which sets substantial limitations in the realistic use of PeLEDs for displays or solid-state lighting.13-17 To enhance the light emission efficiency of metal halide perovskites, the nanoscale-sized and structured dimensional confinement strategies have been developed to compensate for the limitation of low exciton binding energy of three dimensional (3D) perovskite.1, 8, 18 Presently, the most effective metal halide perovskite emitters in green and red light spectral range are nanocrystals and quasi-two-dimensional (quasi-2D) perovskites.19, 20 Especially, the reduced dimensional structure of quasi-2D perovskite has a general formula of B2An-1PbnX3n+1, where B, A, X, and n represent bulky cations with long-chain ammonium groups, small cations (methylammonium, formamidindium, or cesium), halide ions (I-, Br-, or Cl-), and the number of PbX4 octahedral layers within a crystallite, respectively.21 In previous works, efficient green and red PeLEDs based on quasi-2D perovskites with multilayers of octahedral PbX4 (n ≥ 3) have been demonstrated to exhibit outstanding device performances.22-26 Theoretically, quasi2D perovskites can also enable blue light emission by reducing the number of inorganic 3


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octahedral PbX4 layers lower than three (n < 3) to tune its bandgap into blue light spectral range.27-29 However, blue PeLEDs based on reduced dimensional quasi-2D perovskites (n < 3) showed very poor performances so far.30, 31 This is due to the fact that the enlarged exciton binding energy in further reduced quasi-2D perovskites would cause fast exciton quenching in inorganic layers32,

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and the enriched insulating long-chain ammonium groups would also

inhibit charge carrier transport in blue PeLEDs.29 As a result, reasonable values of inorganic layer number (n ≥ 3) should be ensured to obtain high light emission efficiency and good charge carrier transport in PeLEDs. On the other hand, it has been demonstrated that the bandgaps of metal halide perovskites can be feasibly tuned by changing the ratio of halide anions in perovskites.34 Along with this line, the mixed Cl and Br anions design in quasi-2D perovskites with multilayers of inorganic octahedral PbX4 (n ≥ 3) is promising to achieve efficient blue PeLEDs. Herein, we report a facile solution-processable approach to fabricate multilayered quasi-2D CsPbBrxCl3-x perovskite films with high PLQYs (up to 42% at 486 nm) for efficient blue PeLEDs. By increasing the ratio of chloride to bromide, the PL emission peak of as-fabricated quasi-2D CsPbBrxCl3-x perovskite films can be tuned from green (504 nm) to blue (470 nm) with narrow full widths at half-maximum (fwhm) (~24 nm). Moreover, NiOx/LiF was adopted as a novel hole transport layer (HTL), which not only exhibited better wetting capability to hydrophilic perovskite precursor in dimethylsulfoxide (DMSO) but also resulted in much lower photoluminescent quenching compared to other HTLs. Based on these merits, we achieved a blue PeLED with a maximum EQE of 0.52 % and a brightness of 1446 cd m-2 at an 4


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electroluminescence emission wavelength of 490 nm. Furthermore, the PeLEDs with the emission peak at 481 and 473 nm were also demonstrated with EQE of 0.25% and 0.16% and brightness of 509 and 217cd m-2, respectively.

Figure 1. a) Schematic fabrication procedure of quasi-2D CsPbBrxCl3-x perovskite films by spin coating of precursor solution with a followed thermal annealing treatment. b) UV-vis absorption and PL spectra of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film. Insets show the photos of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film under ambient and UV light (365 nm) irradiation. c) PXRD patterns of as obtained quasi-2D CsPbBrxCl3-x (30% Cl) film. d) Typical TEM image showing the representative nanostructures of quasi-2D CsPbBrxCl3-x (30% Cl) (inset: the HRTEM of quasi-2D CsPbBrxCl3-x (30% Cl) nanocrystal. e) Crystal structural model of as-obtained quasi-2D CsPbBrxCl3-x perovskite.

