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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. Moreover, the organic amino-acid precursor additives appear to passivate surface defects, reducing radiative recombination.
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.”
Researchers have set a new efficiency record for LEDs based on perovskite semiconductors, rivalling that of the best organic LEDs (OLEDs).
Compared to OLEDs, which are widely used in high-end consumer electronics, the perovskite-based LEDs, developed by researchers at the University of Cambridge, can be made at much lower costs, and can be tuned to emit light across the visible and near-infrared spectra with high colour purity.
The researchers have engineered the perovskite layer in the LEDs to show close to 100% internal luminescence efficiency, opening up future applications in display, lighting and communications, as well as next-generation solar cells. These perovskite materials are of the same type as those found to make highly efficient solar cells that could one day replace commercial silicon solar cells. While perovskite-based LEDs have already been developed, they have not been nearly as efficient as conventional OLEDs at converting electricity into light.
Earlier hybrid perovskite LEDs, first developed by Professor Sir Richard Friend’s group at the University’s Cavendish Laboratory four years ago, were promising, but losses from the perovskite layer, caused by tiny defects in the crystal structure, limited their light-emission efficiency. Now, Cambridge researchers from the same group and their collaborators have shown that by forming a composite layer of the perovskites together with a polymer, it is possible to achieve much higher light-emission efficiencies, close to the theoretical efficiency limit of thin-film OLEDs. Their results are reported in the journal Nature Photonics.
“This perovskite-polymer structure effectively eliminates non-emissive losses, the first time this has been achieved in a perovskite-based device,” said Dr Dawei Di from Cambridge’s Cavendish Laboratory, one of the corresponding authors of the paper. “By blending the two, we can basically prevent the electrons and positive charges from recombining via the defects in the perovskite structure.”
The perovskite-polymer blend used in the LED devices, known as a bulk heterostructure, is made of two-dimensional and three-dimensional perovskite components and an insulating polymer. When an ultra-fast laser is shone on the structures, pairs of electric charges that carry energy move from the 2D regions to the 3D regions in a trillionth of a second: much faster than earlier layered perovskite structures used in LEDs. Separated charges in the 3D regions then recombine and emit light extremely efficiently.
“Since the energy migration from 2D regions to 3D regions happens so quickly, and the charges in the 3D regions are isolated from the defects by the polymer, these mechanisms prevent the defects from getting involved, thereby preventing energy loss,” said Di.“The best external quantum efficiencies of these devices are higher than 20% at current densities relevant to display applications, setting a new record for perovskite LEDs, which is a similar efficiency value to the best OLEDs on the market today,” said Baodan Zhao, the paper’s first author.
While perovskite-based LEDs are beginning to rival OLEDs in terms of efficiency, they still need better stability if they are to be adopted in consumer electronics. When perovskite-based LEDs were first developed, they had a lifetime of just a few seconds. The LEDs developed in the current research have a half-life close to 50 hours, which is a huge improvement in just four years, but still nowhere near the lifetimes required for commercial applications, which will require an extensive industrial development programme. “Understanding the degradation mechanisms of the LEDs is a key to future improvements,” said Di.
Stable perovskite films promise improved LED efficiency
The development of metal halide perovskite devices able to act as LEDs with acceptable quantum efficiency has been a focus of research for some time, but poor operational stability has proven to be a challenge. Performance has been limited particularly by the large perovskite grain sizes, which negatively impact the electro-luminescence and encourage so-called trap states at the grain boundaries, where the desired movement of electrons and holes is limited.
A team at City University of Hong Kong has now developed a manufacturing process capable of producing smooth, pinhole-free and small-grained perovskite films, and demonstrated that both quantum efficiency and operational lifetime can benefit. The work was reported in Nature Communications.
The breakthrough relates in particular to perovskites based on inorganic cesium cations, or CsPbX3, where X can be chlorine, bromine or iodine. These materials exhibit better thermal and chemical stability than other metal halide perovskites, but have proved to have poor electro-luminescence, due to the large perovskite grain sizes.
Using cesium trifluoroacetate (TFA) as the cesium source rather than the more common cesium bromide in a one-step solution coating process proved to be the key, due to its effect on the crystallization rate of the perovskite films. Surface defects in the film were suppressed as a result.
“TFA ions are preferentially bound to the surface Pb2+ ions in the CsPbBr3,” noted the team in its published paper. “Compared to the conventional CsBr route, the TFA-derived films show a flatter energy landscape with a more homogeneous energy level distribution for charges, more stable crystal structure, better optical properties, and suppressed ion migration.”
Record operational lifetime
Testing the performance of the modified perovskite films as LEDs, the project was able to produce green LEDs demonstrating a current efficiency of 32 cd A−1, corresponding to an external quantum efficiency of 10.5 percent. More importantly, the all-inorganic perovskite LEDs demonstrated a record operational lifetime, with a half-lifetime of over 250 hours at an initial luminance of 100 cd m−2.
This represents a 17-fold improvement in operational lifetime compared with perovskite LEDs derived from CsBr, according to the University, and could now indicate a route towards inorganic lead-halide perovskite films able to act as highly efficient LEDs.”Our study suggests that the high color-purity and low-cost all-inorganic lead halide perovskite films can be developed into efficient and stable LEDs via a simple optimization of the grain boundaries,” commented Andrey Rogach of City University, a co-author of the paper.
“I foresee significant application potential of such films, as they are easy to fabricate and can be easily deposited by printing to realise various optoelectronic devices.”
