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Opportunities and Challenges in Perovskite Light-Emitting Devices Xiaofei Zhao,† Jun De Andrew Ng,†,‡ Richard H. Friend,§ and Zhi-Kuang Tan*,†,‡ †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, 117574, Singapore § Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
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‡
ABSTRACT: Lead halide perovskite semiconductors first gained prominence in photovoltaic technology, where they demonstrated impressive photon-to-electron conversion efficiencies in solution-processed devices. The benefits of perovskites are, however, more vividly expressed in light-emitting applications. This new class of halide semiconductors displays intense fluorescence, and possesses narrow emission line-width that could be easily tuned across the entire visible spectrum. These qualities allow excellent color reproduction that matches or exceeds the best commercial display devices. Here, we provide a brief review on the history and the new developments in perovskite light-emission and highlight some of the opportunities and challenges in this fledging technology. In particular, we study the advances in light-emitting diodes and photoluminescent devices and discuss the device lifespan, ion migration and spectral instability issues that would need to be circumvented in order for perovskites to be successful in next-generation display technologies. KEYWORDS: perovskite, light-emitting devices, color display, device physics, optoelectronics, nanocrystal
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Recently, lead halide perovskites has emerged as a highly promising class of semiconductor for optoelectronic device applications. They were first shown to be useful as light absorbers in dye-sensitized solar cells6−8 but were later proved to be effective standalone semiconductors in solar cells9 and light-emitting diodes (LED).10−12 The many advantages of perovskites include their low-cost, facile synthesis, solutionprocessability, direct bandgap, ease of bandgap tuning, and high luminescence quantum yield that are comparable to traditional high-temperature, high-vacuum processed III−V semiconductors. In particular, the narrow line-width emission from lead halide perovskites enables their devices to display rich and saturated colors across the entire visible spectrum.10,11 Although there may be concerns over the presence of lead in perovskite semiconductors, the quantities that are used in lightemitting devices are typically small (L90) 14 h (L50)
14 12 21 27 28 29 43 30 30 32 44 45
38 40 0.7 h (L50)
41 39
and 2D perovskites. Conventional 3D perovskites possess good carrier transport properties, but their radiative recombination under low-level injection is suppressed by defective traps, resulting in low luminescence yield.22,23 2D layered-perovskites that contain interdigitated layers of lead halide and long organic cations possess strong exciton binding energy and tend to be more stable against moisture due to its significant organic content. However, they exhibit poor charge transport orthogonal to the layers, and were previously unsuccessful in room-temperature electroluminescence. Quasi-2D perovskites contain slabs of few unit-cell thick 3D perovskite layers that are separated by large organic cations (see Figure 2a). Their electronic bandgap varies with the number of 3D layers in each slab, and they generally possess both enhanced stability and improved optoelectronic performance.24−26 The group of Sargent et al. achieved efficient NIR perovskite LEDs with an EQE of 8.8% (Figure 2a),27 where large PEA cation was implemented into their perovskite layer to give quasi-2D layered structure. A range of quantum-confined quasi-2D layer thicknesses were achieved, but these structures showed rapid energy funneling toward smaller bandgap grains
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RECENT ADVANCES IN QUASI-TWO-DIMENSIONAL PEROVSKITE LIGHT-EMITTING DEVICES Important advances in PeLED performance were recently achieved using quasi-two-dimensional (2D) perovskites. Quasi-2D perovskites combine the advantages of bulk-3D C
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Figure 2. (a) Different n values of quasi-2D perovskite and the carrier transfer process. Reproduced with permission from ref 27. Copyright 2016 Nature Publishing Group. (b) The schematic diagram of nanoplates. Reproduced with permission from ref 31. Copyright 2017 American Chemical Society. (c) Photoluminescence image of different n value quasi-2D perovskite films and the PLQY of films treated and untreated by TOPO. Reproduced with permission from ref 32. Copyright 2018 Nature Publishing Group.
