Important technical barriers to inexpensive, durable electronics and solar cells made out of non-toxic chemicals have been overcome by researchers in the University of Minnesota and the National Renewable Energy Laboratory.
“What this research means is that we are one step closer to producing more pure and more stable electronic ink with non-toxic chemicals,” said Uwe Kortshagen, co-author of the paper announcing the breakthrough . “The bigger goal here is to find a way that this research can benefit everyone and make a real difference.”
“Imagine a world where every child in a developing country could learn reading and math from a touch pad that costs less than $10 or home solar cells that finally cost less than fossil fuels,” said Kortshagen.
Nanometer-sized Silicon Crystals
The researchers developed a unique method to create a specialized ink from non-toxic nanometer-sized crystals of silicon, often called electronic ink. Their electronic ink could produce inexpensive electronic devices with techniques that essentially print it onto inexpensive sheets of plastic, as if they were silk-screening a t-shirt.
Almost, but it’s not quite that easy. The research team first needed to solve fundamental problems of silicon electronic inks. The first problem is the need of organic “soap-like” molecules, called ligands, for making inks with a good shelf life. But the ligands cause disadvantageous residues in the films after printing, leading to films with electrical properties too poor for electronic devices.
Secondly, nanoparticles are often deliberately implanted with impurities, a process called “doping,” to enhance their electrical properties.
The researchers’ new method uses an ionized gas, called nonthermal plasma, to not only produce silicon nanocrystals, but also to cover their surfaces with a layer of chlorine atoms. This surface layer of chlorine induces an interaction with many widely used solvents that allows production of stable silicon inks with excellent shelf life without the need for organic ligand molecules.
The team also discovered that these solvents lead to doping of films printed from their silicon inks, which gave them an electrical conductivity 1,000 times larger than un-doped silicon nanoparticle films. The researchers have a provisional patent on their findings.
Lance M. Wheeler, Nathan R. Neale, Ting Chen, Uwe R. Kortshagen. Hypervalent surface interactions for colloidal stability and doping of silicon nanocrystals.
Nature Communications, 2013; 4 DOI: 10.1038/ncomms3197
Image courtesy of University of Minnesota : Argon gas atoms flowing through the glass tube of a plasma reactor.