Perovskite LED (perovskite LED) is a new generation of light-emitting technology with great potential in display, lighting, communication and other fields. The production cost of perovskite LED is low, and the technical advantages are significant: it has similar characteristics as OLED, such as thin and flexible, but also has similar color purity and spectrum adjustability as II-V semiconductor LED. After only a few years of development, the efficiency of perovskite leds is comparable to mature luminous technologies.
However, similar to perovskite solar cells, the instability of perovskite leds is the biggest challenge in realizing industrial applications. At present, the life of high-performance perovskite LED is generally in the order of 10-100 hours. OLED technology needs at least 10,000 hours of service life when it enters industrialization. There are significant challenges in this direction because perovskite semiconductors can be inherently unstable: their crystal structure has significant ionic properties, and ions tend to move in response to an applied LED electric field, resulting in material degradation.
Recently, a team led by Professor David Di and researcher Baodan Zhao from the State Key Laboratory of Modern Optical Instruments, College of Opto-Electronics, Zhejiang University, and the International Research Center for Advanced Photonics, Haining International Campus, made an important breakthrough in this direction. Using a bipolar molecular stabilizer, they have achieved an extremely long working life in perovskite leds that can meet the needs of practical applications.
The corresponding authors are Professor Di Dawei and Researcher Zhao Baodan, and the first author is Guo Bingbing, a master student at Zhejiang University. The collaborators include Professor Li Cheng team from Xiamen University, Professor Hong Zijian team from Zhejiang University, and Professor Li Weiwei team from Nanjing University of Aeronautics and Astronautics. The related research paper Ultrastable near-infrared perovskite light-emitting diodes was published in Nature Photonics on 8 August (Guo et al, Nat.Photon. (2022), doi:10.1038/s41566-022-01046-3).
These perovskite leds were driven by a constant current of 5 mA/c square meters for five months (3,600 hours) without a decrease in brightness.This is very exciting, and it goes beyond what perovskite leds are. These devices are very stable, and some of the ongoing tests seem unlikely to be completed in a year or more. In order to obtain lifespan data within a reasonable experimental period, accelerated LED aging experiments are needed.
These near-infrared perovskite leds exhibit extremely long service life. For example, the estimated T50 lifetime (the time required to reduce the initial radiance to 50%) of the device is 32675 hours (3.7 years) for an initial radiance of 2.1 W sr-1 m-2 (current 3.2 mA/c㎡). The luminous power provided by this radiance is comparable to that of commercial green OLeds at high brightness of 1000 cd/m2. At 0.21W sr-1 m-2, a low radiance (1/10 of the above) or 0.7 mAc square meters, the T50 life is estimated to be 2.4 million hours (about 270 years).
We felt it was necessary to perform a reliable lifetime analysis of this novel LED, and for this purpose we collected 62 device lifetime data points covering the widest possible current density range of 10-200 mA/c square meters in an accelerated aging experiment. Guo Bingbing said. The electroluminescence external quantum efficiency (EQE) and energy conversion efficiency (ECE) of the device reached 22.8% and 20.7% respectively, which is the highest efficiency of near-infrared perovskite LED at present.
The authors found that these perovskite luminescent materials have very stable crystal structure. The crystal structure of the material did not change after more than 322 days, Zhao said. This indicates that the bipolar molecular stabilizer helped perovskite maintain its initial crystalline phase with excellent photoelectric properties. As a comparison, the crystal structure of the untreated control perovskite samples changed significantly and degraded within two weeks.
Ion migration in perovskite is one of the important factors leading to instability, and this problem becomes more serious under the influence of LED applied voltage. Our experiments and calculations show that bipolar molecules chemically bond or interact with ions at the grain boundaries of perovskite.This may be the reason why ion migration in our perovskite is difficult, Guo said. The electrical and optical experiments we carried out with our collaborators all showed the inhibition of the ion movement phenomenon,Zhao added.
The device lifetime results indicate that there is no genetic defect in the stability of perovskite materials. New semiconductors, such as metal halide perovskites, are widely believed to be inherently unstable, especially for high electric fields such as LED applications, said Dedavide. Our results show that achieving stable perovskite devices is not mission impossible.;
The extremely long device life is expected to boost confidence in the perovskite LED sector, as it already meets the basic stability requirements of commercial OLeds. These near-infrared leds can be used for near-infrared display, communications and biological applications. Although visible light perovskite devices with similar long life are still to be developed, the realization of ultra-stable perovskite leds paves the way for the introduction of perovskite luminescence technology into industrial applications.