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Low-Bandgap Fused Ring Electron Acceptors towards High-Efficiency Organic Solar Cells

Periodic Reporting for period 1 - WONDER (Low-Bandgap Fused Ring Electron Acceptors towards High-Efficiency Organic Solar Cells)

Reporting period: 2019-06-01 to 2021-05-31

Energy crisis becomes an urgent problem nowadays. Organic solar cells (OSCs) can convert sunlight into electricity directly. Owing to their advantages of low-cost, semi-transparency and flexibility, OSCs become the most promising candidate to release the energy crisis in the near future. The past several years has witnessed the rapid growth in the field of OSCs based on non-fullerene acceptors (NFAs), with intensive efforts being devoted to materials development, device engineering and understanding of device physics. However, there is a lack of reviews concerning the latest development of OSCs, which is critical for the community to have a scope of NFA OSCs and trigger new thought for further development. As a result, we summarized a comprehensive review on emerging approaches in enhancing the efficiency and stability in NFA OSCs.
Efficiency and stability are two key factors for commercialized OSCs. One objective is to summarize the emerging approaches for high short-current density (JSC), fill factor (FF) as well as open-circuit voltage (VOC). The other objective is to conclude the emerging approaches for enhanced stability of NFA OSCs.
1. Investigate the current development of NFA OSCs via plenty of literatures.
2. Divide the part of Emerging Approaches in Enhancing the Efficiency of NFA OSCs in two parts:
① Emerging Strategies for High JSC and FF
② How to maximize VOC?
3. Divide the part of Emerging Approaches in Enhancing the Device Stability in NFA OSCs in four parts:
① NFA molecular structure
② Morphology
③ Film Processing
④ Interfacial Contacts and Device Configuration
4. Give a status analysis and highlight some aspects that might need further attention for breakthroughs
An overview of the results:
I. A wide range of narrow bandgap NFAs have been developed with great efforts to modify the donor moiety and electron-withdrawing end groups. These new NFAs, blended with donors having complementary absorption, are now increasingly closing the gap between OSCs and perovskite/inorganic solar cells, especially in terms of JSC and FF. In order to realize industrial applications, it is of critical importance to decrease the cost of NFAs, which can preferably be processed from green solvents and show good thickness tolerance. Along this line, more efforts should be dedicated to the simplification of the synthetic routes (for reduced cost), modification of the structures (for processing with green solvents), and improvement of the crystallinity of NFAs (for high mobility and hence thickness tolerance).
II. The energy losses have also been significantly decreased in NFA-based OSCs compared with those based on fullerene derivatives. In order to further decrease the energy losses to a level close to that of high-efficiency inorganic and perovskite solar cells, the key is to develop NFAs with strong photoluminescence, so that the non-radiative recombination losses can be suppressed. However, the relationship between the molecular structures and emission properties of NFAs remains unclear, and hence further efforts are needed to guide the design of highly emissive NFAs that maintain efficient charge generation at the same time. Meanwhile, it is quite interesting to note that some third components or additives can help to reduce non-radiative losses, although the underlying mechanism needs further clarification. It might be intriguing to have a deep understanding of device physics in this case, so that we can also suppress the energy losses from the point of device engineering. In addition, the relationship between the morphology and energy losses, though discussed in some previous reports on fullerene-based OSCs, might also deserve further investigation in OSCs based on NFAs.
III. The stability of NFA OSCs is closely related to the organic materials and the morphology. The photo-oxidation of NFAs accounts for the degradation of devices, as they contain more active photo-oxidation reaction sites than the fullerene counterparts. It has been demonstrated that the photostability of NFAs can be improved by choosing the end groups and the donor moiety rationally. The structure-stability relationship requires more investigations so that the photostability of NFAs can be further enhanced. In terms of the morphology stability, it is important to optimize the miscibility of D/A materials or introduce a third component to adjust the interaction of D/A materials. In addition, developing efficient and low-cost encapsulation is also very important for the commercial application of OSCs.

Exploitation and dissemination of the results:
I have taken the following three approaches to effectively disseminate the research results achieved from this project. Firstly, publication in the high-impact journal of Advanced Energy Materials (IF: 25.245) will facilitate to disseminate the knowledge gained from this project. This will help me and EU to achieve a good scientific reputation and stimulate other researchers for more novel ideas. Secondly, I have presented these results in national and international conferences via oral presentations or posters. The conferences include the European Conference on Molecule Electronics meeting in Linkoping (2019), Sweden, and the Energy Materials Conference (2021) in Hefei, China. Thirdly, I have also utilized academic social media like MaterialsViews and ResearchGate to spread the research results generated in this project.
Enhancing the future career prospects of the researcher after the fellowship
The researcher has realized her medium-term goal of realizing research independence and becoming a research team leader in a Chinese top university Central South University, and establishing long-term cooperative relations between China and Sweden. This fellowship also laid the foundation to realize her long-term goal of becoming a world-known expert in the energy research field. The training through this project has diversified my scientific competencies and skills, which are significant to my future career prospects. Through the project, I have acquired new knowledge and skills on device physics, including EL, FTPS-EQE spectra, time-resolved photoluminescence, and transient absorption spectroscopy, which has complemented my knowledge on OSCs and enabled me to start new research fields in the future.
The training through this fellowship has also enabled the researcher to gain transferable skills, which are also valuable to my future academic career. I can better manage my research team and attract funding after attending courses on collaboration, leadership, grant writing and project management. Benefit from the international environment, I have communicated with top scientists, and built my own research network through this fellowship. Moreover, I have disseminated my research results and build collaborations and academic reputation by attending national and international conferences. In a word, the obtained scientific and transferable qualifications through this project have accelerated my career development, helping me to realize research independence in the medium-term and become a world-known expert in optoelectronics in the long-term.
TOC of the review