15 septembre 2020

[QLEDs] – Bilateral interface engineering for efficient and stable perovskite QLEDs

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Perovskite QLEDs - Codex International

Although the first cadmium selenide (CdSe) QD-based light-emitting devices (QLEDs) were developed in 1994, the first perovskite QLEDs were reported only more than 20 years later.

Perovskite QLEDs possess the features of wide gamut and real color expression; that’s why they have been considered as very promising candidates for next-generation high-quality lighting and displays. However, device efficiency and stability of perovskite QLEDs has always been a major challenge.
In recent years, researchers have achieved remarkable results in this area. So far, the external quantum efficiency (EQE) of perovskite QLED has exceeded 20% in the green and red band. However, the blue LED is still in a relatively sluggish state and stability is still an issue for all perovskite QLEDs.
« Although numerous surface passivation strategies on colloidal quantum dots have been tried, this problem cannot be avoided; which led to the assumption that dealing with QD film directly will be more effective, » Jizhong Song, a Professor in the College of Materials Science and Engineering at Nanjing University of Science and Technology, tells Nanowerk. « However, most of the reported work only focused on one part of the interface, especially the top-side interface, when doing interface processing. In our new work, we propose a bilateral passivation strategy through passivating the top and bottom interface of QD film with organic molecules. »
As Song and his collaborators report in Nature Communications (« A bilateral interfacial passivation strategy promoting efficiency and stability of perovskite quantum dot light-emitting diodes »), this technique greatly enhances device performance and stability compared to single interface processing.

Besides QLEDs, this kind of bilateral passivation strategy can be widely applied to other types of perovskite materials, and other optoelectronic devices including solar cells, and photodetectors.
Interface molecular passivation has been widely used in perovskite-based devices, resulting not only in improved effective radiation recombination, but also enhanced stability. However, most work today only focused on the top surface of the perovskite film.
« Our group has been committed to the research of perovskite QLEDs since our first report of all organic halide perovskite QLEDs back in 2015, and in recent years we have done a lot of work to improve the their performance, » says Song. « The motivation for this recent work happened more or less by chance: When we tried to improve the performance of the device through interface processing, we found that the introduced organic molecules on either the top or bottom interface could improve the device performance. So we wanted to see what would happen when both interfaces were passivated at the same time? Obviously, the result is amazing. »
This is the first time that bilateral passivation has been applied to perovskite QLEDs.
The researchers note that it is well known that, since the perovskite layer is at the center of the sandwich structure in practical optoelectronic devices, both the top and bottom surface of the QD film may experience the interface problems of defects and other deposited materials affecting the carrier behavior inside the film.
« What we found is that the interface treatment on both sides of perovskite QD film is effective to improve the efficiency and stability of the film and consequently QD-based LEDs, » Song points out.
The team’s findings advances the field of perovskite QLED in two aspects. Firstly, the sharp attenuation of fluorescence is always a severe problem when the colloidal QDs transform into the QD solids; this is because massive defects are inevitably introduced during the film-forming process. This may lead to the formation of non-radiative recombination centers, which will deteriorate the efficiency and stability of QD-based device. The proposed bilateral passivation strategy for QD film can effectively solve this problem.
Secondly, interface passivation has always been an important method for planar devices. However, most research work only focused on one side of the interface, especially the top-side. Whether from the perspective of passivating defects or matching energy levels, simultaneous regulation of both top and bottom interfaces will be helpful for improving device performance.
The researchers report that their passivated QD films exhibit high exciton recombination features with a photoluminescence quantum yield of 79%, and the corresponding LEDs have a high electro-optic conversion efficiency with an EQE of 18.7%.
« Interestingly » Song notes, « the passivation approach makes the QD materials and LEDs exhibit a higher stability. For example, the T50 operational lifetime (the time taken for the current efficiency to drop to half its initial value) of 15.8 hours for QLEDs based on QD films passivated by a phosphine oxide molecule is a factor of 20 longer than the control devices (0.8 hours). »
Notwithstanding these results, the stability of perovskite QLEDs still remains far from the requirements of actual commercial production and application.
« We made a bit of progress in this work and our next stage steps will be to improve the stability of perovskite QLEDs on the basis of our current research, » Song concludes.

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