Understanding lattice strain-controlled charge transport in organic semiconductors: A computational study

The softness and anisotropy of organic semiconductors offer unique properties. Recently, solution-sheared thin-films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-P) with nonequilibrium single-crystal domains have shown much higher charge mobilities than unstrained ones (Nature2011, 480, 504). However, to achieve efficient and targeted modulation of charge transport in organic semiconductors, a detailed microscopic understanding of the structure-property relationship is needed. In this work, motivated by the experimental studies, the relationship between lattice strain, molecular packing, and charge carrier mobility of TIPS-P crystals is elucidated. By employing a multiscale theoretical approach combining nonequilibrium molecular dynamics, first-principles calculations, and kinetic Monte Carlo simulations using charge-transfer rates based on the tunneling enabled hopping model, charge-transport properties of TIPS-P under various lattice strains are investigated. Shear-strained TIPS-P indeed exhibits one-dimensional charge transport, which agrees with the experiments. Furthermore, either shear or tensile strain lead to mobility enhancement, but with strong charge-transport anisotropy. In addition, a combination of shear and tensile strains could not only enhance mobility, but also decrease anisotropy. By combining the shear and tensile strains, almost isotropic charge transport could be realized in TIPS-P crystal with the hole mobility improved by at least one order of magnitude. This approach enables a deep understanding of the effect of lattice strain on charge carrier transport properties in organic semiconductors.