From Perylene Diimide Polymers to Fused‐Ring Electron Acceptors: A 15‐Year Exploration Journey of Nonfullerene Acceptors

Abstract

Comprehensive Summary Fullerene derivatives are classic electron acceptor materials for organic solar cells (OSCs) but possess some intrinsic drawbacks such as weak visible light absorption, limited optoelectronic property tunability, difficult purification and photochemical/morphological instability. Fullerene acceptors are a bottleneck restricting further development of this field. Our group pioneered the exploration of novel nonfullerene acceptors in China in 2006, and initiated the research of two representative acceptor systems, rylene diimide polymer and fused‐ring electron acceptor (FREA). FREA breaks the theoretical efficiency limit of fullerene‐based OSCs (~13%) and promotes the whole field to an unprecedented prosperity with efficiency of 20%, heralding a nonfullerene era for OSCs. In this review, we revisit 15‐year nonfullerene exploration journey, summarize the design principles, molecular engineering strategies, physical mechanisms and device applications of these two nonfullerene acceptor systems, and propose some possible research topics in the near future. What is the most favorite and original chemistry developed in your research group? Fused‐ring electron acceptor for photovoltaics. How do you get into this specific field? Could you please share some experiences with our readers? In 2006 when I came back to ICCAS from USA and started my independent academic career, I was interested in organic photovoltaics (OPV) which I had never touched before since I believed it is useful and should have a bright future. OPV converts renewable solar energy to clean electricity through photovoltaic effect. The active layer in OPV device consists of electron donor and electron acceptor. Although fullerenes were prevailing acceptor materials in OPV at that time, I doubted if this choice is correct considering their fatal flaws such as weak absorption. I was curious about why no research groups in China explored fullerene alternatives before 2006. In 2006 our group initiated the nonfullerene OPV research of China. In 2015 we invented the milestone molecule ITIC and pioneered fused‐ring electron acceptor (FREA). Around 300 research groups in >20 countries have utilized the FREA to fabricate high‐efficiency OPV devices with champion efficiency of >20%. The FREA has subverted previously predominant fullerenes, and is inaugurating a new era of the OPV field. How do you supervise your students? I expect that my students should have some essential personalities such as curiosity, creativity, devotion, persistence and diligence. I encourage them to do original, unique and leading research. I endow them with cheerful academic atmosphere such as self‐motivation, open‐mindedness and cross‐cooperation. I would like to provide them with warm human solicitude when they need help or suffer from bitterness. What is the most important personality for scientific research? Curiosity; uniqueness; persistence.

Molecular engineering of Y‐series acceptors for nonfullerene organic solar cells

“…A third component has been utilized in the fabrication of ternary OSCs, as the additional photovoltaic material can function as an optical complement to enhance light harvesting, an energetic agent to promote charge generation and transport, a morphology regulator to finely tune the microstructures in the blends, and a stabilizer to suppress phase separation during the device aging process. 47,78 Developing ratio-insensitive ternary OSCs with improved reproducibility needs to be explored. Furthermore, the development of new donor materials that are well matched with Y-series acceptors can further enhance efficiency.…”

“…Developing new Y‐series NFAs with absorption extended over 1000 nm will be beneficial for achieving high‐performance semitransparent OSCs. A third component has been utilized in the fabrication of ternary OSCs, as the additional photovoltaic material can function as an optical complement to enhance light harvesting, an energetic agent to promote charge generation and transport, a morphology regulator to finely tune the microstructures in the blends, and a stabilizer to suppress phase separation during the device aging process 47,78 . Developing ratio‐insensitive ternary OSCs with improved reproducibility needs to be explored.…”

“…For in-depth information relating to the physical mechanisms of Y-series nonfullerene acceptors, readers are referred to other existing works. [45][46][47]…”

“…The following discussion will focus on chemical synthesis as a powerful tool in the development of new Y‐series acceptors. For in‐depth information relating to the physical mechanisms of Y‐series nonfullerene acceptors, readers are referred to other existing works 45–47 …”

Development of non-fullerene electron acceptors for efficient organic photovoltaics

“…Constructing twisted PDI dimers is one of the most synthetically accessible approaches to suppress the over-aggregation, and constructing multidimensional PDI based NFAs has enabled multidimensional charge transport, similar to fullerenes. As the optical, electronic and morphological properties of PDI based NFAs are sensitive to their intrinsic molecular geometry, conjugation length and electron distribution, judicious consideration should be taken when designing new PDI based NFAs and more effort is required to understand the relationship between the molecular structure, optoelectronic properties, and photoelectric performances [104].…”