Perovskite LED has an external quantum efficiency of over 20%, setting the world's highest record.

Recently, Professor Wei Zhan, Professor of Huaqiao University, Professor Xiong Qihua from Nanyang Technology University in Singapore and Professor Edward H. Sargent of University of Toronto in Canada have made significant breakthroughs in the research of perovskite light emitting diodes.

The researchers used perovskite composition control strategy to obtain smooth and compact Perovskite Thin Films with excellent optical and electrical properties. By adding barrier layer to improve the electron hole injection equilibrium, the external quantum efficiency (EQE) of perovskite light emitting diodes was over 20%, which refreshed the world's highest perovskite light emitting diode. At the same time, stability has also been greatly improved, far beyond international counterparts.

The relevant research results are published in the international top academic journals Nature (Dol:10.1038/s41586-018-0575-3), entitled "Perovskite Light-Emitting Diodes with External Quantum Efficiency Exceeding 20%".

Introduction of results

Perovskite semiconductors have achieved great success in the field of solar cells. The photoelectric conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells has been from the initial 3.8% to the current 23.3% authentication efficiency. Perovskite materials have great potential in flat panel display and solid state lighting because of their low preparation cost, solution preparation, high fluorescence quantum efficiency, high purity and adjustable color. Since 2014, Richard H. Friend and Zhi-Kuang Tan and others have reported the perovskite light emitting diodes which can work at room temperature for the first time. The MAPbI3-X and MAPbBr3 (MA = CH3NH3+) as the light-emitting layer of the perovskite LED with near infrared light and green light have obtained the EQE of 0.76% and 0.1% respectively.

Since then, perovskite LED has attracted more and more researchers to study, and has made continuous breakthroughs. However, the highest external quantum efficiency (EQE) of green and red perovskite LED reported at present is 14.36% and 11.7% respectively, and the stability of perovskite LED devices is poor, far lower than that of commercialized organic light-emitting diodes (OLEDs) and inorganic quantum dot light-emitting diodes (QLEDs) (EQE:25%). Perovskite LED has great room for improvement in efficiency and stability.

In this study, the researchers used CsPbBr3 and MABr (MA = CH3NH3+) in the polarity solvent DMSO solubility is large difference, by adding MABr additives in CsPbBr3 perovskite precursor fluid, and accurately regulating the amount of additive MABr, successfully achieved the composition distribution control of calcium titanium mine, and got the surface compact and smooth. Moreover, the perovskite thin film with CsPbBr3@MABr shell core structure has excellent photoelectric properties, and the device EQE is more than 17%.

By comparing the pure electron and pure hole devices, the researchers found that the electron and hole injection in the device were unbalanced. Excessive electron injection restricted the further improvement of the device performance. In this regard, the researchers introduced polymethyl methacrylate (PMMA) insulating materials between the emitting layer and the electron transporting layer, blocking the excessive electricity. The injection of the daughter improves the injection balance between the electrons and holes in the device, and further improves the efficiency of the device. Finally, the perovskite LED device with a EQE exceeding 20% and a stability exceeding 100h (T50 > 100h) is far superior to that of the international counterparts.

Brief introduction to pictures and texts

Figure 1. Optical characterization of different perovskite types

(A) images of CsPbBr3, MAPbBr3 and mixed perovskite 1 under fluorescent lamps and ultraviolet lamps.

UV visible absorption curves of (B) CsPbBr3 and perovskite mixtures with different mixing ratios;

(C) the fluorescence curves of CsPbBr3, MAPbBr3 and mixed perovskite 1 PL curves (excitation wavelength 400nm, 4uw) (D) CsPbBr3, MAPbBr3 and mixed perovskite 1.

Fig. two component distribution control improves the PL of calcium titanium mine.

(A) distribution maps of different components: single layer CsPbBr3, laminated CsPbBr3/MABr and CsPbBr3@MABr core-shell structure;

(B) PL pictures of different perovskite under ultraviolet lamp;

(C) deep analysis of CsPbBr3@MABr core-shell structure by two ion mass spectrometry (SIMS);

(D) focused ion beam (FIB) cutting, surface sputtering C as the protective layer of the CsPbBr3@MABr shell core structure TEM cross section (the white part shows that the MABr shell structure encased the CsPbBr3 grain).

Fig. three perovskite LED device and its performance characterization

(A) structural schematic diagram of perovskite LED device, PEDOT:PSS and B3PYMPM respectively serve as hole transport layer (HTL) and electron transport layer (ETL).

(B) work diagram of perovskite LED device;

(C) CE-V curves of CsPbBr3, MAPbBr3 and mixed perovskite 1 as luminescent layers.

J-V curves of (D) CsPbBr3 and mixed pure perovskite 1 pure electron hole devices;

(E) statistical chart of current efficiency distribution of devices;

(F) EQE-V curve of mixed perovskite 1 with best performance.

Fig. four insertion of PMMA barrier layer in calcium titanium layer and electron transport layer to further improve device performance

(A) pure electron pure hole device J-V curve with PMMA barrier layer inserted in the calc titanium layer and electron transport layer;

(B) the schematic diagram of the device for inserting the PMMA barrier layer into the calc titanium layer and the electron transport layer;

(C) distribution of current efficiency distribution after insertion of PMMA barrier layer; perovskite LED with the best performance

(D) L-J-V curves and (E) EQE-L curves;

(F) perovskite LED life test curve.

Summary

The researchers used CsPbBr3 and MABr to obtain a high Perovskite Thin film with CsPbBr3@MABr core shell structure and high fluorescence quantum efficiency (PLQY) by spin coating on the basis of the solubility difference of polar solvent DMSO. It is pointed out that the addition of MABr contributes to the nucleation and growth of CsPbBr3, effectively passivating the surface defects of CsPbBr3, reducing the radiation free recombination, and the MABr on CsPbBr3 can achieve the effect of balanced charge injection.

By inserting PMMA insulating material between the luminescent layer and the electron transporting layer, the researchers further improved the electron hole injection balance in the device. The final perovskite luminescent two stage tube EQE reached 20.3% and the stability exceeded 100 hours, making the development of perovskite LED reached a new height.

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