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 cm2·V–1·s–1 of hole and 0.31 cm2·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 cm2·V–1·s–1 for hole and 0.16 cm2·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.