Harnessing Dielectric Confinement on Tin Perovskites to Achieve

Jun 6, 2019 - Tin perovskite nanomaterial is one of the promising candidates to replace organic lead halide perovskites in lighting application. Unfor...
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Harnessing Dielectric Confinement on Tin Perovskites to Achieve Emission Quantum Yield up to 21% Jin-Tai Lin,† Chen-Cheng Liao,‡ Chia-Shuo Hsu,† Deng-Gao Chen,† Hao-Ming Chen,*,† Ming-Kang Tsai,*,‡ Pi-Tai Chou,*,†,§ and Ching-Wen Chiu*,† †

Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan Department of Chemistry, National Taiwan Normal University, No. 88, Section 4, Ting-Zhou Road, Taipei 11677, Taiwan § Center for Emerging Materials and Advanced Devices, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan Downloaded via BUFFALO STATE on July 20, 2019 at 07:19:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Tin perovskite nanomaterial is one of the promising candidates to replace organic lead halide perovskites in lighting applications. Unfortunately, the performance of tin-based systems is markedly inferior to those featuring toxic Pb salts. In an effort to improve the emission quantum efficiency of nanoscale 2D layered tin iodide perovskites through fine-tuning the electronic property of organic ammonium salts, we came to unveil the relationship between dielectric confinement and the photoluminescent properties of tin iodide perovskite nanodisks. Our results show that increasing the dielectric contrast for organic versus inorganic layers leads to a bathochromic shift in emission peak wavelength, a decrease of exciton recombination time, and importantly a significant boost in the emission efficiency. Under optimized conditions, a leap in emission quantum yield to a record high 21% was accomplished for the nanoscale thienylethylammonium tin iodide perovskite (TEA2SnI4). The as-prepared TEA2SnI4 also possessed superior photostability, showing no sign of degradation under continuous irradiation (10 mW/cm2) over a period of 120 h.

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INTRODUCTION Emerging emissive nanomaterials have received considerable attention due to their unique photophysical properties such as enlarged exciton binding energy and enhanced light absorption cross section with respect to the corresponding bulk materials.1 Among these nanomaterials, lead halide perovskite (CsPbX3 where X = Cl, Br, I) nanocrystals have shown excellent photoluminescence performance, including high photoluminescence quantum yield (PLQY), narrow full width at halfmaximum (fwhm), and tunable emission covering the visible range.2−11 As a result, lead halide perovskite nanocrystals have been widely studied for their potential applications in lightemitting diodes12−15 and lasers.16−18 Despite these vastly developed research fields, the toxicity remains a major obstacle for commercializing lead halide perovskite-based innovations. As for the replacement, the demand for nontoxic elementbased perovskite materials continues to grow in the past few years.19,20 Among these nontoxic alternatives, divalent tin cation has been considered as a good candidate to replace Pb2+, and the potential application of tin halide perovskites in optoelectronic devices has also been investigated.21−30 However, Sn2+ undergoes facile oxidation to its tetravalent state, creating a high defect density in the perovskite lattice. These defects would generate trap states, leading to rapid nonradiative relaxation of excitons. It is thus much more © 2019 American Chemical Society

challenging to achieve high PLQY and stability with tin perovskites.31,32 Although the high PLQY (88−95%) can be accomplished by the self-trapped state emission of tin-layered perovskites,33,34 the broad emission (∼135 nm fwhm) may not be suitable for display application that requires high color purity. Currently, the highest PLQY value of direct band narrow emissions of tin-based 2D perovskite nanomaterials can only reach 6.4%,35 which is still far inferior to lead-based perovskite nanomaterials (∼100%).4,36 Both experimental and theoretical studies on quantum dots have shown that the emission intensity is positively correlated to their exciton binding energy (Eb), which can be fine-tuned by quantum confinement. For example, improved PLQY of an AlGaN quantum well could be realized by increasing the exciton binding energy through reducing the width of the quantum well.37 A similar phenomenon also has been observed in the luminescent perovskite materials. Pullerits reported that a high PLQY of lead perovskite nanoparticles can be attributed to their exciton binding energy, which is approximately 5 times higher than that of bulk crystals.38 Likewise, nanoscale 2D layered tin perovskites35,39,40 always exhibit higher PLQY than their corresponding 3D nanocrystals41 due to the increased Eb Received: March 27, 2019 Published: June 6, 2019 10324