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The preparation procedure of quasi-2D CsPbBrxCl3-x perovskite films is shown in Figure 1a. Typically, the perovskite precursor solution was prepared by dissolving PbBr2, PbCl2, CsBr, and phenylbutylammonium bromide (PBABr) (PbBr2:PbCl2:CsBr:PBABr = 0.7:0.3:1.25:0.72, named as quasi-2D CsPbBrxCl3-x (30% Cl)) in DMSO at a concentration of 0.2 M. Then the prepared precursor solution was spun-cast onto the glass substrate at 4000 round per minute (rpm) for 90 s in a N2-filled glovebox followed by thermal annealing at 80 oC for 5 min. After annealing, the transparent precursor film turned to green and exhibited bright blue light emission under the UV light (365 nm) irradiation (inset in Figure 1b). The optical properties of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite films were characterized by steady-state PL and UV-vis absorption spectroscopy. As shown in Figure 1b, the PL spectrum displays an emission peak centered at 486 nm with a narrow fwhm of 21 nm and the absolute PLQY is as high as 42%. Moreover, we compared the PLQYs of the quasi-2D CsPbBrxCl3-x (30% Cl) films with various ration of PBABr (10%, 40%, 72% and 100%). As shown in Figure S1, the peak position of quasi-2D CsPbBrxCl3-x (30% Cl) film blue-shifted from 496 to 472 nm with the PBABr ration increasing from 10% to 100%, demonstrating the strong effect of PBA cations to impede the crystal growth of quasi-2D perovskite film. The PLQY values of these perovskite films improved with the increase of PBABr ratio from 10% to 72%, which are 0.5%, 13.4% and 42%, respectively. Then the PLQY value dropped to 35% when the ration of PBABr increased to 100%. Although the perovskite film with 100% PBABr shows relatively high PLQY (35%), the full with at half maximum (fwhm) of its photoluminescence spectrum is wider (26 nm) than that of perovskite films with 10% PBABr (20 nm), 40% PBABr (20 nm) and 72% PBABr (21 nm). Overall, the quasi-2D CsPbBrxCl3-x (30% Cl) film with 72% PBABr 6


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shows not only highest PLQY (42%) but also narrow fwhm (21 nm), which is the best candidate for the fabrication of PeLED. The UV-vis absorption spectrum indicates multiple excitonic absorption features of as-fabricated quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film at ~387 nm (3.2 eV), 413 nm (3.0 eV), 438 nm (2.83 eV), and 468 nm (2.65 eV) corresponding to the perovskite phases with n = 1, 2, 3, and 4, respectively.35 In addition, the lowest bandgap absorption (< 2.5 eV) was also found in the spectrum, which is similar to the band-edge absorption of bulk 3D perovskites and corresponds to the perovskite phase with much larger n value (≥ 10). These results suggest that the obtained quasi-2D CsPbBrxCl3-x (30% Cl) perovskite films comprise layered perovskite phases with a series of n values larger than three.36 To assess the crystal structure and layered nature of resulting quasi-2D perovskite films, the powder X-ray diffraction (PXRD) characterization was conducted. As shown in Figure 1c, two prominent broadened peaks at 15.5° and 30.6° can be assigned to the (020) and (040) planes, respectively.26 The broadened peak of (020) plane indicates the existence of layered quasi-2D perovskite phase.4 It also implies that the quasi-2D perovskite crystals in the asfabricated film are oriented with the (020) plane being parallel to the substrate surface. To further gain more information about the quasi-2D perovskite nanocrystals in as-fabricated film, the samples were scratched from the substrate and characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). As revealed in Figure 1d, the morphologies of quasi-2D perovskite nanocrystals are square or rectangular with domain sizes of 10–40 nm (Figure S2). From the HRTEM image (inset in Figure 1d), the lattice spacing of 4.1, 4.1, and 5.8 Å corresponding to (200), (020) and (101), respectively, can be identified, suggesting that the crystal structure of as-obtained quasi-2D 7


