Polymer solar cells have outstanding advantages such as simple structure and preparation process, low cost, light weight, and flexibility to be fabricated into a flexible device, and have become a research hotspot at home and abroad in recent years. Substituting the fullerene derivative acceptor with an n-type organic semiconductor material can overcome the shortcomings such as weak light absorption in the visible light region, difficulty in energy level control, and poor stability of morphology in the presence of fullerene receptors, and has attracted the attention of researchers in recent years. . A variety of excellent non-fullerene receptors have been designed, such as the narrow band gap n-type polymer acceptor N2200 and the organic semiconductor acceptor ITIC.
Institute of Chemistry, Chinese Academy of Sciences Key Laboratory of Organic Solids Research Institute Li Yongji Research Group developed a series of intermediate band gap two-dimensional conjugated polymers based on thiophene-substituted benzodithiophene (BDTT) and benzotriazole (BTA) units. The donor material reduces the HOMO energy level through side chain engineering, enhances interchain interactions, and increases hole mobility. The energy conversion efficiency of non-fullerene polymer solar cells based on such polymers as donors and ITIC as acceptor reached 11.4%.
The researchers found in previous work that the two hydrogen atoms on the BTA unit were replaced by fluorine atoms, and the HOMO energy level of the polymer was shifted down by 0.13 eV while the hole mobility was significantly increased (Chem. Mater. 2012, 24, 3247- 3254). By using this type of polymer in the visible-near infrared region to absorb the ITIC receptors, non-fullerene polymer solar cells using these polymers as donors and ITIC as acceptors were prepared. Thanks to the lower HOMO level and higher hole mobility of the fluorine-substituted polymer J51, the open circuit voltage (Voc) of the J51 device increases from 0.71 V to 0.82 V compared to the fluorine-free polymer J50. The energy conversion efficiency (PCE) increased from 4.80% to 9.26% (Adv. Mater. 2016, 28, 8288-8295). Considering that the Voc of 0.82 V is still not high enough, the researchers further reduced the polymerization by introducing a thioalkyl into the thiophene conjugated side chain of the BDTT unit based on previous experience (Energy Environ. Sci. 2014, 7, 2276-2284). The HOMO level of the material also shifts its absorption spectrum to a redshift. It has also been found that the resulting polymer J61 is more favorable for aligning molecules parallel to the direction of the substrate, and these factors are favorable for improving the photovoltaic performance. The device Voc and PCE based on J61 and ITIC were increased to approximately 0.9 V and 9.52%, respectively (J. Am. Chem. Soc. 2016, 138, 4657-4664), and the efficiency was further improved by 10.57% (J. Am Chem. Soc. 2016, 138, 15011-15018).
Silane-based side chains have received little attention in the construction of conjugated polymer photovoltaic materials. Although trimethylsilane is usually used as a protecting group in the construction of organic functional groups, little attention has been paid to the electronic effect of the silyl chain. Researchers at the Organic Solids Laboratory for the first time incorporated silane groups on the thiophene conjugated side chains of BDTT and fluorine-substituted BTA copolymers to modulate the electronic structure and photovoltaic performance of the polymer. It was found that the obtained polymer J71 has a lower HOMO energy level due to the interaction of the silicon atom σ* and the π orbitals on the aromatic system. In addition, the longer C-Si bonds also allow the trisubstituted alkyl chains on the silicon atoms to be further from the backbone, increasing the ordered packing of the molecules and the crystallinity of the polymer. The Voc of non-fullerene polymer solar cells prepared by blending J71 and ITIC was further increased to 0.94 V, and the device efficiency reached 11.41%. This system still obtains efficient separation of the receptor exciton charge when the difference in the HOMO energy level of the acceptor is only 0.11 eV, thereby obtaining a photovoltaic device having both a high open circuit voltage and a high short-circuit current. The driving force of the exciton charge separation in polymer solar cells is of great significance, and it also provides a new idea and new approach for the molecular design of future highly conjugated polymer donor photovoltaic materials, and will be used for the development of organic photovoltaics. Play a catalytic role. This work has been published in Nature-Communications (Nat. Commun. 2016, 7, 13651).
Figure: Side-chain engineering of non-fullerene polymer solar cell donor photovoltaic materials and their HOMO energy levels and device performance
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