Organic solar cells (OSC) are a promising, cost-effective alternative to established photovoltaic technologies, offering a pathway to sustainable energy generation. The recent record efficiencies (>19%) are attributed to the development of non-fullerene small molecule acceptors, such as Y5. However, despite their promising efficiencies, OSCs based on NFA small molecules exhibit low thermal and mechanical stability, posing a significant obstacle to their commercialization of OSCs. To overcome these limitations, the polymerization of small molecule acceptors emerged as a promising strategy in recent years [1]. Nevertheless, the introduction of new small molecule-based polymer acceptors has brought into focus challenges related to microstructure and molecular orientation.

In this study, we compare the molecular orientation of two Y5-based polymer acceptors, PF5-Y5 [2] and PYT [3], as neat films and in blends with the polymer donor PBDB-T at two ratios of donor: acceptor, 1:0.75 and 1:10. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy with variable angle of X-rays was employed to investigate the molecular orientation of these thin films spin-coated from CB. Spectra were acquired using partial electron yield (PEY), total electron yield (TEY), and fluorescence yield (FY), providing depth-dependent information about the films. For the blends, the spectral shape of the carbon K-edge spectra in PEY and TEY modes resembled the one of the neat donor PBDB-T films, indicating that the blend film surface is PBDB-T-rich, even at a tenfold higher acceptor concentration. Identifying the acceptor component in the carbon K-spectra required deeper subsurface probing using TEY and FY modes, along with an analysis of the angular dependence of the spectra. To assess whether the polymer donor influences the acceptor’s molecular orientation, we utilized the element specificity of NEXAFS spectroscopy. Nitrogen was employed as a fingerprint element for the polymer acceptors [4]. N K-edge NEXAFS spectra revealed that the acceptors in PBDB-T: PF5-Y5(1:0.75), PBDB-T: PF5-Y5 (1:10), and PBDB-T: PYT (1:10) blends retain the face-on orientation observed in neat acceptor films. However, for the PBDB-T: PYT (1:0.75) blend, a reduced dichroic ratio was observed for the first nitrogen 1s → π* resonance at 398.56 eV in PEY spectra, indicating that the donor PBDB-T, influences the molecular orientation of PYT in the film's surface layer.