In the past decade, researchers have used a new material made of lead-halide perovskites as the semiconducting absorber layer in solar cells. While these perovskites have dramatically increased solar cells' efficiency for converting solar energy to electricity, they not only contain toxic lead, but also are unstable in light, moisture and heat and breaks down in a matter of days, leaking lead into groundwater.
With a three-year, $480,000 grant from the National Science Foundation, Rohan Mishra, assistant professor of mechanical engineering & materials science, and Pratim Biswas, assistant vice chancellor, the Lucy & Stanley Lopata Professor and chair of the Department of Energy, Environmental & Chemical Engineering, at Washington University in St. Louis' School of Engineering & Applied Science are studying whether a nontoxic element — bismuth, lead's neighbor on the periodic table — is a safer and equally efficient substitute for lead in perovskites.
It's a big task: There are about 30,000 compounds of bismuth oxide perovskites that can be potentially synthesized, but only about 25 are experimentally known, Mishra said.
Together, the two teams will use an efficient strategy of quantum-mechanical calculations, machine learning, data mining, state-of-the-art synthesis techniques and characterization to rapidly discover and optimize these new compounds.
"We will use quantum-mechanical calculations — performed on some of the world's most powerful supercomputers — and combine them with materials informatics to efficiently search through the 30,000 compounds and then suggest which of these could be stable and could be potentially useful for semiconducting applications," Mishra said.
Once his team makes its predictions and finds some candidates, Biswas' team takes on the challenging process of synthesizing them in the lab using an innovative, scalable electrospray technique that disperses the elements into a fine, uniform aerosol.
"The aerosol techniques allow us to make complex materials very easily," Biswas said. "I can tune my processing conditions very precisely to get the composition I want without having to go through trial and error. This electrospray process allows us to get them to be extremely stable with the desired crystal structures."
Much of this work has been done by doctoral students: Arashdeep Thind in Mishra's lab and Shalinee Kavadiya in Biswas' lab.
Once the thin films are created, Mishra's team will analyze them at the atomic scale using powerful electron microscopes at Oak Ridge National Laboratory.
"We see if the atoms are distributed the way we predicted or if they have defects or imperfections," Mishra said. "We take this information to make more accurate models for the quantum-mechanical calculations to see which defects are bad and which need to be mitigated, then take it back to Prof. Biswas' lab, where they develop new strategies to avoid the defects."
Ultimately, they would design materials that are tolerant of defects and still perform well. Biswas' team also will make some devices with solar cells based on these materials to demonstrate that they could be as good or better than existing materials, but easier to make, less expensive and much more stable.