Strain induced polymorphism and band structure modulation in low-temperature 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene single crystal

Organic semiconductors are inherently soft, making it possible to increase their mobilities by strains. Such a unique feature can be exploited directly in flexible electronics for improved device performance. The 2,7-dioctyl[1]benzothieno[3,2-b][1]-benzothiophene derivative, C8-BTBT is one of the best small-molecule hole transport materials. Here, we demonstrated its band structure modulation under strains by combining the non-equilibrium molecular dynamics simulations and first-principles calculations. We found that the C8-BTBT lattice undergoes a transition from monoclinic to triclinic crystal system at the temperature below 160 K. Both shear and uniaxial strains were applied to the low-temperature triclinic phase of C8-BTBT, and polymorphism was identified in the shear process. The band width enhancement is up to 8% under 2% of compressive strain along the x direction, and 14% under 4% of tensile strain along the y direction. The band structure modulation of C8-BTBT can be well related to its herringbone packing motifs, where the edge to face and edge to edge pairs constitute two-dimensional charge transport pathways and their electronic overlaps determine the band widths along the two directions respectively. These findings pave the way for utilizing strains towards improved performance of organic semiconductors on flexible substrates, for example, by bending the substrates.