Effect of Bulky Atom Substitution on Backbone Coplanarity and Electrical Properties of Cyclopentadithiophene-based Semiconducting Polymers.

Herein, we investigate the effect of atomic substitution on the optoelectronic properties of a coplanar donor-acceptor (D-A) semiconducting polymer (SPs), PCDTBT, prepared using cyclopentadithiophene (CDT) and 2,1,3-benzothiadiazole (BT) moieties. By substituting a carbon atom in the BT unit of PCDTBT with C-F or C-Cl, two random D-A SPs (PCDTFBT and PCDTClBT, respectively) were prepared, and their optoelectronic properties were thoroughly investigated. Density functional theory calculations demonstrated that PCDTFBT has a slightly smaller dihedral angle (ϴ = 0.6°) than PCDTBT (ϴ = 1.9°) in its lowest-energy conformation, implying efficient charge transport through the coplanar backbone of PCDTFBT. However, PCDTClBT shows the lowest energy at a relatively larger dihedral angle (ϴ = 139°) due to the steric hindrance induced by bulky chlorine atoms in the backbone, thereby leading to thin-film morphology, which is unfavorable for charge transport. Consequently, PCDTFBT yields the highest field-effect mobility (μ) of 0.57 cm2 V-1 s-1 , slightly higher than that of PCDTBT (μ = 0.33 cm2 V-1 s-1 ), whereas the μ-value of PCDTClBT (0.02 cm2 V-1 s-1 ) is an order of magnitude lower than those of PCDTBT and PCDTFBT devices. In addition to the enhanced μ, the fluorinated analog, PCDTFBT, extended the device lifetime of organic field-effect transistors over 12 days without any encapsulation layers. The results of this study provide design guidelines for air-stable D-A SPs. This article is protected by copyright. All rights reserved.