Scientists at Osaka Metropolitan University have developed a single organic molecule that naturally forms the internal p/n bonds needed to convert sunlight into electricity, offering a potential shortcut to more efficient organic thin-film solar cells. The study shows how careful molecular design and self-assembly can generate stable nanoscale p/n heterojunctions without the need to physically mix separate p-type and n-type materials.
Solar cells generate electricity when photons create charge carriers in a semiconductor and an internal electric field at the ap/n junction drives these charges apart. In conventional devices, these bonds form at the interface between individual p-type and n-type materials, but small variations in processing can disrupt the interface, leading to inconsistent performance and reduced efficiency.
Organic thin-film solar cells use carbon-based semiconductors instead of silicon, making them lightweight, flexible and suitable for printing on window films, building materials and even fabrics. Despite these advantages, their energy conversion efficiency lags behind that of silicon, partly because it is difficult to reproducibly develop an optimal interface between p-type and n-type domains at the nanoscale. Researchers can tune the electronic properties and morphology of organic materials, but the required precision remains a challenge in real devices.
To address this problem, the Osaka team explored a strategy that integrates both semiconductor types into a single molecular system that self-assembles into p/n heterojunctions at the nanoscale. In such single-component systems, subtle differences in solvent or temperature can drive the formation of competing aggregate structures, making it difficult to obtain well-defined and functionally optimal interconnect architectures. The researchers therefore focused on controlling supramolecular assembly pathways to select a specific nanoscale structure with desired electronic behavior.
The team designed a donor-acceptor donor molecule called TISQ that combines a squaraine-based p-type segment with a naphthalene diimide n-type segment in one molecular backbone. Amide linkages connect these segments and promote hydrogen bonding, allowing TISQ molecules to self-organize into ordered aggregates. This architecture was intended to encourage the spontaneous formation of built-in nanoscale p/n heterojunctions through self-assembly alone, without external templates or complex processing.
Experiments have shown that TISQ can self-assemble into two different types of supramolecular aggregates depending on the solvent environment. In polar solvents, TISQ forms nanoparticle-like J-type aggregates via a cooperative nucleation elongation process. In less polar solvents, the molecule instead assembles into fibrous H-type aggregates via an isodesmic, stepwise mechanism where each added molecule contributes similarly to the growing structure.
These different aggregate morphologies exhibit distinctly different electronic behavior under illumination. Measurements showed that the J-type aggregates produce almost double the photocurrent response as the H-type aggregates, highlighting how nanoscale packing and supramolecular architecture directly influence charge separation and transport. The results link solvent-driven self-assembly to a measurable change in photoresponse in a single-component organic material.
To assess the relevance of the device, the researchers integrated TISQ as the sole photoactive component into organic thin-film solar cells. In these test devices, TISQ self-assembled into nanoscale p/n heterojunctions, demonstrating that the molecular design can autonomously generate functional internal interfaces suitable for photovoltaic operation. The work provides proof of concept that a single, carefully designed molecule can provide both p-type and n-type functionality and self-organize into an electronically active compound.
The authors describe this as a bottom-up approach for translating self-organization at the molecular level into electronic function at the macroscale. By correlating specific supramolecular structures with photocurrent responses, the study provides a framework for using self-assembly to systematically connect nanoscale p/n heterojunction architectures with device-level performance. This concept could extend beyond solar cells and include other organic optoelectronic devices, including photodetectors and light collection systems.
Although the energy conversion efficiency of the prototype TISQ devices remains low and not yet suitable for practical deployment, the work clarifies how subtle changes in self-assembly at the nanoscale can strongly influence the photocurrent in a single-component organic system. The researchers aim to refine molecular design strategies and assembly control to improve both junction quality and charge transport, expanding the design space of organic thin-film solar cells and related optoelectronic materials. The findings are reported in Angewandte Chemie International Edition.
