Lead-free Antimony-based Light-Emitting Diodes through Vapor-Anion

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Functional Inorganic Materials and Devices

Lead-free Antimony-based Light-Emitting Diodes through Vapor-Anion Exchange Method Anupriya Singh, Nan-Chieh Chiu, Karunakara Moorthy Boopathi, Yu-Jung Lu, Anisha Mohapatra, Gang Li, Yang-Fang Chen, Tzung-Fang Guo, and Chih Wei Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10602 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Applied Materials & Interfaces

Lead-Free Antimony-based Light-Emitting Diodes through Vapor-Anion Exchange Method Anupriya Singh,a,b,c Nan-Chieh Chiu,a,d Karunakara Moorthy Boopathi,a Yu-Jung Lu,a,b Anisha Mohapatra,a Gang Li,e Yang-Fang Chen,b,c Tzung-Fang Guo,*,d and Chih-Wei Chu*,a,f,g

aResearch

Center for Applied Science, Academia Sinica, Taipei 115, Taiwan (R.O.C.)

bDepartment

of Physics, National Taiwan University, Sec. 4, Roosevelt Road, Taipei 106,

Taiwan (R.O.C.) cNano

Science and Technology, Taiwan International Graduate Program, Academia Sinica and

National Taiwan University, Taiwan (R.O.C.) dDepartment

eDepartment

of Photonics, National Cheng Kung University, Tainan 70101, Taiwan (R.O.C.) of Electronic and Information Engineering, The Hong Kong Polytechnic

University, Hung Hom, Kowloon, Hong Kong, China fCollege

of Engineering, Chang Gung University, Taoyuan City, Taiwan (R.O.C.)

gDepartment

of Materials Science and Engineering, National Tsing Hua University, Hsinchu

30013, Taiwan (R.O.C.)

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ABSTRACT: Hybrid lead halide perovskites continue to attract interest for use in optoelectronic devices as solar cells and light emitting diodes. Although challenging, the replacement of toxic lead in these systems is an active field of research. Recently, the use of trivalent metal cations (Bi3+, Sb3+) that form defect perovskites A3B2X9 has received great attention for the development of solar cells, but their light-emissive properties have not previously been studied. Herein, an all-inorganic antimony-based two-dimensional perovskite, Cs3Sb2I9, was synthesized using solution process. Vapor-Anion exchange method (V-AEM) was employed to change the structural composition from Cs3Sb2I9 to Cs3Sb2Br9 or Cs3Sb2Cl9 by treating CsI:SbI3 spin-coated films with SbBr3 or SbCl3, respectively. This novel method facilitates the fabrication of Cs3Sb2Br9 or Cs3Sb2Cl9 through solution processing with no need to use poorly soluble precursors (e.g., CsCl, CsBr). We go on to demonstrate electroluminescence from device employing Cs3Sb2I9 emitter sandwiched between ITO/PEDOT:PSS and TPBi/LiF/Al as hole and electron injection electrodes, respectively. A visible–infrared radiance of 0.012 W·Sr–1·m–2 was measured at 6V when Cs3Sb2I9 was the active emitter layer. These proof-of-principle devices suggest a viable path toward lowdimensional, lead-free A3B2X9 perovskite optoelectronics.

Keywords: Halide perovskites, lead-free, antimony, inorganic, Vapor-Anion exchange method, light emitting diodes


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1. INTRODUCTION In a quest to search for cheap and abundant energy, different resources like dye sensitizers, organic materials, and inorganic−organic hybrid perovskites have been investigated and received great attention for their attractive electrical and optical properties.1-10 Owing to the high material quality and notably strong photoluminescence, the discovery of Pb based perovskites have attracted researchers as a new generation functional materials for light-emitting diodes (LEDs).1119

