Synthesis and Photovoltaic Properties of Two-Dimensional Low-Bandgap Copolymers Based on New Benzothiadiazole Derivatives with Different Conjugated Arylvinylene Side Chains

Graphical 2D Photovoltaic copolymers : New benzothiadiazole acceptors with different conjugated arylvinylene side chains have been designed and used to build 2D-like low-bandgap copolymers (see figure). It was found that their optical and electrochemical properties can be easily modified by changing the structures of the conjugated arylvinylene side chains. The photovoltaic properties imply that all the prepared copolymers are promising candidates for roll-to-roll manufacturing of efficient polymer solar cells. A new series of 2,1,3-benzothiadiazole (BT) acceptors with different conjugated aryl-vinylene side chains have been designed and used to build efficient low-bandgap (LBG) photovoltaic copolymers. Based on benzo[1,2- b :3,4- b ′]dithiophene and the resulting new BT derivatives, three two-dimensional (2D)-like donor (D)–acceptor (A) conjugated copolymers have been synthesised by Stille coupling polymerisation. These copolymers were characterised by NMR spectroscopy, gel-permeation chromatography, thermogravimetric analysis and differential scanning calorimetry. UV/Vis absorption and cyclic voltammetry measurements indicated that their optical and electrochemical properties can be facilely modified by changing the structures of the conjugated aryl-vinylene side chains. The copolymer with phenyl-vinylene side chains exhibited the best light harvesting and smallest bandgap of the three copolymers. The basic electronic structures of D–A model compounds of these copolymers were also studied by DFT calculations at the B3LYP/6-31G* level of theory. Polymer solar cells (PSCs) with a typical structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS)/copolymer:[6,6]-phenyl-C 61 (C 71 )-butyric acid-methyl ester (PCBM)/calcium (Ca)/aluminum (Al) were fabricated and measured under the illumination of AM1.5G at 100 mW cm −2 . The results showed that the device based on the copolymer with phenyl-vinylene side chains had the highest efficiency of 2.17 % with PC 71 BM as acceptor. The results presented herein indicate that all the prepared copolymers are promising candidates for roll-to-roll manufacturing of efficient PSCs. Suitable electronic, optical and photovoltaic properties of BT-based copolymers can also be achieved by fine-tuning the structures of the aryl-vinylene side chains for photovoltaic application.