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Towards Stable Perovskite Light-Emitting Diodes

Periodic Reporting for period 1 - Stable PeLEDs (Towards Stable Perovskite Light-Emitting Diodes)

Reporting period: 2019-09-01 to 2020-08-31

Recent years, perovskite light-emitting diodes (LEDs) have achieved great progress, with external quantum efficiency (EQE) exceeding 20%. However, the poor stability becomes obstacle to the practical application of perovskite LEDs. The degradation of perovskite LEDs working under electrical stress can be caused by Auger recombination, ion migration, Joule heating or deterioration of perovskite material, etc. This project aims to demonstrate fundamental degradation mechanism of perovskite LEDs under electrical stress. Then high-performance perovskite LEDs can be fabricated.
This project demonstrates that the ion migration under electrical stress is the main reason for degradation in three-dimensional (3D) FAPbI3 perovskite LEDs. By introducing bifunctional-molecule 3-chlorobenzylamine (3Cl-BA) additive into the perovskite precursor solution to suppress the detrimental ion migration, bright near-infrared perovskite LEDs with a peak EQE of 16.6% were achieved, which exhibits a record half-lifetime of 49 h at room temperature under a constant current density of 100 mA cm-2. Furthermore, this project reveals that the already existed heterogeneous distribution of halides in the as-deposited perovskite films is the main reason for the halide phase separation during operation of LED device. By simply introducing zwitterionic surfactants to improve the homogeneity of the halides in the precursor solution, we can overcome the phase segregation issue and obtain spectrally-stable single-phase blue-emitting perovskites.
This project not only increases the knows on how to design stable perovskite LEDs, but also makes significant contribution to the development of perovskite LEDs. It suggests a design strategy of additive to achieve high-quality perovskites, which is also important for perovskite solar cell community, where the ion migration and phase segregation need to be minimized to suppress the hysteresis and enhance device stability.
To reveal the degradation mechanism of 3D perovskite, we used FAPbI3 as a model system and fabricated perovskite LEDs. By performing the simultaneous measurement of PL and electroluminescence (EL) on working LEDs, excitation-intensity-dependent photoluminescence quantum efficiencies (PLQEs) measurement and PL measurement of FAPbI3 film heated at different temperatures, the effects of Auger recombination and Joule heating on device stability were all excluded, suggesting the ion migration of 3D FAPbI3 perovskites under electrical stress is the main reason for device degradation and efficiency roll-off. Based on this understanding, we introduced a bifunctional-molecule 3-chlorobenzylamine (3Cl-BA) additive into the perovskite precursor solution to assist the oriented growth of perovskites through both the amino and chloride groups. This results in near-infrared perovskite LEDs with a peak external quantum efficiency of 16.6%, which sustains 80% of its peak value at a high current density of 460 mA cm-2. Moreover, the device reaches a record half-lifetime of 49 h at room temperature under a constant current density of 100 mA cm-2.
Furthermore, to verify the phase segregation issue in mixed-halide perovskites, by using high-resolution cathodoluminescence microscopy, we directly revealed that the phase segregation in mixed-halide perovskite is caused by the intrinsic and microscopical phase heterogeneity in as-deposited perovskite films, which is resulted from precursor solution with nonuniform mixture of halides. The first-principles calculations indicate that the halide separation in the as-made materials predisposes to undergo much more substantial halide segregation under electrical excitation in LEDs, whereas the more uniform initial halide distribution materials are much more resistant to subsequent halide segregation. Therefore, uniform initial mixed-halide perovskite material is essential for stable perovskite LEDs.
The project results will be disseminated via publications, conference and public engagement. One work has been submitted to Nature Communications and the other manuscript is planned to be submitted to Advanced Materials. Due to the Covid-19 situation, the researcher took some online conferences, including the Translational Perovskite e-conference. For commercialization, the University of Cambridge has a dedicated office (Cambridge Enterprise) for the commercialization of research, which has a very successful track record in transferring technology from lab to industry. Also, the host group has close collaboration with the company of Heliochrome Ltd, which was set up to develop light-emitting devices made with lead halide perovskites and related materials.
This project provides deep understanding of the degradation mechanism of perovskite LEDs, and demonstrates high-efficiency and long-lifetime perovskite LEDs by materials design and device engineering. The NIR perovskite LEDs reaches a record half-lifetime of 49 h at room temperature under a constant current density of 100 mA cm-2. It suggests the real promise of commercialization for perovskite LEDs. And the success of commercialization of perovskite LEDs will completely change the life-style of modern society, i.e. large-area, flexible and low-cost lighting and displays.