Researchers have shown that titanium is an attractive choice to replace toxic lead in perovskite thin film solar cells. Image: Padture Lab/Brown University.A class of materials called perovskites has emerged as a promising alternative to silicon for making inexpensive and efficient solar cells. But for all their promise, perovskites are not without their downsides. Most contain lead, which is highly toxic, and include organic materials that are not particularly stable when exposed to the environment.
Now a group of researchers at Brown University and the University of Nebraska – Lincoln (UNL) has come up with a new titanium-based material for making lead-free, inorganic perovskite solar cells. In a paper published in Joule, the researchers show that the material makes a particularly good candidate, especially for use in tandem solar cells. These are arrangements in which a perovskite solar cell is placed on top of a solar cell made of silicon or another established material to boost the overall efficiency.
"Titanium is an abundant, robust and biocompatible element that, until now, has been largely overlooked in perovskite research," said the senior author of the new paper, Nitin Padture, a professor in Brown's School of Engineering and director of the Institute for Molecular and Nanoscale Innovation. "We showed that it's possible to use titanium-based material to make thin-film perovskites and that the material has favorable properties for solar applications which can be tuned."
Interest in perovskites, a class of materials with a particular crystalline structure, for clean energy emerged in 2009, when they were shown to be able to convert sunlight into electricity. The first perovskite solar cells had a conversion efficiency of only about 4%, but that has quickly skyrocketed to near 23%, rivalling traditional silicon cells. And perovskites offer some intriguing advantages. They're potentially cheaper to make than silicon cells and they can be partially transparent, raising the possibility of new technologies like windows that generate electricity.
"One of the big thrusts in perovskite research is to get away from lead-based materials and find new materials that are non-toxic and more stable," Padture said. "Using computer simulations, our theoretician collaborators at UNL predicted that a class of perovskites with cesium, titanium and a halogen component (bromine or/and iodine) was a good candidate. The next step was to actually make a solar cell using that material and test its properties, and that's what we've done here."
The team made semi-transparent perovskite films that had a bandgap – a measure of the energy level of photons the material can absorb – of 1.8 electron volts, which is considered to be ideal for tandem solar cell applications. This material had a conversion efficiency of 3.3%, which is well below that of lead-based cells, but a good start for an all-new material, the researchers say.
"There's a lot of engineering you can do to improve efficiency," said Yuanyuan Zhou, an assistant professor (research) of engineering at Brown and a co-author of the paper. "We think this material has a lot of room to improve."
Min Chen, a PhD student of materials science at Brown and the first author of the paper, used a high-temperature evaporation method to prepare the films, but says the team is investigating alternative methods. "We are also looking for new low-temperature and solvent-based methods to reduce the potential cost of cell fabrication," he said.
The research showed that the material has several advantages over alternative candidates for lead-free perovskites. One contender is a material made largely from tin, but tin rusts easily when exposed to the environment. Titanium, on the hand, is rust-resistant. The titanium-perovskite also has an open-circuit voltage – a measure of the total voltage available from a solar cell – of over 1 volt. Other lead-free perovskites generally produce a voltage smaller than 0.6 volts.
"Open-circuit voltage is a key property that we can use to evaluate the potential of a solar cell material," Padture said. "So, having such a high value at the outset is very promising."
The researchers say that the material's relatively large bandgap compared to silicon makes it a prime candidate to serve as the top layer in a tandem solar cell. The titanium-perovskite upper layer would absorb the higher-energy photons from the sun that the lower silicon layer can't absorb because of its smaller bandgap. Meanwhile, lower energy photons would pass through the semi-transparent upper layer to be absorbed by the silicon, thereby increasing the cell's total absorption capacity.
"Tandem cells are the low-hanging fruit when it comes to perovskites," Padture said. "We're not looking to replace existing silicon technology just yet, but instead we're looking to boost it. So if you can make a lead-free tandem cell that's stable, then that's a winner. This new material looks like a good candidate."
This story is adapted from material from Brown University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.