A Figure Eight
Opens Into an X

The shape of a molecule can determine how electrons find their way through a solid. Fullerenes—the hollow carbon structures widely used in electron-transport layers—offer useful three-dimensional pathways, but their production cost and limited scope for chemical modification have driven a search for alternatives. A Japanese team has now made one by cutting open a molecule that begins as a twisted figure eight.

The research was led by Associate Professor Norihito Fukui of Nagoya University’s Graduate School of Engineering, working with groups at Kyoto University, Tokyo Metropolitan University and Osaka University. The team modified its previously developed figure-eight π-conjugated molecule, known as CBBC, to produce a chiral X-shaped n-type semiconductor.

In laboratory devices, the material’s best reported perovskite solar cell reached 20.6% power-conversion efficiency, close to the 21.2% obtained with the fullerene derivative PCBM under the study’s comparison conditions. A film made from one optical isomer also showed spin selectivity of about 80%.

Why Open a Figure-Eight Molecule?

Most organic π-conjugated molecules are relatively flat. Fullerenes are different: their curved, three-dimensional form can support electron transport in several directions through a solid. The drawback, according to the joint university release, is that fullerene materials are costly to synthesize and difficult to diversify through chemical modification.

The Nagoya group had previously made CBBC by oxidatively cleaving a bond inside commercially available dibenzochrysene. CBBC twists into a figure-eight form, contains electron-accepting carbonyl groups and can be synthesized on a comparatively large scale with high enantioselectivity, the researchers report.

For the new material, the team added electron-accepting functional groups around the CBBC framework. Starting with brominated CBBC, it produced compound 1 in three steps and also synthesized two related compounds. Two linear electron-accepting sections cross in three dimensions, giving compound 1 the description “chiral X-shaped.”

From Molecular Shape to Solar-Cell Test

Compound 1 showed a lowest unoccupied molecular orbital, or LUMO, level of about −4.6 electron volts, indicating strong electron-accepting ability. It dissolved in heated tetrachloroethane and could be deposited as a thin film by spin coating. The resulting film’s electron mobility was nearly twice that of the linear reference molecule used in the study.

The researchers interpret the mobility gain as evidence that the two crossed linear sections interact effectively within the solid. Their theoretical calculations supported that explanation, although the interpretation remains a model of how molecular packing and electronic interaction produced the measured result.

When compound 1 served as the electron-transport layer in an inverted perovskite solar cell, the best reported device reached 20.6% efficiency. The PCBM reference reached 21.2%. That is a close device-level comparison under the study’s conditions; it is not proof of equal lifetime, manufacturability or performance in full-size modules.

The Same X Also Filters Electron Spin

Chirality means that two structures can be mirror images yet cannot be placed exactly on top of one another, like left and right hands. In some chiral materials, electrons with one spin orientation pass more readily than electrons with the opposite orientation—a phenomenon known as chirality-induced spin selectivity, or CISS.

A thin film containing only one optical isomer of compound 1 produced a spin-selectivity value of about 80%. The team evaluated the effect from the change in current when the magnetization direction of a ferromagnetic electrode was reversed. The university release describes the value as high relative to earlier chiral electron-transport materials.

The finding gives the molecule two experimental roles: moving charge in a solar-cell layer and selecting electron spin in a separate measurement. It does not yet demonstrate a complete low-power semiconductor or memory device.

A Platform Candidate, Not Yet a Commercial Material

The paper establishes CBBC as a candidate framework for non-fullerene electron-transport materials. The team argues that the short synthesis and scope for structural modification could support further material development. Those are documented laboratory advantages and institutional expectations, not evidence that commercial production will be cheaper.

Important engineering questions remain outside the announced results: long-term device stability, performance over large areas, compatibility with scalable coating methods, solvent replacement, batch reproducibility and the cost and yield of every upstream step. The use of heated tetrachloroethane in the reported film process also means that an industrial route would require its own safety and environmental assessment.

The solar-cell and spin-selectivity results should likewise be kept distinct. One establishes electron transport in a photovoltaic stack; the other shows CISS behavior in a magnetization-dependent current measurement. Integrating both functions into a practical device would be a separate development program.

The research is most persuasive when stated narrowly. A molecule derived from a figure-eight framework can be made into a three-dimensional, chiral electron transporter; in the reported experiments, it approached a standard fullerene derivative in solar-cell efficiency and strongly selected electron spin. Whether that molecular elegance becomes an inexpensive technology is now a question for scale, durability and process engineering.