However, relatively low stability and the facile dissociation of excitons due to long exciton

diffusion lengths and small exciton binding energies limits their efficiency.20-21 These problems were tackled by using long organic cation to form a layered two-dimensional (2D) organicinorganic perovskite structures. 2D layered structures offer larger exciton binding energies and shorter exciton diffusion lengths, because excitons are more strongly confined in these structures.22 Nevertheless, the high toxicity of Pb remain problematic and recently, the development of Pb-free halide perovskites has attracted the interest of academic and industrial scientists.23 To address the issue of toxicity, bismuth (Bi) and antimony (Sb) have been used as replacements for Pb, due to the isoelectronic nature and presence of ns2 lone pairs of electrons in Sb3+ and Bi3+ ions, similar to Pb2+.24-26 Furthermore, Sb and Bi based solar cells have exhibited relatively strong air- and moisture-stability—a critical requirement for optoelectronic devices.2631

These trivalent metal cations (Sb3+, Bi3+) form A3M2X9 structures with 2/3 occupancy of the M

sites in the A3M3X9 perovskite formula; therefore, they can be considered “defect perovskites.”3233

These defect perovskites, also referred to as “perovskite-like structures,” retain the most

important feature of perovskite crystal structures: networks of BX6 octahedra, BX5 pyramids, and BX4 squares linked at their vertices.34



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Experimentally, A3B2X9 (B = Bi or Sb) crystallizes in zero-dimensional (0D) or 2D polymorphs, with the hexagonal phase consisting of bi-octahedral face-sharing (M2I9)3− clusters (0D) and a phase composed of corrugated layers with partially corner sharing MX6 octahedra (2D). The 0D dimer phase is readily synthesized through low-temperature solution-processing,31, 35

but the 2D layered phase is less thermodynamically favorable.28, 30 Nevertheless, perovskites

based on the 0D dimer phase suffer from intrinsic problems relating to low-symmetry–induced indirect band gaps, strong quantum-confinement effects (causing oversized band gaps), and inferior hopping-like carrier transport, all of which are unfavorable for optoelectronics.36 Jiang et al. succeeded in preparing a layered phase of organohalide Sb-based defect perovskite by incorporating Cl in the solution.37 In previous reports, vapor‐based deposition has been manifested to be a promising approach to give well-controlled reaction rate for high quality and large‐scale perovskite active area.38-39 Recently, vapor-treatment at higher temperature has made it possible to obtain layered Cs3Sb2I9 through solution-processing, suggesting its use in other optoelectronic applications.28 In principle, a good light-harvesting material should be a good light-emitting material,40 but the materials used for light-emitting devices and photovoltaic devices require different design principles. Efficient recombination of excitons through a radiative pathway is needed for light-emitting devices, while effective separation of excitons is critical for converting photons to electrical power for solar cells. Nevertheless, A3B2X9 structures have never been used previously as active materials in light emitting diodes (LEDs), due to their high contents of deep-level defects—an intrinsic property of these structures.41 In addition to these intrinsic material defects, structures featuring Sb and Bi face the problem of poor morphology.42-43 The light-emissive properties of Cs3Sb2X9 have investigated by synthesizing quantum dots (QDs) and nanocrystals.44-45 Zhang et al. prepared stable and bright luminescent 4 ACS Paragon Plus Environment

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Cs3Sb2Br9 QDs and reported a high photoluminescence quantum yield (PLQY) of 46% at 410 nm.45 Furthermore, they tuned the inorganic perovskite QD (Cs3Sb2X9) emission wavelength from 370 to 560 nm through the use of halide exchange reactions. Previous research into the light-emitting properties on Pb-free halide perovskites have been based on nanocrystals, with an emphasis on their synthesis with suitable physical properties.41 Electroluminescence (EL) has not yet been achieved in Pb-free perovskite nanocrystals, presumably because the poor material properties (low emissive quantum yields, large optical bandwidths, and low environmental stability) have discouraged efforts to fabricate actual electroluminescent devices. To obtain the EL, synthesis of non-aggregated, monodispersed and highly crystalline oxide nanocrystals is required.46 Also, a commonly reported problem with nanocrystal based films is rather poor control over the thickness and micro-cracking of the films.47 So, development of a method to obtain thin films by solution process offers a unique advantage for the production of adherent films to explore the emissive properties of these Pb-free perovskites. Precursor solubility in polar solvents plays a crucial role in the solution based thin film production. There are certain broad variations in solubility in polar solvents, amongst the metal halides. For a large cation like Cs+, solubility increases from fluoride to iodide (CsI > CsBr > CsCl > CsF).48 This poor solubility of Cs based precursors hinders the fabrication as well as the tuning of emission wavelength. It is therefore advantageous to develop a method that enables the halide change with no need to dissolve poor soluble precursor (CsBr or CsCl) to tune the band gaps of the perovskites. In this study, we tuned the wavelengths of solution-processed Cs3Sb2I9 thin films by using novel Vapor-Anion exchange method (V-AEM) involving the exchange of the halide anion (I–) by Br– or Cl– ions. The resulting films exhibited wide photoluminescence (PL), with a full width at half maximum (FWHM) of 120 nm and were studied for their PL at various 5 ACS Paragon Plus Environment


