Fig. 1. Submicron structure of perovskite LED, with efficiency exceeding 20%, developed by Jianpu Wang and colleagues. (Image credit: Jianpu Wang, Nanjing Tech University). Reprinted by permission from: Cao et al., Nature (2018), https://doi.org/10.1038/s41586-018-0576-2. © 2018.
Fig. 1. Submicron structure of perovskite LED, with efficiency exceeding 20%, developed by Jianpu Wang and colleagues. (Image credit: Jianpu Wang, Nanjing Tech University). Reprinted by permission from: Cao et al., Nature (2018), https://doi.org/10.1038/s41586-018-0576-2. © 2018.
Fig. 2. Perovskite compositional distribution management boosts efficiency of LED developed by ZhanhuaWei and colleagues. (A) Illustration of the single-layered, bi-layered, and quasi-core-shell structures; (B) photos of the three as-prepared perovskite films under ultraviolet light; and (C) photos of perovskite LED devices showing six uniform and bright pixels and a logo ‘Pero-LED’. (Image credit: Zhanhua Wei, Huaqiao University). Reprinted by permission from: Lin et al., Nature (2018), https://doi.org/10.1038/s41586- 018-0575-3. © 2018.
Fig. 2. Perovskite compositional distribution management boosts efficiency of LED developed by ZhanhuaWei and colleagues. (A) Illustration of the single-layered, bi-layered, and quasi-core-shell structures; (B) photos of the three as-prepared perovskite films under ultraviolet light; and (C) photos of perovskite LED devices showing six uniform and bright pixels and a logo ‘Pero-LED’. (Image credit: Zhanhua Wei, Huaqiao University). Reprinted by permission from: Lin et al., Nature (2018), https://doi.org/10.1038/s41586- 018-0575-3. © 2018.

Perovskites have generated huge interest in recent years because of their potential for solid-state lighting and displays, despite lagging behind other state-of-the-art technologies in efficiency and longevity. Now two independent teams have reported light-emitting diodes (LEDs) based on perovskites that have surpassed a milestone in efficiency [Cao et al., Nature (2018), https:// doi.org/10.1038/s41586-018-0576-2; Lin et al., Nature (2018), https://doi.org/10.1038/s41586-018-0575-3].

Lighting and displays have been revolutionized in recent years by the advent of energy-efficient LEDs based on organics and quantum dots. Organic semiconductor LEDs provide cheaper, more efficient, flexible displays and devices, with high-quality color output and wide viewing angles. Perovskite-based LEDs could push efficiency even further by offering very low-cost solution processing using readily available low-tech printing technologies and low overall embodied energy (the energy consumed over the entire lifetime of a device).

Two teams have simultaneously demonstrated perovskite-based LEDs with external quantum efficiency, which is a measure of the number of photons produced per electron used, exceeding 20%. This milestone achievement for perovskite LEDs has been achieved using two quite different routes.

Jianpu Wang and Wei Huang’s team at Nanjing Tech University, Zhejiang University, Nanjing University of Posts and Telecommunications, and Northwestern Polytechnical University report organometal halide perovskite LEDs with peak EQEs of 20.7% (at a current density of 18 mA/cm2).

This was achieved by simply introducing additives to the perovskite precursors with little additional cost, which facilitate the passivation of surface defects and the formation of submicrometerscale structures,” explains Wang.

Like organic LEDs, a significant proportion of light generated by a perovskite emitting layer remains trapped inside the device, in an effect known as ‘outcoupling’. The team’s solution processing approach produces randomly oriented tile-like perovskite platelets 100–500 nm in size on the surface of the substrate embedded in a thin (8 nm) organic layer. The researchers believe that the concave-convex sub-micron structure created by the high-index perovskite and low-index organic layer extract the light trapped inside the waveguide structure more efficiently (Fig. 1). Moreover, the organic amino-acid precursor additives appear to passivate surface defects, reducing radiative recombination (Fig. 2).

The EQE values of 20.7% and energy conversion efficiencies of 12% (at a high current density of 100 mA/cm2) achieved by the devices compare favorably to the best-performing organic LEDs, say the researchers. Their approach effectively tackles the outcoupling problem without resorting to diffraction gratings or physically buckling the device.

“In principle, the EQE of these LEDs could reach over 30%,” says Wang. “This could be achieved by optimizing the additives and fabrication process.”

Zhanhua Wei, Qihua Xiong, Edward H. Sargent, and their teams at Huaqiao University, Nanyang Technological University, and the University of Toronto have also reached the 20% EQE milestone with a green-emitting metal halide perovskite LED that demonstrates an operational lifetime of over 100 h. While this is still not sufficient for practical applications, it improves on previously reported perovskite devices by 1–2 orders of magnitude.

The key is the introduction of a MABr additive (where MA is CH3NH3) during the simple, one-step spin-coating process, which forms a protective shell around the perovskite (CsPbBr3), to maximize the efficiency of the light generation process.

"The MABr shell passivates the nonradiative defects that would otherwise be present in CsPbBr3 crystals, boosting the photoluminescence quantum efficiency, while the MABr capping layer enables balanced charge injection,” explains Wei.

The passivating layer, together with an electron-blocking poly(methyl methacrylate) (PMMA) layer, ensures that no charge is wasted in nonradiative recombination. This strategy, called compositional distribution management, produces high-quality perovskite films with passivated defects.

“There is still plenty room for improvement in terms of EQE,” says Wei, “[and] we believe device stability to be the key obstacle to overcome. However, we have great confidence in the future of perovskite-based real applications. With this rapid improvement in performance, we believe we can get perovskite-based products into daily life in the relatively near future.”

Wang agrees that the recent findings offer real promise for perovskite LEDs in applications requiring high efficiency, high brightness, and large area at low cost.

“With these papers, perovskite LEDs cross the 20% threshold, which is the starting point for them to compete with organic LEDs,” comments Daniel Congreve, Rowland Fellow at the Rowland Institute at Harvard. “Both groups provide simple yet effective methods for improving the quality of the materials, innovations which I expect will drive further improvements in efficiency and stability.”

The results of Cao et al. and Lin et al. show just how far perovskite LED research has come in a few short years, he adds.

“Exceeding 20% is a remarkable achievement and an important milestone for these materials,” says Congreve. “At the same time, there is a lot of work on the road ahead. We still need more efficient red and blue emitters, with blue being a particular challenge, and despite admirable steps forward in stability shown in these papers there is still quite a way to go to achieve commercial viability.”

This article was originally published in Nano Today 23 (2018) 1-2.