Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX
pubs.acs.org/journal/apchd5
Mn2+ Doping Enhances the Brightness, Efficiency, and Stability of Bulk Perovskite Light-Emitting Diodes Mahesh K. Gangishetty, Samuel N. Sanders, and Daniel N. Congreve* Rowland Institute at Harvard 100 Edwin H. Land Blvd, Cambridge, Massachusetts United States
Downloaded via UNIV OF SOUTHERN INDIANA on April 10, 2019 at 12:36:59 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Interest in organic−inorganic hybrid perovskite (ABX3) LEDs has exploded over the past several years, yet significant gains in stability, efficiency, and brightness are required before commercialization is possible, particularly for blue devices. The perovskite composition has been shown to play a crucial role in its performance, yet to date nearly all existing reports focus on tuning the A-site composition. Here, we find that doping the B-site with manganese allows us to achieve bright, efficient, and stable LEDs regardless of A or X composition. By doping with Mn, we demonstrate ultrabright sky-blue, green, and red perovskite LEDs with a maximum brightness of 11800, 97000, and 1470 cd/m2 and quantum efficiencies of 0.58%, 3.2%, and 5.1%, respectively. Crucially, these devices show excellent operational stability, with the sky-blue devices lasting for 20 min and red devices over 5 h with strong spectral stability. Moreover, the green devices showed over 1% efficiency even at higher current densities, ∼2000 mA/cm2. Mn doping allows for universal improvement in perovskite performance and stability, opening the door to a huge number of applications. KEYWORDS: lead halide perovskites, light-emitting diodes, Mn doping, stability
L
co-workers applied a similar phenomenon to FAPbBr3 perovskites and achieved an EQE of 14% for n = 3 structures. By using a combination of Cs and MA, You and co-workers achieved ultrabright and highly efficient green LEDs with EQEs of 10% and a brightness of 91000 cd/m2.22 In a similar way, Lee and co-workers employed a combination of Cs and FA to improve the performance of FA-based perovskite LEDs.14 Recently, Zhao et al. achieved EQEs over 20% using quasi 2D/3D perovskites, and Cao et al. employed surface passivating additives to achieve similar efficiencies from submicron perovskite grains in the red region.23,24 Along with the red devices, the green LEDs also reached a record EQE of >20%, and to achieve such high EQEs, Lin et al. used compositionally graded perovskite structures.25 Despite these impressive results, however, perovskite materials face several large hurdles before large-scale commercialization can be considered. By far, the most important challenge is the stability of these materials. Most high-performance devices have lifetimes on the order of seconds to minutes.26 This lack of stability is largely attributed to ion migration within the perovskite and is a prohibitive barrier to commercialization. For mixed halide blue and red devices, the problem is even more severe, as the electroluminescence spectra of these devices have been shown
ead halide perovskites have shown rapid progress in lightemitting diodes due to their exceptional optoelectronic properties.1,2 They adopt an ABX3 structure, where A represents the cation methylammonium, formamidinium, or cesium, or a mixture of the three, B represents lead, and X represents a halide. Since the first report on efficient perovskite LEDs from the Friend group,3 many different nanostructures for these materials have emerged, including colloidal nanocrystals,3−8 quasi 2D,9−13 and bulk 3D,14−16 all of which have shown great promise for LEDs. Many strategies have been developed to improve the efficiency of perovskite LEDs, often focusing on the nanoscale morphology of the perovskite, including nanograin engineering, 5,17,18 ligand engineering, 6,7,9,19,20 and crystal pinning.10,13,17,21,22 So far, perovskite thin films with nanograins and multiple quantum-well structures have shown attractive external quantum efficiencies (EQEs) in green and red LEDs. To obtain high-quality perovskites, the composition of the A cation has been shown to play a crucial role. Sargent and coworkers employed aromatic phenylethylammonium (PEABr) cations to achieve multiple quantum wells, with the number of layers of MAPbBr3 perovskite nanoplatelets depending the concentration of PEABr.13 Their energy-funneling landscapes, with n = 5 layers, led to EQEs of 8% in green perovskite LEDs.9 Rand et al. incorporated larger alkyl ammonium ions at the A-site to obtain nanograins down to 10 nm, leading to EQEs over 10% for green and ∼9% for red LEDs.17 You and © XXXX American Chemical Society
Received: January 26, 2019
A
DOI: 10.1021/acsphotonics.9b00142 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
