Abstract
Light-emitting diodes (LEDs), which convert electricity to light, are widely used in modern society—for example, in lighting, flat-panel displays, medical devices and many other situations. Generally, the efficiency of LEDs is limited by nonradiative recombination (whereby charge carriers recombine without releasing photons) and light trapping1,2,3. In planar LEDs, such as organic LEDs, around 70 to 80 per cent of the light generated from the emitters is trapped in the device4,5, leaving considerable opportunity for improvements in efficiency. Many methods, including the use of diffraction gratings, low-index grids and buckling patterns, have been used to extract the light trapped in LEDs6,7,8,9. However, these methods usually involve complicated fabrication processes and can distort the light-output spectrum and directionality6,7. Here we demonstrate efficient and high-brightness electroluminescence from solution-processed perovskites that spontaneously form submicrometre-scale structures, which can efficiently extract light from the device and retain wavelength- and viewing-angle-independent electroluminescence. These perovskites are formed simply by introducing amino-acid additives into the perovskite precursor solutions. Moreover, the additives can effectively passivate perovskite surface defects and reduce nonradiative recombination. Perovskite LEDs with a peak external quantum efficiency of 20.7 per cent (at a current density of 18 milliamperes per square centimetre) and an energy-conversion efficiency of 12 per cent (at a high current density of 100 milliamperes per square centimetre) can be achieved—values that approach those of the best-performing organic LEDs.
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Acknowledgements
This work is supported financially by the Joint Research Program between China and the European Union (2016YFE0112000); the Major Research Plan of the National Natural Science Foundation of China (91733302); the National Basic Research Program of China-Fundamental Studies of Perovskite Solar Cells (2015CB932200); the Natural Science Foundation of Jiangsu Province, China (BK20150043, BK20150064, BK20180085); the National Natural Science Foundation of China (11474164, 11474249, 61634001); the National Science Fund for Distinguished Young Scholars (61725502, 61725503); and the Synergetic Innovation Center for Organic Electronics and Information Displays. We thank D. Di and B. Zhao for cross-checking the LED measurement system and Y. Zhao for helpful discussions. We thank M. Winton and N. Greenham for proof reading.
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Nature thanks A. Urban and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Contributions
J.W. had the idea for and designed the experiments. J.W. and W.H. supervised the work. Y.C. carried out device fabrication and characterizations, with the assistance of Y.M., H.Cao and Y.K.; Y.C., W.Z., M.X., Y.Wang, Z.F., D.K., Q.P., M.Y. and Y.H. conducted the optical measurements. Y.Wei and H.L. carried out AFM measurements. H.Chen, G.L. and Y.J. carried out FTIR characterizations. H.T. carried out high-resolution TEM and STEM characterizations with the assistance of Y.C., Y.Wei and K.D.; J.G. carried out optical simulations of the device with the assistance of K.P.; D.D. supervised the optical simulation. J.W., N.W. and Y.C. analysed the data. N.W. wrote the first draft of the manuscript and J.W. and W.H. provided major revisions. All authors discussed the results and commented on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Images of perovskite films.
a, Photograph of a perovskite film on a 2 cm × 2 cm glass/ZnO-PEIE substrate, alongside a coin. The perovskite film is shiny and uniform. b, Optical microscope images with different magnifications. The scale bars represent 30 μm. c, SEM images with different magnifications. The scale bars represent 3 μm. d, High-magnification SEM images of randomly selected regions. The scale bars represent 3 μm. e, AFM images with different magnifications. The scale bars represent 2 μm. The images show the submicrometre-scale structure of the perovskite film.
Extended Data Fig. 2 Characterization of our perovskite films and perovskite LEDs fabricated with different annealing times.
a, SEM images of perovskite films. The scale bars represent 1 μm. The images show that as the annealing time increases, the crystallites grow from small particles and become larger and more faceted. When the annealing time is more than 6 min, similar submicrometre-scale structures form. b, XRD spectra. Crystallinity is enhanced as the annealing time increases, in agreement with the SEM images. c, Excitation-intensity-dependent PLQEs. Trap densities gradually decrease as the annealing time increases, resulting in PLQEs of more than 60% when the annealing time is between 16 and 20 min. d, Time-resolved photoluminescence (PL) decay transients (at a carrier density of 1.0 × 1013 cm−3). The films show longer PL decay lifetimes at longer annealing times. e, Dependence of current density and radiance on the driving voltage. The circles denote a bunch of curves; the arrows show the y axis to which a given bunch belongs. f, EQE versus current density. g, Peak EQE versus annealing time. An average peak EQE of more than 18% can be maintained with annealing times of between 14 and 20 min. Error bars correspond to the standard deviation.
Extended Data Fig. 3 Formation of the organic layer surrounding the submicrometre structures.
a, Dehydration reaction of 5AVA on top of the ZnO-PEIE surface36,37. b, Grazing-angle reflectance FTIR spectra of perovskite films at various annealing times. As the annealing time increases, the peak at 3,400–3,300 cm−1 (νO–H in 5AVA) decreases; simultaneously, peaks appear at 1,620 cm−1 and 1,550 cm−1, which can be assigned as amide I band (νC = O) and amide II band (δN–H), respectively. These spectra indicate dehydration reactions of 5AVA, leading to the formation of an organic layer during annealing.
Extended Data Fig. 4 Characterizations of perovskite films and LEDs with various 5AVA amounts.
