Organic solar cells (OSCs) are the building blocks for an emerging renewable energy technology that offers a low-cost, flexible, and lightweight alternative to conventional photovoltaics. Record power conversion efficiencies of OSCs have exceeded 19% to date. Nevertheless, a comprehensive understanding of the molecular orientation and microstructure in the active layer remains essential to advancing the field. This thesis addresses these challenges by investigating the factors that govern the molecular orientation and microstructure of electron donors and acceptors in OSCs.

In the first study, we investigate the role of processing solvent in determining the molecular orientation and morphology of planar Y-type small molecule acceptors in spin-coated films. Chlorobenzene (CB), chloroform (CF), and ortho-xylene (o-XYL) were identified by means of Hansen Solubility Parameters (HSP) as suitable solvents for thin-film processing. Using Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, we showed that CF promotes a face-on orientation of the conjugated plane, in contrast to films processed from CB and o-XYL, where no preferential orientation was found. Furthermore, with support from density functional calculations, the distinct spectral signatures in the carbon K-edge spectra of Y6, compared to Y5, could be understood. The fluorine substitution in Y6 influences its electronic structure by shifting contributions from carbon 1s® π* transitions to higher photon energies.

In a second study, we investigate the Y5-based polymer acceptors, PF5-Y5 and PYT, in neat and blend films with the donor polymer PBDB-T. The dichroic ratios, extracted from series of NEXAFS spectra taken at variable incident angle of the X-ray, demonstrate that PF5-Y5 and PYT exhibit a face-on molecular orientation even in films processed from CB. For blend films, nitrogen K-edge NEXAFS spectra were used to probe the acceptor molecules selectively. The results revealed that increasing the donor: acceptor ratio in PBDB-T: PYT blends reduces the degree of orientation of the acceptor, suggesting that the polymer donor influences the acceptor orientation in this blend.

This work demonstrates the importance of solvent selection and polymer design in optimizing the molecular orientation and microstructure in spin-coated photoactive layers for solar cells, offering guidelines for the development of high-performance organic solar cells.

Organic solar cells (OSCs) are lightweight, flexible, and potentially scalable alternatives to traditional silicon solar panels, in which molecular semiconductors are used to convert solar energy into electricity. With record efficiencies exceeding 19%, OSCs have become a promising renewable energy technology. However, further research is required to understand how molecular orientation and microstructure in the photovoltaic layer affect device performance.

The photovoltaic layer is typically a solution-processed blend of electron donor and electron acceptor molecules. Controlling the molecular orientation is crucial for achieving high-efficiency devices. In this work, we use X-ray absorption spectroscopy to investigate the molecular orientation and electronic structure of small molecules, as well as polymer donors and acceptors in films consisting of one or several components. The results show that the choice of solvent used to process the small molecule acceptor layer plays an important role in determining their molecular orientation. Additionally, we demonstrate a novel approach that enables selective probing of the molecular orientation of one of the materials in a blend.