Probing molecular orientation of donors and acceptors in all-polymer blend films by near-edge x-ray absorption fine structure spectroscopy

Abstract The molecular orientation is crucial for the efficiency of organic solar cells. A face-on orientation, in which the π−π 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.