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perovskite was dominated by orthorhombic phase.37, 38 It seems that the PBA cations impeded further crystal growth of quasi-2D CsPbBrxCl3-x perovskites through surface passivation. The crystal structural model of as-obtained quasi-2D perovskite is depicted in Figure 1e. Through varying the ratio of lead chloride to lead bromide in the precursor solution, a series of uniform and bright light emissive quasi-2D perovskite films were obtained (Figure 2a and 2b, under ambient and UV light irradiation, respectively). The PL spectra in Figure 2c show that the PL emission peak of pristine quasi-2D CsPbBr3 perovskite film is at 504 nm and a systematic blue shift in PL emission spectra from 504 to 470 nm is achieved as the ratio of PbCl2 to PbBr2 increases from 0% to 50%. This blue-shift in PL emission can be attributed to the bandgap tuning via mixed Cl- and Br- anions characteristics of metal halide perovskites.34 Furthermore, the PL peak positions followed a linear function with the variation of chloride fraction in as-fabricated quasi-2D CsPbBrxCl3-x films (Figure 2d). Although the absolute PLQYs decreased with increasing the chloride amount in the film, the obtained quasi-2D CsPbBrxCl3-x perovskite film still exhibited high PLQY of 27% at 470 nm (Figure 2e, pink line). The high PLQY of as-obtained quasi-2D CsPbBrxCl3-x perovskite film is due to the quantum confinement effect via the formation of quantum well structure between large bandgap phase (smaller n-phases) and small bandgap phase (n→∞ phase or 3D perovskite).23 Differently from the change in absolute PLQYs, the fwhm of the PL spectra of as-fabricated quasi-2D perovskite films remained narrow (21–23 nm) as the emission peak changed from 504 to 470 nm (Figure 2e, blue line). A blue-shift of the absorption edge of as-fabricated quasi2D perovskite film with the increase of Cl amount was also observed (Figure 2f). Moreover, the multiple absorption peaks suggest that all these quasi-2D CsPbBrxCl3-x perovskite films 8


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Figure 2. a, b) Photos of a series of quasi-2D CsPbBrxCl3-x perovskite films with different chloride content under ambient and UV light (365 nm) irradiation, respectively. c) Normalized PL spectra for a series of quasi-2D CsPbBrxCl3-x perovskite films with different chloride content. d) The linear relationship between PL peak positions with different chloride content for quasi-2D CsPbBrxCl3-x perovskite films. e) PLQYs and fwhms of quasi-2D CsPbBrxCl3-x perovskite films with different chloride content. f) UV-vis absorption spectra of as-obtained quasi-2D CsPbBrxCl3-x films with different chloride content.

contain a series of phases with different numbers of inorganic PbX4 layers. To further confirm the number range of inorganic layers, the thickness of as-formed quasi-2D CsPbBrxCl3-x perovskite was measured by atomic force microscopy (AFM). As shown in Figure S3, the 9


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thickness of nanoplatelets varied from 2 to 10.48 nm, which implied that the layered number values (n) are in the range of about 3–18. The PXRD patterns of these quasi-2D CsPbBrxCl3-x perovskite films (Figure S4) show that as the chloride content increased the dominant (040) peaks in the diffraction patterns slightly shifted to higher scattering angles (from 30.3 o to 30.7o) due to the lattice shrinkage with the replacement of bromide ions by smaller sized chloride ions.39 Furthermore, the intensity of dominant peak (040) become a little stronger with the increase of Cl content, which implied that the crystalline of as-formed quasi-2D perovskite film improved a little with the increase of content of Cl. To find a suitable device structure for quasi-2D CsPbBrxCl3-x perovskite films to achieve highly efficient blue light PeLEDs, we compared several HTLs including poly(9vinlycarbazole)

(PVK),

poly(3,4-ethylenedioxythiophene)-polystyrene

sulfonate

(PEDOT:PSS), NiOx and NiOx/LiF. As shown in Figure 3a-3d, the contact angles of a DMSO droplet on the surface of PVK, PEDOT: PSS, NiOx, NiOx/LiF were measured at first, respectively. The results clearly showed that the PVK layer exhibited the highest contact angle of 52.9o (Figure 3a), while the PEDOT:PSS performed the best wettability to DMSO with the contact angle about 0o (Figure 3b). The inorganic hole-transport layer, NiOx and NiOx/LiF, both exhibited more affinity to DMSO than that of PVK and the contact angles are 17.2o and 22.9o, respectively (Figure 3c-3d). Moreover, other commonly used HTLs, poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) and poly[N,Nʹ-bis(4-butylphenyl)-N,Nʹbis(phenyl)-benzidine] (Poly-TPD), also exhibited the high contact angle of 53.5o and 54.2o,

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Figure 3. The contact angle images for perovskite precursor solution dropped on a) PVK, b) PEDOT: PSS, c) NiOx, d) NiOx/LiF, respectively. Photos of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite films deposited on e) PVK, f) PEDOT: PSS, g) NiOx, h) NiOx/LiF under ambient conditions and UV light (365 nm) irradiation, respectively. i) PL spectra comparison of quasi2D CsPbBrxCl3-x (30% Cl) perovskite films deposited on PEDOT: PSS, NiOx and NiOx/LiF. j) PL decay trace comparison of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite films deposited on PEDOT: PSS, NiOx and NiOx/LiF.