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temperatures. We fabricated electroluminescent devices with Cs3Sb2I9 systems as the emitter layer and observed visible-infrared radiance of 0.012 W·Sr–1·m–2 at 6V. To the best of our knowledge, this paper is the first to report the use of Pb-free A3B2X9-based halide perovskites as active layers in LEDs. 2. EXPERIMENTAL SECTION 2.1 Fabrication of Cs3Sb2X9 films through Vapor-Halide Exchange Method A single precursor solution was prepared by mixing 0.25 M of SbI3 and 1 M of CsI in DMSO and then continuous stirring the mixture for 6 h at 70 °C. This solution was dropped onto the PEDOT:PSS-coated substrates and spin-coated for 40 s at 8000 rpm. These films (two samples at a time) were moved directly to a glass bottle (diameter: 4 cm; height: 6 cm) preheated at 200 °C. This bottle was covered with a cap after adding 10 µL (45 wt%) of SbI3 in DMF in the corner of the bottle. The films were annealed at 200 °C for 15 min to remove residual solvent and to induce layer-Cs3Sb2I9 perovskite crystallization (Figure S1). The temperature was decreased gradually to 150 °C before bringing the sample to room temperature, to avoid quenching. The limited solubility of CsBr and CsCl precursors makes it difficult to use them to prepare films of Cs3Sb2Cl9 and Cs3Sb2Br9 through solution-processing. Here, the V-AEM was used to synthesize films having the chemical composition Cs3Sb2X9 (X = I, Br, Cl). The CsI:SbI3 spin-coated films were treated with SbBr3 or SbCl3 in place of the SbI3 vapor, leading to exchange of the I– anions by Br– or Cl– anions, respectively. As the heat of vaporization for SbBr3(16.5 kcal/mol) and SbCl3(13.6 kcal/mol) are lower than SbI3 (20.4 kcal/mol),49 so the concentration of solution added in the bottle is increased to 80% SbBr3 or SbCl3 (in DMF), to form films of Cs3Sb2Br9 or Cs3Sb2I9, respectively. Note that the precursor (CsI:SbI3) for spin-coating was the same when preparing all of the films; the only thing changed was the solution used for the halide exchange in the second step.

2.2 Fabrication of Light Emitting device 6 ACS Paragon Plus Environment

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ITO substrates were cleaned with DI water and ethyl alcohol and then kept in an oven overnight. UV-Ozone treatment was performed for 20 min. PEDOT:PSS was spin-coated for 60 s at 4000 rpm over the substrates and then the samples were annealed at 150 °C for 30 min. Active layer is then prepared onto it. TPBi (50 nm) was thermally evaporated through an open mask at a rate of 1.0–1.5 A/s. Devices were transferred to a patterned mask and LiF (1.5 nm) was deposited at a rate of 0.1–0.3 A/s. Finally, Al (60 nm) was deposited. The active area of each device was 10 mm2. 3

