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Associate Professor Huang Jianhua of Huaqiao University: Photovoltaic properties of halogenated B ← N polymer acceptor materials
2020-01-08 Source: Polymer Technology

B ← N polymer materials are widely used as electron acceptor materials in organic solar cells because of their higher electron affinity, stronger light absorption ability, and lower front-line orbital energy levels. However, compared with the classic imide polymer acceptor material, the photoelectric conversion efficiency of the B ← N polymer acceptor is still low. Therefore, it is necessary to carry out in-depth and systematic structural design and cutting, establish the structure-effect relationship of materials, and provide theoretical guidance for designing higher-performance materials. Halogenation, such as fluorination and chlorination, is widely used in the design and synthesis of organic photovoltaic materials. Due to the strong electronegativity and polarity of fluorine and chlorine elements, the introduction of them into organic conjugated molecules can significantly change the photoelectric properties and aggregation properties of the materials, thereby achieving performance regulation. In current reports, fluorination and chlorination have been studied more in polymer donor materials. In contrast, less research has been done on the halogenation of polymer acceptor materials. In particular, systematic comparative studies of non-halogenated, fluorinated and chlorinated polymer acceptor materials have not been reported. The systematic halogenation research is of great significance for elucidating the structure-property relationship of polymer receptor materials and then designing higher performance receptor materials.

In the previous research, the team of Associate Professor Huang Jianhua of Huaqiao University explored the effect of intermolecular B ← N coordination on the photoelectric properties of conjugated molecules (Dyes Pigms., 2018, 153, 1-9; New J. Chem., 2018, 42 , 18961-18968). Furthermore, this B ← N bond as a whole was introduced into the polycyclic conjugated molecular skeleton instead of the CC bond, thereby obtaining a class of conjugated electron-deficient unit BNIDT containing the B ← N bond (Tetrahedron, 2018, 74, 4308-4314; Tetrahedron Lett., 2019, 60, 151286). This unit was further used to construct a series of polymer acceptor materials, and its application in all-polymer solar cells was studied. The photoelectric conversion efficiency of 8.78% was obtained, which refreshed the efficiency of B ← N polymer acceptor materials. Records (Adv. Mater., 2019, 31, 1904585; Polymers, 2019, 11, 1369).

Recently, the group used BNIDT as the building unit and selected three commercial conjugated units: benzodithiophene (BDT), fluorinated benzodithiophene (BDT-F), and chlorinated benzodithiophene (BDT- Cl) was copolymerized with them to obtain three polymers BN-BDT, BN-BDT-F and BN-BDT-Cl with the same backbone structure and non-halogenated, fluorinated and chlorinated side groups. They systematically compared the effects of non-halogenated, fluorinated, and chlorinated side groups on the properties of such polymer acceptor materials with B ← N bonds in the backbone.

Figure 1. (a) Chemical structures of three B ← N-containing polymers and PBDB-T; (b) Three polymer synthesis routes.

In this DA copolymer, generally, the lowest empty orbit (LUMO) is mainly determined by the electron-deficient unit (BNIDT), and the highest occupied orbit (HOMO) is mainly determined by the electron-rich unit (BDT). The study found that the introduction of electron-drawing halogens into the BDT resulted in polymers ranging from BN-BDT (-5.46 eV) to BN-BDT-F (-5.71 eV) and BN-BDT-Cl (-5.74 eV). The HOMO gradually decreases. The LUMO energy levels of the three polymers are very similar (-3.79 vs. -3.77 vs. -3.76 eV). Therefore, when the commercial polymer PBDB-T is selected as the donor material to match the three polymer acceptors, the LUMO difference (ΔLUMO) between the donor and acceptor is similar, reaching about 0.76 eV. The HOMO difference (ΔHOMO) between donor and acceptor increased gradually (0.18 vs. 0.43 vs. 0.46 eV). Different ΔHOMOs represent the driving force for hole separation. Since the ΔHOMO between BN-BDT and PBDB-T is the smallest, it can be considered that its driving force for hole separation is the lowest, which will be detrimental to charge separation and electron / hole balance transport.

Figure 2. Energy level diagrams for BN-BDT, BN-BDT-F, BN-BDT-Cl, and PBDB-T

OC )相近,均为0.96 V左右。 The selection of PBDB-T and three polymer receptors to prepare all-polymer solar cells indicates that the open-circuit voltage ( V OC ) of the three cells is similar, all about 0.96 V. SC )和填充因子(FF)逐渐提高,从而器件的效率从1.60%显著提高到3.71%,进而到4.23%。 From non-halogenated BN-BDT to fluorinated BN-BDT-F to chlorinated BN-BDT-Cl, the short-circuit current ( J SC ) and fill factor (FF) of the device have gradually increased, so that the device efficiency has increased from 1.60 % Increased significantly to 3.71% and then to 4.23%. Through device physical characterization and SCLC testing, it was found that the charge separation efficiency of all-polymer solar cell devices from BN-BDT to BN-BDT-F and BN-BDT-Cl gradually improved, the charge recombination loss gradually decreased, and hole / electron migration The rate gradually increases and the transmission balance improves. This explains the reason for its efficiency improvement. OC的前提下,显著提高其J SC和FF,从而实现光电转换效率的逐步提升。 This research proves that the introduction of halogens F and Cl on the electron-rich unit in the polymer acceptor material can significantly increase its J SC and FF without changing V OC , thereby achieving a gradual improvement in photoelectric conversion efficiency. This is the first systematic non-halogenation, fluorination and chlorination research report on polymer acceptor materials.

Figure 3. JV curve of an all-polymer solar cell based on three B ← N bond polymers

This work was recently published in ACS Applied Materials & Interfaces (2019, DOI: 10.1021 / acsami.9b20214). The first author of the thesis is Meng Huifeng, a master student of Huaqiao University and the Chinese Academy of Sciences. Li Yongchun, a graduate student of Huaqiao University, is the co-first author. Corresponding author is Associate Professor Huang Jianhua of Huaqiao University, and co-corresponding author is Professor Li Xuemei of Linyi University and Researcher Zhan Chuanlang of Institute of Chemistry, Chinese Academy of Sciences.

Paper link: http://pubs.acs.org/doi/10.1021/acsami.9b20214

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