Researchers in California have extended the spectral range of perovskite light-emitting diodes into the blues. How did they do it? By cracking the mystery of why the optical and electronic properties of the materials change when current flows through them.
Painstaking measurements revealed that current-induced heating deformed cells in the semiconductor crystals. That observation enabled the team to make the first single-crystal perovskite diodes, says group leader Peidong Yang, a chemistry professor at the University of California at Berkeley.
Earlier perovskite light-emitting diodes (LEDs) only emitted red or green light, so this new finding immediately extends the capabilities of the compounds across the visible spectrum, which is essential if they’re ever to be used in electronic displays. In the long term, the discovery points to potential ways to control instabilities that had threatened to limit practical use of the materials.
What are perovskites?
Perovskites are materials that share the cubic crystalline structure of calcium titanate (CaTiO3), a mineral called perovskite that’s found in nature. Varieties containing metals such as lead or tin, and halogens such as fluorine, chlorine, bromine, or iodine, are semiconductors that have attracted wide attention over the past decade for their potential use in solar cells, LEDs, and transistors.
This is a blue-emitting halide perovskite crystal. Photo: Peidong Yang/UC Berkeley
The materials are inexpensive, and they can be easily fabricated into semiconductor devices using solution chemistry rather than the high-temperature processes needed by silicon. However, the metal-halide perovskites lacked the stability of conventional compound semiconductors such as gallium arsenide, raising doubts about potential applications.
Metal-halide perovskites are particularly attractive for solar cells because they can efficiently convert absorbed sunlight into electricity. Last year, Oxford PV, a spinout of the University of Oxford in England, made solar cells that converted a record 28 percent of light energy into electricity, beating the 26.7 percent record for silicon solar cells. The company is now developing hybrid solar cells containing light-absorbing layers of both metal-halide perovskite and silicon.
The ability to deposit metal-halide perovskites from solution onto a substrate without the high-temperature processing needed for silicon has spurred interest in them as transistors. Development is still in the early stages, but a year ago, Aram Amassian at North Carolina State University in Raleigh made field-effect transistors by depositing a thin layer of a perovskite on silicon dioxide.
Earlier experiments showed that perovskite semiconductors could efficiently convert electrical energy into red and green light in LEDs. However, the emission did not reach blue wavelengths (although, in theory, the compounds should have been able to emit blue light).
The reason for that failure, says Yang, is that depositing thin films of metal-halide perovskites from solution produces regions with different band-gap energies. When electrons and holes move through such material, they tend to migrate to the regions that have the smallest band-gap energies, a funneling effect that causes electrons and holes only to recombine where they produce low-energy red or green light.
Inspired by graphene
Yang's group solved that problem by making a single crystal LED with a layered structure like that of graphene. The semiconducting perovskite was a blend of cesium, lead, and bromine, with ammonium and methyl groups added to form an organic spacer between metal-halide perovskite layers.
The crystal structure of the blue-emitting halide perovskite changes when heated from room temperature (300 Kelvin) to 450 K, the typical operating temperature of an electronic device. The structural change alters the wavelength of light, shifting it from blue to blue-green. Image: Peidong Yang/UC Berkeley
To separate the layers to yield single-crystal LEDs, they borrowed the "micro-mechanical defoliation" technique pioneered by graphene developers. They applied sticky tape to the thin film to pull off a single crystal with the same band-gap throughout it. The layers separated easily, and glowed with blue light when current was applied. In the 24 January issue of Science Advances, they report producing blue light at 416, 445, and 473 nanometers from areas with one, two, or three layers of cells, respectively.
The group also found that the instability of the perovskite compounds stemmed from the presence of the halides, the same atoms that allowed the crystals to be fabricated from solution. Halides form ionic bonds with other atoms, which are much weaker than the covalent bonds present in silicon or semiconductors such as gallium arsenide or gallium nitride. The weaker bonds are more vulnerable to heat, humidity, and other environmental effects.
Joule heating from current flow through the semiconductor changes the arrangement of atoms in the crystal. At room temperature (300 K) bonds are bent, but as the temperature increases, the bonds straighten to form right angles. The fundamental building blocks are not decomposing, but the crystalline lattice is soft, says Yang. So the change is "a structural deformation rather than chemical instability."
That would be good news for other metal-halide perovskite applications. Electrical and electronic applications inevitably require current flow, but that's not a fatal flaw. "We will have to take care of heat management" to control the deformation, says Yang, but the material should not crumble away. In fact, the sensitivity of metal-halide perovskites to heat, moisture, and chemicals could make them valuable sensors.
Jeff Hecht writes about lasers, optics, fiber optics, electronics, and communications. Trained in engineering and a life senior member of IEEE, he enjoys figuring out how laser, optical, and electronic systems work and explaining their applications and challenges. At the moment, he’s exploring the challenges of integrating lidars, cameras, and other sensing systems with artificial intelligence in self-driving cars. He has chronicled the histories of laser weapons and fiber-optic communications and written tutorial books on lasers and fiber optics.