to change and redshift as ions migrate throughout the material.7,27 Therefore, both high operational stability, where the device can emit light for a long period of time, and high spectral stability, where the device emits a consistent color, are required. Further, for color rendering applications, efficient red, green, and blue devices must all be demonstrated. High efficiencies have been consistently shown in green and red devices, yet blue has lagged far behind. Despite some success in perovskite nanocrystals,4,28 there have been only a handful of bulk blue electroluminescent devices reported29−31 due to the poor quality of the chloride-based materials, necessitating a way to fix the materials issues present in the chloride system. Here, we use Mn doping in quasi-bulk 3D perovskites to attack both of these issues. Mn doping has been well studied in perovskite nanocrystals32−36 but, so far, has only been deployed at high dilution in bulk material.37 By doping at the B site, the amount of lead (Pb) in perovskite LEDs is reduced, and the stability and performance of nanostructured perovskites improves.32,33 Further, our group has found that doping Mn into nanocrystals delivers substantial enhancement in both optoelectronic and device performance of blue nanocrystals.38 However, significant doping in bulk structures has yet to be investigated. Upon doping sky-blue, green, and red perovskite films with Mn, we observe a significant improvement in the morphology of the perovskite layers as well as the optical properties of the films. The doped films form highly crystalline layers that demonstrate a strong brightness and lifetime increase relative to undoped samples. Devices show significant improvements in efficiency, brightness, and stability as compared to those without Mn. In particular, mixed halide red and sky-blue devices both demonstrate excellent spectral stability on top of their high operational stability. We show that Mn doping is a universal method toward high optoelectronic performance in perovskite LEDs.
Figure 1. Mn-doped perovskites. (a) Perovskite crystal structure. (b) Images of the films used in this work demonstrate improved film uniformity and brightness with increased Mn doping at the B-site. (c) XRD of green films. The films show strong peaks at high Mn content, indicating that the materials become very crystalline. (d) PLQY measurements show a strong brightness increase with Mn doping.
crystallites. In contrast, the undoped sample shows additional peaks corresponding to the (110) and (210) planes of the cubic phase, indicating the formation of polycrystalline 3D perovskite phases.39 Little evidence of 2D perovskite phases are observed in XRD, and the results are in agreement with the absence of excitonic peaks in the UV−vis absorption spectra (Figure S1). Similar XRD patterns are observed for red and sky-blue films, see Figure S2. To quantify this brightness improvement, we measured the photoluminescence quantum yield (PLQY) of the materials as shown in Figure 1d. For all three colors, a striking increase in PLQY is observed with the introduction of Mn doping. To understand the origin of the PLQY enhancement, we recorded time-resolved PL (Figure S3) and observed a significant difference in the excited state behavior after doping with Mn. Both doped and control samples yield biexponential fluorescence decays, with the control lifetimes at 3.4 and 25 ns, whereas the Mn doping increased these lifetimes to 4.4 and 29 ns, respectively. Furthermore, the relative amplitude of the longer lifetime component of the decay increased from 22% to 35% after adding Mn. These enhancements in radiative lifetime upon Mn doping are consistent with the increased PLQY obtained from the doped films. Although the exact mechanism for these lifetime and luminescence enhancements is still under investigation, they hint at a reduction in the density of nonradiative trap states after doping with Mn. In addition to XRD, the Mn doping significantly influences the morphology of the perovskite, which can be seen clearly in the SEM images in Figure 2. From elemental mapping using energy dispersive X-ray spectrometer (EDS), we confirmed the presence of Mn throughout the perovskite layer (see Figures S4 and S5). In Figure 2, we observe that the perovskite transitions naturally from large grains without Mn (a−c) to uniform thin films with small crystalline grains upon Mn incorporation (d−f). Without doping, the crystallites are
■
RESULTS AND DISCUSSION We fabricate perovskite films by mixing the precursors in DMF or a DMF/DMSO mixture. A total of 20% of PEABr with respect to the A cation is added in order to passivate grain boundaries. The films are fabricated with a standard antisolvent dripping during the spinning process. We introduce Mn into bulk-like perovskite crystals by replacing a percentage of PbBr2 with MnBr2, allowing us fine control over the Mn doping percentage. Upon doping, an immediate increase in photoluminescence is observed for all three perovskite colors, Figure 1b. Further, we see a strong improvement in macroscale film crystallinity and brightness as Mn is introduced into the films. For red and sky-blue samples, MnBr2 precursor was still used as the Mn source, and the color was adjusted by replacing the remaining PbBr2 with PbI2 or PbCl2, respectively. This gives a mix of halide ions in the film, tuning the emission peak wavelength. As with the green samples, Mn incorporation leads to large film quality and photoluminescence improvements in the red and sky-blue materials. To confirm that the sample remains in the perovskite crystal structure, we turned to X-ray diffraction, Figure 1c. We find that all perovskite films adopt the cubic crystal phase. The samples became significantly more crystalline with the addition of Mn. In fact, at the highest Mn doping level of 30%, intense peaks of only the (100) and (200) planes are evident, indicating the formation of highly crystalline cubic perovskite B
DOI: 10.1021/acsphotonics.9b00142 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
Figure 2. SEM of perovskite thin films. The films move from small crystallites (a−c, y = 0) to thin films (d−f, y = 0.30) with nanocrystalline grains with increased Mn doping. Scale bars are 1 μm.
