image: The molecular structure of GTA, the normalized PCE curves of GTA-based IS-PSCs under various stretch and stretch-release cycles.
Credit: by Yang Bai et al.
Recently, a research team led by Professor Zhi-Guo Zhang from Beijing University of Chemical Technology (BUCT), in collaboration with Professor Ye Long from Tianjin University, has made a breakthrough in the field of flexible polymer solar cells (PSCs). Their research has revealed the inherent trade-off of efficiency, stability and stretchability via acceptors materials structural regulation, providing critical insights for the bright future of flexible organic photovoltaics. The discovery has been published in National Science Review in an article entitled “Simultaneous enhancement of efficiency, stability and stretchability in binary polymer solar cells with a three-dimensional aromatic-core tethered tetrameric acceptor”.
What about the polymer solar cells (PSCs)?
Polymer Solar Cells (PSCs) is a promising emerging solar technology for powering wearable, portable devices, and such innovative areas. Compared with those of rigid solar panels based on inorganic materials (silicon, perovskite etc.), PSCs is made from organic (carbon-based) materials, which endow the benefits of lightweight, thin, flexible, affordable, and non-toxic.
Over the past five years, the power conversion efficiencies (PCE) of PSCs have improved significantly to over 20%. This progress is largely attributed to the development of a series of non-fullerene small molecular acceptors (SMAs). However, the stability and mechanical properties of PSCs devices are still not ideal for commercial applications, and the inherent trade-off of these performances are also lack of sufficient understanding for further advancements.
What about the scientific discovery?
As PSCs rely on a bulk heterojunction (BHJ) structure of polymer donors and SMAs to convert light into electricity, their performance in terms of morphological stability, mechanical robustness, and device efficiency, is inherently tied to these BHJ structures. High-efficiency PSCs often incorporate highly rigid and crystalline SMAs, which typically embrittle the conjugated BHJ films. Additionally, the thermodynamic relaxation of SMAs due to their low glass transition temperature (Tg) raises critical concerns regarding operational stability. Thus, a fundamental challenge is to enhance the morphological stability and mechanical robustness of PSCs without compromising their efficiency.
In this work, the authors designed a tethered giant tetrameric acceptor (GTA) with increased molecular weight that promotes entanglement of individual SMA units, The key to this design is using tetraphenylmethane as the linking core to create more free volume of acceptor component, and produce a three-dimensional and high C2 symmetry structure, which successfully regulate s their aggregation and relaxation behavior. Thus the sub-Tg relaxation of the polymer donor can be maintained, thereby preserving film robustness.
As a result, the PM6:GTA-based blend films exhibit a nearly 150% improvement in crack onset strain compared to their PM6:Y6 counterparts. The GTA-based intrinsically stretchable device retained 88% of their initial PCE under 15% strain and approximately 76% after 150 stretch-release cycles, whereas the Y6-based devices failed. Moreover, the PM6:GTA-based devices achieved a significantly higher PCE of 18.71% and demonstrated outstanding photostability, maintaining over 90% of their initial PCE after more than 1000 hours of operation. Notably, this represents the first case of a non-fullerene acceptor simultaneously enhancing efficiency, stability, and stretchability in binary polymer solar cells.
What about the further advancements?
This research highlights the advantages of a tethered design strategy using three-dimensional core-tethered SMAs, which increase Tg and suppress the thermodynamic relaxation of the acceptor component. In such design, multi-scale supramolecular interactions of the sub SMA units, allow precise regulation of aggregation and relaxation properties in PSC films, improving efficiency, stability, and mechanical properties. Further exploration of molecular architecture could enable flexible PSCs to offer a sustainable and durable energy solution for wearable technologies.
This study was conducted in collaboration with Professor Long Ye from Tianjin University and Dr. Yang Bai from Huanghuai University.
About the paper
Yang Bai, Saimeng Li, Qingyuan Wang, Qi Chen, Ze Zhang, Shixin Meng, Yu Zang, Hongyuan Fu, Lingwei Xue, Long Ye* and Zhi-Guo Zhang*. Simultaneous enhancement of efficiency, stability and stretchability in binary polymer solar cells with a three-dimensional aromatic-core tethered tetrameric acceptor. Natl. Sci. Rev., nwaf019. (* Corresponding authors)
Link to the paper: https://doi.org/10.1093/nsr/nwaf019.
About Authors
Zhi-Guo Zhang: Prof. Zhang received his Ph.D. degree from Wuhan University, and completed a joint training program at the National University of Singapore. Following this, he pursued postdoctoral research at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS), subsequently advancing to an associate research faculty position. He is currently a Full-Professor at the College of Materials Science and Engineering, Beijing University of Chemical Technology. Professor Zhang's research group specializes in:
- High-performance polymer solar cells
- Physics and engineering of organic photovoltaic devices
- Molecular design and synthesis of novel conjugated organic semiconductors
E-mails: zgzhangwhu@iccas.ac.cn
Long Ye: Prof. Ye has been a Professor at the School of Materials Science and Engineering, Tianjin University, since October 2019. He received his Ph.D. from the Institute of Chemistry, Chinese Academy of Sciences (advisor: Prof. Jianhui Hou) in July 2015. From August 2015 to September 2019, he worked as a postdoctoral researcher and was later promoted to Research Assistant Professor in Prof. Harald Ade’s group at the Department of Physics, North Carolina State University. His current research focuses on the morphological and mechanical characterization of semiconducting polymers and their blends in solar cells and transistors, as well as the polymer physics of conjugated polymers.
E-mail: yelong@tju.edu.cn
Yang Bai: He received his B.Sc., M.Sc. and Ph.D. degrees from Beijing University of Chemical Technology, and currently a Full-Lecturer at the College of Chemistry and Pharmaceutical Engineering, Huanghuai University. He mainly focuses on the novel flexible-functional conjugated organic materials for polymer solar cells.
Contact: https://www.researchgate.net/profile/Yang-Bai-135
E-mails: baiyang@huanghuai.edu.cn