respectively, which indicated their poor wettability to DMSO (Figure S5). The photos of quasi2D CsPbBrxCl3-x (30% Cl) perovskite films formed on different HTL are shown in Figure 3e3h. In consistent with the results of contact angle measurements, the quasi-2D CsPbBrxCl3-x (30% Cl) perovskite films on PEDOT:PSS, NiOx and NiOx/LiF are uniform and continuous, whereas the perovskite precursor in DMSO is hard to spread out on the surface of PVK layer to form uniform film. To clarify the quality of the films formed on different hole transport layers, SEM characterizations were carried out to show the morphologies of perovskite films 11


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deposited on PEDOT:PSS, NiOx, NiOx/LiF and PVK. As shown in Figure S6, the perovskite films deposited on different substrates are all pin-hole free regardless of the wettability between precursor solution and hole transport layers. However, the perovskite film formed on PEDOT:PSS layer exhibited the best uniformity and smooth surface due to its best affinity to precursor solution. By contrast, the film deposited on PVK layer was relatively rough. Therefore, PVK is not a good choice as a HTL in our case. Although PEDOT:PSS exhibited outstanding wettability with the perovskite precursor solution, as-formed quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film showed very low PL intensity and the absolute PLQY was as low as 1% (Figure 3i, red line). Correspondingly, the quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film also behaved short average PL lifetime about 1.22 ns derived from the timeresolved photoluminescence (TRPL) spectrum (Figure 3j, red line and Table S1). In addition, the PEDOT:PSS as an anode buffer layer in thin film PeLEDs may lead to stability issues due to the hygroscopic and acidic nature of PEDOT:PSS.40 On the contrary, NiOx as a metal oxide HTL owns superior stability. However, the perovskite film directly deposited onto NiOx layer displayed strong PL quenching due to the charge transfer induced by defects on the surface of NiOx layer.41 To avoid this PL quenching, an inert LiF intermediate layer (1 nm) was inserted between NiOx and quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film. As shown in Figure 3i, the PL intensity of as-fabricated quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film was largely enhanced and the absolute PLQY increased from 9.5% to 18% after the LiF interlayer modification (Figure 3i, pink line and blue line). To further show the difference of hole injection and exciton quenching effect of NiOx and NiOx/LiF HTLs on quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film, the TRPL spectra were carried out as well (Figure 3j, pink line and 12


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blue line). The average PL lifetime increased from 12.17 to 12.39 ns after introducing a 1 nm of LiF interlayer between perovskite and NiOx films (Table S1), implying that LiF layer can effectively passivate the NiOx HTL and suppress the exciton quenching induced by the defect trap states on the surface of NiOx. To further confirm the passivation effect of LiF layer on NiOx HTL, the confocal PL microscopy images of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite films on both NiOx and NiOx/LiF HTLs were collected. Mapping images of the PL decay time are presented in Figure S7 (a) and (b) for quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film on NiOx and NiOx/LiF, respectively. It is clearly seen that the perovskite film on NiOx/LiF HTL exhibited much more homogeneous spatial distribution of PL decay time and a longer average PL decay time in comparison to that on the NiOx HTL indicating the good passivation effect of LiF interlayer. The high PLQYs of as-fabricated quasi-2D CsPbBrxCl3-x perovskite films encouraged us to explore their applications in blue PeLEDs. We fabricated blue PeLEDs with a structure as depicted in Figure 4a, which is consisted of glass/indium tin oxide (ITO)/NiOx/LiF/quasi-2D CsPbBrxCl3-x/1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi)/LiF/Al. Except for NiOx and perovskite layers, all other layers were deposited by thermal evaporation in vacuum. AFM measurement (Figure S8) illustrates that the root-mean-square (rms) surface roughness of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film is ∼1.4 nm. The relatively smooth surface feature of as-formed perovskite film is in agreement with the uniform film observed by scanning electron microscopy (SEM) (Figure S6c). As shown in cross-sectional SEM image (Figure 4b), the thicknesses of NiOx, quasi-2D CsPbBrxCl3-x, TPBi and Al layers are ~40, ~30

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Figure 4. a) Device structure of as-fabricated PeLEDs. b) Cross-sectional SEM image of asfabricated PeLED devices. c) EL spectrum at an operation voltage of 5 V and their corresponding PL emission spectrum for quasi-2D CsPbBrxCl3-x (30% Cl) based PeLED. Inset shows the photo of corresponding devices at an applied voltage of 5 V. d) Current density, e) Luminance and f) EQE versus driving voltage characteristics for the quasi-2D CsPbBrxCl3-x (30% Cl) perovskite based PeLED using HTLs of pristine NiOx and with different LiF interlayers, respectively.