Figure 3. (a) EL spectra (straight line) and PL spectra (dashed line) of inorganic perovskite quantum dots. Reproduced with permission from ref 34. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. (b) Cross-linked mechanism of perovskite quantum dots by TMA treatment. Reproduced with permission from ref 11. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA. (c) Ligand exchange by replacing with DDAB in perovskite quantum dots. Reproduced with permission from ref 35. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.
al. reported the use of BA halides to achieve nanometer-sized grains and low-dimensional layered perovskite structure.30 The resulting exciton confinement led to NIR and green-emitting LEDs with EQE of 10.4% and 9.3%, respectively. This series of reports have consistently demonstrated that the implementation of a small amount of long-chain organic ammonium cations and the formation of quasi-2D perovskites are very helpful toward exciton confinement, and this represents a promising new strategy toward improving the efficiency and stability in perovskite LEDs.
and exhibited efficient radiative recombination under low excitation. Green-emitting LEDs with an EQE of 7.4% were also demonstrated using the same concept, using PEA2MAn−1PbnBr3n+1 as the perovskite emitter.28 At around the same time, the group of Wang et al. reported quasi-2D perovskite LEDs with a NIR emission EQE of 11.7%.29 The group employed NMA as large cationic additives to achieve quasi-2D layered structures, and attributed the enhanced LED performance to efficient exciton confinement and radiative recombination. In another related work, the group of Rand et D
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Figure 4. (a) Microencapsulation method for perovskite-polymer phosphor film. Reproduced with permission from ref 49. Copyright 2016 WileyVCH Verlag GmbH & Co. KGaA. (b) TEM images of perovskite−polystyrene nanocomposite and its application in a white light-emitting photoluminescent device. Reproduced with permission from ref 15. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.
crystals. Sargent et al. demonstrated a two-step ligand exchange process,35 where they replaced the more common long chain oleyl ligands with short chain DDAB (Figure 3c) for improved charge transport. The blue LEDs fabricated through this protocol produced high EQE of 1.9%. The same group also developed a new approach to synthesize perovskite nanocrystals with cross-linkable and polymerizable ligands.36 They employed 4-vinyl-benzyl-dimethyloctadecylammonium chloride, which contains a styryl group and could be cross-linked via heating with a radical initiator. This method allows the next layer to be deposited by spin-coating without damaging the perovskite nanocrystal layer. The LEDs with a structure of ITO/TiO2/Al2O3/cross-linked perovskite nanocrystals/F8/ MoO3/Ag achieved a luminance of over 7000 cd m−2. In a series of subsequent work, the EQE of red-emitting perovskite nanocrystal LEDs was improved to 6.3% by Yu’s group using a polyethylenimine post-treatment method.37 Zeng et al. applied a hexane/ethyl acetate solvent treatment to control ligand density and achieved an EQE of 6.27% for the green LEDs.38 The group also achieved a higher EQE of 11.6% through a room-temperature synthesized nanocrystal that employs a combination of three ligand types.39 Kido et al. reported an alternative solvent treatment method using butyl acetate and achieved an EQE of 8.73%.40 More recently, Rogach et al. reported the passivation of CsPbI3 nanocrystals through silver doping, and achieved a visible red electroluminescence of 690 nm at a respectable EQE of 11.2%.41 Another recent work by Demir et al. employs the organic− inorganic MAPbBr3 nanocrystals in their green LEDs, and attained a high EQE of 12.9% at luminance levels above 1000 cd m−2 through the optimization of the electron-injection layer.42 The advances in this domain are still primarily attained through the enhancement of nanocrystal luminescence and the improvement of charge injection into the ligand-encapsulated crystals. Future progress in perovskite nanocrystal LEDs may come through the development of new device structures via all-solution processing to fully realize the advantage of a colloidal nanocrystal system.
