Perovskites advance toward vibrant, low-cost displays
Metal halide perovskites are exciting materials for low-cost, efficient solar cells, and devices based on these materials are getting close to commercialization. A string of recent advances now shows that perovskites also hold promise for next-generation displays and cheap, energy-efficient lighting.
A group of researchers led by Jin Young Kim recently reported in the journal Joule a simple, reversible method to coax a rainbow of colors out of perovskite nanocrystals. Meanwhile, two independent articles in the journal Nature report perovskite-based light-emitting diodes (LEDs) that emit light with a record-breaking efficiency of over 20 percent.
The vivid, high-definition screens in today’s high-end TVs use two different technologies. In OLED TVs, organic LEDs produce red, green, and blue lights at each tiny pixel. Meanwhile, some companies are now making QLED TVs, in which quantum dots, which are semiconductor nanocrystals, act as color filters to produce a combination of colors from white LED arrays.
Perovskites are easy to process from solutions and could lead to inexpensive quantum dot or LED displays that could be sprayed or printed on flexible substrates using conventional techniques. Perovskite nanocrystals emit much purer colors than OLEDs and QLEDs, says Jin Young Kim, professor of energy and chemical engineering at the Ulsan National Institute of Science and Technology in Korea, and author of the Joule article.
Nanocrystals of the perovskite cesium lead halide (CsPbX3) emit sharp, saturated colors that can easily be tuned by changing the halogen element in the material. The material emits red, green, and blue when it is rich in iodine, bromine, and chlorine, respectively. But different color materials have to be synthesized separately.
Kim and his colleagues report an easy method to tune the ratio of halide atoms in a perovskite nanocrystal solution to make it emit colors across the entire visible wavelength spectrum of 400–700 nm. The researchers make CsPbX3 nanocrystals in solution and then add a haloalkane solvent to the solution as a halide source. When they add trioctylphosphine (TOP) to the solution, the halogen-carbon bond in the solvent breaks, allowing the solvent and the perovskite to exchange halogen atoms. The solution changes color as the perovskite’s composition changes.
For example, adding tribromomethane and TOP to CsPbCl3 nanocrystals gave CsPbClxBry nanocrystals, changing the color from red to green. Likewise, adding trichloromethane or ethyl iodide to a solution of green CsPbBr3 nanocrystals gave CsPbBrxCly or CsPbBrxIy, shifting the color to blue or to red respectively.
The researchers made red, green, and blue LEDs with these nanocrystals using conventional fabrication methods. The devices glowed very saturated, pure colors, Kim says. “We believe that our development is a solution toward the commercialization of perovskite LEDs.”
The two LED devices reported in Nature, meanwhile, are made of a 200-nm-thick perovskite semiconductor layer sandwiched between two electrodes. One group, led by Jianpu Wang and Wei Huang of Nanjing Tech University in China, used submicrometer-sized crystals of formamidinium lead iodide for the perovskite layer. This structuring increases the fraction of light that comes out of the emissive layer to 30%, compared with 22% for a device made with a flat perovskite layer.
The other group, led by Zhanhua Wei of Huaqiao University in China, Qihua Xiong of Nanyang Technological University in Singapore, and Edward Sargent of the University of Toronto in Canada, used crystals of CsPbBr3 capped by methyl ammonium bromide (MABr). The MABr fills gaps between the crystals, quenching the defects that would otherwise trap electrons and holes and reduce photon generation. The top MABr layer helps to balance the electrons and holes injected into the perovskite layer, so that the charges are most efficiently used to generate photons.
Both devices boast an external quantum efficiency (EQE)—the number of photons produced per electron absorbed—of over 20%. The highest reported EQEs so far for red and green LEDs are around 12% and 14% respectively. The EQE of 20% already rivals those of the best-performing OLEDs, write Paul Meredith and Ardalan Amin in a News & Views article accompanying the Nature papers. And it has been achieved in less than four years since the report of the first perovskite LED, write Meredith and Amin, “suggesting that there is plenty of room for even further improvement in their performance.”