A new study from Cornell researchers has revealed an unexpected obstacle to improving charge transport in hybrid perovskites, a promising class of semiconductor materials used in energy conversion and electronic devices.

Published June 17 in Advanced Energy Materials, the research shows that certain conjugated organic molecules which have long been thought to enhance electrical conductivity in layered perovskites can instead trap charge carriers and dramatically reduce conductivity. The findings could help guide the design of next-generation perovskite materials for applications ranging from thermoelectric energy conversion to optoelectronics.

The paper’s first author is Prithwish Biswas, a postdoctoral researcher in the lab of Zhiting Tian, professor in Cornell’s Sibley School of Mechanical and Aerospace Engineering. Biswas, Tian and colleagues investigated how the molecular structure of organic ligands – molecules whose electrons can move relatively freely – influences charge transport in two-dimensional hybrid organic-inorganic perovskites, materials composed of alternating inorganic semiconductor layers and organic molecular layers.

Scientists have increasingly explored the use of conjugated organic ligands because they are expected to improve conductivity by allowing electrical charges to travel more easily through a material. In theory, these ligands should outperform conventional insulating ligands by reducing the barriers that impede carrier movement.

But when the researchers compared two specific types of perovskite structures, they found the opposite effect. A Dion-Jacobson perovskite with conjugated ligands counterintuitively exhibited electrical conductivity more than an order of magnitude lower than a related Ruddlesden-Popper perovskite containing an insulating organic ligand. The surprising result persisted even after both materials were chemically doped to increase carrier concentrations.

“Rather than giving up when we got the unexpected results, we investigated the electronic energy levels to understand why the semiconducting ligand performed worse than the insulating ligand,” Tian said.

By combining structural, optical and electronic measurements, the researchers discovered that the conjugated material’s organic layer contained an energy level positioned about 1.5 electron volts above the valence band of the inorganic semiconductor layer.

“The energy mismatch causes it to function as a hole trap, reducing the number of free carriers available for conduction,” Biswas said. “Rather than contributing to electrical conduction, the carriers migrate into the organic layer and become effectively immobilized.”

The results highlight a challenge for researchers developing high-performance layered perovskites. By minimizing the energy offset between organic and inorganic components, researchers may be able to harness the advantages of conjugated molecules without sacrificing conductivity.

“This is a finding that should be of significant interest to the community because it provides important guidance for the future design of ligands,” Tian said. “Simply introducing conjugated molecules is not enough to improve transport properties. Instead, future materials will need carefully engineered organic ligands whose electronic energy levels align closely with those of the inorganic semiconductor framework.”

The research was supported by the U.S. Department of Energy and made use of capabilities at the Cornell Center for Materials Research and the Cornell NanoScale Science and Technology Facility.

Chris Dawson is a communications coordinator for Duffield Engineering.