Thin Film Structure of Thieno[3,2-b]thiophene-Based Mixed Ionic/electronic Conductor from Grazing Incidence X-ray Scattering and Molecular Dynamics Simulation
We have designed and synthesized a dual-conducting molecule (BTTT/dEO4) containing 2,5-bis(thien-2-yl)thieno[3,2-b]thiophene (BTTT) as the electron-conducting segments attached with tetra(ethylene oxide) (EO4) at both ends as the ion-conducting segments. We adopted a combination of experimental grazing incidence wide-angle X-ray scattering (GIWAXS) and molecular dynamics (MD) simulations to better understand the molecular packing of BTTT/dEO4 films, particularly when loaded with the ion conducting salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We observed a layered structure with highly ordered BTTT layers and less ordered EO4 layers. In BTTT layers, our analysis reveals a 2D centered rectangular lattice and a herringbone motif within film plane, and estimates the molecular tilt toward the film. When BTTT/dEO4 molecules were mixed with LiTFSI, we calculated several geometric parameters to characterize the positional and orientational rearrangements of BTTT segments as well as the anisotropic film expansion.