Modulation of Interchain Packing Orientation by Varying a Branch Point of Alkyl Substituent: Molecular Dynamics Simulation and Electrical Properties

Designing organic semiconductor thin films with controlled molecular orientation is crucial for enhancing device performance. In this study, we tackle this challenge by analyzing the mechanism underlying molecular orientation from a molecular design perspective. We synthesize two types of p-type semicrystalline polymers, PPDT2FBT-2C and PPDT2FBT-5C, which differ in their side-chain branch points. PPDT2FBT-5C, with increased distance between the main chain and the branching point, promotes strong preaggregation in solution, leading to a shift in dominant orientation from face-on to edge-on. Molecular dynamics simulations provide insights into the mechanism of orientation change in thin films. For highly aggregated PPDT2FBT-5C polymers, the length of the π–π packing cluster is sufficiently large, increasing the surface area of the alkyl side chains within the cluster. This facilitates polymer interaction with the substrate and enables stacking in the edge-on orientation. The distinct orientations observed in PPDT2FBT-2C and PPDT2FBT-5C correlate well with the electrical properties of horizontal and vertical organic field-effect transistors and solar cells, suggesting a strategy for developing tailored materials with specific molecular alignment to optimize various optoelectronic devices.