A team of researchers at Cornell University has uncovered a surprising problem in a promising class of materials known as hybrid perovskites.
The discovery could help scientists build better materials for future technologies, including solar cells, energy-harvesting devices, and advanced electronics.
Hybrid perovskites have attracted enormous interest in recent years because they are efficient at moving electrical charges and can be made relatively cheaply.
They are already being explored for use in solar panels, light-emitting devices, sensors, and thermoelectric systems that convert heat into electricity.
These materials have a layered structure. They are made of inorganic semiconductor layers separated by organic molecular layers.
Scientists have been trying to improve how electricity flows through these materials by changing the organic molecules between the layers.
One promising approach has been to use special molecules called conjugated organic ligands. These molecules contain electrons that can move relatively freely, so researchers believed they would help electrical charges travel more easily through the material.
In theory, this should have increased electrical conductivity and improved performance.
However, the Cornell team found something completely unexpected.
The researchers compared two different perovskite structures. One contained the supposedly beneficial conjugated molecules, while the other used a more traditional insulating organic material.
To their surprise, the material containing the conjugated molecules performed much worse. Its electrical conductivity was more than ten times lower than the material containing the insulating molecules.
The scientists even chemically modified both materials to increase the number of charge carriers available for conduction. Yet the unexpected result remained the same.
Instead of accepting the surprising finding and moving on, the team decided to investigate further. They carefully examined the electronic structure of the materials to understand why the supposedly superior molecules were reducing performance.
Their analysis revealed the problem.
The conjugated molecules created an energy level that acted like a trap for charge carriers. In semiconductors, charge carriers are particles that transport electricity through a material. In this case, the charge carriers moved into the organic layer and became effectively stuck there.
Lead author Prithwish Biswas explained that the trapped charges could no longer contribute to electrical conduction. Rather than helping electricity flow, the conjugated molecules were actually immobilizing the charges and reducing conductivity.
The researchers discovered that this trapping effect was caused by a mismatch between the energy levels of the organic and inorganic parts of the material.
The finding highlights an important challenge for scientists developing next-generation perovskites. Simply adding molecules that appear to conduct electricity is not enough. The different components of the material must also have compatible energy levels.
The researchers believe their work provides valuable guidance for future material design. By carefully engineering organic molecules whose energy levels align better with the inorganic semiconductor layers, scientists may be able to avoid these hidden charge traps.
Such improvements could lead to more efficient perovskite-based technologies, including better solar cells, more effective heat-to-electricity converters, and new generations of electronic and optoelectronic devices.
Sometimes, understanding why something fails is exactly what opens the door to future breakthroughs.
