《Adv. Sci》 from the research group of Professor Feng He from SUSTech: New progress has been made in the substitution of chlorine for donor materials for polymer solar cells

2021-01-26

Bulk heterojunction (BHJ) polymer solar cells (PSC) have attracted much attention due to their unique advantages such as low toxicity, light weight, translucency, and solution processing to prepare large-area flexible devices. In recent years, with the rapid development of new organic photovoltaic materials, especially thanks to the development of fused ring electron acceptor materials, single-junction polymer solar cells have achieved unprecedented breakthroughs, making the efficiency (PCE) of single-cell solar cells exceed By 18%. Therefore, polymer solar cells have shown broad development prospects and huge commercial application potential. However, compared with the vigorous development of acceptor materials, the design and development of new organic polymer donor materials is particularly important for improving energy conversion efficiency. Recently, Professor Feng He from the Department of Chemistry of Southern University of Science and Technology designed and developed a chlorine substitution based on the study of chlorine substitution at different positions in the Benzo[1,2-b:4,5-c']dithiophene-4,8-dione nucleus. The polymer donor material PBBD-Cl-β, found that the precise positioning of the position of chlorine substitution plays a vital role in the mutual accumulation of polymer molecules, and the efficiency of its single-cell device reached 16.2%. The related research was titled "Chlorinated Benzo[1,2-b:4,5-c']dithiophene-4,8-dione Polymer Donor: A Small Atom Makes A Big Difference" on January 4, 2021 in the field of materials Published online in the internationally renowned journal Advanced Science (IF=15.84) (Adv. Sci. 2020, 2003641).

The thieno[3,4-b]thiophene (TT) unit has a quinone resonance structure, so that the molecule has a better plane, thereby enhancing the interaction between the molecules. At the same time, polymers based on TT units can broaden the absorption spectrum of molecules and extend the absorption spectrum to a long wavelength range. Recently, the author used Friedel-Crafts acylation reaction to embed 1,4-cyclohexanedione on the basis of TT to prepare a new fused-ring central core TTDO. Compared with the synthesis of TT unit, the synthesis of TTDO is not only simple, but also has a high yield. In addition, TTDO inherits the quinone resonance of the TT unit, and further improves the electron withdrawing ability, which is conducive to enhancing DA polymer Intramolecular charge transfer (Adv. Mater. 2020, 1907059). Then, on this basis, the author studied the interaction between the polymer molecules and the interaction of chlorine substitution at different positions of the Benzo[1,2-b:4,5-c']dithiophene-4,8-dione core. According to the influence of the photovoltaic performance of the corresponding polymers, the polymers PBBD, PBBD-Cl-α and PBBD-Cl-β were prepared, and their synthetic routes are shown in Scheme 1.

 

Scheme 1. Synthesis of monomers (TTO, TTO-Cl-α, TTO-Cl-β, M1, M2 and M3) and polymers (PBBD, PBBD-Cl-α and PBBD-Cl-β).

