Efficient light-emitting diodes based on oriented perovskite nanoplatelets
Jieyuan Cui, Yang Liu, Yunzhou Deng, Chen Lin, Zhishan Fang, Chensheng Xiang, Peng Bai, Kai Du, Xiaobing Zuo, Kaichuan Wen, Shaolong Gong, Haiping He, Zhizhen Ye, Yunan Gao, He Tian, Baodan Zhao, Jianpu Wang, Yizheng Jin
11 Efficient light-emitting diodes based on oriented perovskite nanoplatelets
Jieyuan Cui , Yang Liu , Yunzhou Deng , Chen Lin , Zhishan Fang , Chensheng Xiang , Peng Bai , Kai Du , Xiaobing Zuo , Kaichuan Wen , Shaolong Gong , Haiping He , Zhizhen Ye , Yunan Gao , He Tian , Baodan Zhao , Jianpu Wang and Yizheng Jin Centre for Chemistry of High-Performance & Novel Materials, State Key Laboratory of Silicon Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, China. Centre for Chemistry of High-Performance & Novel Materials, State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Centre of Electron Microscope, State Key Laboratory of Silicon Material, School of Material Science and Engineering, Zhejiang University, Hangzhou 310027, China. China State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China . X-Ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, US. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Centre for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China. Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan, 430072, China . Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
These authors contributed equally to this work. *Corresponding author email address: [email protected]
Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent (EL) devices ideal for large-area display and lighting applications . Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost photon outcoupling of planar LEDs . However, LEDs based on anisotropic perovskite nanoemitters remains to be inefficient (external quantum efficiency, EQE <5%) , due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of the assembled nanostructures. Here we demonstrate efficient EL from an in-situ grown continuous perovskite film comprising of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet films is ~84%, substantially higher than that of isotropic emitters (67%). The nanoplatelet film shows a high PLQY of ~75%. These merits enable LEDs with a peak EQE of 23.6%, representing the most efficient perovskite LEDs.
Photon emission characteristics in semiconductors are mediated by the TDMs. The optical TDMs of inorganic nanostructures with reduced dimensions, such as nanoplatelets and nanorods, are highly anisotropic . This unique structure-property relationship is of interest for planar LEDs because the outcoupling efficiency of the devices is fundamentally correlated to the orientation of emissive TDMs. In general, TDMs that are horizontally-oriented with respect to the electrode interface are favoured for light outcoupling while the vertically-oriented TDMs largely contribute to the energy loss (See Fig. S1 and Fig. S2 for detailed illustrations). Metal halide perovskite is an emerging class of solution-processed semiconductors with intriguing optoelectronic properties, such as high PLQY and tuneable emission wavelength . Since the first report of the room-temperature operating perovskite LEDs in 2014 , remarkable progress has been made on device efficiency . We note that these state-of-the-art planar perovskite LEDs are based on films with isotropic TDMs (Table S1, supplementary information). Enhancing the ratio of horizontally oriented TDMs is expected to further improve light outcoupling and boost the upper limit for the EQEs of perovskite LEDs. Here we report efficient LEDs based on in-situ grown perovskite films, which simultaneously demonstrate high ratios of horizontal TDMs and high PLQYs. Our devices consist of multilayers of nickel oxide (NiO, ∼
7 nm), poly(9,9-dioctylfluorene-co- N -(4-butylphenyl)-diphenylamine) /poly(9-vinlycarbazole) (TFB/PVK, ∼
38 nm), perovskite (~9 nm), 2,2′,2″-(1,3,5-benzinetriyl)tris(1-phenyl-1 H -benzimidazole) (TPBi, ∼
48 nm), lithium fluoride (LiF, ∼
1 nm) and aluminum (Al, ∼
