Caption: Cross section of the interface between a lead chalcogenide nanoparticle and its embedding cadmium chalcogenide matrix. When integrated into optoelectronic devices, it is enough to have a single atom in the wrong place at the interface (represented by the glowing blue color) to jeopardize their performance. (Illustration: Peter Allen, Institute for Molecular Engineering, University of Chicago)

After taking a closer look inside of solar cell nanostructures, researchers have noticed some defects at the atomic level.

Researchers from the U.S. Department of Energy’s Argonne National Laboratory and the University of Chicago have identified various defects including that heterogeneous nanostructured materials have structural defects that can affect the performance of solar cell devices and found that the root cause of the defects was two commonly used semiconductor materials—lead selenide and cadmium selenide.

“We are interested in understanding quantum dots and nanostructures and how they perform for solar cells,” Giulia Galli, Liew Family professor of Molecular Engineering at the University of Chicago and co-author of the paper, said in a statement. “We are doing modeling, using both classical molecular dynamics and first principle methods, to understand the structure and optical properties of these nanoparticles and quantum dots.”

The researchers looked at heterostructured nanoparticles—particularly a colloidal quantum dot in which lead selenide nanoparticles are embedded in cadmium selenide.

“Experiments have suggested that these heterostructured nanoparticles are very favorable for solar energy conversion and thin-film transistors,” Márton Vörös, Aneesur Rahman Fellow at Argonne and co-author on the paper, said in a statement.

Colloidal quantum dot energy conversion efficiencies currently hover around 12 percent in the lab but the researchers hope to predict quantum dot structural models that go beyond 12 percent and possibly reach 20 percent efficiency.

The researchers developed a computational strategy to investigate at the atomic level the effect of the structure of the interfaces on the materials’ optoelectronic properties. They used classical molecular dynamics and first principles method that do not rely on any fitted parameters to build computational models of embedded quantum dots.

They used this model to run a series of simulation to characterize the quantum dots and find that atoms are displaced at the interface and their corresponding electronic states—called trap states—can jeopardize solar cell performance.

The researchers also used the model to predict a new material that does not have trap states and should perform better in solar cells.

“Using our computational framework, we also found a way to tune the optical properties of the material by applying pressure,” Federico Giberti, postdoctoral research scholar at the University of Chicago's Institute for Molecular Engineering and first author of the paper, said in a statement.

The next step for the researchers will be to use the new computational framework to investigate other materials and structures.

“We believe that our atomistic models, when coupled with experiments, will bring a predictive tool for heterogeneous nanostructured materials that can be used for a variety of semiconducting systems,” Giberti said. “We are very excited about the possible impact of our work.”

The study was published in Nano Letters.