Recently, He Feng, associate professor of the Department of Chemistry of Southern University of Science and Technology, published the latest research results in the top journal joule in the field of energy, introducing a high-efficiency organic solar cell receptor material synthesized by the team to locate trifluoromethyl substitution, which can form a three-dimensional network structure with more electron jump transmission nodes through the synergy of h/j aggregation, which can greatly improve the transmission of charge between molecules and greatly improve the performance of the device.
Organic non-fullerene small molecule receptor materials have received more and more attention in recent years due to their simple design and synthesis, strong absorption in the visible light and even near-infrared regions, and adjustable energy levels, and their research has also made breakthrough progress. In particular, small molecule acceptor materials based on thick ring cells can exceed 16% of the energy conversion efficiency of single-section solar cells by optimizing their thick ring cell structure, adjusting alkyl side chains, and introducing halogen atoms. Among them, the introduction of halogen atoms can effectively regulate the absorption spectrum and energy level distribution of such small molecules, which is a very simple and effective way to improve the performance of non-fullerene organic solar cell devices. Most small molecule acceptor materials absorb spectral redshifts after the introduction of fluorine, chlorine or bromine atoms, reduced homo and lumo energy levels, increased electron mobility, improved crystallinity, and thus better device performance. However, the effect of trifluoromethyl functional groups on organic solar cell receptors has been rarely reported compared to the introduction of individual halogen atoms.
The research group successfully designed and synthesized trifluoromethylated end groups ic-cf3–m, and used the recrystallization separation strategy to obtain the trifluoromethyl localization-substituted end group ic-cf3–γ, and the two were codensified with bt-2cho, and two narrow band gap small molecules btic-cf3–m and btic-cf3–γ were obtained. The study found that compared with the btic-f-m substituted by the fluorine atom and the btic-cl-m substituted by the chlorine atom, the btic-cf3–γ absorption spectrum has a greater redshift amplitude, and its absorption edge reaches 951nm, corresponding to the optical band gap of 1.3ev, which belongs to the category of ultra-narrow bandgap receptors, which greatly benefits from the super electron absorption ability of trifluoromethyl.
Figure 1. Schematic diagram of the molecular structure of the receptor and its absorption spectrum and energy level structure
Figure 2. Btic-cf3–γ single crystal structure of the acceptor molecule
By studying the single crystal structure of the btic-cf3–γ molecule, the team found that the s∙∙∙o=c configuration interlocking y-plane structure in the btic-cf3–γ molecules, so that the two end bases of the center are on the same side of the skeleton. For common small molecule receptor systems, the intermolecular arrangement is mainly formed by the accumulation of π-π interactions between the end groups and the end groups to form a molecular arrangement dominated by j aggregation. In the single crystal of btic-cf3–γ, the researchers found that there is not only j aggregation between end groups and end groups, but also h aggregation formed by accumulation between thick ring cores, so the accumulation between btic-cf3–γ molecules has the molecular arrangement stacking characteristics of j- aggregation and h-aggregation interaction, which can form more intermolecular nodes and are more conducive to efficient transport of carriers. In addition, the introduction of trifluoromethyl did not destroy the accumulation between molecules because of the steric hindrance effect, but instead formed a strong multiple interaction between trifluoromethyl and sulfur atoms. The synergy of these multiple molecular interactions and h/j aggregation allows btic-cf3–γ to form a three-dimensional network structure with more electron-skipping nodes, which is similar to the isotropic transport in fullerene materials, which can greatly accelerate the transfer of charge between molecules and significantly improve device performance.
In the device study, the researchers blended btic-cf3–γ with the donor pbdb-tf, and the energy conversion efficiency of the prepared solar cell devices reached 15.59%, while the efficiency of btic-f-m based fluorine-substituted btic-f-m and chlorine-substituted btic-cl-m devices was only 13.61% and 13.16%, respectively, thanks to the ultra-narrow band gap of btic-cf3–γ and the absorption spectrum of significant redshifting. It is worth noting that the efficiency of 15.59% is the highest value reported in the current literature in the ultra-narrow band gap material (band gap less than or equal to 1.30 ev). After the btic-cf3–γ is blended with y6, pbdb-tf to prepare a three-component device, the energy conversion efficiency reaches 16.50%, which further shows the great application value of btic-cf3–γ in the structure of ternary systems and other devices.
In this study, the team successfully introduced trifluoromethyl into the thick ring electron acceptor, obtained the ultra-narrow band gap receptor btic-cf3–γ, and applied it to the solar electronic device, which greatly improved the energy conversion efficiency of the device, fully reflected the advantages of spectral redshift and ultra-narrow band gap, and showed great potential prospects in the application of multivariate systems, translucent devices and laminated devices. More importantly, the single crystal structure of btic-cf3–γ helps researchers understand the stacked forms of such molecules and the interactions between molecules from the molecular level, and also provides a favorable basis and guidance for further design of new high-performance materials.
Lai Hanjian, a 2017 SUSTech-HIT Joint Training Ph.D. student, is the first author of the paper, Zhao Qiaoqiao, Chen Ziyi, postdoctoral fellows of the Department of Chemistry of SUSTech, and Chen Hui, assistant professor of the Institute of Frontier and Interdisciplinary Sciences, are the co-first authors of the article, He Feng, associate professor of the Department of Chemistry, is the corresponding author, and SUSTech is the first unit and the only communication unit. In addition, Dr. Zheng Nan of South China University of Technology, Zhang Yuanzhu, Associate Professor of the Department of Chemistry of SUSTech, Chao Pengjie, PhD student of SUSTech-Peking University Lianpei in 2016, Mo Daize, PhD student of SUSTech-University of Macau in 2016, Zhu Yulin, PhD student of SUSTech-Harbin Institute of Technology in 2018, and Lang Yongwen, a 2017 undergraduate student of SUSTech, also made important contributions to the article.
This research is strongly supported by the National Natural Science Foundation of China, Shenzhen Basic Research Program, Shenzhen Nobel Prize Scientist Laboratory, Guangdong Innovation and Entrepreneurship Team Project and Southern University of Science and Technology Analysis and Testing Center.
Article links:
https://doi.org/10.1016/j.joule.2020.02.004