Charge transfer-induced solution-state crystallization assembly of conjugated polymer into nanowire for organic optoelectronic devices

Abstract

The control of the nanoscale morphology of conjugated polymers is one of the essential parameters in determining the electrical performance of organic thin film devices. Having one-dimensional (1D) nanostructure, especially, have been attracted much attention due to its potential for fabricating high electrical performance optoelectronic devices with the characteristics of the highly efficient charge transport along the anisotropic stacking direction, high mechanical stretchability, and providing a long percolation network for charge carriers. Along this line, the pre-assembly of conjugated nanowires (NWs) in the solution state has attracted attention as an alternative to the use of a post-annealing process due to its easy fabrication and available of property transfer to as-cast films with well-defined morphology without an additional annealing process. As representative methods for solution-state crystallization-driven self-assembly, the whisker, mixed solvent, and vapor diffusion methods have been used extensively. However, these methods have an inconvenience of controlling solubility or temperature to induce the self-assembly of CPs. The self-assembly, normally, is directed through noncovalent interactions, such as hydrogen bonding, van der Waals forces, and π-π interactions. In this work, I report a solution process for crystallization-driven self-assembly of CPs by the addition of doping additive. Recently, the solution process for the fabrication of efficient thin film devices using doping additives is also drawing attention as well as the crystallization-driven self-assembly process. In this regard, a simple blending of CPs and doping additive using a single good solvent with aging induce the crystallization-driven self-assembly of conjugated polymer into 1D NWs by charge transfer between two species, which leads doping and self-assembly of CPs at the same time. The mechanism underlying the formation of NWs by addition of additive has been confirmed by using π-π stacking derived additive, controlling the molecular parameter, and direct observation of dynamics of the assembly. Direct solution state assembly has been clearly visualized by cryo- and in situ LC-TEM. The resultant CPs/additive NWs based film showed higher conductivity than that of as cast films. This charge-transfer induced solution-state crystallization assembly method can provide a new and universal approach for the self-assembly of conjugated polymers and with a potential to fabricate highly conductive thin film with the simple process.