Recently, the research group of Associate Professor He Feng of the Department of Chemistry of our university designed and synthesized a polymer donor material pbtt-f based on benzo [1,2-b:4,5-c'] dithiophene-4,8-dione, and prepared a single-section polymer solar cell with an energy conversion efficiency of 16.1%. The research was published online in advanced materials (if:21.950), a top journal in the field of materials.
Humans and the entire Earth's biosphere live under the sun's light and shelter. The energy that the sun sends to human beings every year is equivalent to 10 billion degrees of electrical energy, and it does not produce pollution, which is the most ideal energy source for human beings. Therefore, the progress of organic solar cell technology is of great significance to the sustainable development of mankind.
The active layer of a non-fullerene bulk heterojunction polymer solar cell is mainly composed of a polymer donor and a thick ring electron acceptor. Since the discovery of the thick ring electron acceptor itic, the efficiency of polymer solar cells has improved rapidly. However, compared with electron acceptor materials, the relevant research of polymer donors is still lagging behind.
Thiophene [3,4-b] thiophene (tt) units, because of their quinone resonance structure, can effectively move the corresponding polymer absorption spectrum to a long wavelength range. The representative polymer donor material ptb7 containing the tt unit achieved a world record efficiency of 7.4% in the fullerene era. Subsequently, ptb7-th with a two-dimensional side-chain structure was blended with itic, which increased the efficiency of non-fullerene solar cells to 6.8%, which is also an important breakthrough in the process of efficiency innovation of non-fullerene polymer solar cells.
Figure 1. (a) the structure of polymer pbtt-f, ;(b) synthetic route of monomers and polymer pbtt-f
The key indicator to measure the performance of solar cells is their photoelectric conversion efficiency, so improving the conversion efficiency of cells by designing and synthesizing new polymers has become the key to research. Recently, the team prepared a new polymer donor material, pbtt-f (Figure 1). The polymer core building unit ttdo has the characteristics of simple synthesis and high yield, which is suitable for a large number of preparations; in addition, ttdo further improves the electron absorption capacity on the basis of inheriting the quinone resonance of the tt unit, which is conducive to enhancing the charge transfer in the d-a type polymer molecule, making this type of material have a good application prospect in organic solar cells.
Figure 2. (a) the chemical structural formula of bdto and ttdo and their theoretically calculated UV-Vis absorption spectra; (b) UV/VIS absorption spectra of bdto and ttdo in solution; (c) the single crystal structure of the ttdo element and its vertical projection view; (d) the single crystal structure of the bdto unit and its vertical projection view; (e) the accumulation between the molecules of ttdo; (f) the accumulation between bdto molecules
Through theoretical simulations, the research group found that the absorption spectrum of ttdo was significantly redshifted relative to bdto, which was similar to the actual measured absorption spectrum results (Figures 2a, 2b). At the same time, it can be found from the single crystal results of the two (Fig. 2c-2f) that there is obvious partial overlap between ttdo molecules, while there is almost no overlap between bdto molecules, and the distance between ttdo molecules is also smaller than that of bdto, these results show that the interaction between ttdo molecules is stronger than that of bdto, which explains the reason for the redshift of the ttdo spectrum well.
Figure 3. Photovoltaic performance: (a) pbtt-f: j–v curve of y6 cells; (b) device stability test
As shown in Figure 3, in terms of device preparation, the research group selected a receptor material y6 blended with pbtt-f absorption complementary energy level to prepare a single-section inverted device, and obtained a photoelectric conversion efficiency of 16.1%. When the thickness of the pbtt-f:y6 blend film is increased to 190 nanometers, the device can still achieve an energy conversion efficiency of 14.2%, which is of great significance for the large-area preparation of polymer solar cells. The device also shows good stability and good universality for different receptor materials and interfaces.
Two-dimensional grazing incident wide-angle X-ray scattering (2d-giwaxs) experiments show that the pbtt-f:y6 hybrid membrane has a clear face-up arrangement in the out-of-plane direction, which is conducive to the efficient vertical transport of carriers between the electrodes in the active layer (Figures 4a-4d). In further experiments, the research group explored the morphology of the pbtt-f:y6 hybrid membrane through atomic force microscopy (afm) and transmission electron microscopy (tem) and found that it has obvious phase separation regions, each region has two-dimensional continuous fiber network structure interconnected, and each region has obvious phase separation and nanofiber structure (Figure 4e-4f). This facilitates the separation of excitons and the transport of carriers, which is conducive to increasing current density and fill factor, and ultimately improving device efficiency.
This work provides new ideas for the design of polymer donors, showing that pbtt-f is a very promising polymer, providing another new and efficient donor material choice for a large number of receptor materials.
Figure 4. Two-dimensional swept-incidence wide-angle X-ray scattering pattern: (a) pbtt-f; (b) y6; (c) pbtt-f:y6 mixed membrane; (d) giwaxs extra-surface orientation curve; (e) afm height map of the pbtt-f:y6 mixed membrane under optimized conditions; and (f) atem plot of the pbtt-f:y6 mixed membrane under optimized conditions
Sustech-Peking University jointly trained doctoral student Chao Pengjie, research assistant professor Chen Hui of the research group as the co-first author of the paper, and He Feng as the only corresponding author of the paper.
The research has been strongly supported by the National Natural Science Foundation of China, the introduction of innovative scientific research teams from Guangdong Province, the basic research of shenzhen science and technology innovation commission, as well as the Shenzhen Grubbs Research Institute and the Analysis and Testing Center of Southern University of Science and Technology.
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