Magnetic-field guided solvent vapor annealing for enhanced molecular alignment and carrier mobility of a semiconducting diketopyrrolopyrrole-based polymer

Abstract

Controlling the molecular orientation of organic semiconductors is crucial to improving the performance of electronic devices. In this article, we describe a straightforward method to achieve large-area highly aligned films of a diketopyrrolopyrrole-bithiophene polymer (PDPP2TBT) by solvent vapor annealing (SVA) of the as-spun films under a high magnetic field. The structural characterizations disclose that the chain backbones of PDPP2TBT in the films are highly aligned to the applied magnetic field during SVA; meanwhile, the films exhibit high crystallinity. A mechanism is proposed to explain the magnetic alignment, based on the reformation of chain aggregates in the wet film exposed to solvent vapor. Field-effect transistors (FETs) based on the magnetically aligned PDPP2TBT films exhibit an enhancement of hole mobility (a maximum value of 1.56 cm² V⁻¹ s⁻¹) by a factor of 6 compared to the unaligned devices, as well as mobility anisotropy of three. Temperature-dependent FET mobility measurement reveals a remarkable lowering of thermally activated energy for carrier hopping in the aligned film. The results indicate the formation of a rapid intra-chain conduction pathway parallel to the chain alignment direction, which originates from the alignment-induced backbone extension and enhanced order of inter-chain packing.