DOI: 10.1021/jacs.9b03148 J. Am. Chem. Soc. 2019, 141, 10324−10330

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Journal of the American Chemical Society of the 2D structure.42 Therefore, raising the exciton binding energy could be an effective strategy to further enhance the emission intensity of tin perovskite-based nanomaterials. Unfortunately, the exciton binding energy of monolayered 2D tin perovskites cannot be further increased by reducing the lattice dimensions, because the quantum confinement effect has reached a maximum along the packing direction. In an aim to boost emission efficiency one must resort to a different strategy in enhancing Eb of the nanoscale 2D layered tin perovskites. Studies of emissive quantum dots also demonstrate that exciton binding energy of semiconducting materials, in addition to quantum confinement, can be harnessed by the dielectric confinement effect. Dielectric confinement is caused by the mismatch of dielectric constants between a semiconductor and its surrounding. During the synthesis of nanoparticles, organic surfactants are applied to control the topology of nanoparticles and prevent aggregation. The surrounding organic layer with a low dielectric constant is less polarizable and hence decreases the screening of the hole− electron Coulomb interaction, resulting in an increase of the exciton binding energy. For the 2D perovskites, we also anticipate that the discrepancy of dielectric constants between the inorganic framework (semiconductor) and the organic layers (surrounding) should give rise to the dielectric confinement imposed on the exciton, which can be modulated by changing the composition of the organic cations.43 Theoretically, the correlation among Eb of 2D and 3D excitons and the dielectric constant of media is expressed by the following equation:44 ji ε zy = 4jjj w zzz E b3D j εb z k {

as an effective approach to attain highly emissive and stable nanoscale 2D layered tin perovskites. To prove the concept, herein, we investigated a series of organic ammonium cations to fine-tune the dielectric confinement in 2D tin iodide perovskites. As a result, a correlation of increasing strength of dielectric confinement resulting in the increase of PLQY has been established. Among various ammonium cations incorporated in nanoscale 2D layered tin perovskites, TEA2SnI4 (TEA: thienylethylammonium) showed the largest εw/εb ratio and hence the highest PLQY. Further optimization of TEA2SnI4 by appropriate aliphatic acid additives achieved a record-high emission quantum yield of up to 21% and long-term stability that revealed no sign of degradation after continuous irradiation for 120 h. Details of the experimental results, theoretical approaches, and discussion are elaborated in the following sections.

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RESULTS AND DISCUSSION Prior to the systematic study of dielectric confinement on the optical properties of tin perovskites, proper selection of ammonium cations is indispensable. Also, the packing defect and structural variation could potentially be the dominant factors governing the photophysical properties of the perovskite nanomaterials. As reported in our previous study,35 2D layered tin perovskite nanodisks prepared from aliphatic ammonium cations all exhibited poor morphology due to the lack of interlayer interactions. These structural defects resulted in not only poor stability of the nanodisk but also low PLQY values ( pFPEA2SnI4 > PEA2SnI4. This correlates well with the experimental result of most red-shifted emission wavelengths being ascribed to the TEA2SnI4 nanodisks (see eq 2). As a result, the optical band gap as a function of the ratio of dielectric constant (εw/εb) exhibits a linear behavior (see Figure 5a), which decreased as εw/εb increases, supporting the

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CONCLUSION In summary, the mismatch of dielectric constant of inorganic frameworks and organic cations was found to play a crucial role in the photoluminescent properties of aromatic ammonium tin perovskite nanodisks, such as emission peak position, exciton relaxation dynamics, and PLQY values. The correlations among these photoluminescent properties and dielectric confinement effect were then established semiempirically. The enhancement of dielectric confinement leads to a red shift of the emission peak wavelength, a shortening of the exciton recombination time, and an increase in PLQY. Upon further fine-tuning with pentanoic acid as the additive, TEA2SnI4, which possesses the largest dielectric contrast between an organic ammonium cation and a semiconducting tin iodide layer in this study, shows a record-high PLQY value of 21%, one of the highest achieved to date for tin halide perovskite nanomaterials with direct band narrow emissions.

