Singlet fission breakthrough could lead to ultra-efficient solar cells

A team of Japanese researchers has discovered a new technique to tweak molecular orientation that could dramatically improve energy transfer when absorbing light. While in its infancy, this discovery could lead to the development of much-improved energy-harvesting technologies like solar cells.

The discovery involves a process called singlet fission (SF). This is a process wherein an exciton absorbs light, splitting and generating an additional exciton in the process.

For context, excitons are particle-bound pairs of a negatively charged electron and a positively charged “hole.” These pairs are held together by Coulombic attraction (attraction of oppositely charged particles) and can move within molecular assemblies.

SF results from the absorption of a single particle of light, or photon, in molecules called chromophores (molecules that absorb specific wavelengths of light). Controlling the molecular orientation and arrangement of chromophores is essential for achieving high efficiency in singlet fission materials.

A significant discovery

To date, SF studies have focused on solid materials, and little detailed work has been done on finding ways to manipulate molecular organization to maximize the efficiency of the SF process.

However, a team of Japanese researchers from Kyushu University is set to change this. Led by Professor Nobuo Kimizuka, the team has successfully demonstrated that SF can be promoted by introducing chirality into chromophores.

Chirality is a term that refers to a property of molecules that makes them non-superimposable in their mirror images. This occurs due to the specific arrangement of atoms within the molecular structure.

Chirality is important in fields like organic chemistry because different chiral forms, or ‘enantiomers,’ can have distinct properties and behaviors. This difference makes them important in fields like pharmacology and materials science.

Chromophores are parts of a molecule responsible for its color. They absorb light at specific wavelengths, which corresponds to certain colors we can see, and this is due to their unique arrangement of electrons.

“We have discovered a novel method to enhance SF by achieving chiral molecular orientation of chromophores in self-assembled structures,” Kimizuka explains.

Chirality is the key

The researchers explored the self-assembly characteristics of aqueous nanoparticles derived from ion pairs of tetracene dicarboxylic acid and various chiral or non-chiral amines. They identified that the counterion (an ion with a charge opposite to another ion in the solution)—specifically, the ammonium molecule—played a crucial role in this process.

The team found that the counterion influenced several factors, including the molecular orientation of the ion pairs, the structural regularity, the spectroscopic properties, and the strength of the intermolecular coupling between the tetracene chromophores. As a result, it was revealed that the counterion was key in controlling the alignment of the chromophores and the related singlet fission (SF) process.

When analyzing their technique, the team achieved a remarkable triplet yield (an SF measurement) of 133%, indicating high SF efficiency. Achiral (non-chiral) molecules acting as a control didn’t show similar results, proving the impact of chirality.

“Our research offers a novel framework for molecular design in SF research and will pave the way for applications in energy science, quantum materials, photocatalysis, and life science involving electron spins,” Kimizuka concluded.

“Furthermore, it inspires us to continue exploring SF in chiral molecular assemblies in organic media and thin film systems, which are critical for applications in solar cells and photocatalysts.”

The study has been published in the Wiley Online Library.