Figure 3. (a) Cross-sectional SEM image of a perovskite LED. (b) Schematic representation of our device structure. (c) Energy level diagram of Mn doped perovskites. Perovskite energy levels are from UPS spectra and energy levels of the transport layers were adopted from literature (Figure S10).
amorphous structure with a large number of mesoscopic pores. We have also observed similar morphology changes on pure MAPbBr3 perovskites upon the addition of Mn (Figure S8), clearly indicating that the doping effect is universal, regardless of A and X composition. Typically, large crystallites are viewed as problematic for light emission, as the charge confinement is lost and the photoluminescence yield decreases. This was previously resolved by the introduction of crystal pinning.21 For these materials, however, we observed quite bright luminescence, despite the relatively large crystallites. To verify uniform coverage throughout the perovskite layer, we measured the
randomly distributed with large gaps between them, whereas after doping the layer is uniformly covered with a number of crystallites, an important development for efficient devices which require uniform, pinhole-free films. These trends in morphology after Mn doping are similar for all sky-blue, green, and red perovskites. The bromide perovskite forms a combination of rod shaped and spherical crystallites (see Figure S6 for a high-res SEM image), whereas the mixed halide red perovskites form a uniform layer covered with nanosized particles on top. Of particular interest, the sky-blue bromide/ chloride perovskites form an ultrasmooth uniform layer upon doping with Mn, whereas the control films show porous C
DOI: 10.1021/acsphotonics.9b00142 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
Figure 4. Mn-doped perovskite LEDs. (a) The electroluminescence spectrum of all devices presented in this work. (b, d, f) J−V−L curves of the green, red, and sky-blue perovskites, respectively. All colors show a reduction in leakage current and strong brightness increase with Mn addition. (c, e, g) EQE curves of the green, red, and sky-blue perovskites, respectively. Mn addition increases the maximum efficiency by 3×, 40×, and 180× for green, red, and sky-blue devices, respectively. The labels “x” indicates the halide composition and “y” indicates the concentration of Mn2+ in precursors. The composition is PEA0.2Cs0.4MA0.6Pb(1−y)MnyBr3, PEA0.2Cs0.4MA0.6Pb(1−y)MnyBr0.9I2.1, and PEA0.2Cs0.4MA0.6Pb(1−y)Mny(BrCl)3 for green, red, and sky-blue perovskites, respectively.
