Tin-Based Multiple Quantum Well Perovskites for Light-Emitting

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Tin Based Multiple-Quantum-Well Perovskites for Light-Emitting Diodes with Improved Stability Ying Wang, Renmeng Zou, Jin Chang, Zewu Fu, Yu Cao, Liangdong Zhang, Yingqiang Wei, Decheng Kong, Wei Zou, Kaichuan Wen, Ning Fan, Nana Wang, Wei Huang, and Jianpu Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03700 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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The Journal of Physical Chemistry Letters

Tin Based Multiple-Quantum-Well Perovskites for Light-Emitting Diodes with Improved Stability Ying Wang1, Renmeng Zou1, Jin Chang1*, Zewu Fu1, Yu Cao1, Liangdong Zhang1, Yingqiang Wei1, Decheng Kong1, Wei Zou1, Kaichuan Wen1, Ning Fan1, Nana Wang1, Wei Huang1,2, Jianpu Wang1* 1Key

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 2Shaanxi

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

127 West Youyi Road, Xi’an 710072, China

E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Tin-based halide perovskites have attracted considerable attention for non-toxic perovskite lightemitting diodes (PeLEDs), but the easy oxidation of Sn2+ and non-uniform film morphology cause poor device stability and reproducibility. Herein, we report a facile approach to achieve efficient and stable lead-free PeLEDs by using tin-based perovskite multiple quantum wells (MQWs) for the first time. Based on various spectroscopic investigations, we find that the MQW structure not only facilitates the formation of uniform and highly emissive perovskite films, but also suppresses the oxidation of Sn2+ cations. The tin-based MQW PeLED exhibits a peak external quantum efficiency of 3% and a high radiance of 40 W sr-1 m-2 with a good reproducibility. Significantly, these devices show excellent operational stability with over 2 h lifetime under a constant current density of 10 mA cm-2, which is comparable to lead based PeLEDs. These results suggest that perovskite MQWs can provide a promising platform for achieving high performance lead-free PeLEDs.

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Solution-processable metal halide perovskites have attracted much attention for the electroluminescence (EL) applications owing to their high photoluminescence quantum efficiencies (PLQEs), high color purity, and good charge mobilities.1-4 In past four years, the external quantum efficiencies (EQEs) of lead-based halide perovskite light-emitting diodes (PeLEDs) have rapidly ascended from 0.76% to over 20%.1,

5-7

Despite the significant

improvement in device efficiency, the toxicity of lead has raised considerable concern, which motivates the development of environmentally benign lead-free alternatives.8 Recent studies have shown that non-toxic tin is able to replace lead in halide perovskites and give decent device efficiencies.8-12 The lead-free PeLEDs with a maximum EQE of 0.72% were initially demonstrated by three-dimensional (3D) CH3NH3Sn(Br1-xIx)3 perovskites.8 A relatively high EQE of 3.8% was also achieved for CsSnI3-based PeLEDs.11 However, these 3D tin halide perovskite LEDs suffer from extremely poor stability and reproducibility owing to the facile oxidation of Sn2+ to Sn4+ and the fast crystallization rate during the film-forming process.13, 14 The oxidation of Sn2+ can cause high level of Sn vacancies and p-type self-doping, which leads to metallic behavior of perovskites and deterioration of device performances.10,

15

The fast

crystallization speed impedes the homogeneous film growth, resulting in poor surface coverages and large leakage current in LED devices.11 Efforts have been made in solar cells to suppress the oxidation of Sn2+ and improve the film morphology,10,

16, 17

but the film qualities are not

satisfying for LED applications which usually require much thinner films with higher uniformity.2, 3 Recently, it has been shown that low-dimensional perovskites formed by replacing certain amount of small cations (e.g., CH3NH3+ and Cs+) with bulky ones can significantly improve the stability of perovskite films.3,

18-21

Two-dimensional (2D) perovskite such as (PEA)2SnIxBr4-x


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(PEA = phenylethylammonium) has shown better stability than the 3D counterparts under ambient air conditions.21 The enhanced stability is attributed to the protecting effect of bulky organic cations against the oxygen or moisture diffusion into the perovskite films.21,

22

Nevertheless, at room temperature the 2D perovskite-based LEDs exhibit very low emission efficiencies due to the strong exciton-phonon interaction.21,