~40 and ~100 nm, respectively. The flat-band energy levels of different functional layers in asfabricated PeLED is depicted in Figure S9. The fabricated PeLED emitted bright blue light under the operation of 5 V (inset in Figure 4c). The normalized electroluminescence (EL) and PL spectra of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite based PeLED are plotted in Figure 4c. The EL spectrum is centered at 490 nm with a narrow fwhm of 21 nm (Figure 4c, cyan line) and a little red shift (4 nm) of EL spectrum comparing to PL spectra was observed due to halide separation induced by the electrical field.7 According to the CIE 1931 standard color matching functions, the emitted blue light can be indexed by (x, y) chromaticity coordinates as (0.057, 0.34) (Figure S10). To show the influence of LiF interlayer on the performances of asfabricated PeLEDs, we tried different thickness of LiF layer on the NiOx HTL. The current 14


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density-voltage, luminance-voltage and EQE-voltage curves of representative PeLEDs with different thickness of LiF interlayers are shown in Figure 4d, e, and f, respectively. The full set of devices characteristics can be found in Table S2. As shown in Figure 4d, the current density of PeLEDs decreased at low driving voltage range (< 2.8 V) with the insertion of LiF interlayer, which is attributed to the insulator nature of LiF. The luminance of all PeLEDs containing LiF interlayers is higher than that of the device with only pure NiOx. A maximum brightness of 1446 cd m-2 at 5.8V is achieved when the thickness of LiF is 1 nm (Figure 4e). Accordingly, the EQE of PeLED devices is enhanced with the insertion of LiF layer and the highest EQE of 0.52% is achieved with 1 nm of LiF as the interlayer (Figure 4f). To clarify the role of LiF interlayer in our PeLEDs, we measured the current density–voltage (J–V) characteristics of hole-only devices with/without LiF interlayer (ITO/NiOx/LiF/quasi-2D CsPbBrxCl3-x (30% Cl)/MoO3/Al and ITO/NiOx/quasi-2D CsPbBrxCl3-x (30% Cl)/MoO3/Al) and electron-only device (ITO/TPBi/quasi-2D CsPbBrxCl3-x (30% Cl)/TPBi/Al) (Figure S11). The results showed that the current density of both hole-only devices are higher than that of electron-only device, which can be attributed to the high hole mobility of NiOx transport layer.42,

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Therefore we consider that our fabricated PeLEDs are hole-dominate devices.

Further comparison of hole-only device with/without LiF interlayer indicates that the current density of hole-only device with LiF interlayer is lower than that of device without LiF interlayer, which means more balanced charge injection achieved by LiF interlayer. The more balanced charge injection with LiF interlayer would enable more efficient PeLEDs, which has been demonstrated in Figure 4e and f. Moreover, the average peak EQE of 22 devices based on quasi-2D CsPbBrxCl3-x (30% Cl) and 1 nm of LiF interlayer is 0.43% with a relative standard 15


