Using near-edge X-ray absorption fine structure (NEXAFS) and resonant Auger spectroscopy (RAS) in conjunction with the core-hole clock methodology the electronic structure, molecular ordering and orientation and charge transfer dynamics in the femtosecond time scale of 2,7-di-tert-pentylbenzothieno[3,2-b]benzothiophene (di(Me)2Pr-BTBT), 2,7-di-iso-propylbenzothieno[3,2-b]benzothiophene (diiPr-BTBT) and 2,7-di-tert butylbenzothieno[3,2-b]benzothiophene (ditBu-BTBT) films were investigated. Total electron yield (TEY) and fluorescence yield (FY) NEXAFS spectra were recorded with the aim of determining the preferred molecular orientation of the oligomers at the surface and in the bulk. Angular dependent sulfur 1s NEXAFS spectra for diiPr-BTBT and ditBu-BTBT films deposited onto FTO (fluorine doped tin oxide) point to well-organized films with a preferred edge-on geometry, while for the di(Me)2Pr-BTBT film little variation is seen, indicating that the film matches its herringbone crystal packing. Films prepared on ITO (indium tin oxide) and silicon were also investigated by TEY and FY NEXAFS. Greater film ordering is observed in the bulk. The electron charge transfer time following sulfur K-edge main resonance was calculated. The ditBu-BTBT films showed the lowest charge transfer time as well as greater molecular organization, pointing to an increased coupling, as compared to the other oligomers.
- near-edge x-ray absorption fine structure
- charge transfer dynamics
- organic field effect transistor