23, 24

By contrast, quasi-2D

perovskites with self-assembled multiple quantum well (MQW) structures have shown not only good stability but also high PLQEs in lead-based perovskites, thus leading to high performance PeLEDs.3, 4, 25 The previous achievements in lead-based MQW PeLEDs encourage us to explore the lead-free tin-based perovskite MQWs and their application in LEDs. In this work, uniform tin-based perovskite films with high PLQEs are realized by partially replacing the Cs+ cations of 3D CsSnI3 perovskite with bulky PEA cations (Figure 1a). The oxidation of Sn2+ is significantly suppressed by the MQW structure, which leads to lead-free PeLEDs with a peak EQE of 3% and a high radiance of 40 W sr-1 m-2. More importantly, the device shows a record lifetime of over 2 h under a constant current density of 10 mA cm-2, which is comparable to lead based PeLEDs. This work suggests that tin-based perovskite MQWs are promising candidates for achieving high performance lead-free PeLEDs. The tin-based perovskite films are prepared by using a similar method of our previous work on lead-based analogues.19 Precursors of phenylethylammonium iodide (PEAI), CsI and SnI2 are dissolved in a mixed solvent of N,N-dimethylformamide and dimethyl sulfoxide (volume ratio, 4:1) with various molar ratios. Additional 10 mol% SnF2 is added to reduce the Sn vacancies without influencing the perovskite lattice structure.10 After spin-coating the solution on poly(9vinylcarbazole) (PVK) substrates, the films are annealed at 65 °C for 5 min in a nitrogen glovebox (see details in Experimental Section). The general formula of our tin perovskites is


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(PEAI)x(CsI)y(SnI2)z, where x:y:z is the PEAI/CsI/SnI2 molar ratio in precursor solutions. Based on the molar ratios, a ternary diagram is plotted and shown in Figure 1b, which presents the 2D, 3D, and typical MQW perovskites.

Figure 1. (a) A schematic of (PEAI)x(CsI)y(SnI2)z perovskite MQWs composed of various perovskites with different inorganic layer numbers (n). The x:y:z stands for the PEAI/CsI/SnI2


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molar ratio in precursor solutions. The yellow, big blue, red, gray, and small blue spheres represent Cs, Sn, I, C, and N atoms, respectively. (b) A ternary diagram of (PEAI)x(CsI)y(SnI2)z perovskites with various compositions. The black and blue spheres represent the 3D and 2D perovskites, respectively. The light blue to red spheres represent typical MQW perovskites. (c) X-ray diffraction patterns of the 2D, 3D, and representative MQW perovskite films. Diffraction peaks of the n = 1 (black dashed lines), n = 2 (red dashed lines), and n → ∞ (black asterisks) perovskites are labeled. (d) Absorption and PL spectra of the 2D, 3D, and MQW perovskite films, respectively.

The crystallographic structure of the 2D, 3D, and representative MQW (x:y:z = 2:3:4, 2:5:6, 3.5:5:4.5) perovskite films are determined by the X-ray diffraction (XRD) technique (Figure 1c). The 2:0:1 film displays well-defined (0 0 2l) (where l = 1 – 6) diffraction peaks at 5.4°, 10.8°, 16.2°, 21.7°, 27.2° and 32.8°, confirming the formation of 2D perovskite (PEA)2SnI4.26 The 2:3:4, 2:5:6, and 3.5:5:4.5 films exhibit strong diffraction peaks at ≈14.3° and 28.8°, which are consistent with the black orthorhombic phase (B-γ) CsSnI3,27 indicating the formation of large-n QWs which are close to 3D B-γ CsSnI3. Meanwhile, diffraction patterns of the (PEA)2SnI4 (n = 1 QW) and (PEA)2CsSnI7 (n = 2 QW) perovskites are also observed in these films (see analysis in Table S1). Therefore, the XRD results suggest that the 2:3:4, 2:5:6, and 3.5:5:4.5 perovskite films contain both small-n and large-n QW perovskites and form self-assembled MQW structures, which is consistent with the previous reported lead based perovskite MQW structures.3, 18, 19, 28 The optical absorption and emission spectra of the tin-based perovskite films are presented in Figure 1d. The absorption spectra show that the 2D (PEA)2SnI4 film exhibits a strong absorption