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deviation of 13%, indicating good reproducibility of the device performance (Figure S12). The best device performance achieved by the 1 nm of LiF interlayer can be attributed to the balance between carrier transport and efficient emission of quasi-2D CsPbBrxCl3-x perovskite film as demonstrated by the aforementioned PL and TRPL analysis. In order to demonstrate the versatility of as-proposed NiOx/LiF interlayer to achieve colortunable PeLEDs in more deep-blue spectral range, we increased the chloride content in asfabricated quasi-2D CsPbBrxCl3-x perovskite film to adjust the EL emission wavelength. As shown in Figure S13, the morphologies of quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) perovskite nanocrystals are still square or rectangular. The size distributions are collected in Figure S14, where the main sizes of quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) are 10–46 nm and 10-55 nm, respectively. The AFM and SEM measurements of quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) perovskite films were conducted. As shown in Figure S15, the root-mean-square (rms) surface roughness of quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) perovskite films are ∼3.5 and 5.3 nm, respectively, indicating their smooth surfaces. The relatively uniform and dense films were also revealed by SEM images as shown in Figure S16. The fabricated devices based on quasi-2D CsPbBrxCl3-x (40% Cl) and (50% Cl) perovskites displayed EL spectra centered at 481 and 473 nm with narrow fwhm of 22 nm and 21 nm (Figure 5a), corresponding to CIE of (0.094, 0.16) and (0.11, 0.10), respectively (Figure S10). The photos of quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) based PeLEDs under the operation of 5 V show bright blue light emissions of as-fabricated PeLEDs (inset in Figure 5a). The current density-voltage, luminance-voltage and EQE-voltage 16


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characteristics of as-fabricated PeLED devices were also revealed in Figure 5b-d, respectively. The quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) based PeLEDs displayed the maximum brightness of about 509 and 217 cd m-2, respectively (Figure 5c). In addition, the maximum EQE of 0.25% and 0.16% were reached at the voltage of 5.6 V and 5.4 V in the EL emission wavelength of 481 and 473 nm for quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) perovskites based PeLEDs, respectively (Figure 5d). Devices based on quasi-2D CsPbBrxCl3-x (40% Cl) showed an average peak EQE of 0.20%

Figure 5. a) EL spectra of quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) based PeLEDs at an applied voltage of 5 V. Inset are the photos of corresponding devices at an applied voltage of 5 V. b) Current density, c) Luminance and d) EQE versus driving voltage characteristics for the devices, respectively, based on the quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) perovskite films using NiOx/LiF (1 nm) as the HTL.

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with a relative standard deviation of 16% (Figure S17) and devices based on quasi-2D CsPbBrxCl3-x (50% Cl) showed an average peak EQE of 0.11% with a relative standard deviation of 23% (Figure S18). We summarized the performances of our fabricated blue PeLEDs devices performances and previously reported blue PeLEDs (Table S3), showing that our quasi-2D CsPbBrxCl3-x based blue PeLEDs exhibit much improved brightness and EQEs in comparison to recently reported blue PeLEDs. We further tested the EL spectral stability of our quasi-2D CsPbBrxCl3-x PeLEDs under different operational conditions. Firstly, we collected the EL spectra of PeLEDs based on quasi2D CsPbBrxCl3-x (30%Cl), quasi-2D CsPbBrxCl3-x (40%Cl) as well as quasi-2D CsPbBrxCl3-x (50% Cl) at 9 V. As shown in Figure 6a, the EL peak of the device based on quasi-2D CsPbBrxCl3-x (30% Cl) under 9V is 490 nm, indicating no shift in EL spectrum compared to that at 5 V (Figure 4c). There are also only 1 and 2 nm EL peak changes for devices based on quasi-2D CsPbBrxCl3-x (40%Cl) and quasi-2D CsPbBrxCl3-x (50%Cl) at 9 V (Figure 6b-c) in comparison to devices at 5 V (Figure 5a). In contrast, the spectral instability of the PeLEDs was observed under varying bias. It is found that EL peaks of all devices have a tendency to be red-shift. As the driving voltage increased from 5 to 13 V, the EL peak of device based on quasi-2D CsPbBrxCl3-x (30% Cl) slightly red-shifted 8 nm (from 490 to 498 nm) due to the halide ions separation caused by the strong electrical field (Figure 6d). With the increase of Cl content, the red-shift seems more striking where EL peaks of devices based on quasi-2D CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) moved about 14 and 17 nm when the driving voltage increased from 5 to 13 V, respectively (Figure 6e-f). We also collected the spectral instability of our fabricated PeLEDs under a continuous driving voltage of 9 V and 18


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Figure 6. EL spectrum at an operation voltage of 9 V for (a) quasi-2D CsPbBrxCl3-x (30% Cl) based PeLED, (b) quasi-2D CsPbBrxCl3-x (40% Cl) based PeLED and (c) quasi-2D CsPbBrxCl3-x (50% Cl) based PeLED. EL spectra of (d) quasi-2D CsPbBrxCl3-x (30% Cl) based PeLED, (e) quasi-2D CsPbBrxCl3-x (40% Cl) based PeLED and (f) quasi-2D CsPbBrxCl3-x (50% Cl) based PeLED operated under varying bias. EL spectra of (g) quasi-2D CsPbBrxCl3-x (30% Cl) perovskite based PeLED, (h) quasi-2D CsPbBrxCl3-x (40% Cl) perovskite based PeLED and (i) quasi-2D CsPbBrxCl3-x (50% Cl) perovskite based PeLED operated at 9 V with different time.