Results and Discussion

The V-AEM to synthesize Cs3Sb2X9 (X = I, Br, Cl) films is schematically given in Figure 1 and real-time photographs are given in Figure S1. Films treated with the SbI3 vapor were reddishorange in color, but were greenish-yellow after SbBr3 vapor-treatment and transparent-blue after SbCl3 vapor-treatment, as revealed in the three-dimensional (3D) real-time photos of the films (Figure 1a). Figure 1b-d present scanning electron microscopy (SEM) images of films treated with the three different antimony halides. The SbI3-treated film featured a uniform coverage with a low density of pinholes, whereas the films treated with SbBr3 and SbCl3 featured relatively more pinholes. These pinholes for Br and Cl presumably arose because of the different vapor pressure (arising from different heat of vaporization) of SbI3, SbCl3 and SbBr3; these features would presumably be controlled through further optimization of different annealing time and concentration of SbX3 (in DMF). To confirm the changes in the compositions of these films, X-ray photoelectron spectroscopy (XPS) revealed that the films treated with SbBr3 and SbCl3 did not feature any peaks for I atoms, but clearly indicated the presence of Br and Cl, respectively (Figures 2a–c). The stoichiometric ratios were determined using energy-dispersive X-ray spectroscopy (EDXS) 7 ACS Paragon Plus Environment


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and elemental XPS which are given in Supporting Information in Figure S2 and Figure S3, respectively. XPS revealed an iodine deficiency in the film, with an I-to-Sb molar ratio of 4.01 (Table S1)—similar to the value of 3.9 determined using EDX, but inconsistent with the stoichiometric ratio of 4.5. The synthesized Cs3Sb2X9 featured an anion-deficient stoichiometry, consistent with that proposed by Saparov et al. using generalized gradient approximation calculations for the iodine counterpart.30 This iodine-deficiency produces the deep level defects in the structure and results in non-radiative transitions which contributes to the suppressed photoluminescence for Cs3Sb2I9 as compared to CH3NH3PbI3 whose prominent defects comes from the shallow levels.20,30 Cs3Sb2I9 can crystallize in either dimer (P63/mmc, 194) or layer (P3̅m1, 164) form depending upon the crystallizing conditions (e.g., temperature, composition).37,50 X-ray diffraction (XRD) confirmed (Figure 2d) the successful crystallization of Cs3Sb2X9 in the layer form at 200 °C.22 The effect of halide replacement on the lattice was evident in the XRD patterns (Figure 2d) of the films at various compositions. Replacement of I– anions with smaller Br– and Cl– anions leads to change in d-spacing and causes a shift in the value of 2θ. The average crystallite sizes for Cs3Sb2X9 films were estimated using the Scherrer equation and was found to be ~ 270 nm to 320 nm (Table S2). Furthermore, inorganic perovskites should tackle the issue of stability, because improved thermal stability would make them attractive for use in various optoelectronic devices.51,52 The air-stability of the thin films can be monitored by different methods like XRD, absorption and PL.20,53,54 We monitored the presence of layered polymorph of Cs3Sb2I9, synthesized using our optimized conditions (Figure S4) by using XRD. The films exhibited good stability for up to 30 days; thereafter, modest evolution of CsI occurred as an impurity. Absorption and PL intensity of Cs3Sb2I9 have also been observed in open air for 30 hrs 8 ACS Paragon Plus Environment

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as is shown in figure S4. The impact of the compositional change, due to anion exchange, on the absorption was checked using UV–Vis spectroscopy (Figure 2e), revealing a clear blue-shift of the exciton peak. The absorption curves matched well with those expected from the films’ colors as displayed in Figure 1a. To further characterize the optical properties of the Cs3Sb2X9 films featuring the various anions, their PL spectra were measured (Figure 2f). PL spectra with blueshifts were observed after Br- and Cl-treatment, consistent with the replacement of I– anions by Br– and Cl– anions. The PL spectra of all three of our perovskites featured similar FWHMs of 120 nm—a value that is much wider than that of the ABX3 perovskite. Because the anionexchanged perovskites exhibited similar behavior, we focus primarily hereafter on the Cs3Sb2I9 perovskites. The absorption spectrum of Cs3Sb2I9 features a small signal near 2.0 eV, which we attribute to the exciton peak, while its normalized PL spectrum featured the exciton peak centered at 750 nm. As suggested previously, layer-type Cs3Sb2I9 perovskite is a direct band gap semiconductor and, therefore, it should have a narrow PL peak because of its direct transitions.28, 30, 37

.