Figure 5. (a) Correlation plot of band gaps of tin perovskite nanodisks versus εw/εb. Here, the red line represents linear regression with the square of the sample correlation coefficient r2 = 0.99991. (b) Correlation plot of exciton radiative lifetime versus (εb/εw)2. The linear regression possesses r2 = 0.99829. (c) Correlation plot of PLQY versus (εw/ εb)2. (d) Photostability test of TEA2SnI4 nanodisks in degassed toluene with 0.1 volume % pentanoic acid solution under continuous 375 nm (10 or 150 mW cm−2) illumination.

relationship expressed in eq 2. The plot of radiative lifetime τr (1/kr) versus (εb/εw)2 (Figure 5b) illustrates a linear behavior; that is, the decrease of (εb/εw)2 results in a decrease of τr. In other words, the increase of binding energy gives rise to a shorter radiative lifetime. This result may not be surprising given the relationship between the exciton binding energy and radiative lifetime. Feldmann has reported that the radiative lifetime is proportional to the inverse of the exciton binding energy for quantum wells.47 A similar phenomenon was also observed in other 2D perovskite systems,48 in which the radiative lifetime decreases as the exciton binding energy increases. In theory, the increase of exciton binding energy should facilitate the electron−hole recombination rate and hence decrease the emission radiative lifetime. This, in combination with the decrease of the nonradiative decay rate due to the increase of binding energy, i.e., decrease of the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b03148. Additional synthesis, characterization data, and computational details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. 10328

DOI: 10.1021/jacs.9b03148 J. Am. Chem. Soc. 2019, 141, 10324−10330

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Journal of the American Chemical Society *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

CsPbI3 Perovskite Quantum Dots Achieving Near 100% Absolute Photoluminescence Quantum Yield. ACS Nano 2017, 11, 10373. (12) Jizhong, S.; Jianhai, L.; Xiaoming, L.; Leimeng, X.; Yuhui, D.; Haibo, Z. Quantum Dot Light-Emitting Diodes Based on Inorganic Perovskite Cesium Lead Halides (CsPbX3). Adv. Mater. 2015, 27, 7162. (13) Guangru, L.; Rocca, R. F. W.; K, D. N. J. L.; Sai, B.; C, J. T.; Francisco, d. l. P.; Shaocong, H.; Caterina, D.; Feng, G.; H, F. R.; C, G. N.; Zhi-Kuang, T. Highly Efficient Perovskite Nanocrystal LightEmitting Diodes Enabled by a Universal Crosslinking Method. Adv. Mater. 2016, 28, 3528. (14) Jianhai, L.; Leimeng, X.; Tao, W.; Jizhong, S.; Jiawei, C.; Jie, X.; Yuhui, D.; Bo, C.; Qingsong, S.; Boning, H.; Haibo, Z. 