Role of the Solvent\non the Orientation of Y‑Type\nAcceptor Molecules in Spin-Coated Films

In this study, a\nrational processing strategy is presented,\naiming\nto achieve well-ordered thin films of the molecular electron acceptors\nY5 and Y6 by the choice of solvent as a key parameter. The thin films\nwere spin-coated from chlorobenzene (CB), chloroform (CF), and <i>ortho</i>-xylene (<i>o</i>-XYL) solutions. The film\nmorphology and molecular orientation were investigated by atomic force\nmicroscopy (AFM) and near-edge X-ray absorption fine structure (NEXAFS)\nspectroscopy, respectively. A homogeneous and smooth morphology was\nachieved when CF was used as the processing solvent. However, using\nCB and <i>o</i>-XYL resulted in significantly rougher films\nwith larger structures. The dichroism observed in NEXAFS spectra using\na linearly polarized incident X-ray beam and recorded in total electron\nyield (TEY) mode is indicative of a preferential face-on molecular\norientation at the surface of Y5 and Y6 thin films processed from\nCF solution. In contrast, NEXAFS spectra of thin films processed from\nCB and <i>o</i>-XYL do not show any dependence on the electric\nfield polarization direction of the incident X-ray beam, implying\nthe absence of molecular orientation in those films. To understand\nthe nature of the electronic transitions responsible for the absorption\nresonances in the NEXAFS spectra at the N and C K-edges, the natural\ntransition orbitals corresponding to these electronic transitions\nwere determined by time-dependent density functional theory (TD-DFT)\ncalculations, confirming the face-on orientation of the molecules\nin the films processed from the CF solution. The fact that face-on\noriented films are only achieved using CF is attributed to the superior\nsolubility of Y5 and Y6 in this solvent and the lower degree of preaggregation\nin solution.