Structural Insight into Aggregation and Orientation of TPD-Based Conjugated Polymers for Efficient Charge-Transporting Properties

Abstract

In this study, we obtained a new structural insight into the charge-transporting properties in TPD-based polymers that cannot be solely explained in terms of the type of orientation. We synthesized two types of copolymers comprising mono-TPD or bis-TPD as the accepting unit. Although the planarity and energy levels are similar with the mono-TPD unit, the aggregation state is quite different, and the X-aggregation tendency seems to be stronger when the bis-TPD unit is incorporated. In the case of TPD1, an effective π- π orbital overlap is found to originate from the H-aggregates, and 3D charge transport pathways are formed with a bimodal orientation of edge-on and face-on, resulting in an efficient charge transportation (1.84 cm 2 ·V -1 ·s -1 of hole and 0.31 cm 2 ·V -1 ·s -1 of electron). In contrast, despite the well-aligned edge-on orientation of TPD2, it exhibited a relatively very low mobility and splitted emission characteristics in photoluminescence spectra because of the tilted intermolecular stacking pattern with an X-shape (0.015 cm 2 ·V -1 ·s -1 for hole and 0.16 cm 2 ·V -1 ·s -1 for electron). An overall characterization of the semiconducting polymers was performed, and it was found that the type of aggregation in the final thin films, such as H- or X-aggregation, is indeed important and perhaps more important than the orientation to obtain polymers with a high charge carrier mobility. © 2019 American Chemical Society.