The success of the quasi-2D approach was further demonstrated by Jin et al., where PBA cation was employed to achieve EQE of 7.3% and 10.4% in red and green-emitting perovskite LEDs, respectively (Figure 2b).31 More recently, the EQE of quasi-2D green perovskite LEDs is enhanced to 14.36% (Figure 2c) through the implementation of TOPO, which helps in passivating the surfaces of quasi-2D perovskites.32
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RECENT ADVANCES IN PEROVSKITE NANOCRYSTAL LIGHT-EMITTING DEVICES Perovskite nanocrystal (CsPbX3) synthesis was demonstrated by Kovalenko and co-workers using a hot-injection approach33 and sets the foundation for the development of perovskite nanocrystal light-emitting devices. These nanocrystals exhibit high PLQY (up to 90%), narrow line widths of 12−42 nm, and tunable emission across the entire visible spectrum (410−700 nm) by both halide composition variation and quantum size effect. Earlier work on perovskite nanocrystal LEDs by Zeng et al. relied on a ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al device structure and demonstrated a modest luminance and EQE of 946 cd m−2 and 0.12% in their green-emitting device (Figure 3a).34 A common problem encountered in the fabrication of nanocrystal or quantum dot devices is the need for a vacuum-evaporated organic charge transport layer (e.g., TPBi), which negates the advantage of solutionprocessability in nanocrystals. In another report, Tan et al. employed a TMA vapor-based cross-linking method to treat the CsPbX3 nanocrystal films (Figure 3b), thereby allowing the above organic layer to be solution-deposited.11 This new approach renders the nanocrystal films insoluble toward further solution processing, and additionally passivates the nanocrystal surface defects, leading to improved PL and EL performance. The perovskite nanocrystal LEDs were fabricated in the structure of ITO/ZnO/CsPbX3/TFB/MoO3/Ag, and showed vivid emission across the entire visible spectrum. A high luminance and EQE of 206 cd m−2 and 5.7% were achieved in the red-emitting device. The choice of ligands is important for the attainment of both high PLQY and good charge transport in perovskite nanoE
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Figure 5. (a) Cross-sectional SEM images of the device with MAPbBr3 film treated at 60 °C. Reproduced with permission from ref 52. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA. (b) The normalized EL intensity of perovskite LEDs the different device structure. Reproduced with permission from ref 47. Copyright 2017 American Chemical Society. (c) Radiance of quasi-2D device under different operation time. Reproduced with permission from ref 46. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.
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PEROVSKITES AS PHOTOLUMINESCENT PHOSPHORS Perovskites’ high photoluminescence quantum yield and narrow spectral line width make them excellent candidates as color-enhancement phosphors for displays. The material is also significantly more stable under photo excitation compared to electrical excitation. Lead halide perovskites meet the wide color space standard by the International Telecommunication Union (ITU), also known as Rec. 2020, for ultrahigh definition (UHD) television. A typical method for the fabrication of perovskite phosphors involve embedding the perovskite nanocrystals into a polymer matrix.15,48−51 Such a polymer encapsulation method provides the perovskite nanocrystals with enhanced stability against moisture, air, light, and elevated temperature. It is likely that any near-term application of perovskite in display devices will rely on such a photoluminescent approach due to the greater ease of implementation and the robustness against degradation or burn-in issues that are commonly reported for electroluminescent displays. Several implementation methods for photoluminescent devices have been reported. The group of Zhong et al. obtained their photoluminescent films by casting a precursor solution of perovskites and PVDF in DMF, which forms luminescent nanocrystals in a PVDF matrix upon drying.48 Dong et al. employed a polymer swelling/deswelling strategy where they deposited a perovskite precursor solution in DMF onto sheets of polymer substrate, causing the polymer to swell and assimilate the solution, and subsequently form perovskite crystals upon drying and deswelling (Figure 4a). Their perovskite samples in polystyrene and polycarbonate possessed superior stability and were able to survive stress tests in boiling water.49 More recently, Tan et al. reported a photopolymerization approach to obtain a nanocomposite of singly dispersed perovskite nanocrystals in a polymer matrix.15 They dispersed presynthesized perovskite nanocrystals in styrene or methyl methacrylate monomers and illuminated the mixture with white light (Figure 4b). This resulted in the photoactivation of the perovskites, which were able to directly initiate a freeradical polymerization of the vinyl monomers and produce a luminescent nanocomposite. The spatial dispersion of their nanocrystals in the polymer matrix prevented fluorescence quenching by energy transfer and gave rise to high photoluminescence quantum yield in a solid film.
