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.

In this study, we evaluate the sulphur L₁-edge, as a more accessible alternative soft X-ray to the sulphur K-edge that provides comparable spectroscopic information and as a simpler spectroscopic probe than the sulfur L₂,₃ edges for studying organic semiconductors. Angle-dependent NEXAFS measurements were performed on neat films of PYT, PF5 Y5, supported by TD-DFT calculations. Sulphur atoms in distinct chemical environments show nearly identical L₁ edge spectral signatures in all polymers. PYT shows a clear dichroic response consistent with previously reported orientation trends from carbon K‑edge measurements, whereas PF5‑Y5 shows reduced dichroism due to its more complex electronic structure. The TD-DFT calculations reveal that the sulfur L₁-edge spectral features are not trivial to interpret. In particular, the intense low-energy resonance is composed of multiple transitions with mixed character, including significant σ*(S–C) contributions together with π*-type states associated with the conjugated backbone. This mixed nature closely parallels observations at the sulfur K-edge and demonstrates that the dominant peak cannot be assigned to a purely π* or σ* transition. At higher excitation energies, the unoccupied electronic structure becomes increasingly complex, with strong mixing of states with different symmetry, and sulfur-centered contributions originating from the hybridization of thiophene sulfur lone pairs. Owing to the third-row character of sulfur, these higher-energy states exhibit more diffuse and multi-lobed spatial distributions, reflecting increased angular-momentum mixing. As a result, a straightforward separation into π* and σ* resonances is no longer adequate in this energy range. These results establish practical guidelines for interpreting sulfur L₁-edge spectra and demonstrate that its reliability as an orientation probe depends sensitively on the electronic structure of the specific 

Organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) typically exhibit highly intermixed active-layer morphologies with characteristic domain sizes on the order of a few tens of nanometers. Atomic force microscopy (AFM)-based techniques, such as photoconductive AFM (pc-AFM), Kelvin probe force microscopy (KPFM), and PeakForce quantitative nanomechanical mapping (PFQNM) enable spatially resolved measurements of local electrical and mechanical properties. While interpreting nanoscale contrast in highly intermixed NFA-based blends remains challenging, particularly in assigning electrical or mechanical heterogeneity to compositional variations, the complementarity of the different signals is helpful to understand this more complete picture. 

Here, we use the benchmark donor–acceptor system PBDB-T:Y6 to assess the capabilities and limitations of AFM-based nanoscale characterization in intermixed NFA-based blend films. By systematically comparing surface potential, photocurrent, and nanomechanical maps for films obtained under different (post-)processing conditions, we monitor how nanoscale electrical and mechanical heterogeneity develops and how local photovoltaic behaviour is expressed at the nanoscale. We then evaluate whether these signals can be reliably attributed to specific structural features.