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. (15) Zhang, X.; Lin, H.; Huang, H.; Reckmeier, C.; Zhang, Y.; Choy, W. C. H.; Rogach, A. L. Enhancing the Brightness of Cesium Lead Halide Perovskite Nanocrystal Based Green Light-Emitting Devices through the Interface Engineering with Perfluorinated Ionomer. Nano Lett. 2016, 16, 1415. (16) Yue, W.; Xiaoming, L.; Jizhong, S.; Lian, X.; Haibo, Z.; Handong, S. All-Inorganic Colloidal Perovskite Quantum Dots: A New Class of Lasing Materials with Favorable Characteristics. Adv. Mater. 2015, 27, 7101. (17) Xu, Y.; Chen, Q.; Zhang, C.; Wang, R.; Wu, H.; Zhang, X.; Xing, G.; Yu, W. W.; Wang, X.; Zhang, Y.; Xiao, M. Two-PhotonPumped Perovskite Semiconductor Nanocrystal Lasers. J. Am. Chem. Soc. 2016, 138, 3761. (18) Fu, Y.; Zhu, H.; Stoumpos, C. C.; Ding, Q.; Wang, J.; Kanatzidis, M. G.; Zhu, X.; Jin, S. Broad Wavelength Tunable Robust Lasing from Single-Crystal Nanowires of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). ACS Nano 2016, 10, 7963. (19) Yang, B.; Chen, J.; Yang, S.; Hong, F.; Sun, L.; Han, P.; Pullerits, T.; Deng, W.; Han, K. Lead-Free Silver-Bismuth Halide Double Perovskite Nanocrystals. Angew. Chem., Int. Ed. 2018, 57, 5359. (20) Yang, B.; Mao, X.; Hong, F.; Meng, W.; Tang, Y.; Xia, X.; Yang, S.; Deng, W.; Han, K. Lead-Free Direct Band Gap Double-Perovskite Nanocrystals with Bright Dual-Color Emission. J. Am. Chem. Soc. 2018, 140, 17001. (21) Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Lead-free solid-state organic−inorganic halide perovskite solar cells. Nat. Photonics 2014, 8, 489. (22) Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A.-A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; Petrozza, A.; Herz, L. M.; Snaith, H. J. Lead-free organic-inorganic tin halide perovskites for photovoltaic applications. Energy Environ. Sci. 2014, 7, 3061. (23) Weiqiang, L.; Dewei, Z.; Yue, Y.; R, G. C.; Changlei, W.; J, C. A.; Philip, S.; Weiwei, M.; Kai, Z.; Ren-Gen, X.; Yanfa, Y. Lead-Free Inverted Planar Formamidinium Tin Triiodide Perovskite Solar Cells Achieving Power Conversion Efficiencies up to 6.22%. Adv. Mater. 2016, 28, 9333. (24) Liao, Y.; Liu, H.; Zhou, W.; Yang, D.; Shang, Y.; Shi, Z.; Li, B.; Jiang, X.; Zhang, L.; Quan, L. N.; Quintero-Bermudez, R.; Sutherland, B. R.; Mi, Q.; Sargent, E. H.; Ning, Z. Highly Oriented LowDimensional Tin Halide Perovskites with Enhanced Stability and Photovoltaic Performance. J. Am. Chem. Soc. 2017, 139, 6693. (25) Lee, S. J.; Shin, S. S.; Kim, Y. C.; Kim, D.; Ahn, T. K.; Noh, J. H.; Seo, J.; Seok, S. I. Fabrication of Efficient Formamidinium Tin Iodide Perovskite Solar Cells through SnF2−Pyrazine Complex. J. Am. Chem. Soc. 2016, 138, 3974. (26) Hemant, K. M.; Sabba, D.; Lin, L. W.; P, B. P.; Ramanujam, P. R.; Tom, B.; Chen, S.; Hong, D.; Ramamoorthy, R.; Mark, A.; Michael, G.; G, M. S.; Nripan, M. Lead-Free Halide Perovskite Solar Cells with High Photocurrents Realized Through Vacancy Modulation. Adv. Mater. 2014, 26, 7122.

