Two dimensional photovoltaic copolymers based on new benzothiadiazole acceptors with diphenylamine-vinylene side chains

Abstract

A novel acceptor of 2,1,3-benzothiadiazole (BT) with diphenylamine-vinylene side chains was firstly designed and synthesized for building efficient low bandgap (LBG) photovoltaic copolymers in this paper. Based on benzo[1,2-b;3,4-b]dithiophene and the designed new BT acceptor, two dimensional (2D) like conjugated copolymers have been prepared by Stille coupling polymerization. The resulting copolymers were characterized by NMR, elemental analysis, gel permeation chromatography, differential scanning calorimetry and thermogravimetric analysis. UV-vis absorption and cyclic voltammetry measurements indicate that these copolymers have good optical and electrochemical properties. A copolymer with thiophene modified benzo[1,2-b;3,4-b]dithiophene exhibits better light harvesting and a smaller bandgap of the two copolymers. The electronic structures of the model compounds were also studied by DFT calculations at the B3LYP/6-31G* level. The polymer solar cells (PSCs) were fabricated and measured with a typical structure of ITO/PEDOT:PSS/copolymer:PCBM/Ca/Al. The primary results showed that the device based on the copolymer with thiophene modified benzo[1,2-b;3,4-b]dithiophene had a higher efficiency of 1.88%, attributed to the smaller bandgap and improved active film morphology. All the prepared copolymers are promising candidates for efficient PSCs with high V_oc.