Opportunities and Challenges in Perovskite Light-Emitting Devices

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Opportunities and Challenges in Perovskite Light-Emitting Devices Xiaofei Zhao, Andrew Jun De Ng, Richard H. Friend, and Zhi Kuang Tan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00745 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Opportunities and Challenges in Perovskite LightEmitting Devices Xiaofei Zhao1, Andrew Jun De Ng1,2, Richard H. Friend3, Zhi-Kuang Tan1,2* 1

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore

2

Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, 117574, Singapore 3

Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK KEYWORDS: Perovskite; light-emitting devices; color display; device physics; optoelectronics; nanocrystal

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

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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.

After decades of research and development, organic light-emitting diodes (OLEDs)1-2 and semiconductor quantum dots (QDs)3-5 have emerged as the leading technologies in the consumer flat-panel displays industry. Electroluminescent OLEDs, which consist of individually-lit pixels, offer superior contrast, energy savings, wider viewing angles and thinner profile due to the absence of a backlight module. However, the synthesis of OLED emitters are typically complex and many require the incorporation of heavy metal complexes (Ir, Pt) for triplet harvesting in electro-phosphorescent devices. These, coupled with the expensive vacuum-based thermal evaporation of organics, contribute to high manufacturing costs. Quantum dots, on the other hand, possess narrow and tunable emission, which greatly improves color reproduction in both electroluminescent and photoluminescent devices. However, quantum dots require wellcontrolled synthesis under stringent conditions to achieve core-shell nanostructures and monodisperse size distribution for good color purity and high luminescence efficiency. The use of toxic elements in certain quantum dots, such as cadmium selenide, also brings about safety and environmental concerns when applied in consumer electronic products. 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 cells,6-8 but were later proved to be effective standalone semiconductors in solar cells9 and light-emitting diodes (LED).10-12 The many advantages of


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perovskites include their low-cost, facile synthesis, solution-processibility, direct bandgap, ease of bandgap tuning, and high luminescence quantum yield that are comparable to traditional hightemperature, 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 light-emitting devices are typically small (L90) 0.5

-

35

14 h

[46]

(L50)

MoO3/Ag


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CsPbBr3 nanocrystal

ITO/Modified PEDOT:PSS/PolyTPD/perovskite/TPBi/LiF/Al

512

8.73

-

-

-

[40]

Silver-doped CsPbI3 nanocrystal

Ag/ZnO/PEI/perovskite/TCTA/ MoO3/Au/MoO3

690

11.2

1106

-

0.7 h

[41]

FA-doped CsPbBr3 nanocrystal

ITO/PEDOT:PSS/PTAA/perovs kite/TPBi/LiF/Al

515

11.6

55800

-

-

[39]

CsPbBr3 nanocrystal

Au/pMgNiO/perovskite/PMMA/nMgZnO/n+-GaN

522

2.39

3809

-

10 h

[47]

MAPbBr3 nanocrystal

ITO/PEDOT:PSS/perovskite/B 3PYMPM:TPBi/B3PYMPM: Cs2CO3/Al

524

(L50)

(L80) 12.9

22830

-

0.1 h

[42]

(L50)

Perovskites as photoluminescent phosphors Perovskites’ high photoluminescence quantum yield and narrow spectral linewidth 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 ultra-high 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 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 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


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drying.48 Dong et al. employed a polymer swelling/de-swelling 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 photo-polymerization approach to obtain a nanocomposite of singly-dispersed perovskite nanocrystals in a polymer matrix.15 They dispersed pre-synthesized perovskite nanocrystals in styrene or methyl methacrylate monomers and illuminated the mixture with white light (Figure 4b). This resulted in the photo-activation of the perovskites, which were able to directly initiate a free-radical 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.


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Figure 4. (a) Micro-encapsulation method for perovskite-polymer phosphor film. Reproduced with permission from ref 49. Copyright 2016 Wiley-VCH 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. 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.