ORCID

Chia-Shuo Hsu: 0000-0002-7767-8413 Deng-Gao Chen: 0000-0001-6406-2209 Hao-Ming Chen: 0000-0002-7480-9940 Ming-Kang Tsai: 0000-0001-9189-5572 Pi-Tai Chou: 0000-0002-8925-7747 Ching-Wen Chiu: 0000-0001-7201-0943 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Ministry of Science and Technology of Taiwan (MOST 107-2113-M-002-007; MOST 107-2119-M-002-002; MOST 107-2113-M-003-007) and National Taiwan University (NTU-108L880104). Computational resources are supported by the National Center for HighPerformance Computing of Taiwan and the Center for Cloud Computing in National Taiwan Normal University.

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REFERENCES

(1) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L. Colloidal nanoplatelets with twodimensional electronic structure. Nat. Mater. 2011, 10, 936. (2) Kovalenko, M. V.; Protesescu, L.; Bodnarchuk, M. I. Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science 2017, 358, 745. (3) 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. (4) Koscher, B. A.; Swabeck, J. K.; Bronstein, N. D.; Alivisatos, A. P. Trap-Free CsPbBr3 Colloidal Nanocrystals by Postsynthetic Thiocyanate Surface Treatment. J. Am. Chem. Soc. 2017, 139, 6566. (5) Ahmed, T.; Seth, S.; Samanta, A. Boosting the Photoluminescence of CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals Covering a Wide Wavelength Range by Postsynthetic Treatment with Tetrafluoroborate Salts. Chem. Mater. 2018, 30, 3633. (6) Yu, T.; Eva, B.; F, A. M.; Aurora, M.; Z, M. K.; A, H. V.; Pablo, D.; Sara, B.; S, U. A.; Lakshminarayana, P.; Jochen, F. Highly Luminescent Cesium Lead Halide Perovskite Nanocrystals with Tunable Composition and Thickness by Ultrasonication. Angew. Chem., Int. Ed. 2016, 55, 13887. (7) Xiaoming, L.; Ye, W.; Shengli, Z.; Bo, C.; Yu, G.; Jizhong, S.; Haibo, Z. CsPbX3 Quantum Dots for Lighting and Displays: RoomTemperature Synthesis, Photoluminescence Superiorities, Underlying Origins and White Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 2435. (8) Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-g.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533. (9) Sun, S.; Yuan, D.; Xu, Y.; Wang, A.; Deng, Z. Ligand-Mediated Synthesis of Shape-Controlled Cesium Lead Halide Perovskite Nanocrystals via Reprecipitation Process at Room Temperature. ACS Nano 2016, 10, 3648. (10) Park, Y.-S.; Guo, S.; Makarov, N. S.; Klimov, V. I. Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots. ACS Nano 2015, 9, 10386. (11) Liu, F.; Zhang, Y.; Ding, C.; Kobayashi, S.; Izuishi, T.; Nakazawa, N.; Toyoda, T.; Ohta, T.; Hayase, S.; Minemoto, T.; Yoshino, K.; Dai, S.; Shen, Q. Highly Luminescent Phase-Stable 10329

DOI: 10.1021/jacs.9b03148 J. Am. Chem. Soc. 2019, 141, 10324−10330

Article

Journal of the American Chemical Society

multiple quantum well crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 125421. (45) Takagahara, T. Effects of dielectric confinement and electronhole exchange interaction on excitonic states in semiconductor quantum dots. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 4569. (46) Mitzi, D. B.; Dimitrakopoulos, C. D.; Kosbar, L. L. Structurally Tailored Organic-Inorganic Perovskites: Optical Properties and Solution-Processed Channel Materials for Thin-Film Transistors. Chem. Mater. 2001, 13, 3728. (47) Feldmann, J.; Peter, G.; Göbel, E. O.; Dawson, P.; Moore, K.; Foxon, C.; Elliott, R. Linewidth dependence of radiative exciton lifetimes in quantum wells. Phys. Rev. Lett. 1987, 59, 2337. (48) Cheng, B.; Li, T.-Y.; Maity, P.; Wei, P.-C.; Nordlund, D.; Ho, K.-T.; Lien, D.-H.; Lin, C.-H.; Liang, R.-Z.; Miao, X.; Ajia, I. A.; Yin, J.; Sokaras, D.; Javey, A.; Roqan, I. S.; Mohammed, O. F.; He, J.-H. Extremely reduced dielectric confinement in two-dimensional hybrid perovskites with large polar organics. Commun. Phys. 2018, 1, 80.