To examine the cause of the wide PL peak, we performed a low-temperature PL analysis

(Figure 3a). At 4.7 K, a narrower PL peak appeared, centered at 650 nm for Cs3Sb2I9 with an FWHM of approximately 80 nm, that we attribute to direct band gap transitions. In addition, another peak appeared centered near 700 nm and having an FWHM of 120 nm. We attribute this broad peak to the defect transitions—a most challenging intrinsic problem that has yet to be solved. Similar trends were shown by anion exchanged films for Br and Cl (figure 3a). The strong Raman scattering of Cs3Sb2I9 revealed strong coupling between the excited electrons and the Raman active vibrations of the lattice, which reflected the lattice’s sensitivity to local fluctuations at room temperature.55 This behavior of forming local charges affects the efficient 9 ACS Paragon Plus Environment


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movement of the charges, because the carrier movement will cause deformation of the lattice, leading to increases in the effective masses of the charge carriers.56 Strong electron–phonon coupling leads to charge-localization and the emission of phonons, resulting in a change in the energy of the emitted photons. A simple model (Figure 3b) is presented to understand the phenomenon of the interaction of the radiative transitions with phonons in the lattice at the room temperature. The absorption transition upon excitation occurs from 1 to 2 and after interaction with phonons, the emission occurs from 3 to 4. The energy of radiative transition changes from “E34” to “E34 ± Ehɷ ” due to subsequent emissions of phonons (Ehɷ). The resulting luminescence spectrum (Figure 2f) is the convolution of multiple Gaussians, with the primary transition as E34, convoluted with multiple phonon peak offsets. Ultraviolet photoelectron spectroscopy (UPS) provided the valence band maximum (VBM) and Fermi energy (Ef) of the Cs3Sb2X9 film (Figure 3c). The value of Ef of Cs3Sb2I9 was estimated to be –4.8 eV by subtracting the intercept at a binding energy of 16.4 eV (cutoff region) with the UV photon energy (He 1 excitation, 21.2 eV). Linear fitting of the UPS spectrum in the long tail generated an extrapolation of 0.5 eV, which corresponds to the distance between the value of Ef and the VBM. Using the band gap of 2.05 eV measured from the absorption spectrum, we calculated the VBM to be at –5.3 eV and conduction band minimum (CBM) to be at –3.25 eV. The value of Ef positioned between the intrinsic Fermi level and the valence band confirmed the p-type nature of our Cs3Sb2I9 film. In the similar way CBM and Ef for anion exchanged films Cs3Sb2Br9 and Cs3Sb2Cl9 were also calculated and are been shown in figure 3d. Unlike the I and Br counterparts, the Cs3Sb2Cl9 showed the n type behavior. This change in behavior from p to n can be explained because of prominent presence of donor-like defects such as halogen vacancy (VX), cation interstitial (Csi, Sbi).30 10 ACS Paragon Plus Environment

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We pursued the realization of LEDs having the structure glass/ITO/PEDOT:PSS/layerCs3Sb2I9/2,2´,2´´-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi)/lithium fluoride (LiF)/Al (Figure 4a). Apart from TPBi and LiF/Al, which were deposited under vacuum, all of the other layers were solution-processed through spin-coating. The presence of all of the deposited layers was evident in a cross-sectional SEM image, and the thickness of the active layer was approximately 90 nm (Figure 4b). The band alignment for the LED device is provided in Figure 4c. The band was aligned to ensure electron/hole recombination in the active layer, with the release of photons corresponding to the emission wavelengths, as suggested by the PL emission. The device based on Cs3Sb2I9 was primarily optimized, and its EL was measured using radiance spectroscopy. Consistent with the PL emission peaks, the device prepared using the Cs3Sb2I9 film as the active layer provided a red emission (inset to Figure 4e). Figure 4d displays the normalized EL intensity of a working device under the optimized conditions for Cs3Sb2I9. The EL for this device was in the same range as the PL obtained at room temperature, but the FWHM of the signal in the EL spectrum was wider than that in the PL spectrum. It is common for LEDs to suffer from parasitic EL signals that arise from radiative recombination processes occurring not only in the bulk of the active material but also at the interfaces or even in the electron–hole selective contacts. This effect usually limits the performance of the ultimate devices and leads to significant deterioration of the EL signal. Figure 4e presents the current density and radiance plotted on a log scale with respect to applied bias for the devices featuring Cs3Sb2I9 as the emitter. The log J–V curve clearly reveals an increase in the current density within the potential range of approximately 2 V, accompanied by a sudden increase of the radiance showing a mean value of 0.012 W.sr−1·m−2 at 6 V (current density of 1080 mA/cm2). Figure S5 shows the J-V curve and real time photographs of working LEDs for the anion 11 ACS Paragon Plus Environment