RESEARCH CHALLENGES i. Device Stability and Lifespan. The reported efficiencies of perovskite LEDs have improved significantly over the years, but device stability and lifespan remain poor in comparison to OLEDs or QD-LEDs. Device stability issues is probably the most significant problem that will hinder the adoption of this technology, and should probably receive greater attention from the research community. The group of Song investigated the effect of thermal annealing on the stability of perovskite LEDs and reported that the devices show improved stability when annealed at 60 °C for 2 h (Figure 5a), although luminance (∼400 cd m−2) still decreased by 75% under continuous DC bias operation at 3.4 V.52 The improved stability is attributed to the high crystalline and continuous morphology without vertical cracks. In other work by the same group, polyethylenimine, ethylenediamine, and phenylmethylamine were employed to passivate defect sites in MAPbBr3, suppress PL blinking and improve device stability.44,53 The perovskite LEDs treated by amine retained 70% of the initial luminance after 3.9 h (14000 s) at driving current density of 20 mA cm−2. The authors attributed the stability to the amine’s ability to prevent electrode corrosion, caused by the ion migration from the perovskite layer to the metal electrode. In a report by Shan et al., the authors employed n-MgZnO and p-MgNiO as the electron and hole injection layers to fabricate all-inorganic perovskite nanocrystal LEDs with improved lifespan, using a Au/p-MgNiO/CsPbBr3QDs/ PMMA/n-MgZnO/n+-GaN device configuration.47 The unencapsulated device under ambient air condition retained 80% of its initial luminance after 10 h when a constant bias of 10 V was applied (Figure 5b). Layered perovskite has been demonstrated to provide good stability against light, humidity and heat in perovskite solar cells,24−26 and has inspired several work in light-emitting devices. In the report by Wang et al., which employs 1naphthalenemethylamine in the layered perovskite, their device EQE decreased to 50% of the initial value after 2 h under a constant current density of 10 mA cm−2. This is respectable compared to their 3D perovskite LEDs which last about a minute under similar test conditions.29 In another work by the group, they reported an improvement in the stability of redemitting perovskite LEDs through the incorporation of Cs. In this case, the device EQE decreased to 50% of the initial value F
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Figure 6. (a) Performance of perovskite device measured from different sweep direction. Reproduced with permission from ref 10. Copyright 2014 Nature Publishing Group. (b) Morphology of device before and after operation under electrical field and the EQE of device under several voltage scans. Reproduced with permission from ref 58. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA.
Figure 7. (a) PL spectra of MAPb(Br0.6I0.4)3 film after several cycles of illumination follows by 5 min in the dark. Reproduced with permission from ref 60. Copyright 2015 The Royal Society of Chemistry. (b) Evolution of EL spectra over time during LED operation and after resting. Reproduced with permission from ref 11. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.
