What is a perovskite?
A perovskite is a material that has the same crystal structure as the mineral calcium titanium oxide, the first-discovered perovskite crystal. Generally, perovskite compounds have a chemical formula ABX3, where ‘A’ and ‘B’ represent cations and X is an anion that bonds to both. A large number of different elements can be combined together to form perovskite structures. Using this compositional flexibility, scientists can design perovskite crystals to have a wide variety of physical, optical, and electrical characteristics. Perovskite crystals are found today in ultrasound machines, memory chips, and now – solar cells.
A schematic of a perovskite crystal structure. (Wikimedia Commons)
Clean energy applications of perovskites
All photovoltaic solar cells rely on semiconductors — materials in the middle ground between electrical insulators such as glass and metallic conductors such as copper — to turn the energy from light into electricity. Light from the sun excites electrons in the semiconductor material, which flow into conducting electrodes and produce electric current.
Silicon has been the primary semiconductor material used in solar cells since the 1950s, as its semiconducting properties align well with the spectrum of the sun’s rays and it is relatively abundant and stable. However, the large silicon crystals used in conventional solar panels require an expensive, multi-step manufacturing process that utilizes a lot of energy. In the search for an alternative, scientists have harnessed the tunability of perovskites to create semiconductors with similar properties to silicon. Perovskite solar cells can be manufactured using simple, additive deposition techniques, like printing, for a fraction of the cost and energy. Because of the compositional flexibility of perovskites, they can also be tuned to ideally match the sun’s spectrum.
In 2012, researchers first discovered how to make a stable, thin-film perovskite solar cell with light photon-to-electron conversion efficiencies over 10%, using lead halide perovskites as the light-absorbing layer. Since then, the sunlight-to-electrical-power conversion efficiency of perovskite solar cells has skyrocketed, with the laboratory record standing at 25.2%. Researchers are also combining perovskite solar cells with conventional silicon solar cells – record efficiencies for these “perovskite on silicon” tandem cells are currently 29.1% (surpassing the record of 27% for conventional silicon cells) and rising rapidly. With this rapid surge in cell efficiency, perovskite solar cells and perovskite tandem solar cells may soon become cheap, highly efficient alternatives to conventional silicon solar cells.
A cross-section of a perovskite solar cell. (Clean Energy Institute)
What are some current research objectives?
While perovskite solar cells, including perovskite on silicon tandems, are being commercialized by dozens of companies worldwide, there are still basic science and engineering challenges to address that can improve their performance, reliability, and manufacturability.
Some perovskite researchers continue to push conversion efficiencies by characterizing defects in the perovskite. While perovskite semiconductors are remarkably defect-tolerant, defects still –negatively affect performance — especially those occurring at the surface of the active layer. Other researchers are exploring new perovskite chemical formulations, both to tune their electronic properties for specific applications (like tandem cell stacks), or further improve their stability and lifetime. READ MORE...
Researchers at the RIKEN Center for Emergent Matter Science (CEMS) in Japan have found a simple and affordable way to store ammonia, an important chemical in a range of industries. The discovery could also help in establishing a hydrogen-based economy.
Ammonia, chemically written as NH3, is widely used across industries ranging from textiles to pharmaceuticals and is an important component in the manufacture of fertilizers. For its current use, ammonia is stored in pressure-resistant containers after liquefying it at temperatures of -27 Fahrenheit (-33 degrees Celsius).
Alternate methods of storing ammonia in porous compounds have been explored. The storage and retrieval process can be achieved at room temperature, but the storage capacity of these compounds is limited.
A research team led by Masuki Kawamoto at RIKEN CEMS has now found that perovskites, crystalline structures associated with improving energy conversion efficiencies of solar panels, can also serve as an excellent medium for the storage and retrieval of ammonia.
Perovskite as an ammonia carrier
Kawamoto's team found that the perovskite ethyl ammonium lead iodide (EAPbI3) reacts with ammonia at room temperature and pressure to make lead iodide hydroxide, or Pb(OH)I. Ethyl ammonium lead iodide has a one-dimensional columnar structure but, after reacting with ammonia, forms a two-dimensional layered structure.
Ammonia is a highly corrosive gas, but the chemical reaction with the perovskite allows for its safe storage that does not need any special equipment to store it either. The retrieval process is also very straightforward. Under vacuum, ethyl ammonium lead iodide can be heated to 122 Fahrenheit (50 degrees Celsius) to release ammonia gas. READ MORE...
