A self-assembling shortcut to better organic solar cells

Osaka Metropolitan University scientists have created a molecule that naturally forms p/n junctions, structures that are vital for converting sunlight into electricity. Their findings offer a promising shortcut to producing more efficient organic thin-film solar cells.

Solar cells convert sunlight directly into electricity. Within each cell, two semiconductors — p-type and n-type — form a p/n junction, where the photovoltaic effect performs the conversion.

Organic thin-film solar cells use carbon-based semiconductors instead of the traditional silicon, making them lightweight, flexible, and economical. They can be incorporated into inks for printing on everyday surfaces such as window films and even clothing. Their efficiency, however, still lags behind that of conventional silicon solar cells.

Achieving high performance depends critically on creating an optimal interface between the p-type and n-type materials. Organic solar cells are known for their tunable electronic properties, allowing researchers to change their alignment and morphology to improve the interface; however, optimally tuning them remains challenging.

“The ‘p/n heterojunction’ must be precisely tuned to enable the rapid separation and transport of charges generated when light is absorbed,” said Takeshi Maeda, associate professor at Graduate School of Engineering, Osaka Metropolitan University, and lead author of the study.

“Traditionally, the interfaces are created by physically mixing p-type and n-type molecules,” he continued. “But even subtle changes in processing conditions can lead to inconsistent mixing, unstable structures, and reduced device performance.”

To address these challenges, an alternative strategy has been actively explored in which p-type and n-type semiconductor components are integrated within a single molecule, allowing nanoscale p/n heterojunctions to be formed solely through molecular self-assembly. However, self-assembly of single molecules is inherently complex. Small differences in solvent or temperature can give rise to multiple competing aggregate structures, making it difficult to reliably obtain well-defined and optimal nanoscale p/n heterojunctions.

Against this background, the team focused on controlling molecular self-assembly to selectively form a well-defined nanoscale p/n heterojunction from a single molecular system.

They designed a donor–acceptor–donor molecule, called TISQ, that integrates a squaraine-based p-type segment (donor) and a naphthalene diimide n-type segment (acceptor) within a single molecule. Linked by amide groups that promote hydrogen bonding, TISQ can spontaneously self-assemble into distinct nanoscale structures, potentially offering a more stable route to built-in p/n heterojunctions.

“We found that TISQ forms either J-type or H-type aggregates depending on the solvent. Both show different electronic behaviors, especially in how efficiently they transport charges when light hits them,” Maeda said.

In polar solvents, TISQ forms nanoparticle-like J-type aggregates through a cooperative nucleation–elongation process. In low-polarity solvents, it assembles into fibrous H-type aggregates via an isodesmic, or stepwise, mechanism.

Measurements showed that J-type aggregates exhibit nearly double the photocurrent response of H-type aggregates.

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

“This bottom-up approach provides a platform for exploring how molecular self-organization can be translated into electronic functionality, including organic solar cells and a wide range of organic optoelectronic devices, from photodetectors to light-harvesting systems,” Maeda said.

While the power conversion efficiency of the fabricated cells remains low and will require further research before it can be practically applied, the study demonstrates 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 study was published in Angewandte Chemie International Edition.

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