Combinatorial Study of Temperature-Dependent Nanostructure and Electrical Conduction of Polymer Semiconductors: Even Bimodal Orientation Can Enhance 3D Charge Transport

Abstract

Temperature-dependent (80-350 K) charge transport in polymer semiconductor thin films is studied in parallel with in situ X-ray structural characterization at equivalent temperatures. The study is conducted on a pair of isoindigo-based polymers containing the same p-conjugated backbone with different side chains: one with siloxane-terminated side chains (PII2T-Si) and the other with branched alkyl-terminated side chains (PII2T-Ref). The different chemical moiety in the side chain results in a completely different film morphology. PII2T-Si films show domains of both edge-on and face-on orientations (bimodal orientation) while PII2T-Ref films show domains of edge-on orientation (unimodal orientation). Electrical transport properties of this pair of polymers are also distinctive, especially at high temperatures (>230 K). Smaller activation energy (E-A) and larger pre-exponential factor (mu(0)) in the mobility-temperature Arrhenius relation are obtained for PII2T-Si films when compared to those for PII2T-Ref films. The results indicate that the more effective transport pathway is formed for PII2T-Si films than for the other, despite the bimodally oriented film structure. The closer pi-pi packing distance, the longer coherence length of the molecular ordering, and the smaller disorder of the transport energy states for PII2T-Si films altogether support the conduction to occur more effectively through a system with both edge-on and face on orientations of the conjugated molecules. Reminding the 3D nature of conduction in polymer semiconductor, our results suggest that the engineering rules for advanced polymer semiconductors should not simply focus on obtaining films with conjugated backbone in edge-on orientation only. Instead, the engineering should also encounter the contribution of the inevitable off-directional transport process to attain effective transport from polymer thin films.