Role of the Solvent on the Orientation of Y-Type Acceptor Molecules in Spin-Coated Films

In this study, a rational processing strategy is presented, aiming to achieve well-ordered thin films of the molecular electron acceptors Y5 and Y6 by the choice of solvent as a key parameter. The thin films were spin-coated from chlorobenzene (CB), chloroform (CF), and ortho-xylene (o-XYL) solutions. The film morphology and molecular orientation were investigated by atomic force microscopy (AFM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, respectively. A homogeneous and smooth morphology was achieved when CF was used as the processing solvent. However, using CB and o-XYL resulted in significantly rougher films with larger structures. The dichroism observed in NEXAFS spectra using a linearly polarized incident X-ray beam and recorded in total electron yield (TEY) mode is indicative of a preferential face-on molecular orientation at the surface of Y5 and Y6 thin films processed from CF solution. In contrast, NEXAFS spectra of thin films processed from CB and o-XYL do not show any dependence on the electric field polarization direction of the incident X-ray beam, implying the absence of molecular orientation in those films. To understand the nature of the electronic transitions responsible for the absorption resonances in the NEXAFS spectra at the N and C K-edges, the natural transition orbitals corresponding to these electronic transitions were determined by time-dependent density functional theory (TD-DFT) calculations, confirming the face-on orientation of the molecules in the films processed from the CF solution. The fact that face-on oriented films are only achieved using CF is attributed to the superior solubility of Y5 and Y6 in this solvent and the lower degree of preaggregation in solution.