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peak at 2.05 eV, corresponding to the exciton absorption of n = 1 QWs.21 The 2:3:4 film exhibits an additional absorption shoulder at 1.89 eV, which can be attributed to the exciton absorption of n = 2 QWs. The exciton emission peaks of the n =1 and n = 2 QWs are observed at 1.99 eV and 1.83 eV, respectively, which are consistent with the absorption peaks. The exciton features of the 2:5:6 and 3.5:5:4.5 films are not evident in the absorption spectra, but can be observed in the emission spectra on semi-log scale. The dominant photoluminescence (PL) peaks of the 2:3:4, 2:5:6, and 3.5:5:4.5 films are observed at 1.38-1.36 eV, which are slightly blue-shifted to that of the 3D CsSnI3 (1.34 eV). The blue-shifted PL peaks can be attributed to the large-n QWs, which is similar to the lead-based counterparts.3, 29 Therefore, the above optical characterization verifies the existence of MQW structure in the 2:3:4, 2:5:6, and 3.5:5:4.5 tin perovskite films.

Figure 2. SEM and AFM images of (a, f) the 2D perovskite film; (b, g) the 2:3:4 MQW perovskite film; (c, h) the 2:5:6 MQW perovskite film; (d, i) the 3.5:5:4.5 MQW perovskite film; and (e, j) the 3D perovskite film. The AFM image sizes are 5 × 5 μm and the scale bars are 60 nm.


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To investigate the morphology of our tin-based perovskite films, scanning electron microscope (SEM) and atomic force microscope (AFM) measurements are carried out. The SEM images show that the 2D (PEA)2SnI4 film consists of large size (> 1 μm2) grains with a high surface coverage, while the 3D CsSnI3 film is discontinuous with a low surface coverage (Figure 2a, e). This can be attributed to the different crystallization rates and growth orientations of the 2D and 3D perovskites determined by the PEA and Cs+ cations, respectively.30 The PEA and Cs+ coexisted MQW films show composition-dependent grain sizes and surface coverages (Figure 2bd). Notably, the 3.5:5:4.5 MQW film exhibits the smallest grain size and the highest surface coverage without pin-holes. AFM images show that the 3.5:5:4.5 film exhibits the lowest surface roughness of 2.1 nm in comparison with other perovskite films (Figure 2f-j).

Figure 3. XPS Sn 3d spectra of the 2D, 3.5:5:4.5 MQW, and 3D tin-based perovskite films. Black line: measured results; blue line: Sn2+ state; red line: Sn4+ state; dashed green line: sum of deconvolution curves.


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Since the properties of tin-based perovskite films are highly related to the oxidation of Sn2+ cations, the chemical state analysis is conducted for Sn by using X-ray photoelectron spectroscopy (XPS). As shown in Figure 3, the Sn 3d doublets are fitted with an area ratio of 3:2 corresponding to the degenerate spin states of Sn 3d5/2 and Sn 3d3/2.31 The Sn 3d doublets are further deconvoluted into Sn4+ states at 495.8 and 487.4 eV and Sn2+ at 494.4 and 486.0 eV, respectively.13 The relative contents of Sn4+ in the 2D and 3.5:5:4.5 MQW films are much lower than that in 3D CsSnI3, suggesting the suppressed oxidation of Sn2+ in MQW structures. The enhanced chemical stability can be attributed to the blocking effect of bulky PEA against the moisture or oxygen ingression.22

Figure 4. (a) Excitation-intensity-dependent PLQEs of the 3D and MQW tin perovskite films. (b) Time-resolved PL spectra of the tin perovskite films (excitation: 633 nm, 2.17 nJ cm-2). The instrument response (IR) is also presented.

The light-intensity-dependent PLQEs of the 2:3:4, 2:5:6, 3.5:5:4.5 and 3D perovskite films are measured by the 445 nm continuous wave laser excitation (Figure 4a). It is shown that the 3.5:5:4.5 MQW perovskite film exhibits PLQEs up to 18%. We note that this is the highest


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PLQE value for the tin-based perovskite films to date.8, 21 In addition, all the MQW films present relatively high PLQEs even at excitations as low as ~0.05 mW cm-2, indicating that the trapinduced non-radiative recombination is not significant. By contrast, the 3D CsSnI3 perovskite shows high PLQEs only at high excitations when the non-radiative recombination centers are filled. The PLQE feature of our tin-based MQW perovskites are similar with that of lead-based analogues, which give high PLQEs due to the cascade energy transfer and exciton confinement in large-n QWs.3 The time-resolved PL spectra show that MQW films have a slower PL decay than the 3D film and the PL lifetime (time to decay by 1/e) of the 3D and 3.5:5:4.5 MQW films are calculated as ~0.8 ns and ~1.8 ns, respectively (Figure 4b), which also suggest that the trap assisted non-radiative recombination are suppressed in the MQW structures. The tin-based PeLED devices are fabricated with the architecture as indium tin oxide (ITO)/poly(9-vinylcarbazole)

