Potential solar cell material can be both hard and insulating

A hybrid perovskite that is both stiff and thermally insulating—unusual combination of properties—could pave the way for improved solar cells (ACS Nano 2024, DOI: 10.1021/acsnano.3c12172).

In many materials, including ceramics and polymers, those properties don’t normally go hand in hand: the harder the material, the more thermally conducting it is, and vice versa. But in this case, researchers created three separate materials that became more insulating as they got stiffer.

The materials were all 2D hybrid metal halide perovskites. A perovskite is any material with a particular crystal structure; the 2D hybrid versions are thin films made up of alternating organic and inorganic layers. Scientists are pursuing perovskites as the future of solar cells because they can convert a higher percentage of light to electricity.

Researchers achieved this mix of properties by altering the organic ligands between the inorganic layers. To do this, they added benzene rings to the ends of some of or all the ligands’ carbon chains.

Benzene rings are large and heavy, says Jun Liu of North Carolina State University, who co-led the research with Qing Tu of Texas A&M University and Wei You of the University of North Carolina at Chapel Hill. Their atomic structure makes them stiff, he says, and because they’re heavy, the vibrations of one have a hard time syncing up with the next one, which slows the transfer of heat. Two of the materials the team developed had ligands composed of different combinations of carbon chains and benzene rings. The third had benzene rings on all its ligands.

Stiff, insulating thin films could add protection between layers of solar cells or other electronics, with the hardness reducing scratching or slippage and the insulation preventing overheating. Liu suggests that the films could be useful in satellites, which also have layers that must resist damage and which need to deal with large changes in temperature as they move in and out of sunlight in space.

Team members also found that they could select chiral molecules as starting points for making the ligands. To their surprise, they found that having chiral chains allowed the material to keep the same stiffness and thermal conductivity even as they altered the composition of the organic layers. That may be particularly important for solar cells, because it could enable researchers to alter the chemistry of the material to improve its optoelectronic properties.

Liu says this paper didn’t push the limits of what can be done with these materials. His team is working on increasing their thermal insulation to approach the thermal conductivity of air. He also wants to work with photovoltaic researchers to see whether this work can be combined with advances in optical or electronic efficiency. “If you want the solar cell material to be commercially competitive, you have to consider many different properties,” he says.

Zhitang Tian, who studies energy transport in materials at Cornell University and was not involved with this project, says, “Typically, thermal conductivity and elastic modulus go hand in hand. Fundamentally, it is interesting to see these exceptions.”