architecture is shown in Figure 3b. We followed our standard fabrication procedure, which we repeat briefly here. More details can be found in the Methods and Materials. First, ITO on glass is cleaned via solvent cleaning and oxygen plasma. PEDOT:PSS and perfluorinated ionomer (PFI, used for green only) is then spun-cast. The perovskite layer is spun, followed by transfer to a thermal evaporator, where 40 nm TPBi, 1 nm LiF, and 60 nm aluminum are then deposited. The devices are then packaged with UV-cured epoxy and a glass coverslip. The corresponding perovskite energy levels are derived from UPS spectra (Figure S10) and shown in Figure 3c. From these
SEM of the cross-section using a lamella prepared by focused ion beam (FIB). The cross-sectional SEM image of a Mndoped perovskite LED is shown in Figure 3a. The perovskite crystallites are densely packed without any pin holes across the thin layer from the HTL/perovskite interface to the ETL/ perovskite interface. Such dense packing of crystallites indicates that the growth is uniform throughout the perovskite layer after doping with Mn. Thus, the addition of Mn to these bulk films promotes uniform and crystalline grains, along with the improved optical properties. We then turned to LED fabrication to see if these improvements could translate to devices. The device D
DOI: 10.1021/acsphotonics.9b00142 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
Figure 5. Stability of perovskite LEDs. (a) For X = Br3, the addition of Mn greatly increases the lifetime, from an L50 of 98%), PbBr2 (Sigma, >99.99%), MnBr2 (Sigma, >98%), PEABr (phenyethylammonium bromide, Dyesol >98%)), PbI2 (Sigma, >99.99%), MAI (Sigma, >98%), and CsPbI3 were dissolved in DMF. A total of 0.3 M CsPbBr3 was dissolved in DMSO by constantly stirring for 2 h. These are mixed in appropriate ratios to maintain the final composition of PEA0.2Cs0.4MA0.6Pb(1−y)MnyBr3 and PEA0.2Cs0.4MA0.6Pb(1−y)MnyBr0.9I2.1 for green and red, respectively, where y = 0, 0.15, and 0.3. The precursor solutions were filtered with 0.2 μm PTFE syringe filter before using. The thickness of perovskite layers are 103 ± 17 and 91 ± 7 nm, respectively, for undoped and 30% Mn-doped bromide perovskites. For the sky-blue devices, 0.3 M (MACl2 + PbCl2), 0.3 M (MACl2 + PbBr2), and 0.3 M (MACl2 + MnBr2) were dissolved in 1:1 DMF/DMSO and used as precursors. These are mixed in proper ratios to obtain final composition of PEA0.2Cs0.4MA0.6Pb(1−y)Mny(BrCl)3, where y = 0, 0.15, or 0.3. Higher concentration >45% of MnBr2 in the precursors were insoluble at device relevant concentrations. Device Fabrication. All ITO substrates were cleaned via sonication in detergent, water, and acetone, before submersion in boiling IPA. The glass was treated with O2 plasma at 200 W using 0.5 Torr O2 gas. On these cleaned substrates, a thin layer of PEDOT:PSS (Clevios PVP AI 4083, filtered using 0.2 μm PVDF filter) was spin-coated at 4000 rpm with a ramp of 2000 rpm/sec for 45 s. Then, the films were annealed at 145 °C for 30 min in a nitrogen glovebox. On these PEDOT layers, a thin layer of PFI (Sigma, Nafion perfluorinated resin solution in 5 wt % in lower aliphatic alcohols and water) was coated using 15 μL/mL PFI stock solution in isopropanol, for the green devices only. The films were annealed at 145 °C for 30 min. After cooling, perovskite precursors were spun at 1000 rpm for 10 s and ramped up to 2800 rpm for 45 s. After ∼20 s, a 90 μL of chloroform was dripped on the spinning perovskite layer. The dripping time for control films were maintained exactly at 20 s to achieve the best quality. For the iodide perovskites, no PFI layer was used, and the 30% Mn doping required a mild heating at 40 °C for 5 min to achieve complete formation of the perovskite, whereas the control red films did not require any annealing. All these thin films were taken into a deposition chamber to deposit 40 nm TPBi, 1 nm LiF, and 60 nm Al layers, respectively. Device Measurements. All devices were packaged before measurement. Electroluminescence spectra were taken with an Ocean Optics QE Pro and 500 uA sourced to the device from a Keithley 2400. J−V−L curves were taken using an HP4145A and a calibrated Thorlabs photodiode physically held just above the face of the device. The device (1 mm radius) is much smaller than the detector, which is smaller than the glass substrate. When combined with the black material construction of the holder, this prevents the collection of waveguided light and overestimation of the EQE.42 Stability curves were determined in the same setup by sourcing a constant current and measuring the electroluminescence over time. Materials Characterization. XRD measurements were performed on Bruker D2Phase diffractometer using a Cu Kα
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.9b00142. Absorption spectra, time-resolved PL, high resolution SEM, and UPS spectra of Mn-doped green perovskites, XRD of sky-blue and red perovskites, EDAX elemental mapping on SEM images, J−V−L, EQE, and stability data of MAPbBr3 perovskites (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Daniel N. Congreve: 0000-0002-2914-3561 Author Contributions
M.K.G. fabricated and measured all devices, S.N.S. contributed to scientific discussions, D.N.C. and M.K.G. performed materials characterization, and D.N.C. oversaw the research. All authors wrote the manuscript. Notes
The authors declare the following competing financial interest(s): Harvard University has filed a patent partially based on this work.