(PVK,

~6 nm)/perovskites

(~25 nm)/1,3,5-tri(m-pyrid-3-yl-

phenyl)benzene (TmPyPB, ~45 nm)/lithium fluoride (LiF, ~1.2 nm)/aluminum (Al, ~100 nm). Detailed fabrication methods can be found in the experimental section. Figure 5a shows the energy level diagram of the device. The bandgap of the tin perovskite MQWs is estimated from the optical spectra, and the energy levels of other layers are taken from the literature.32, 33 The EL emission peaks are consistent with the dominant PL peaks of the perovskite films (Figure 5b). The EL peaks of the 3.5:5:4.5 MQW-based PeLED is observed at 920 nm, which is blue-shifted (~10 nm) to that of the 3D CsSnI3 PeLED. Notably, the EL emission peak does not change at various bias voltages (Figure S1), suggesting the good operational stability of our lead free MQW PeLEDs.


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Figure 5. Device characteristics of the tin-based perovskite LEDs. (a) Energy level diagram of the PeLEDs. The energy level values besides the MQWs are taken from the literature.32, 33 (b) EL spectra of the PeLEDs based on the 2:3:4, 2:5:6, 3.5:5:4.5 and 3D perovskites. (c) Dependence of


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the current density and radiance on the driving voltage. (d) EQE values versus current density. For the 3.5:5:4.5 MQW-based PeLED, a maximum EQE of 3% is achieved. (e) Histograms of peak EQEs measured from 88 devices. (f) Stability data of the 3.5:5:4.5 MQW device tested at a constant current density of 10 mA cm-2 with an initial radiance of 0.6 W sr-1 m-2.

The current density-voltage-radiance (J-V-R) and EQE characteristics of tin-based PeLEDs are also presented in the Figure 5. The leakage current densities and turn-on voltages of the MQWbased PeLEDs are much lower than that of the 3D CsSnI3-based device (Figure 5c). For devices prepared from the 2:3:4, 2:5:6, and 3.5:5:4.5 MQW films, the peak EQE values are 0.24%, 1.52% and 3.01%, respectively (Figure 5d). The optimized device performance can be attributed to the excellent film quality of the 3.5:5:4.5 MQWs, such as pinhole-free morphology with the lowest surface roughness (Figure 2) and the highest PLQE (Figure 4a). Notably, the 3.5:5:4.5 MQW-based PeLEDs shows a high radiance of 40 W sr-1 m-2, and a good reproducibility (Figure S2). The EQE histogram for 88 P0.27C0.38S0.35-based devices shows an average peak EQE of 2.5% (Figure 5e), and a relative standard deviation of 10%. Significantly, our tin-based MQW PeLEDs exhibit excellent operational stability. The lifetime (T50) of tin-based MQW PeLEDs is over 2 h under a constant current density of 10 mA cm-2 with an initial radiance of 0.6 W sr-1 m-2 (Figure 5f). This lifetime is comparable with the lead-based MQW PeLEDs under the same driving condition,3, 20 and is a record for tin-based PeLEDs. In comparison, the lifetime of 3D tin-based PeLED is only ~10 min with an initial radiance of 0.44 W sr-1 m-2 (Figure S3), which is much shorter than that of MQW devices. Moreover, the radiance and EQE values of MQW device under the forward and reverse scan are coincident, while those of 3D device show serious