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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 1-naphthalenemethylamine 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 red-emitting perovskite LEDs through the incorporation of Cs. In this case, the device EQE decreased to 50% of the initial value 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


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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 minutes under the same driving voltage. They attributed the improved device stability to the intrinsic stability of the phase-pure layered perovskite films.

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) The

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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(ii) Ion migration effects in perovskites Much experimental and theoretical evidence has confirmed that ion migration exists in halide perovskite materials, in particular under photo-excitation 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 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 non-radiative trap-mediated 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


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Figure 6. (a) The performance of perovskite device measured from different sweep direction. Reproduced with permission from ref 10. Copyright 2014 Nature Publishing Group. (b) The 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.

(iii) Spectral instability and micro-phase 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 phase separation of the perovskite into I-rich and Br-rich domains through halide migration.60


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A similar spectral instability was observed by Tan et al. in electrically-driven mixedhalide 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 multi-crystal length-scales. Halpert et al. observed the same spectral shifts in their perovskite nanocrystal LEDs, and suggested this process to be electric-fielddriven.61 However, the fact that spectral shifts could also occur through photo-excitation 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 towards 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 photo-excited charges induce a localised 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 iodiderich 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


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spectral shift.64 Kuno et al. proposed, through thermodynamic considerations, that the phase segregation is driven by the bandgap reduction from the formation of iodide-rich domains.65

Figure 7. (a) PL spectra of MAPb(Br0.6I0.4)3 film after several cycles of illumination follows by 5 minutes in the dark. Reproduced with permission from ref 60. Copyright 2015 The Royal Society of Chemistry (b) The 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. 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, i.e. balanced injection, transport and recombination of electron 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


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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 has, so far, 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 seconds of operation.66 A combination of materials development and device engineering would therefore be required for the realisation 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 nano-structuring or ion-fixing precursors, and new device architectures, such as deployment of ion-blocking interlayers, are possible avenues towards 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.


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Concluding remarks 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.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions X.Z, A.J.D.N and Z.K.T. jointly wrote the perspective. Z.K.T and R.H.F. guided the work. Funding Sources 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-143-000-691114, R-143-000-A10-133). Notes The authors declare no competing financial interest.


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ABBREVIATIONS ITO: indium tin oxide PEDOT:PSS: poly(ethylene dioxythiophene):polystyrene sulfonate F8: poly(9,9-dicotylfluorene) TiO2: titanium dioxide MoO3: molybdenum oxide PEI: polyethyleneimine ZnO: zinc oxide TFB: poly(9,9-dioctyl-ﬂuorene-co-N-(4-butylphenyl)diphenylamine) PIP: polyimide precursor PVP: polyvinyl pyrrolidine MA: methylammonium FA: formamidinium PEA: phenylethylammonium NMA: 1-naphthalenemethylamine BA: n-butylammonium PBA: phenylbutylammonium TOPO: trioctylphosphine oxide


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PVK: poly(9-vinlycarbazole) TPBi: 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) LiF: lithium fluoride TMA: trimethylaluminum DDAB: di-dodecyl dimethyl ammonium bromide Al2O3: aluminum oxide SOCP: PEDOT:PSS mixed with PFI (perfluorinated ionomer) CBP: 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl Poly-TPD: Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine TCTA: Tris(4-carbazoyl-9-ylphenyl)amine PTAA: poly (bis(4-phenyl)(2,4,6-trimethylphenyl) amine) PMMA: Poly(methyl methacrylate) FPMAI: fluorophenylmethylammonium iodide PVDF: polyvinylidene fluoride PCBM: [6,6]-phenyl-C61-butyric acid methyl ester B3PYMPM: 4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine


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For Table of Contents Use Only Opportunities and Challenges in Perovskite Light-Emitting Devices Xiaofei Zhao, Andrew Jun De Ng, Richard H. Friend, Zhi-Kuang Tan

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.


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Opportunities and Challenges in Perovskite Light-Emitting Devices

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