Study of Bulk and Thin-Film Polymorphism of NDI Derivatives—Annealing and Deposition Procedures to Access Elusive Polymorphs

In the last decades, organic semiconductors (OSCs) have attracted a lot of attention due to their possible employment in solution-processed optoelectronic and electronic devices, such as organic field-effect transistors (OFETs). One of the big advantages of solution-processing is the possibility to process into flexible substrates at low cost. Organic molecular materials tend to form polymorphs, which can exhibit very different conductive, luminescent, and mechanical properties. In most cases, the control of the crystal structure is decisive to maximise the performances of the final devices.<br/>Bis(naphthalene diimide) (NDI) derivatives are a particularly interesting family of organic materials. NDIs possess high electron affinity, good charge carrier mobility, and excellent thermal and oxidative stability, making them promising candidates for applications in organic electronics, photovoltaic devices, and flexible displays. Recently, the structure-properties relationship and the polymorphism of these molecules have gained considerable attention.<br/>We have performed a full thermal characterization of bulk NDI-C6, which presents four bulk polymorphs (Form α, Form β, Form γ, and Form δ) and two different thermal pathways depending on whether the material is melted or not, by combining different characterization techniques and we determined the crystal structure of Form γ at 54°C from powder. The analysis of the thermal expansion principal axis shows that Form α and Form γ possess negative thermal expansion (X1) and massive positive thermal expansion (X3) which are correlated to the thermal behaviour observed. C₂F₃-Cl₄NDI polymorphism was also studied, and it was possible to identify and solve the crystal structure of two polymorphs and a solvate obtained by slow evaporation experiments with this molecule, although in organic semiconductors the appearance of solvates is very uncommon. Thin films of both NDI derivatives were produced and characterized, both structural and electrical properties, since different polymorphs can present very different behaviours. In the case of NDI-C6 a new metastable form that was not observed in bulk, called Form ε, was found by spin coating deposition of the toluene solution of NDI-C6 on the Si/SiO₂ substrate. This new phase could also be obtained by quenching NDI-C6 after the melting on a Si/SiO₂ substrate, which suggests Form ε is a surface induced polymorph.<br/><b>Acknowledgement </b><br/>This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 811284 (UHMob).