Perovskites


Most of the solar panels ( for example: RENOGY 150 WATT SOLAR PANEL ) commercially deployed are made from the silicon cells described above. But research into other materials and strategies is underway in laboratories around the world.

Some of the most promising recent research for silicon alternatives has involved materials called perovskites. The mineral perovskite (CaTiO3) was named in 1839 in honor of Count Lev Aleksevich Perovski (1792-1856), a Russian mineralogist. It can be found on every continent and in the clouds of at least one exoplanet. The word “perovskite” is also used for synthetic compounds that have the same orthorhombic crystal structure as the naturally occurring mineral (or a closely related one) and share a structurally similar chemical formula. Depending on which elements are used, perovskites can display a wide variety of useful properties, such as superconductivity, giant magnetoresistance, and photovoltaic activity. Their use in PV cells has generated a great deal of optimism, as they have shown an unprecedented increase in efficiency from 3.8 percent to 20.1 percent in the past seven years of laboratory research. This rapid rate of progress inspires confidence that further gains are likely, especially as the factors limiting efficiency are becoming clearer. Recent experiments at Los Alamos showed that solar cells made from certain perovskites attained efficiencies similar to silicon’s, while potentially being cheaper and easier to work with. The secret to perovskite’s appeal is the ability to routinely grow defect-free crystals of millimeter-scale in a thin film. This is a huge size for a perfect crystal lattice, which allows the conduction electron to travel through the crystal without interference. This crystal quality partly compensates for the somewhat less-than-ideal band gap of about 1.4 eV, compared with silicon’s nearly optimal 1.1 eV.Much of the research directed toward increasing the efficiency of perovskite cells involves searching for ways to eliminate as many crystal defects as possible. The ultimate goal is to manufacture an entire cell’s perovskite layer in the form of a perfect crystal lattice. Researchers at MIT have recently made significant progress on this. They have discovered how to “heal” the defects in a film made from particular perovskites by exposing it to intense light. The advantage over previously devised methods of defect removal, involving chemical baths or electrical current, is that no physical contact with the film is required.Whether or not perovskites will lead to revolutions in solar panel cost or aggregate efficiency is still unknown. While they’re easy to make, so far most perovskites decompose rapidly, which sharply limits their utility.Research is underway in many places to attack the decomposition problem. A collaboration between scientists in China and Switzerland invented a novel way to form perovskite cells that obviates the need for hole motion (see above). Since the layer devoted to hole conduction is susceptible to degradation, the material will be far more stable once that layer can be eliminated. A recent report from Berkeley Labs describes how perovskite solar cells might one day attain their theoretical maximum 31 percent efficiency while still being cheaper to manufacture than silicon cells. The Berkeley researchers measured the conversion efficiency of individual grain surfaces using photoconductive atomic force microscopy. They found, to their surprise, that different facets have markedly different efficiencies, with some exhibiting the maximum possible.The researchers now believe that they can find a way to grow bulk films with only the most efficient facets interfacing with the electrodes. This could lead to the entire cell running at 31 percent. If this pans out, it would amount to a revolutionary advance in solar cell technology.