In order to study the influence of chlorine atoms on the structure and properties of polymers, especially the influence of chlorine substitution on intermolecular interactions, the author cultivated TTO, TTO-Cl-α and TTO-Cl-β single crystals and passed X-ray single crystals. Crystal diffraction confirmed its single crystal structure. The single crystal structures of TTO, TTO-Cl-α and TTO-Cl-β are shown in Figure 1. As shown in Figure 1a, the π-π stacking distance (dπ-π) of the side view of the chlorine-free TTO is 3.55Å, and the distance between H‧‧S (dH-S) is 5.93Å. The top view of Fig. 1b shows that intermolecular stacking is carried out along the long axis of the molecule, and the overlapping part is approximately the area of ​​a single thiophene ring. Since the chlorine atom has an empty 3d orbital, the Cl atom can accept the lone electron of the heteroatom and the π electron of the aromatic ring, thus forming the intermolecular Cl‧‧‧S and Cl‧‧‧π non-covalent interaction. The side view of TTO-Cl-α in Fig. 1c shows that, compared with TTO, due to the weaker π-π stacking and Cl‧‧S non-covalent interaction in TTO-Cl-α, TTO-Cl-α The π-π stacking distance (dπ-π= 3.48Å) and the Cl‧‧S distance (dCl-S=4.4Å) decrease at the same time. It can be seen from the top view of TTO-Cl-α in Figure 1d that TTO-Cl -α is arranged side by side along the short axis of the molecule, and the alkyl chains are all facing out of the plane. At the same time, because Cl atoms are arranged side by side in the same direction, TTO-Cl-α slides significantly in the direction of the short axis of the molecule due to the steric repulsion of chlorine atoms. At the same time, this molecular arrangement relies on weak π-π stacking and Cl‧‧S non-covalent interlocking interaction to maintain. As shown in Figure 1e, when the β-position H atom of thiophene in TTO is replaced by Cl, the molecular arrangement of TTO-Cl-β changes significantly again. First, reduce the distance between H and S atoms in the TTO unit (dH-S=5.93Å) to the new distance between Cl and S in the TTO-Cl-β unit (dCl-S = 3.81Å) , Which indicates that there is a strong non-covalent interaction between Cl‧‧S and Cl‧‧‧π in TTO-Cl-β. In addition, it is precisely because of this strong non-covalent interaction between Cl‧‧‧S and Cl‧‧‧π that the single crystal structure of TTO-Cl-β achieves a smaller π-π stacking distance. Compared with TTO (3.55Å), the π-π stacking distance between TTO-Cl-β is 3.23Å, indicating that TTO-Cl-β has smaller spatial repulsion and tighter molecular packing. As shown in Figure 1d, the TTO-Cl-β molecule has rotated about 90°, but still maintains almost the same area overlap with TTO, which should be attributed to the smaller spatial repulsion and stronger Cl‧‧S and Cl ‧ ‧‧Π non-covalent interactions. Based on these results, the single crystal structures of TTO, TTO-Cl-α and TTO-Cl-β directly indicate the existence of strong non-covalent interactions between Cl‧‧S and Cl‧‧S and Cl‧‧‧π Non-covalent interactions can significantly enhance the π-π stacking effect between molecules, so the chlorination at the appropriate position plays an important role in the molecular arrangement.

 

Figure 1. Single-crystal structure: (a) side view of TTO, (b) top view of TTO, (c) side view of TTO-Cl-α and (d) top view of TTO-Cl-α (e) side view of TTO-Cl-β and (f) top view of TTO-Cl-β. Note: here the alkyl chain (2-ethylhexyl) were omitted for clarity.

 

Figure 2. (a) J−V curves; (b) EQE curves; (c) Jsc vs the light intensity; (d) hole and electron mobility of the optimized blend films.

Finally, a single-cell solar cell device was prepared by blending the receptor material BTP-eC9, which is complementary to the corresponding polymer absorption spectrum and energy level matching, as shown in Figure 2, based on the polymer PBBD and PBBD-Cl-α The photoelectric conversion efficiencies of 10.06% and 13.35% were obtained respectively, while the efficiency of the PBBD-Cl-β:BTP-eC9 device was as high as 16.2%. At the same time, the PBBD-Cl-β:BTP-eC9 device better suppressed the carriers The combination of fluorine has a high and balanced mobility, indicating that the substitution position of the chlorine atom has an important influence on the photovoltaic performance of the polymer, which provides a new idea for the rational design and synthesis of efficient chlorine-substituted polymer donors.

Chao Pengjie, a PhD student jointly cultivated by Southern University of Science and Technology and Peking University, is the first author of the paper, and Chen Hui, a research assistant professor at Southern University of Science and Technology, is the co-first author. The corresponding author is Professor Feng He from the Department of Chemistry of Southern University of Science and Technology.

Paper link:

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.202003641