Recently, Professor He Feng's research group of the Department of Chemistry of Southern University of Science and Technology has achieved fruitful research results in multiple research directions such as device structure of organic solar cells, synthesis of polymer photovoltaic materials, and interface engineering, and has published many papers in flagship journals of materials and energy such as Advanced Materials, Advanced Functional Materials and Joule, which has promoted the development of organic solar cell research.
The active layer of bulk heterojunction organic solar cells (OSCs) contains corresponding donor and acceptor materials, and its photoelectric properties are highly dependent on the optical physical properties and compatibility of donors and acceptors. In recent years, with the rapid development of non-fullerene receptors, from ITIC to Y6 and its derivatives, the photoelectric conversion efficiency (PCE) of OSCs has exceeded 18%. However, for some special and efficient photovoltaic material systems, when the compatibility in some solvents is not good, the bulk heterojunction (BHJ) structure is used, and good device performance cannot be obtained, so the planar heterojunction (PHJ) device structure is required.
Figure 1. Schematic diagram of the device structure of a quasi-planar heterojunction organic solar cell.
The PHJ active layer containing the donor/acceptor material usually employs a continuous spin coating process, and due to the swelling of the solvent and the diffusion of molecules, tiny nanoscale heterojunction (BHJ) regions may be created at the intermediate junction of the PHJ film. Therefore, the structure is defined as a quasiplanar heterojunction (Q-PHJ). Although the efficiency of the current Q-PHJ organic solar cell lags behind that of the BHJ device, it still has certain advantages. A separate bilayer structure is formed to the active layer of the /acceptor, resulting in a D/A interface capable of efficiently dissociating excitons. With the 3D network receptor structure receptor BTIC-BO-4Cl with long-range exciton diffusion can transfer excitons and charges in multiple directions, the long lifespan and diffusion distance of the excitons ensure that most excitons diffuse to the D/A interface for dissociation, which can be used to prepare excellent Q-PHJ organic solar cells. Based on the special material properties of the polymer donor D18 and the receptor BTIC-BO-4Cl, up to 17.6% of the PCE of the Q-PHJ organic solar cell was prepared by testing the prerequisites for the preparation of Q-PHJ organic solar cells. A comprehensive study of BHJ and Q-PHJ devices based on D18 and BTIC-BO-4Cl has obtained an important guiding basis for the preparation of efficient and stable Q-PHJ organic solar cells. The study shows that in some unique photovoltaic material systems, the Q-PHJ structure can replace the organic solar cells with excellent BHJ structure preparation performance, providing new research ideas for the design of photovoltaic materials and device preparation. The relevant research results are now published in the international flagship journal Advanced Materials, the first author of the research paper is Chen Hui, research assistant professor of the Department of Chemistry of SUSTech, and Tingxing Zhao, postdoctoral fellow, other authors include Li Long, postdoctoral fellow of the Department of Mechanical and Energy Engineering of SUSTech, and Guo Liang, assistant professor of the Department of Mechanical and Energy Engineering of SUSTech, and He Feng, corresponding author, sustech is the first unit of the paper.
Figure 2. Structure and device properties of dπ-pπ conjugate systems of metal-nano graphene.
In the research direction of interface engineering of organic solar cells, the research team designed and synthesized a series of metal-nano graphene dπ-pπ conjugate systems through interdisciplinary and mutual cooperation between the research groups, and effectively combined nano-graphene and carbon dragon complexes through metal Kaby reaction, and obtained a new large-π conjugate system with the participation of metal d orbitals in π conjugate. And through the design of conjugate extension, the interface molecules of this type are further optimized, and finally the results can be obtained as alcohol-soluble cathode interface layer materials, and effectively improve the efficiency of organic solar cells by more than 18%. Such interface molecules are mainly due to strong and orderly charge transfer, more matched energy level arrangement, better interface contact between the active layer and the electrode, and a more suitable active layer morphology formed after regulation, which greatly promotes the transmission of carriers, blocks the composite of carriers, and ultimately effectively improves the performance of organic solar cells. Such new nano-graphene-carbon dragon complexes are expected to promote the further development of organic solar cells as cathode interface layer materials with great potential. The results were published in Advanced Materials, the flagship journal of international materials, and the first authors of the research paper were Liu Longzhu, a doctoral student in the Department of Chemistry of SUSTech, and Chen Shiyan, a doctoral student at Xiamen University, and He Feng, a corresponding author, Xia Haiping, chair professor of the Department of Chemistry of SUSTech, and Tan Yuanzhi, a professor at Xiamen University.