100 nm) sequentially deposited onto indium tin oxide (ITO)-coated glass substrates (see Methods for details). The perovskite film was deposited from a precursor solution of lithium bromide (LiBr), phenylbutylammonium bromide (PBABr), phenylethyl-ammonium bromide (PEABr), caesium bromide (CsBr) and lead bromide (PbBr ) with a molar ratio of 0.25:0.75:0.25:1.75:1.4 dissolved in dimethyl sulfoxide (DMSO). Fig. 1a shows a cross-sectional view of our device analyzed by aberration-corrected scanning transmission electron microscopy (STEM). The High-angle annular dark-field (HAADF) image indicate a continuous and pinhole-free perovskite film with a thickness of 8.6 ± 1.5 nm (Fig. 1a and Fig. S3). Zoomed-in observations (Fig. 1b) produce an atomic resolution image with well-resolved atom columns, revealing high crystallinity of the perovskite nanocrystal. The crystal structure of the nanocrystal matches that of tetragonal β-CsPbBr . The perovskite crystal is oriented with {001} planes parallel to the substrate surface. Atomic force microscopy characterizations on a perovskite film show a low root-mean-square surface roughness of ~1.2 nm (Fig. S4), which is in line with the cross-sectional observations. The perovskite nanocrystals were transferred and dispersed onto a copper grid for high-resolution transmission electron microscopy (HRTEM) analyses . The results (Fig. 1c and S5) indicate that the perovskite crystals are nanoplatelets with lateral sizes of 25.8 ± 6.8 nm (Fig. 1d). Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements (Fig. 1e) of a perovskite film show discrete diffraction spots. The diffraction spot on q z = 1.065 Å -1 corresponds to a real-space distance of 5.89 Å, which can be assigned as the d-spacing of {001} planes of tetragonal β-CsPbBr . These features suggest that the assemblies of the perovskite nanoplatelets are highly ordered and all nanoplatelets share the same orientation with {001} planes parallel to the substrate. The GIWAXS data, together with the STEM-HAADF and HRTEM observations, suggest that the perovskite films consist of a monolayer of face-on oriented nanoplatelets with thicknesses of 8.6 ± (7 nm) . Photoluminescence (PL) spectrum of the perovskite film shows a symmetric peak centered at 516 nm (Fig. 2a), corresponding to an optical bandgap of 2.41 eV. This value is larger than the bandgap of bulk β-CsPbBr , ~2.36 eV . The excitonic absorption features of quasi-two-dimensional (2D) perovskites in the strong confinement regime, such as n = 2 and n = 3 (n: the number of PbBr octahedral layers within a crystallite) layered perovskites, are absent in the ultraviolet-visible absorption spectrum (Fig. 2a). This result suggests that our film comprising of perovskite nanoplatelets is distinctive from the previously reported perovskite films of multiple quantum wells or quantum dots embedded in quasi-2D phase . Remarkably, the perovskite nanoplatelet film exhibits a high PLQY of ~75% at a low excitation power density of ~0.02 mW/cm . This feature indicates efficient radiative recombination of the photogenerated excitons in the perovskite nanoplatelets. The orientation of TDMs of the perovskite nanoplatelet film is quantified by the ratio of horizontal TDMs, Θ. This parameter is defined as Θ = p ∥ /(p ∥ + p ⊥ ), where p ∥ and p ⊥ stand for contributions from horizontal and vertical components of the optical TDMs, respectively. We use the angle-dependent PL technique, which provides an ensemble measurement (photoexcitation spot size: ~1 mm) of the intensity of p-polarized emission against the detection angle (see Fig. S6 for schematic illustration) . The experimental data (Fig. 3a) is fitted to the pattern simulated using a classical dipole radiation model . The Θ of our perovskite nanoplatelet film is determined to be 84%. This value is substantially higher than that of isotropic emitters (67%). Furthermore, we analysed the light emission of the perovskite film by using back focal plane (BFP) spectroscopy. BFP spectroscopy (see Fig. S7 for schematic illustration) probes a small targeted region of the perovskite nanoplatelet film by employing a laser with a spot size of ~750 nm for photoexcitation . The BFP pattern and the corresponding line-cut data along the p-polarized direction are shown in Fig. 3b and c, respectively. An important feature of the horizontally oriented dipoles is that the p-polarized intensity is minimum at k // = k , where k // is the in-plane wavevector with respect to the substrate and k is the wavevector in vacuum . By fitting the line-cut data in Fig. 3c, Θ is determined to be ~87%. The BFP data of 4 spots from different regions (Fig. S8) demonstrate excellent spatial uniformity of the orientations of TDMs in our film. The results of both measurements unambiguously demonstrate that the emission of our perovskite nanoplatelet film is dominated by horizontal TDMs. EL spectrum of our perovskite nanoplatelet film (Fig. 4a) displays a symmetric peak centred at ~518 nm with a full width at half-maximum of 16 nm (74 meV), representing one of the narrowest emission line widths for high-efficiency perovskite LEDs . The ultra-pure green emission corresponds to Commission Internationale de l’Eclairage (CIE) colour coordinates of (0.09, 0.78) (Fig. S9). The angular emission intensity of our PeLEDs follows the Lambertian profile (Fig. 4b). The EL spectra