A semiconductor is a material with electrical conductivity intermediate in magnitude between that of a conductor and an insulator. Semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. Such devices include transistors and digital and analog integrated circuits. Transition metal oxides also fill this category of materials. But these materials are also relatively rare and expensive. Harnessing the energy of sunlight can be as simple as tuning the optical and electronic properties of metal oxides at the atomic level by making an artificial crystal or super-lattice sandwich, says a Binghamton University researcher in a new study published in the journal Physical Review B which uses more ordinary and commonly available metals.
"Metal oxides are cheap, abundant and green," said Louis Piper, assistant professor of physics at Binghamton University. "And as the study proved, quite versatile. With the right touch, metal oxides can be tailored to meet all sorts of needs, which is good news for technological applications, specifically in energy generation and flat screen displays."
Semiconductors are an important class of materials in between metals and insulators. They are defined by the size of their band gap, which represents the energy required to excite an electron from the occupied shell to an unoccupied shell where it can conduct electricity. Visible light covers a range of 1 (infrared) to 3 (ultraviolet) electron volts. For transparent conductors, a large band gap is required, whereas for artificial photosynthesis, a band gap corresponding to green light is needed. Metal oxides provide a means of tailoring the band gap.
But whilst metal oxides are very good at electron conduction, they are very poor hole conductors. Holes refer to absence of electrons, and can conduct positive charge. To maximize their technologically potential, especially for artificial photosynthesis and invisible electronics, hole conducting metal oxides are required.
Knowing this, Piper has begun studying layered metal oxides systems, which can be combined to selectively dope (replace a small number of one type of atom in the material), or tune (control the size of the band gap). Recent work revealed that a super-lattice of two hole-conducting copper oxides could cover the entire solar spectrum. The goal is to improve the performance whilst using environmentally benign and cheap metal alternatives.
"It’s going to be a case of some serious detective work," said Piper. "We’re working in a world where physics and chemistry overlap. And we’ve reached the theoretical limit of our calculations and fundamental processes. Now we need to audit those calculations and see where we’re missing things. I believe we will find those missing pieces by playing around with metal oxides."
By reinforcing metal oxides’ 'good bits' and downplaying the rough spots, Piper is convinced that the development of new and exciting types of metal oxides that can be tailored for specific applications are well within our reach.
"We’re talking battery storage, fuel cells, touch screen technology and all types of computer switches," said Piper "We’re in the middle of a very important gold rush and its very exciting to be part of that race to strike it rich. But first we have to figure out what we don’t know before we can figure out what we do. One thing’s for sure: metal oxides hold the key. And I believe that we at Binghamton University can contribute to these efforts by doing good science and taking a morally conscious approach."
Indium oxide is one example that could be replaced with the new technology. It is used in some types of batteries, thin film infrared reflectors transparent for visible light (hot mirrors), some optical coatings, and some antistatic coatings. In combination with tin dioxide, indium oxide forms indium tin oxide, a material used for transparent conductive coatings.
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