LEDs, though it is less reported in the light-emission literature.10,30 Hysteresis problems were in fact already discovered in the initial work by Tan et al., where they found that a higher radiance and EQE (Figure 6a) could be obtained when sweeping the device from forward bias to reverse bias than in the reverse direction.10 They attributed this observation to the driving out of the ionic traps under high polarization voltage, which suppresses the nonradiative trapmediated recombination. The same phenomenon was observed by Rand et al., but the hysteresis was successfully reduced after they incorporated BA cations into the 3D perovskites to form layered perovskite.30 In another work, the group studied the influence of electrical stress on the performance of perovskite LEDs and found that the EQE improved after several sweeps, but longer electrical stress eventually led to degradation (Figure 6b).58 Yu et al. showed that low temperature is effective in impeding ion migration and could improve device operational lifespan, but this may not be a practical solution for real-world applications.59 iii. Spectral Instability and Microphase Separation in Mixed-Halide Perovskite. A significant advantage that perovskite semiconductors offer is the ease of bandgap tuning via the control of halide composition (I, Br, Cl) in the material. This advantage is, however, negated when the emission wavelength of mixed-halide perovskite changes upon prolonged photo or electrical excitation. The spectral instability problem was earlier reported by the group of McGehee et al. when they investigated mixed bromide−iodide perovskite for photovoltaics. They found that a 1-sun illumination of the perovskite led to a reversible red-shift in the photoluminescence spectrum and the appearance of a sub-bandgap absorption (Figure 7a), which they have attributed to the
after 5 h under the same operational condition, although they attributed this improvement to the inherent stability of Csbased perovskite films rather than the layered structure.43 Layered perovskites offer an additional advantage of impeding ion motion, which plays a significant role in the stability of perovskite LEDs. Rand et al. reported that the incorporation of butylammonium can hinder ion migration, hence leading to improved stability in perovskite LEDs.30 The iodide-perovskite LED retained 75% of its initial EQE after 5 h (300 min) at a current density of 3 mA cm−2. For the bromide counterpart, the device retained 50% of the initial EQE after 0.8 h (50 min) under the same condition. Both layered perovskite devices possess improved stability over their control devices. In another work by the group, FPMAI additive was shown to be effective in stabilizing the devices, where minimal degradation was observed in the EQE after 10 h at 3 mA cm−2.45 More recently, a report by Mohite et al. suggested that phase purity is important for the operational stability of layered perovskite LEDs.46 Their phase-pure devices could sustain 14 h under continuous operation at 2 V (Figure 5c), but their mixed 2D/3D perovskite LEDs degrade in 1 h and their 3D perovskite LEDs degrade in less than 10 min under the same driving voltage. They attributed the improved device stability to the intrinsic stability of the phase-pure layered perovskite films. ii. Ion Migration Effects in Perovskites. Much experimental and theoretical evidence has confirmed that ion migration exists in halide perovskite materials, in particular under photoexcitation or in an electrical field.54,55 Such ionic movements has been held responsible for the commonly observed current−voltage hysteresis in perovskite solar cells.56,57 This problem exist to a certain extent in perovskite G
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remained elusive. This is likely attributed to the inherently low luminescence quantum yield of larger-bandgap perovskites and the relative difficulty in injecting electrons and holes into the more energetically extreme conduction and valence band levels. A recent paper by Congreve et al. reported a 469 nm device at an EQE of 0.5%, though the device degraded to 10% of its original luminance in less than 10 s of operation.66 A combination of materials development and device engineering would therefore be required for the realization of efficient and reliable blue devices at wavelengths below 470 nm. Finally, fundamental research and engineering solutions are required to address the above-described ion migration and phase separation issues, which are likely the two largest contributors to device instability. Novel material designs, such as nanostructuring or ion-fixing precursors, and new device architectures, such as deployment of ion-blocking interlayers, are possible avenues toward solving these problems. We envision that the future implementation of perovskite LEDs may involve the combination of perovskites and organic semiconductors to exploit their narrow emission width and low ionic conductivity properties, respectively, and consequently achieve devices with superior color reproduction and useful operational lifespan.
