Side chain engineering: The effect on the properties of isoindigo-based conjugated polymers contain different length and structure alkyl chains on nitrogen atom

Organic Electronics (2017), 49, 278-285.

Zhu Lei, Jiang Chao, Chen Guangxin, Zhou Zheng, Li Qifang

Abstract: A series of donor-acceptor (D-A) conjugated polymers based on benzo[1,2-b:4,5-b']dithiophene (BDT) and isoindigo with different alkyl side chains were designed and synthesized.  These polymers named PBDT-TT-IID_O, PBDT-TT-IID_EH, PBDT-TT-IID_BO, PBDT-TT-IID_HD, and PBDT-TT-IID_OD have different length or structure of the alkyl side chains on isoindigo unit.  As the length of the alkyl chain increases, the soly. of the polymer enhances. The results indicate that the alkyl side chains have little effect on the optical properties of individual polymer mols., but have a significant impact on optical, electrochem., film-forming and photovoltaic properties of the polymers in the aggregated state.  PBDT-TT-IID_HD with a sixteen carbon branched chain achieves the best power conversion efficiencies (PCEs) of 6.83% when blending with [6,6]-Ph C71 butyric acid Me ester (PC₇₁BM) as acceptor.  The influence of the alkyl side chain on the short-circuit current is much greater than that of the open circuit voltage. Atomic force microscopy reveals that the side chains changed the morphol. of the active layers and the size of the phase region. For isoindigo based polymers, hexyl-decyl group in the commonly used alkyl chains appear to be a good choice.