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.

The molecular orientation is crucial for the efficiency of organic solar cells. A face-on orientation, in which the pi-pi stacking direction is oriented perpendicular to the substrate, is typically preferred because it enhances vertical charge transport to the electrodes and can additionally modify the position of energy levels. In this study, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy was employed to investigate the molecular orientation of the acceptor polymers PYT and PF5-Y5 and the donor polymer PBDB-T in spin-coated blend films with different donor: acceptor ratios. From the comparison of NEXAFS spectra acquired in partial electron yield (PEY), total electron yield (TEY), and fluorescence yield (FY) modes, depth-dependent information about the orientation of the components in the films can be extracted. We found that the absorption resonances in the PEY carbon K-edge spectra of all the blend films resembled the spectral signatures of PBDB-T, indicating that the surface of these blend films is PBDB-T-rich, even at a 1:10 donor-to-acceptor ratio. To identify the acceptor component in the carbon spectra, deeper subsurface probing was required using TEY and FY modes, alongside analysis of the angular dependence of these spectra. Nitrogen K-edge NEXAFS spectra were employed to selectively probe the acceptor orientation in the blend films, revealing that generally the polymer acceptors retain their face-on orientation observed in neat acceptor films. However, in one blend, a decrease in the dichroic ratio suggests that the donor polymer influences the molecular orientation of the acceptor at the film's surface. This work demonstrates a novel strategy to probe molecular orientation in all-polymer blend films. The approach exploits dichroism at selective absorption edges to access detailed information on the molecular orientation of one component within the blend film.

Sequential deposition (SD) through independent processing of donor and acceptor materials, has emerged as a promising strategy to enable better control over the active layer morphology of organic solar cells. In this work, time-of-flight secondary ion mass spectrometry was employed to investigate the vertical distribution of SD PM6/Y5 films and bulk heterojunction PM6:Y5 films coated from a blend solution in one-step process. The influence of thermal annealing (TA) on the vertical distribution of the components was also evaluated. Our results show that SD inverts the vertical distribution within the active layer, while TA helps to suppress Y5 diffusion into the PM6 layer. Additionally, near edge x-ray absorption fine structure spectroscopy (NEXAFS) was used to investigate the molecular orientation of the donor PM6 and the acceptor Y5, in SD and blend films. Depth-dependent molecular orientation was assessed by comparing NEXAFS spectra acquired in total electron yield and fluorescence yield. Nitrogen K-edge NEXAFS spectra were employed to selectively probe the acceptor orientation. In SD-processed samples, we found that Y5 retains its face-on orientation when deposited on top of PM6, despite the combined effects of film formation dynamics and interfacial intermixing inherent to the process.