■
ACKNOWLEDGMENTS The authors acknowledge the support of the Rowland Fellowship at the Rowland Institute at Harvard University. This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF F
DOI: 10.1021/acsphotonics.9b00142 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
(23) Zhao, B.; et al. High-efficiency perovskite−polymer bulk heterostructure light-emitting diodes. Nat. Photonics 2018, 12, 783− 789. (24) Cao, Y.; et al. Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures. Nature 2018, 562, 249−253. (25) Lin, K.; et al. Perovskite light-emitting diodes with external quantum efficiency exceeding 20%. Nature 2018, 562, 245−248. (26) Himchan, C.; Young-Hoon, K.; Christoph, W.; Hyeon-Dong, L.; Tae-Woo, L. Improving the Stability of Metal Halide Perovskite Materials and Light-Emitting Diodes. Adv. Mater. 2018, 30, 1704587. (27) Vashishtha, P.; Halpert, J. E. Field-Driven Ion Migration and Color Instability in Red-Emitting Mixed Halide Perovskite Nanocrystal Light-Emitting Diodes. Chem. Mater. 2017, 29, 5965−5973. (28) Yao, E. P.; et al. High-Brightness Blue and White LEDs based on Inorganic Perovskite Nanocrystals and their Composites. Adv. Mater. 2017, 29, 1606859. (29) Kim, H. P.; et al. High-Efficiency, Blue, Green, and NearInfrared Light-Emitting Diodes Based on Triple Cation Perovskite. Adv. Opt. Mater. 2017, 5, 1600920. (30) Kumawat, N. K.; et al. Band Gap Tuning of CH3NH3Pb(Br1− xClx)3 Hybrid Perovskite for Blue Electroluminescence. ACS Appl. Mater. Interfaces 2015, 7, 13119−13124. (31) Sadhanala, A.; et al. Blue-Green Color Tunable Solution Processable Organolead Chloride−Bromide Mixed Halide Perovskites for Optoelectronic Applications. Nano Lett. 2015, 15, 6095−6101. (32) Zou, S.; et al. Stabilizing Cesium Lead Halide Perovskite Lattice through Mn(II) Substitution for Air-Stable Light-Emitting Diodes. J. Am. Chem. Soc. 2017, 139, 11443−11450. (33) Arunkumar, P.; et al. Colloidal Organolead Halide Perovskite with a High Mn Solubility Limit: A Step Toward Pb-Free Luminescent Quantum Dots. J. Phys. Chem. Lett. 2017, 8, 4161−4166. (34) Mir, W. J.; Jagadeeswararao, M.; Das, S.; Nag, A. Colloidal MnDoped Cesium Lead Halide Perovskite Nanoplatelets. ACS Energy Lett. 2017, 2, 537−543. (35) Liu, W.; et al. Mn2+-Doped Lead Halide Perovskite Nanocrystals with Dual-Color Emission Controlled by Halide Content. J. Am. Chem. Soc. 2016, 138, 14954−14961. (36) Parobek, D.; et al. Exciton-to-Dopant Energy Transfer in MnDoped Cesium Lead Halide Perovskite Nanocrystals. Nano Lett. 2016, 16, 7376−7380. (37) Jia, L.; et al. Enhancing Optical, Electronic, Crystalline, and Morphological Properties of Cesium Lead Halide by Mn Substitution for High-Stability All-Inorganic Perovskite Solar Cells with Carbon Electrodes. Adv. Energy Mater. 2018, 8, 1800504. (38) Hou, S.; Gangishetty, M. K.; Quan, Q.; Congreve, D. N. Efficient Blue and White Perovskite Light-Emitting Diodes via Manganese Doping. Joule 2018, 2, 2421. (39) Wang, K.-H.; Li, L.-C.; Shellaiah, M.; Wen Sun, K. Structural and Photophysical Properties of Methylammonium Lead Tribromide (MAPbBr3) Single Crystals. Sci. Rep. 2017, 7, 13643. (40) Barker, A. J.; et al. Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films. ACS Energy Lett. 2017, 2, 1416−1424. (41) Saidaminov, M. I.; et al. Suppression of atomic vacancies via incorporation of isovalent small ions to increase the stability of halide perovskite solar cells in ambient air. Nat. Energy 2018, 3, 648. (42) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E. Measuring the Efficiency of Organic Light-Emitting Devices. Adv. Mater. 2003, 15, 1043−1048. (43) de Mello, J.; Wittmann, H.; Friend, R. An improved experimental determination of external photoluminescence quantum efficiency. Adv. Mater. 1997, 9, 230−232.