(27) Lanzetta, L.; Marin-Beloqui, J. M.; Sanchez-Molina, I.; Ding, D.; Haque, S. A. Two-Dimensional Organic Tin Halide Perovskites with Tunable Visible Emission and Their Use in Light-Emitting Devices. ACS Energy Lett. 2017, 2, 1662. (28) Ke, W.; Stoumpos, C. C.; Logsdon, J. L.; Wasielewski, M. R.; Yan, Y.; Fang, G.; Kanatzidis, M. G. TiO2−ZnS Cascade Electron Transport Layer for Efficient Formamidinium Tin Iodide Perovskite Solar Cells. J. Am. Chem. Soc. 2016, 138, 14998. (29) Marshall, K. P.; Walker, M.; Walton, R. I.; Hatton, R. A. Enhanced stability and efficiency in hole-transport-layer-free CsSnI3 perovskite photovoltaics. Nat. Energy 2016, 1, 16178. (30) Lee, S. J.; Shin, S. S.; Im, J.; Ahn, T. K.; Noh, J. H.; Jeon, N. J.; Seok, S. I.; Seo, J. Reducing Carrier Density in Formamidinium Tin Perovskites and Its Beneficial Effects on Stability and Efficiency of Perovskite Solar Cells. ACS Energy Lett. 2018, 3, 46. (31) Wang, A.; Yan, X.; Zhang, M.; Sun, S.; Yang, M.; Shen, W.; Pan, X.; Wang, P.; Deng, Z. Controlled Synthesis of Lead-Free and Stable Perovskite Derivative Cs2SnI6 Nanocrystals via a Facile Hot-Injection Process. Chem. Mater. 2016, 28, 8132. (32) Wang, A.; Guo, Y.; Muhammad, F.; Deng, Z. Controlled Synthesis of Lead-Free Cesium Tin Halide Perovskite Cubic Nanocages with High Stability. Chem. Mater. 2017, 29, 6493. (33) Zhang, X.; Wang, C.; Zhang, Y.; Zhang, X.; Wang, S.; Lu, M.; Cui, H.; Kershaw, S. V.; Yu, W. W.; Rogach, A. L. Bright Orange Electroluminescence from Lead-Free Two-Dimensional Perovskites. ACS Energy Lett. 2019, 4, 242. (34) Wang, A.; Guo, Y.; Zhou, Z.; Niu, X.; Wang, Y.; Muhammad, F.; Li, H.; Zhang, T.; Wang, J.; Nie, S.; Deng, Z. Aqueous acid-based synthesis of lead-free tin halide perovskites with near-unity photoluminescence quantum efficiency. Chem. Sci. 2019, 10, 4573. (35) Chen, M.-Y.; Lin, J.-T.; Hsu, C.-S.; Chang, C.-K.; Chiu, C.-W.; Chen, H. M.; Chou, P.-T. Strongly Coupled Tin-Halide Perovskites to Modulate Light Emission: Tunable 550−640 nm Light Emission (FWHM 36−80 nm) with a Quantum Yield of up to 6.4%. Adv. Mater. 2018, 30, 1706592. (36) Di Stasio, F.; Christodoulou, S.; Huo, N.; Konstantatos, G. Near-Unity Photoluminescence Quantum Yield in CsPbBr3 Nanocrystal Solid-State Films via Postsynthesis Treatment with Lead Bromide. Chem. Mater. 2017, 29, 7663. (37) Shevchenko, E. A.; Nechaev, D. V.; Jmerik, V. N.; Kaibyshev, V. K.; Ivanov, S. V.; Toropov, A. A. Enhanced photoluminescence efficiency in AlGaN quantum wells with gradient-composition AlGaN barriers. J. Phys.: Conf. Ser. 2016, 741, 012118. (38) Zheng, K.; Zhu, Q.; Abdellah, M.; Messing, M. E.; Zhang, W.; Generalov, A.; Niu, Y.; Ribaud, L.; Canton, S. E.; Pullerits, T. Exciton Binding Energy and the Nature of Emissive States in Organometal Halide Perovskites. J. Phys. Chem. Lett. 2015, 6, 2969. (39) Weidman, M. C.; Seitz, M.; Stranks, S. D.; Tisdale, W. A. Highly Tunable Colloidal Perovskite Nanoplatelets through Variable Cation, Metal, and Halide Composition. ACS Nano 2016, 10, 7830. (40) Wong, A. B.; Bekenstein, Y.; Kang, J.; Kley, C. S.; Kim, D.; Gibson, N. A.; Zhang, D.; Yu, Y.; Leone, S. R.; Wang, L.-W.; Alivisatos, A. P.; Yang, P. Strongly Quantum Confined Colloidal Cesium Tin Iodide Perovskite Nanoplates: Lessons for Reducing Defect Density and Improving Stability. Nano Lett. 2018, 18, 2060. (41) Jellicoe, T. C.; Richter, J. M.; Glass, H. F. J.; Tabachnyk, M.; Brady, R.; Dutton, S. E.; Rao, A.; Friend, R. H.; Credgington, D.; Greenham, N. C.; Böhm, M. L. Synthesis and Optical Properties of Lead-Free Cesium Tin Halide Perovskite Nanocrystals. J. Am. Chem. Soc. 2016, 138, 2941. (42) Shinada, M.; Sugano, S. Interband Optical Transitions in Extremely Anisotropic Semiconductors. I. Bound and Unbound Exciton Absorption. J. Phys. Soc. Jpn. 1966, 21, 1936. (43) Jagielski, J.; Kumar, S.; Yu, W.-Y.; Shih, C.-J. Layer-controlled two-dimensional perovskites: synthesis and optoelectronics. J. Mater. Chem. C 2017, 5, 5610. (44) Takagi, H.; Kunugita, H.; Ema, K. Influence of the image charge effect on excitonic energy structure in organic-inorganic 10330

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