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exchanged counterparts as well. The turn on voltage for all the three colors goes well with their respective band gap, as the Cs3Sb2Cl9 is having the highest turn on voltage. The films containing the Br– and Cl– anions had higher resistances, which correlated with lower current densities, at similar applied potentials, presumably because of their poorer morphologies, relative to those of the devices based on the I– anions. Figure 4f presents a very low external quantum efficiency (EQE) of ~10-8 corresponding to Cs3Sb2I9 as the emitter. To the best of our knowledge, this is the first demonstration of electroluminescence exploiting A3B2X9 defect perovskite. Previously reported work for their light emissive properties are listed in Table S3. The low and broad emissive properties of these materials make a huge room for improvement in these materials by controlling the defect states in these materials. The tunability and stability of these materials make them attractive choice in the field of light emitting perovskites.

4

Conclusions In summary, we successfully employed V-AEM to synthesize Pb-free Sb-based 2D

perovskite thin films (Cs3Sb2X9; X = Cl, Br, I). This novel method facilitates the fabrication of Cs3Sb2Br9 or Cs3Sb2Cl9 through solution-processing, with no need to dissolve such poorly soluble precursors as CsCl or CsBr. Electroluminescent devices incorporating Cs3Sb2I9 emitted Vis-IR radiance corresponding to its PL emission wavelengths at room temperature. We report a first example of lead-free perovskite LED based on 2D Cs3Sb2I9 as the emitter. This proof-ofprinciple demonstration of EL offers promising prospects for the development of lead-free perovskite optoelectronics. ASSOCIATED CONTENT Supporting Information 12 ACS Paragon Plus Environment

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The Supporting Information is available free of charge on the ACS Publications website Materials and methods, real time photos showing vapor anion exchange method, EDX and XPS results of anion exchanged films, Stability of films, J-V curve for anion exchanged films (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interest. Acknowledgements

Prof. Chih-Wei Chu thanks the Ministry of Science and Technology (MOST), Taiwan (107-2221-E001 -007-MY3), for financial support. Prof. Tzung-Fang Guo thanks MOST (105-2119-M-006022-MY3) and Dr. Yung-Jung Lu also acknowledges MOST (106-2112-M-001-036-MY3) and Academia Sinica (Grant No. AS-CDA-108-M08) for financial support.



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Figure 1. (a) Schematic representation of the V-AEM for the preparation of layer-type Cs3Sb2X9 (accompanied by real time images of the films). (b–d) SEM images of films of (b) Cs3Sb2I9, (c) Cs3Sb2Br9, and (d) Cs3Sb2Cl9.


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Figure 2. (a–c) XPS survey spectra of the CsI:SbI3 film after (a) SbI3 vapor treatment, (b) SbBr3 vapor treatment, and (c) SbCl3 vapor treatment. (d) XRD patterns, (e) UV–Vis absorption spectra, and (f) normalized PL spectra of films featuring the various anions.



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Figure 3. (a) PL emission spectra of Cs3Sb2I9 film recorded at 4.7 K. (b) Model of non-radiative recombination due to phonons, giving a wider PL at room temperature. (c) UPS spectra for all Cs3Sb2X9 films. (d) Change in valence band and fermi energy levels for anion exchanged films.


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Figure 4. (a) Device architecture. (b) Cross-sectional SEM image of electroluminescent devices featuring Cs3Sb2I9. (c) Energy level alignment of electroluminescent devices. (d) Normalized EL spectrum of the Cs3Sb2I9 film. (e) Current density versus voltage (J−V) and radiance versus voltage characteristics, and (f) the highest external quantum efficiency versus voltage characteristics shown for device incorporating Cs3Sb2I9.



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Table of Content Graphic


Lead-free Antimony-based Light-Emitting Diodes through Vapor-Anion

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