A research team from Osaka Metropolitan University have developed a donor-acceptor-donor molecule that can spontaneously self-assemble into nanoscale structures and offer a more stable route to built-in p/n heterojunctions in organic solar cells.

Scientists from Osaka Metropolitan University have developed a molecular architecture offering a new design strategy toward producing more efficient organic thin-film solar cells.

Their donor-acceptor-donor (DAD) molecule, known as TISQ, integrates a squaraine-based p-type segment and a naphthalene diimide n-type segment within a single molecule. It is capable of naturally forming p/n junctions, otherwise known as the interface between p-type and n-type semiconductors.

The two segments link via amide groups that promote hydrogen bonding, meaning TISQ can spontaneously self-assemble into distinct nanoscale structures, which the scientists believe could offer a more stable route to built-in p/n heterojunctions.

Takeshi Maeda, Associate Professor at the university's Graduate School of Engineering and lead author of the study, explained that depending on the solvent, TISQ can spontaneously organize into nanoparticle-like J-type or nanofiber-like H-type aggregates.

The university’s news release explains polar solvents cause TISQ to form nanoparticle-like J-type aggregates through a cooperative nucleation–elongation process, while low-polarity solvents see it assemble into fibrous H-type aggregates via an isodesmic mechanism. “Both show different electronic behaviors, especially in how efficiently they transport charges when light hits them,” Maeda said.

The team’s research fabricated organic thin-film solar cells incorporating TISQ as a single-component photoactive material in a test of device applicability. The molecule was shown to form nanoscale p/n heterojunctions through self-assembly, which the scientists say highlights the feasibility of molecular designs that autonomously organize into functional electronic structures.

With the power conversion efficiency of the fabricated cells remaining low, the team acknowledged further research is required before it is practically applied. Nevertheless, they concluded that their findings demonstrate how differences in self-assembled nanoscale p/n heterojunction structures directly influence the photocurrent response in a single-component system.

“Our focus is on developing molecular design strategies that use self-assembly to connect nanoscale p/n heterojunction structures with photoelectronic responses in single-component organic systems,” Maeda said. “By deepening this structure–function understanding, we aim to broaden the design space of organic thin-film solar cells and related optoelectronic materials.”

The new molecule is described in the research paper “Solvent-Controlled Supramolecular Polymerization and Morphology-Depended Photoconductivity Modulation in a Squaraine-Naphthalene Diimide-Squaraine Bulk p/n Heterojunction,” available in the journal Angewandte Chemie International Edition.