Award No. 1541959. M.K.G. and D.N.C. are thankful to Greg Lin and Stephan Kraemer for their support in collecting UPS and cross-sectional SEM data. D.N.C. thanks E.M.C. and E.M.C. for their constant love and support. D.N.C. welcomes E.M.C. to the world.
■
REFERENCES
(1) Stranks, S. D.; et al. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341−344. (2) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395−398. (3) Tan, Z.-K.; et al. Bright light-emitting diodes based on organometal halide perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (4) Gangishetty, M. K.; Hou, S.; Quan, Q.; Congreve, D. N. Reducing Architecture Limitations for Efficient Blue Perovskite LightEmitting Diodes. Adv. Mater. 2018, 30, 1706226. (5) Kim, Y.-H.; et al. Highly Efficient Light-Emitting Diodes of Colloidal Metal−Halide Perovskite Nanocrystals beyond Quantum Size. ACS Nano 2017, 11, 6586−6593. (6) Jianhai, L.; 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. (7) Li, G.; et al. Highly Efficient Perovskite Nanocrystal LightEmitting Diodes Enabled by a Universal Crosslinking Method. Adv. Mater. 2016, 28, 3528−3534. (8) Yan, F.; et al. Highly Efficient Visible Colloidal Lead-Halide Perovskite Nanocrystal Light-Emitting Diodes. Nano Lett. 2018, 18, 3157−3164. (9) Quan, L. N.; et al. Tailoring the Energy Landscape in Quasi-2D Halide Perovskites Enables Efficient Green-Light Emission. Nano Lett. 2017, 17, 3701−3709. (10) Byun, J.; et al. Efficient Visible Quasi-2D Perovskite LightEmitting Diodes. Adv. Mater. 2016, 28, 7515−7520. (11) Yang, X.; et al. Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation. Nat. Commun. 2018, 9, 570. (12) Congreve, D. N.; et al. Tunable Light-Emitting Diodes Utilizing Quantum-Confined Layered Perovskite Emitters. ACS Photonics 2017, 4, 476−481. (13) Yuan, M.; et al. Perovskite energy funnels for efficient lightemitting diodes. Nat. Nanotechnol. 2016, 11, 1−27. (14) Cho, H.; et al. High-Efficiency Polycrystalline Perovskite LightEmitting Diodes Based on Mixed Cations. ACS Nano 2018, 12, 2883−2892. (15) Yu, J. C.; Kim, D. B.; Jung, E. D.; Lee, B. R.; Song, M. H. Highperformance perovskite light-emitting diodes via morphological control of perovskite films. Nanoscale 2016, 8, 7036−7042. (16) Yifei, S.; et al. A Strategy for Architecture Design of Crystalline Perovskite Light-Emitting Diodes with High Performance. Adv. Mater. 2018, 30, 1800251. (17) Xiao, Z.; et al. Efficient perovskite light-emitting diodes featuring nanometre-sized crystallites. Nat. Photonics 2017, 11, 108− 115. (18) Xiao, Z.; et al. Mixed-Halide Perovskites with Stabilized Bandgaps. Nano Lett. 2017, 17, 6863−6869. (19) Yu, T.; et al. Highly Efficient Spectrally Stable Red Perovskite Light-Emitting Diodes. Adv. Mater. 2018, 30, 1707093. (20) Chiba, T.; et al. Anion-exchange red perovskite quantum dots with ammonium iodine salts for highly efficient light-emitting devices. Nat. Photonics 2018, 12, 681−687. (21) Cho, H.; et al. Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes. Science 2015, 350, 1222−1225. (22) Zhang, L.; et al. Ultra-bright and highly efficient inorganic based perovskite light-emitting diodes. Nat. Commun. 2017, 8, 15640. G
DOI: 10.1021/acsphotonics.9b00142 ACS Photonics XXXX, XXX, XXX−XXX
