In Situ Analysis of Solvent and Additive Effects on Film Morphology Evolution in Spin‐Cast Small‐Molecule and Polymer Photovoltaic Materials

To elucidate the details of film morphology/order evolution during spin‐coating, solvent and additive effects are systematically investigated for three representative organic solar cell (OSC) active layer materials using combined in situ grazing incidence wide angle x‐ray scattering (GIWAXS) and optical reflectance. Two archetypical semiconducting donor (p‐type) polymers, P3HT and PTB7, and semiconducting donor small‐molecule, p‐DTS(FBTTh2)2 are studied using three neat solvents (chloroform, chlorobenzene, 1,2‐dichlorobenzene) and four processing additives (1‐chloronaphthalene, diphenyl ether, 1,8‐diiodooctane, and 1,6‐diiodohexane). In situ GIWAXS identifies several trends: 1) for neat solvents, rapid crystallization occurs that risks kinetically locking the material into multiple crystal structures or crystalline orientations; and 2) for solvent + additive processed films, morphology evolution involves sequential transformations on timescales ranging from seconds to hours, with key divergences dependent on additive/semiconductor molecular interactions. When π‐planes dominate the additive/semiconductor interactions, both polymers and small molecule films follow similar evolutions, completing in 1–5 min. When side chains dominate the additive/semiconductor interactions, polymer film maturation times are up to 9 h, while initial crystallization times