Efficient, stable and easy to process, the field of organic photovoltaics has ushered in new materials

Recently, Joule published the latest results of the team of Wei Zhixiang, a researcher at the National Center for Nanoscience, and Deng Dan, a project researcher, on the large molecular weight receptor G-Trimer. The study integrates non-halogen solvents with easy processing, high efficiency and high stability, which provides a new class of materials for the development of organic photovoltaics.

The design and synthesis of new structural molecules is the most important driving force for the development of organic electronics. In organic solar cells, conjugated polymers have good film-forming properties, but at the same time, they have shortcomings such as uncontrollable synthesis and large batch differences, resulting in poor repeatability of materials and devices, which is an important bottleneck in their commercialization process.

The conjugated small molecule structure is clear, which solves the problems of synthesis controllability and batch variation, but brings new challenges, especially the strong dependence of charge transport on crystallinity and the optimal active layer morphology is difficult to obtain, which affects the device performance and stability.

Molecular structure and double hole transport mechanism. Photo courtesy of the National Center for Nanoscience

In view of the advantages and disadvantages of small molecules and polymers, Wei Zhixiang, a researcher at the National Center for Nanoscience, and Deng Dan, a project researcher, developed an “N-π-N” type dimer between small molecules and polymers, linking high-efficiency small molecule N-type acceptor materials through conjugated groups, the new dimer material combines the respective advantages of small molecules and polymers to achieve 18.2% device performance, much higher than the corresponding small molecules (16.2%) and polymers (15.8%), and shows more excellent light, Thermal stability.  

In order to further improve the non-halogen solvent processing performance and device efficiency of the acceptor material, the team designed and synthesized G-Trimer, a giant trimer acceptor material with high molecular weight and star-shaped low dipole moment structure, using Y-type acceptor as the structural unit on the basis of the previous work. The material adjusts the aggregation driving force from the traditional strong dipole action to the strong interaction between high molecular weight, effectively regulates the assembly behavior of the acceptor molecules in the preaggregation and film-forming process of the non-halogenated solvent o-xylene, and then achieves the excellent processing performance of the non-halogen solvent and the breakthrough of device efficiency. The forward small-area rigid device with the traditional polymer PM6 as the donor material and the new G-Trimer as the acceptor material obtained an energy conversion efficiency of more than 19% under o-xylene processing, a 1 square centimeter flexible large-area reverse device prepared by scraping in air slit coating obtained an efficiency of 14.25%, and the energy conversion efficiency of a 46.2 square centimeter flexible large-area module could be maintained at 13.25%, which were the highest values reported so far for halogen-free solvent processing.

In addition, the G-Trimer-based reverse device exhibits excellent thermal stability, maintaining an initial efficiency of nearly 90% after continuous heating at 80°C for 4500 hours. Compared with linear dimers, the material exhibits temperature-dependent pre-aggregation characteristics in o-xylene solution and can affect the preaggregation of PM6. The former promotes the orderly aggregation of acceptors, while the latter is conducive to the orderly accumulation of PM6 and the formation of fiber morphology, reduces the energy disorder of the film, and is conducive to the improvement of charge life. Under the condition of low charge separation driving force, there is a two-hole transfer mechanism with efficient local exciton-mediated and delocalized exciton-mediated coexistence, which significantly promotes the charge generation efficiency under low driving force and greatly improves the photon utilization rate. In addition, its high glass transition temperature also allows the device to exhibit excellent thermal stability.

“Therefore, G-Trimer can simultaneously improve the processing performance of green solvents, the efficiency and stability of organic solar devices, and provide important guidance for the design of commercial photovoltaic acceptor materials.” Dundan said. (Source: Zhang Shuanghu, China Science News)

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