Making semiconductor diodes that emit blue light has always been a challenge
Since halide perovskites first drew wide attention in 2009, when Japanese scientists discovered that they make highly efficient solar cells, these easily made, inexpensive crystals have excited researchers. So far, red- and green-emitting diodes have been demonstrated, but not blue. Halide perovskite blue-emitting diodes have been unstable — that is, their color shifts to longer, redder wavelengths with use.
As Yang and his colleagues discovered, this is due to the unique nature of perovskites’ crystal structure. Halide perovskites are composed of a metal, such as lead or tin, 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 crystalizes from sea water. 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 heated from room temperature (about 300 Kelvin) to around 450 Kelvin, a common operating temperature for semiconductors, the crystal’s squashed structure expanded and eventually sprang into a new orthorhombic or tetragonal configuration.
Since the light emitted by these crystals depends on the arrangement of and distances between atoms, the color changed with temperature, as well. A perovskite crystal that emitted blue light (450 nanometers wavelength) at 300 Kelvin suddenly emitted blue-green light at 450 Kelvin.
Yang attributes perovskites’ flexible crystal structure to the weaker ionic bonds typical of halide atoms. Naturally occurring mineral perovskite incorporates oxygen instead of halides, producing a very stable mineral. Silicon-based and gallium nitride semiconductors are similarly stable because the atoms are linked by strong covalent bonds.
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. Electrons get funneled down to those crystals with the smallest bandgap — that is, the smallest range of unallowed energies — before emitting light, which tends to be red.