Using a method known as chemical vapor deposition, scientists in Japan have created perovskite-based LEDs that may one day replace conventional LED lights.
AsianScientist (Aug. 25, 2017) – Scientists in Japan have invented a cheaper method to produce perovskite-based LED lights. They report their results in the Journal of Physical Chemistry Letters. The bipolar electronic properties of perovskite structures give them the ability to convert electricity into light. Light-emitting diode (LED) technology currently relies on semi-conductors that are difficult and expensive to manufacture, hence perovskite LEDs may become the new industry standard in the future due to their lower cost and better conversion efficiency. Moreover, by changing the atomic composition in the perovskite structure, perovskite LEDs can be easily tuned to emit specific colors. The manufacturing of these perovskite LEDs is currently based on dipping or covering the targeted surface with liquid chemicals, a process which is difficult to setup, is limited to small areas and has low consistency between samples. To overcome these issues, scientists from the Okinawa Institute of Science and Technology (OIST) used chemical vapor deposition (CVD) to assemble the first perovskite LED. “Chemical vapor deposition is already compatible with the industry, so in principle it would be easy to use this technology to produce LEDs,” commented Professor Qi Yabing of OIST. “The second advantage in using CVD is a much lower variation from batch to batch compared to liquid-based techniques. The third advantage is that CVD can achieve a uniform surface over very large areas.” The perovskite LED comprises many layers working in synergy. First, an indium tin oxide glass sheet and a polymer layer allows electrons into the LED. The chemicals required for the perovskite layer—lead bromide and methylammonium bromide—are then successively bound to the sample using CVD, in which the sample is exposed to gases instead of liquid. Through this process, the perovskite layer is composed of nanometer-sized grains which play a critical role in the efficiency of the device. “With large grains, the surface of the LED is rough and less efficient in emitting light. The smaller the grain size, the higher the efficiency and the brighter the light,” explained Dr. Meng Lingqiang of OIST. “By changing the assembly temperature, we can now control the growth process and the size of the grains for the best efficiency.” The final step of the development process involved the deposition of two additional layers and a gold electrode, forming a complete LED structure. The result is a flexible, thick, film-like LED with a customizable pattern. The luminance, or brightness, currently stands at 560 cd/m2, comparable to a typical computer screen that emits 100 to 1,000 cd/m2. However, the brightness still falls short of ceiling fluorescent tubes which have a brightness of around 12,000 cd/m2. “Our next step is to improve the luminance a thousand-fold or more,” said Meng. “In addition, we have achieved a CVD-based LED emitting green light but we are now trying to repeat the process with different combinations of perovskite to obtain a vivid blue or red light.”