the EL spectra were recorded intermittently. As shown in Figure 6g, the EL spectra of the devices based on quasi-2D CsPbBrxCl3-x (30% Cl) perovskite moved slowly from 490 nm to 495 nm within 6 min because phase segregation of Br- and Cl- occurred in the perovskite films under the electrical filed.7, 44 Similar trend was also observed to the device based on quasi-2D 19


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CsPbBrxCl3-x (40% Cl) and quasi-2D CsPbBrxCl3-x (50% Cl) perovskite films (Figure 6h-i). It is found that the deeper blue device appeared to be more subjected to the field-induced phase separation (8 nm red-shift of device based on quasi-2D CsPbBrxCl3-x (40% Cl) and 14 nm redshift of device based on quasi-2D CsPbBrxCl3-x (50% Cl) within 5 minutes). In summary, we fabricated highly bright and color-tuable blue PeLEDs based on solutionprocessable quasi-2D CsPbBrxCl3-x perovskite films and novel NiOx/LiF HTLs. Through tuning the chloride content in the precursor solution, we prepared a series of quasi-2D CsPbBrxCl3-x perovskite films with PL emission in the spectral range of 504-470 nm with a PLQY as high as 42% at 486 nm. We also showed that the NiOx/LiF HTL not only performed good affinity to quasi-2D CsPbBrxCl3-x perovskite precursor solution but also effectively suppressed the exciton quenching induced by the defect trap states on the surface of NiOx. In combination of high PLQY of quasi-2D CsPbBrxCl3-x perovskite film and efficient passivation of NiOx/LiF HTL, blue PeLEDs with peak EQE of 0.52%, 0.25% and 0.16% in the emission wavelength centered at 490, 481 and 473 nm were achieved, respectively. The proposed mixed halide anions strategy in solution-processable multilayered quasi-2D CsPbX3 perovskite films will be promising to fabricate efficient blue PeLEDs with tunable colors.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional details on the experimental procedures. Histograms showing the size distribution of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite nanocrystals, maximum EQEs of quasi-2D 20


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CsPbBrxCl3-x based PeLEDs; AFM, XRD and SEM; confocal PL microscopy images, flat-band energy levels and CIE coordinates of blue PeLEDs based on quasi-2D CsPbBrxCl3-x perovskite films; Tables of TRPL parameters for quasi-2D CsPbBrxCl3-x (30% Cl) films derived from different hole transport layer, device parameters of quasi-2D CsPbBrxCl3-x (30% Cl) perovskite film based PeLEDs with different thickness of LiF deposited on NiOx as HTLs and comparison of our CsPbBr3-xClx perovskite based PeLED to recently reported blue PeLEDs. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions H.B.Y. conceived the idea and supervised the project. K.H.W. and Y.D.P. fabricated the devices and conducted most of the measurements. J.G. and S.L.J. conducted measurements of PL lifetimes and PL decay mapping. Q.Z. analyzed the data. B.S.Z., J.S.Y., and J.N.Y. helped with the materials and collected the data. Y.C.Y. drew the fabrication procedure schematic. H.B.Y. wrote the paper. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge the funding support from the National Natural Science Foundation of China (Grant 51571184, 21501165, 21875236), the National Key R & D Program on Nano Science & Technology (Grant 2016YFA0200602, 2018YFA0208702), the Fundamental Research 21


