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The research team designed a donor–acceptor–donor molecule, called TISQ.
Researchers have created a new molecule that naturally forms p/n junctions, structures that are vital for converting sunlight into electricity. The effort by Osaka Metropolitan University scientists offers a promising shortcut to producing more efficient organic thin-film solar cells.
The team pointed out that 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.
“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. But even subtle changes in processing conditions can lead to inconsistent mixing, unstable structures, and reduced device performance.”
To address these challenges, researchers actively explored alternative strategy 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, according to a press release.
The research team 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.
Depending on the solvent, TISQ spontaneously organizes into nanoparticle-like J-type or nanofiber-like H-type aggregates, each with different electronic behaviors.
The team pointed out that 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, according to a press release.
“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.
Prabhat, an alumnus of the Indian Institute of Mass Communication, is a tech and defense journalist. While he enjoys writing on modern weapons and emerging tech, he has also reported on global politics and business. He has been previously associated with well-known media houses, including the International Business Times (Singapore Edition) and ANI.
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