Importance of Short-Range Order in Governing Thin Film Morphology and Electronic Properties of Polymeric Organic Semiconductors

Abstract

Semiconducting polymers provide a ubiquitous platform for a range of applications in molecular electronics and photovoltaics, but the ordered and disordered regions of these materials impart different optoelectronic properties. By resolving local morphology using solid-state magnetic resonance spectroscopy and modeling techniques, here, we demonstrate that the PTB7-Th donor−acceptor (D−A) copolymer and P3HT and MEH-PPV homopolymers exhibit different degrees of the shortrange order, which can be associated with the large differences in their charge carrier mobilities. The high degree of local order in PTB7-Th (84−99%) is facilitated by noncovalent interactions between D and A moieties. In contrast to this, the reduced local order in P3HT (30−44%) and MEH-PPV (39−43%) homopolymers is due to the distortions in the vicinities of backbone and side chain moieties that lead to conformationally tilted polymer chains. Combined solid-state NMR and density functional theory (DFT) modeling allows the degree of backbone torsion in these materials to be determined, and insights into packing interactions are obtained by two-dimensional (2D) 1 H− 1 H, 1 H− 13 C, and 1 H− 19 F correlation NMR spectroscopy. In addition, the different paramagnetic species and hyperfine interactions are analyzed by EPR spectroscopy and are expected to influence the charge carrier mobilities. A detailed analysis of the local structures presented in this study helps explain the morphological anomalies and their impact on bulk charge carrier mobilities and electronic density of states, thus providing essential insights into the morphology−property relationships in polymeric organic semiconductors.