The ratio of 5AVA to FAI to PbI2 is x/2.4/1, where x varies from 0 to 0.9. a, SEM images. The scale bars represent 1 μm. The value of x is given in the top left corner of each image. The reference FAPbI3 perovskite film without 5AVA has low film coverage. Without 5AVA, the perovskites form discrete clusters with random shapes. After adding 5AVA, faceted perovskites with submicrometre structures gradually form. b, XRD spectra. The perovskite films show improved crystallinity with the addition of 5AVA. c, Excitation-intensity-dependent PLQE. After adding 5AVA, PLQEs were greatly enhanced, indicating reduced trap densities. d, Time-resolved PL decay transients (carrier density 1.0 × 1013 cm−3). There is a fast PL decay channel for the perovskite without 5AVA, indicating a high level of trap densities. This fast PL decay channel gradually disappears after adding 5AVA. e, Dependence of current density and radiance on the driving voltage. After adding 5AVA, the leakage current is reduced. f, EQE plotted against current density. g, Peak EQE plotted against 5AVA ratio. Error bars correspond to the standard deviation. After adding 5AVA, the peak EQE increases owing to reduced leakage current and enhanced PLQE. When the 5AVA ratio is increased to 0.9, the EQE decreases, owing to the inferior outcoupling efficiency that results from the more dispersed structural pattern.
Extended Data Fig. 5 Simulation of outcoupling efficiency.
a, Device structure. A typical reference device consists of a metal layer (Au), a 7-nm-thick MoO3 layer, a 40-nm-thick TFB layer, a 50-nm-thick emitting layer (EML), a 30-nm-thick layer of ZnO-PEIE, a 160-nm-thick ITO layer and a semi-infinite glass substrate. In our new device, the EML is replaced by a layer of perovskite squares distributed with a period P and a duty cycle le/P (where le is the length of the perovskite platelets, and le/P = 50%). The height of the convex structure of TFB is denoted as h and the diameter is set to le + 100 nm. b, Discretized map of the perovskite layer. The scale bar represents 1 μm. x and y are the pixel numbers in units of pixel length a. f(x,y) is the discrete function. c, Module of spatial frequency spectrum. Ux and Uy are the spatial frequencies. d, Refractive indices of different layers in our perovskite LEDs. Optical constants (n, k) of the multilayers were determined using an ellipsometer. Here the optical constants of perovskite are from a continuous FAPbI3 film, which are used in the simulation. e, EQE calculated as the period P and the convex height h. f, Calculated outcoupling efficiency as a function of period P with convex height h = 30 nm. The reference is a device made from continuous perovskite film. The simulation shows that the outcoupling efficiency can be more than 25% over a wide range of periods from 310 nm to 900 nm.
Extended Data Fig. 6 EQE versus current density for our perovskite LED device at different temperatures.
Measuring the device EQE at low temperatures minimizes nonradiative recombination so that the EQE reaches a value of 30% at 6 K.
Extended Data Fig. 7 Characterization of our perovskite films and LEDs at different precursor concentrations.
a, SEM images. The scale bars represent 1 μm. As the precursor concentration increases (shown in the top left corner of each image), the size of the crystallites increases and the crystallites become more tightly packed. b, XRD spectra. c, Excitation-intensity-dependent PLQEs. The 7 wt.% film shows the highest PLQEs. d, Dependence of current density on driving voltage. The leakage currents of the devices decrease as the precursor solution becomes more concentrated. e, Dependence of radiance on the driving voltage. As the precursor concentration increases, the turn-on voltage increases and the radiance decreases, probably owing to the poor charge transport of thicker film. f, EQE plotted against current density. g, Peak EQE versus precursor concentration. Error bars correspond to the standard deviation. When the concentration exceeds 10 wt.% the EQE decreases, probably because of a reduced outcoupling-enhancement effect and poor charge transport.
Extended Data Fig. 8 Time-resolved photoluminescence decay transients of perovskites with different precursor concentrations.
a, 5 wt.%. b, 7 wt.%. c, 10 wt.%. d, 15 wt.%. e, 20 wt.%. Charge carrier densities vary as indicated (black, blue, purple and green traces).
Extended Data Fig. 9 Optoelectronic characteristics of perovskite LED devices fabricated with different amino acids in the precursor solution.
a, SEM image of submicrometre-structured perovskites fabricated with 6ACA (chemical structure shown in white). The scale bar represents 1 μm. Inset, FFT pattern in a randomly selected region. The P range of 6ACA is 265–901 nm, yielding a calculated outcoupling efficiency of 28.9% ± 2.5%. b, SEM image of submicrometre-structured perovskites fabricated with 7AHA (chemical structure shown in white). The scale bar represents 1 μm. Inset, FFT pattern in a randomly selected region. The P range of 7AHA is 432–1,430 nm, yielding a calculated outcoupling efficiency of 26.4% ± 3.3%. c, Excitation-intensity-dependent PLQE. The perovskite films with 6ACA and 7AHA have similar PLQEs. d, Dependence of current density and radiance on the driving voltage. e, EQE versus current density. The 6ACA- and 7AHA-based devices reach peak EQEs of 18.2% and 17.3%, respectively. Given that the perovskite films based on 6ACA and 7AHA have similar PLQEs, the EQEs must be affected mainly by the different outcoupling efficiencies that result from the different periodicities of the submicrometre-scale structures. f, Electroluminescence spectra.
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Cao, Y., Wang, N., Tian, H. et al. Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures. Nature 562, 249–253 (2018). https://doi.org/10.1038/s41586-018-0576-2
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DOI: https://doi.org/10.1038/s41586-018-0576-2
Keywords
- Peak EQE
- Organic LEDs
- STEM Tomography
- Perovskite Films
- Photoluminescence Quantum Efficiency (PLQE)
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