phase separation of the perovskite into I-rich and Br-rich domains through halide migration.60 A similar spectral instability was observed by Tan et al. in electrically driven mixed-halide perovskite nanocrystal LEDs (Figure 7b).11 Both the mixed I/Br and Br/Cl devices were shown to red-shift upon minutes of device operation, but the spectral shifts were reversible after longer resting periods. Photoluminescence studies confirm that the spectral shifts originate from the perovskite emissive layer and is not due to other changes in the device during LED operation. The fact that the spectral shifts occurred in nanocrystalline systems raises the question whether the halide migration or phase separation effects occurred within the small confines of the nanocrystal or across larger multicrystal length-scales. Halpert et al. observed the same spectral shifts in their perovskite nanocrystal LEDs, and suggested this process to be electricfield-driven.61 However, the fact that spectral shifts could also occur through photoexcitation suggest that the halide segregation is likely to be driven by excitation rather than an applied field. Rand et al. later showed that the implementation of n-butylammonium cations was helpful toward reducing spectral shifts for several current−voltage sweep cycles.62 A collection of theoretical and experimental work were conducted by various groups to elucidate the origins of the spectral shifts. Ginsberg et al. proposed that photoexcited charges induce a localized strain in the soft perovskite ionic lattice, which promotes halide phase separation and the nucleation of an 8 nm iodide rich cluster.63 Friend et al. showed experimentally that the halide segregation was dependent on the carrier generation profile across the thickness of the film and suggested that the segregation was caused by a defect-assisted migration of more mobile halide (bromide) ions away from the film surface, leading to an iodide-rich surface and the spectral shift. The group also showed that a reduction in halide vacancy defects, through excess halide precursors, could effectively suppress halide segregation and spectral shift.64 Kuno et al. proposed, through thermodynamic considerations, that the phase segregation is driven by the bandgap reduction from the formation of iodiderich domains.65
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CONCLUDING REMARKS
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AUTHOR INFORMATION
The progress in perovskite light emission has been remarkably fast, much of it guided by decades of accumulated knowledge in organic semiconductors and colloidal quantum dots. The strong, narrow-width emission of halide perovskites across the visible spectrum and their ability to be solution-processed allow them to stand out among existing emissive materials and make them particularly attractive for next-generation color display applications. The technology is young and fledging and promises significant rewards in a growing display industry that is constantly seeking improvements and innovations. Research efforts in perovskite light-emitting devices, moving forward, would have to resolutely address device lifespan issues in order to stay technologically relevant and progress into mainstream commercial displays.
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RESEARCH OPPORTUNITIES Perovskite light-emitting device is a young, emerging field and there are numerous opportunities for improvements and breakthroughs. Past advances on perovskite LED has primarily been attributed to the improvements in material luminescence or stability. Current literature shows a deficiency in the investigation and engineering of charge-balance, that is, balanced injection, transport, and recombination of electrons and holes, in perovskite LEDs. More research into this area is likely to play a critical role in the development of efficient and stable devices. We speculate that the accumulation of electrons or holes due to imbalanced injection and transport could lead to premature degradation of the redox-active perovskite semiconductors and could be a major contributing factor to poor device lifespan. Another area that deserves attention is the development of blue-emitting perovskite devices, which is an essential component of any trichromatic display device. Performance in blue devices have, so far, been modest. Pan et al. has reported a sky-blue emitting device at 490 nm with an electroluminescence efficiency of 1.9%.35 Shorter-wavelength deep-blue devices with respectable performance have, so far,
Corresponding Author
*E-mail:
[email protected]. ORCID
Richard H. Friend: 0000-0001-6565-6308 Zhi-Kuang Tan: 0000-0003-1399-1790 Author Contributions
X.Z., J.D.A.N., and Z.K.T. jointly wrote the perspective. Z.K.T and R.H.F. guided the work. Funding
The authors acknowledge funding support from the National University of Singapore and the Ministry of Education of Singapore (R-143-000-639-133, R-143-000-674-114, R-143000-691-114, R-143-000-A10-133). Notes
The authors declare no competing financial interest.
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REFERENCES
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