Scientists at the University of California, Berkeley, have created a blue light-emitting diode (LED) from halide perovskite, a new semiconductor material said to be cheap and easy to manufacture, overcoming a barrier that had previously prevented the employment of the devices. However, in the process, the team also discovered a fundamental property of halide perovskites that may prove a barrier to their widespread use as solar cells and transistors. In a paper appearing in the journal Science Advances, UC Berkeley chemist Peidong Yang and his colleagues show that the crystal structure of the halide perovskites reacts to temperature changes, humidity, and the chemical environment. This reaction disrupted the device’s optical and electronic properties, rendering them inherently unstable if scientists are unable to control the physical and chemical environments. “Some people say this is a limitation,” Yang said. “For me, this is a great opportunity. This is new physics: a new class of semiconductors that can be readily reconfigured, depending on what sort of environment you put them in. They could be a really good sensor, maybe a really good photoconductor, because they will be very sensitive in their response to light and chemicals.” Current semiconductors made of silicon or gallium nitride are very stable over a range of temperatures, primarily because their crystal structures are held together by strong covalent bonds. Halide perovskite crystals are held together by weaker ionic bonds, like those in a salt crystal. “This paper is not just about showing off that we made this blue LED,” Yang said. “We are also telling people that we really need to pay attention to the structural evolution of perovskites during the device operation, any time you drive these perovskites with an electrical current, whether it is an LED, a solar cell, or a transistor. This is an intrinsic property of this new class of semiconductor and affects any potential optoelectronic device in the future using this class of material.” Yang says the challenge of manufacturing blue light-emitting semiconductors isn’t a new one. In 2014, the Nobel Prize for physics was awarded for the breakthrough creation of efficient blue light-emitting diodes from gallium nitride. As Yang and his colleagues discovered, the instability of blue-emitting diodes is due to the nature of perovskite crystal structures. Halide perovskites are composed of a metal, equal numbers of larger atoms, such as cesium, and three times the number of halide atoms, such as chlorine, bromine, or iodine. When these elements are mixed together in solution and then dried, the atoms assemble into a crystal, just as salt crystallizes from seawater. Using a new technique and the ingredients cesium, lead, and bromine, the UC Berkeley and Berkeley Lab chemists created perovskite crystals that emit blue light and then bombarded them with x-rays at the Stanford Linear Accelerator Center (SLAC) to determine their crystalline structure at various temperatures. They found that, when headed from room temperature (about 300 K) to around 450 K, the crystal’s structure expanded and eventually sprang into a new orthorhombic or tetragonal configuration. Due to the light emitted by these crystals depending on the arrangement and distance of the atoms, the color changed with the temperature as well. A crystal emitting blue light at 300 K emitted blue-green light at 450 K. Yang attributes this flexibility to the weaker ionic bonds typical of halide atoms. According to Yang, blue-emitting perovskite diodes have been hard to create because the standard technique of growing the crystals as a thin film encourages formation of mixed crystal structures, each of which emits at a different wavelength. To avoid this, Yang’s co-first authors, Hong Chen, Jia Lin, and Joohoon Kang, grew single, layered crystals of perovskite and used tape to peel off a single layer of uniform perovskite. When incorporated into a circuit and zapped with electricity, the perovskite glowed blue. “We need to think in different ways of using this class of semiconductor,” Yang said. “We should not put halide perovskites into the same application environment as a traditional covalent semiconductor, like silicon. We need to realize that this class of material has intrinsic structural properties that make it ready to reconfigure. We should utilize that.”