at different viewing angles are identical (Fig. S10). The current density−voltage−luminance (J−V−L) curves of a typical device are shown in Fig. 4c. Owing to the pinhole-free morphologies of the nanoplatelet film, the device shows negligible leakage current. The current density and luminance increase rapidly once a turn-on voltage of ~3 V is reached. At 7 V, the device shows a brightness of ~3140 cd/m . The champion device demonstrates a peak EQE of 23.6% (Fig. 4d), which is a record efficiency for perovskite LEDs (Table S1). The statistic of 36 devices (Fig. 4e) indicates an average EQE of 21.3% with a small relative standard deviation of 4.4%, demonstrating the excellent reproducibility of our green LEDs. We performed optical simulations on the perovskite LEDs by using a classical dipole model developed for planar microcavities (see Fig. S11 for details). The result suggests a light outcoupling efficiency of 31.1% for our perovskite devices based on the oriented nanoplatelet film with a Θ of 84% (Fig. S11b). In contrast, a control device based on isotropic TDMs (Θ: 67%) would possess a lower outcoupling efficiency of ~23.4%. Considering that the PLQY of our perovskite nanoplatelet film is ~75%, our optical simulation predicts a maximum EQE of 23.3% (Fig. 4f), which agrees well with the experimental results. Further optimizing the PLQY and enhancing Θ of the perovskite films shall push the upper limit of EQEs to ~40% (Fig. 4f). We highlight that previous work aiming to control the orientations of TDMs focuses on assemblies of anisotropic colloidal nanostructures . High-efficiency EL would require syntheses of anisotropic colloidal nanostructures with high PLQY, controlling the orientations of the anisotropic colloidal nanostructures to realize films with high ratios of horizontal TDMs, minimizing energy transfer between the neighbouring nanoemitters to maintain high PLQYs and efficient and balanced charge injection into the individual nanoemitters. Fulfilling these stringent requirements is challenging and often imposes dilemma in material design and assembly. As a result, the EQEs of the LEDs based on colloidal anisotropic perovskite nanoemitters are currently lower than 5% (see Table S2 for more information). Expanding the scope to all solution-processed planar LEDs utilizing inorganic emitters with oriented TDMs (Table S2), the reported highest EQE is 12.1% . In contrast, our LEDs based on in-situ grown perovskite nanoplatelet films demonstrate high EQEs of up to 23.6%. Two issues, namely the concentrations of the bulky organic ammonium groups and the introduction of LiBr in the precursor solution, are critical for the formation of oriented perovskite nanoplatelet film with high PLQY. Without the use of the bulky organic ammonium groups, the resulting perovskite film shows an optical bandgap of ~2.37 eV (Fig. S12a) and excitation intensity-dependent PLQY (Fig. S12c). These features indicate the formation of three-dimensional CsPbBr crystals. Doubling the concentrations of the bulky organic ammonium groups leads to the formation of perovskite films with strong excitonic absorption peaks at ~430 nm, ~460 nm and ~475 nm, which corresponds to the n = 2, n = 3 and n = 4 layered perovskites, respectively (Fig. S12b). The emission peak of this perovskite film locates at ~501 nm, implying efficient energy transfer from the perovskites with small n values (larger bandgaps) to the emissive centers with smaller bandgaps. BFP measurements on the films processed from the precursor solution with various concentrations of bulky organic ammonium groups indicate that only the oriented perovskite nanoplatelet films possess high ratios of horizontal TDMs (Fig. S13). Further control experiments show that the introduction of LiBr in the precursor solution is beneficial for improving the PLQY of the perovskite film. GIWAXS, angle-dependent PL and BFP measurements (Fig. S14) on the perovskite film processed from the precursor solution without LiBr indicate the formation of oriented nanoplatelets with anisotropic emission (Θ: 84%). The PLQY of this perovskite nanoplatelet film is ~50% (Fig. S15). In conclusion, we have demonstrated that controlling the orientation of TDMs of the perovskite films leads to green LEDs with exceptionally high EQEs of up to 23.6%. Considering the chemical versatility of perovskite materials, our facile approach of in-situ grown nanoplatelet films is readily extended to the fabrication of differently coloured LEDs with high EQEs. Our work represents an effective approach of exploiting anisotropic optical properties from perovskite nanoemitters desirable for optoelectronic applications. Methods
Structural and optical characterization.