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hysteresis (Figure S4). We believe that the excellent stability of MQW-based device is attributed to the enhanced stability of the MQW-structured tin perovskite films. In summary, we show that solution-processed tin halide MQW perovskites are capable of producing efficient lead-free PeLEDs with high stability and good reproducibility. The MQW structure not only facilitates the formation of uniform tin perovskite films with low defect densities and high PLQEs, but also suppresses the oxidation of Sn2+ cations. Our results suggest that the solution-processed MQW perovskites are promising platforms for achieving high performance lead-free PeLEDs, and may also be useful for other lead-free perovskite optoelectronic devices. EXPERIMENTAL METHODS Materials. The phenylethylammonium iodide (PEAI) was prepared by using a similar method to the literature.19 First, 20 mmol phenylethylamine was mixed with 10 mL ethanol in a roundbottom flask, and cooled by ice water bath. Then, 20 mmol hydriodic acid was added dropwise into the flask, followed by stirring for 1 h at room temperature. The white powder was recovered by evaporation, and then recrystallized twice by ethanol and ether. Purified sample was dried at 50 °C overnight in a vacuum oven. The precursor solutions of tin-based perovskite films were prepared by dissolving PEAI, CsI, and SnI2 in a mixed solvent of DMF/DMSO (v/v = 4 : 1). Additional 10% molar ratio of SnF2 was added into the precursor solutions. The concentration of each solution was 7% in weight. All precursor solutions were stirred at room temperature overnight in a glovebox and filtered through a 0.2 μm PTFE filter before use.


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Device Fabrication. Tin-based PeLEDs were fabricated with the device configuration of indium tin

oxide/poly(9-vinylcarbazole)

(PVK)/perovskites/1,3,5-tri(m-pyrid-3-yl-phenyl)

benzene

(TmPyPB)/lithium fluoride (LiF)/aluminum (Al). The PVK layers were deposited from a chlorobenzene solution (6 mg mL-1) at 2000 rpm for 30 s, followed by annealing at 120 °C for 20 min. The perovskite films were deposited on the pre-heated PVK substrates (65 °C) by spincoating the precursor solutions at 4000 rpm for 60 s, followed by annealing at 65 °C for 5 min. The TmPyPB, LiF, and Al layers were deposited by thermal evaporation methods. Characterization. XRD data were collected by a Bruker D8 Advance X-ray Diffraction system operated at 40 kV and 30 mA with Cu Kα radiation. UV-vis absorption spectra were measured by a spectrophotometer with an integrating sphere (PerkinElmer, Lambda 950). PL emission spectra were measured by using a QE65 pro spectrometer and a 445 nm CW laser as an excitation source. PLQE measurements were carried out by using a three-step technique by combining laser, optical fiber, spectrometer, and integrating sphere.34 The time-resolved PL spectra were measured by using Edinburgh Instruments spectrometer (FLS980) with a 633 nm pulsed laser as the excitation source. The SEM images were obtained by using JEOL JSM-7800F SEM. The AFM images were taken by using Park XE7 microscope with a non-contact mode. The XPS spectra were collected by using Thermo ESCALAB250Xi X-ray photoelectron spectrometer. The fabricated PeLEDs were tested on top of an integration sphere at room temperature in a nitrogen glovebox.3 ACKNOWLEDGMENT This work is financially supported by the Major Research Plan of the National Natural Science Foundation of China (91733302), the National Basic Research Program of China-Fundamental Studies of Perovskite Solar cells (2015CB932200), the Joint Research Program between China


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and European Union (2016YFE0112000), the Natural Science Foundation of Jiangsu Province (BK20171002, BK20150043, BK20150064), the National Natural Science Foundation of China (11474164, 61634001, 61875084), the Natural Science Fund for Colleges and Universities in Jiangsu Province of China (16KJB430016), the National Science Fund for Distinguished Young Scholars (61725502), and the Synergetic Innovation Center for Organic Electronics and Information Displays. Supporting Information. Additional XRD analysis, EL spectra, reproducibility of MQW PeLEDs, lifetime of the 3D PeLED, hysteresis effects (PDF) REFERENCES (1) 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. (2) Wang, J.; Wang, N.; Jin, Y.; Si, J.; Tan, Z.-K.; Du, H.; Cheng, L.; Dai, X.; Bai, S.; He, H.; et al. Interfacial Control Toward Efficient and Low-Voltage Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 2311-2316. (3) Wang, N.; Cheng, L.; Ge, R.; Zhang, S.; Miao, Y.; Zou, W.; Yi, C.; Sun, Y.; Cao, Y.; Yang, R.; et al. Perovskite Light-Emitting Diodes Based on Solution-Processed Self-Organized Multiple Quantum Wells. Nat. Photonics 2016, 10, 699-704. (4) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; et al. Perovskite Energy Funnels for Efficient Light-Emitting Diodes. Nat. Nanotechnol. 2016, 11, 872-877.


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Tin-Based Multiple Quantum Well Perovskites for Light-Emitting

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