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Funds for the Central Universities (Grant WK2060190085, WK2340000063), the joint Funds from Hefei National Synchrotron Radiation Laboratory (Grant KY2060000111). H. Y. thanks the support by “the Recruitment Program of Thousand Youth Talents”. We thank the support from USTC Center for Micro and Nanoscale Research and Fabrication. REFERENCES (1) Cho, H. C.; Jeong, S. H.; Park, M. H.; Kim, Y. H.; Wolf, C.; Lee, C. L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; et al. Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes. Science 2015, 350, 1222-1225. (2) Yao, J. S.; Ge, J.; Han, B. N.; Wang, K. H.; Yao, H. B.; Yu, H. L.; Li, J. H.; Zhu, B. S.; Song, J. Z.; Chen, C.; et al. Ce3+ -Doping to Modulate Photoluminescence Kinetics for Efficient CsPbBr3 Nanocrystals Based LightEmitting Diodes. J. Am. Chem. Soc. 2018, 140, 3626-3634. (3) Li, J. H.; Xu, L. M.; Wang, T.; Song, J. Z.; Chen, J. W.; Xue, J.; Dong, Y. H.; Cai, B.; Shan, Q. S.; Han, B. N.; et al. 50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29, 1603885. (4) Xiao, Z. G.; Kerner, R. A.; Zhao, L. F.; Tran, N. L.; Lee, K. M.; Koh, T. W.; Scholes, G. D.; Rand, B. P. Efficient perovskite light-emitting diodes featuring nanometre-sized crystallites. Nat. Photonics 2017, 11, 108115. (5) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696. (6) Wang, K. H.; Wu, L.; Li, L.; Yao, H. B.; Qian, H. S.; Yu, S. H. Large-Scale Synthesis of Highly Luminescent Perovskite-Related CsPb2Br5 Nanoplatelets and Their Fast Anion Exchange. Angew. Chem. Int. Ed. 2016, 55, 8328-8332. (7) Cho, H.; Kim, Y. H.; Wolf, C.; Lee, H. D.; Lee, T. W. Improving the Stability of Metal Halide Perovskite Materials and Light-Emitting Diodes. Adv. Mater. 2018, 30, 1704587. (8) Tan, Z. K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; et al. Bright light-emitting diodes based on organometal halide perovskite. Nat. Nanotechnol. 2014, 9, 687-692. (9) Han, D.; Imran, M.; Zhang, M.; Chang, S.; Wu, X. G.; Zhang, X.; Tang, J.; Wang, M.; Ali, S.; Li, X.; et al. Efficient Light-Emitting Diodes Based on in Situ Fabricated FAPbBr3 Nanocrystals: The Enhancing Role of the Ligand-Assisted Reprecipitation Process. ACS Nano 2018, 12, 8808-8816. (10) Zhang, L. Q.; Yang, X. L.; Jiang, Q.; Wang, P. Y.; Yin, Z. G.; Zhang, X. W.; Tan, H. R.; Yang, Y.; Wei, M. Y.; Sutherland, B. R.; et al. Ultra-bright and highly efficient inorganic based perovskite light-emitting diodes. Nat. Commun. 2017, 8, 15640. (11) Yan, F.; Xing, J.; Xing, G. C.; Quan, L.; Tan, S. T.; Zhao, J. X.; Su, R.; Zhang, L. L.; Chen, S.; Zhao, Y. W.; et al. Highly Efficient Visible Colloidal Lead-Halide Perovskite Nanocrystal Light-Emitting Diodes. Nano Lett. 2018, 18, 3157-3164. (12) Lu, M.; Zhang, X. Y.; Bai, X.; Wu, H.; Shen, X. Y.; Zhang, Y.; Zhang, W.; Zheng, W. T.; Song, H. W.; Yu, W. W.; et al. Spontaneous Silver Doping and Surface Passivation of CsPbl3 Perovskite Active Layer Enable Light22


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For Table of Contents Use Only

Efficient and Color-Tunable Quasi-2D CsPbBrxCl3-x Perovskite Blue Light-Emitting Diodes Kun-Hua Wang1,2, Yande Peng1,2, Jing Ge1,3, Shenlong Jiang 1,3, Bai-Sheng Zhu1,2, Jisong Yao 1,2,

Yi-Chen Yin1,2, Jun-Nan Yang1,2, Qun Zhang1,3, Hong-Bin Yao1,2*

We report a facile solution-processable approach to fabricate quasi-2D CsPbBrxCl3-x perovskite films with high PLQYs (up to 42% at 486 nm) for efficient blue perovskite light emitting diodes (PeLEDs). A series of high brightness of blue PeLEDs with peak luminescence of 1446, 509 and 217 cd m-2 in the emission wavelengths at 490, 481 and 473 nm, respectively, are achieved.

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Efficient and Color-Tunable Quasi-2D CsPbBrxCl3 ... - ACS Publications

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