Perovskite LED is expected to revolutionize lighting and display

In solar cells, a cheap and easy-to-manufacture material called perovskite is very good at converting photons into electrical energy. Now, perovskite will be used for new purposes, converting electrons into light, and its efficiency is comparable to the commercial organic light-emitting diodes (LEDs) commonly found in mobile phones and flat-screen TVs. What will this material do in the future? The researchers published a report in the "Science Progress" magazine last week that they have used a 3D printer to print patterns on a full-color display using perovskite.

Richard Friend, a physicist at the University of Cambridge in the UK, said: "This is an amazing research result and very encouraging." Friend's team developed the first perovskite LED in 2014. The results of this research provide hope for future computer screens and giant displays made of these cheap crystalline materials (made of ordinary raw materials). However, Friend warned that the new perovskite display is not yet commercially viable.

At present, the materials in semiconductor LEDs (including organic LEDs) need to be processed at high temperature in a vacuum chamber to ensure that the resulting semiconductor is pure. In contrast, perovskite can be prepared by simply mixing its chemical components in solution at room temperature. At the same time, only a simple heat treatment is needed to crystallize them. Even if the perovskite crystals eventually have defects, these defects usually do not destroy the light-emitting function of the material.

In most perovskite LEDs, the electrode sandwiched between luminescent materials is responsible for transferring charge. When the charges meet in the center of the "sandwich", the electrons fill this gap and release a little energy in the form of photons.

The color of the photon depends on the chemical composition of the perovskite, which allows researchers to adjust the color of the photon by changing the formula of the perovskite. The first perovskite LED from the Cambridge research team emits near infrared, red or green light, depending on their composition. Since then, the team and other teams have successively produced full-spectrum color perovskite LEDs.

The earliest perovskite LEDs can only convert 0.76% of electrons into photons. This is because the charge passing through the material is trapped between the countless crystals that make up the material. Now many teams have overcome this obstacle. For example, at the end of last year, Friend ’s team reported in the journal Nature-Photonics that by adding a layer of luminescent polymer to guide the charge to bypass surface defects, they have manufactured a red perovskite with an efficiency of 20.1% LED.

A research team led by Edward Sargent, a chemist at the University of Toronto in Canada, took a different approach last year by adding an additive to the perovskite formulation to form a crystalline shell around the perovskite crystals. The researchers reported in the journal Nature that these shells prevent surface defects from trapping charge, resulting in a green perovskite with an efficiency of 20.3%. This is still far below the efficiency of many inorganic LEDs, but it is sufficient for some applications.

Researchers led by Feng Gao, a physicist at Linköping University in Sweden, reported in the online edition of Nature-Photonics on March 25 that they developed a method to solve the defect problem. The researchers targeted the tendency of lead ions at the edge of perovskite crystals to trap electrons. By adding a substance combined with lead, they reduced the capture of electrons by ions, creating a near-infrared LED with an efficiency of 21.6%.

Friend said the results in the past five years were "quite amazing." However, the life span of these perovskite equipment has not exceeded 50 hours, far below the 10,000 hours required for commercial use. Gao said it is unclear why the perovskite crystals will decompose after tens of hours. But he pointed out that the early organic LED life is also very short. Perovskite solar cell manufacturers have largely solved similar life problems by protecting equipment from air and humidity. Gao said: "I am very optimistic that this field can develop rapidly and perovskite LEDs can also be improved."

The latest results of the research team led by Jennifer Lewis, a material scientist at Harvard University, may provide new strategies for building displays. Lewis and her colleagues used a 3D printer to arrange tiny linear perovskite structures in a color display. Lewis says that when the "ink" carrying the nanowires passes through the printer nozzle, the shear forces align them. The common orientation of the nanowires allows each LED to emit light with a single preferred oscillation or polarization.

For their prototype display, Lewis' team did not connect each LED to the electrode; instead, the researchers exposed the entire display to ultraviolet light. Just like applying voltage, ultraviolet light kicks electrons out of their normal state, allowing them to move. Then, they can recombine with the vacancies and emit visible light. But because the emitted light is polarized, Lewis and her colleagues need to use polarization filters to control it.

In one example, the researchers used three different perovskite formulations to create a display, where each pixel contains a red, green, and blue dot side by side, and the nanowire direction of each dot is offset by 60 °. By rotating the polarizing filter, researchers can mix colors or separate a single color.

Sargent said that the perovskite LED still faces many obstacles. But he added, "This work will span the next 10 years and show what cool things we can do." (Zhao Xixi)

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