Planarity control of conjugated polymers for enhancement in their electrical properties

Abstract

Conjugated polymers refer to organic materials whose main backbone is composed of saturated and unsaturated bonds in an alternating arrangement, which enable electrical conductivity. Those conjugated polymers have attracted huge attention as promising materials for organic electronics such as organic solar cells (OSCs) and organic thin-film transistors (OTFTs) due to their solution-processability, flexibility, and low-cost preparation. Structural modification of conjugated polymers can easily induce the change in their optical and electrical properties and thus, is required to improve their performance in the OSCs and OTFTs. Recently, in case of all-polymer solar cells (all-PSCs) which have photoactive layer consisting of both p-type and n-type polymers, the effect of the structural modification on the performance is more critical than that of OSCs based on fullerene derivatives as electron acceptor. Thus, variety of synthetic method for modifying chemical structure of conjugated polymers and corresponding effects on power conversion efficiency (PCE) in OSCs have been carefully investigated. The control of electrical properties can be easily achieved by the structural modification to enhance the planarity of conjugated polymers because the polymer-packing structure and intermolecular interactions between the polymer chains in thin films are strongly dependent on their planarity. However, there is still lack of understanding for synthetic approach to control the planarity of the conjugated polymers. In this paper, (1) the synthetic method to control the planarity of conjugated polymers with the incorporation of single atom in polymer backbone and (2) the calculative ways based on the molecular simulation to expect the polymer-packing structure depending on the planarity of monomers are proposed. A method of lowering the planarity, however, should be also proposed since low solubility can be led by the high level of planarity and thereby the PCE of OSCs may be reduced. There should be optimization between the planarity and the solubility of conjugated polymers to improve the electrical properties in thin film state. Therefore, the synthetic method of adjusting carefully the planarity while increasing the solubility has been developed in this paper. Finally, it is intended that this paper can contribute to improve our understanding on the controlling planarity and the structure-properties relationships.