Figure 3. Molecular structure and photovoltaic properties of polymer receptors.
In the design and synthesis of polymer receptors, the research group used recrystallization to separate two brominated terminal groups at different positions, and synthesized the polymeric monomers g-Br-BTIC and d-Br-BTIC at g and d. The study found that the position of the bromine atom in the polymer monomer is different, and the material obtained by polymerization shows significant differences in the device. Among them, the polymer PBTIC-g-2F2T at the g position can obtain the optimal device efficiency (14.34%). The polymer PBTIC-d-2F2T at the d position exhibits strong aggregation and is difficult to dissolve in some common solvents, such as dichloromethane, trichloromethane, chlorobenzene, o-dichlorobenzene, etc., and it is almost impossible to obtain photovoltaic corresponding in the device. In addition, a hybrid monomer-based polymer material, PBTIC-m-2F2T, was synthesized, and although the material polymerized well solubility, the corresponding device efficiency was only 3.26%. Subsequently, a polymer material without fluorine-substituted g-position, PBTIC-g-2T, was further synthesized. The material exhibited an efficiency of 11.92%, slightly lower than the fluorine-substituted material PBTIC-g-2F2T, suggesting that different link fragments also have a certain impact on the performance of the material. However, it is worth noting that the performance of PBTIC-g-2T is still much higher than that of PBTIC-d-2F2T and PBTIC-m-2F2T, indicating that polymerization at the g position is expected to further provide more efficient polymer receptors. The above results were published in advanced Functional Materials, the flagship journal of international materials, and the first authors of the study were Wang Hengtao and Chen Hui, postdoctoral fellows of the Department of Chemistry of SUSTech, and He Feng, the corresponding author, and SUSTech was the first unit of the paper.
Figure 4. Device performance based on thiophenimide polymer donors
In the design and synthesis of polymer donor materials, through the cooperation between research groups, two novel polymer donor molecules based on naphthienoimide (NTI) units, PNTB and PNTB-2T, were designed and developed. The efficiency of PNTB-based devices is only 3.81%, while the efficiency of PNTB-2T devices is as high as 16.72%, which has great application value for new polymer donors. The researchers then added PC71BM as a third component to the PNTB-2T:Y6-based binary, and the PCE of the tri-component was eventually increased to 17.35%. This study not only develops a new series of polymer donor materials with good high performance reproducibility, but also provides a new strategy for the subsequent synthesis of high-performance polymer donor materials. The results were published in the form of a research paper in Joule, the flagship international journal on energy. Master's students of the Department of Chemistry of Shantou University, Zhang Gongya, Ning Haijun and Chen Hui, are the first authors of the paper, and the corresponding authors are Professors Wu Qinghe and He Feng of the Department of Chemistry of Shantou University.
All of the above research work has been strongly supported by the Scientific Research Startup Fund of SOUTHERN University of Science and Technology, the National Natural Science Foundation of China, the Guangdong Provincial Innovative Research Team, the Shenzhen Science and Technology Innovation Commission, the Shenzhen Grubbs Research Institute and the Analysis and Testing Center of southern University of Science and Technology.
Source: Southern University of Science and Technology
Thesis Link:
https://doi.org/10.1016/j.joule.2021.02.003
https://doi/10.1002/adfm.202100877
https://doi.org/10.1002/adma.202101279
https://doi.org/10.1002/adma.202102778