The cross-sectional samples were prepared by using focused-ion beam equipment (Quata 3D FEG). The TEM observations were conducted using a Cs aberration-corrected scanning transmission electron microscope Titan G2 80-200 ChemiSTEM microscope operated at 200 kV. Atomic force microscopy analyses were conducted on a Cypher-S (Asylum Research) atomic force microscope located in a glovebox filled with nitrogen. GIWAXS measurements were performed at Beamline 12-ID-B of Advanced Photon Source (Argonne National Laboratory, USA). A PerkinElmer detector, XRpad TM ° to 90 ° . The excitation wavelength is 365 nm. The BFP imaging experiments were performed on an optical system as schematically shown in Fig. S7. The excitation wavelength is 457 nm and the diameter of the excitation spot is ~750 nm. The refractive index (n) and extinction coefficient (k) of all layer of the perovskite LEDs were measured by an ellipsometer (J.A.Woollam, USA). Ultraviolet-visible absorption spectra of the samples were collected by using a Carry 5000 (Agilent) spectrophotometer. Steady-state PL spectra were performed on an Edinburgh Instruments (FLS920) spectrometer. Excitation-density-dependent PLQYs of the perovskite films were obtained by following a literature method . The angular dependence of emission intensity of the perovskite LED was measured using a Thorlabs PDA100A detector at a fixed distance of 200 mm from the EL device . All LEDs were measured in a glovebox filled with nitrogen at room temperature. A system consisting of an integration sphere (FOIS-1), a Keithley 2400 source meter and a QE-Pro spectrometer (Ocean Optics) was used for the measurements . The devices were scanned from zero bias at a rate of 0.1 V s −1 . The integral time for each step is 500 ms . The EL characteristics of the LEDs were crosschecked at Zhejiang University (Yizheng Jin group), University of Cambridge (Richard Friend group) and Nanjing Tech University (Jianpu Wang group). References
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Acknowledgements
This work was financially supported by the National Key R&D Program of China (2016YFB0401600), the National Natural Science Foundation of China (51522209, 91833303, Author contributions
Y.J., Y.L. and J.C. conceived the idea and designed the experiments. Y.J. supervised the work. J.C. carried out the device fabrication. J.C. and Y.L. carried out the device characterizations. Y.D. carried out the optical simulation of our perovskite devices. J.C., Y.L., C.L, and Z.F. conducted the optical measurements. P.B. and Y.G. did the BFP measurement. J.C. synthesized the PBABr and NiO x . C.X., K.D., and H.T. carried out the STEM and HRTEM characterizations. X.Z. conducted the GIWAXS measurements. C.L. and Y.D. carried out the AFM experiments. B.Z. cross-checked the LED measurements. K.W. conducted the measurements of angular dependence of EL emission. Y.J., J.C., and Y.L. wrote the first draft of the manuscript. H.H., Z.Y., Y.G. and J.W. participated in data analysis. All authors discussed the results and commented on the manuscript. Additional information
Data Availability
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Competing interests
The authors declare no competing financial interest. Fig. 1 | Structural characterizations of the perovskite nanoplatelet films. a, A cross-sectional STEM-HAADF image showing the continuous and pinhole-free perovskite layer. b, A zoomed-in STEM-HAADF image showing the fine structure of a perovskite nanoplatelets. Inset, the corresponding fast Fourier transform (FFT) pattern. c, A typical HRTEM image of the perovskite nanoplatelets dispersed on a copper grid. Inset, the corresponding FFT pattern. d, Statistics of the size distribution of the nanoplatelets measured by HRTEM. The average size is 25.8 nm and the corresponding standard deviation is 6.8 nm.
The Gaussian fits are provided as a guide to the eye. e, GIWAXS pattern. The two diffraction spots at q z = 1.065 and q y = 1.070 Å -1 correspond to (001) and (010) planes of β-CsPbBr , respectively. Fig. 2 | Optical properties of the perovskite nanoplatelet films. a,
Absorption and PL (405 nm excitation) spectra. b, Excitation-intensity-dependent PLQY. The error bars represent the experimental uncertainties in the PLQY measurements at 0.4 mW/cm and the errors in the determination of relative PL intensities and excitation power. Fig. 3 | Orientations of the TDMs of the perovskite nanoplatelet films. a,
Angle-dependent PL measurements of the perovskite film on a quartz/TFB/PVK substrate. The experimental data (grey squares) are fitted by a classical electromagnetic dipole model (red line), giving a horizontal TDM ratio of 84 ± 4%. b, BFP image of a perovskite film. c, A p-polarized line cut (grey line) along the dashed line in of the BFP image (panel b) . This line cut is fitted with a horizontal TDM ratio of 87% (red solid line). Fig. 4 | Devices characterization of the green LEDs based on the perovskite nanoplatelet films. a,
EL spectrum. Inset, a photograph of a green-emitting device (3.24 mm ). b, Angular distribution of the radiation intensity follows the Lambertian profile. c, Current density-luminance-voltage characteristics of a typical device. d, EQE-voltage relationship of the device with a champion EQE of 23.6%. e, A histogram of peak EQEs from 36 devices. The Gaussian fits are provided as a guide to the eye. f,f,