Domain Orientation and Molecular Ordering in Thin Films of Small Molecule Donor for Organic Photovoltaics

The relationship between the nanomorphology and the charge transport characteristics in solution-processed organic thin films has been the focus of extensive research, as it is key to the further advancement of organic photovoltaics. Understanding the nanostructured domains and their dependence on the applied thermal annealing conditions is one crucial challenge. Here, the conjugated small-molecule donor p-DTS(FBTTh2)2 is used as a model system to demonstrate how isothermal annealing alone, without addition of any processable additives during solution casting, leads to a high ordering of domain structures. Using scanning transmission X-ray microspectroscopy (STXM), the domain structures are probed at the resonance energy of 284.7 eV corresponding to the C1s to Pi* transition. The STXM data analysis reveals the in-plane orientation of the conjugated backbone comprising the heterocyclic aromatic compounds dithienosilole (DTS), bithiophene (BT), and fluorobenzothiadiazole (FBT). The analysis further exhibits the presence of ordered domains accompanied by the disordered boundaries in thin films where the increase in the domain sizes and the high degree of molecular order (anisotropy) is associated to the relatively slow crystallization dynamics of p-DTS(FBTTh2)2 molecules during isothermal annealing at 90 °C. The analysis of the out-of-plane component of the directed resonance intensity shows deviations in the local out-of-plane tilt angles to the lower values, for the most ordered regions in the thin film, suggesting the more out-of-plane orientation of Pi* resonance. These results manifest a promising role of STXM technique in enhancing the knowledge of domain structures in small molecule organic semiconductors.