Organic semiconductors have been attracting great attention in last years due to its potential for application in new electronic devices. Organic semiconducting materials are often solution processable, easy tunable by molecular design and compatible with flexible and lightweight substrates. These properties of organic semiconductors are very advantageous and desirable for production of low cost, large scale, and flexible devices, such as solar cells.
Organic semiconductor molecules in crystals are usually bound by van der Waals forces, which are very weak and therefore allow for forming several different crystalline phases (polymorphs). Resulting crystal structure may strongly depend on crystallization conditions like temperature, substrate and/or substrate treatment, solvent or post-growth treatment. Additionally, electronic properties such as carrier charge mobility corresponding to individual polymorphs may differ in orders of magnitude.
Solution processable small molecule organic semiconductor 5,11-bis(triethyl silylethynyl) anthradithiophene (TES-ADT) has been intensively studied for its high device performance and charge mobility up to 1 cm2 V-1 s-1 [1], however with a poor reproducibility. At least four different polymorphs of TES-ADT were described later [2, 3] and their molecular packing structure is still not fully described. It is very likely that six orders of magnitude wide range of carrier charge mobilities obtained form TES-ADT measurements originates from different polymorph content in samples.
In order to better understand the rich phase behavior of TES-ADT, we performed in situ specular X-ray reflectivity and grazing incidence X-ray diffraction measurements during post-growth thermal annealing of TES-ADT thin film samples on glass substrate prepared at different conditions. We used evacuated annealing chamber mounted on a laboratory diffractometer equipped with a copper rotating anode.
We observed four phases described earlier in ref. [1] and determined its thermal expansion coefficients. We proved that all the phases may also coexist at room temperature. The α-phase appear at room temperature within an hour after all annealing processing. The γ-phase described in ref. [1] was obtained by annealing of amorphous phase, however, it was also observed during fast cooling of TES-ADT melt from 130 °C, where the β-phase appears. We conclude that the relative abundance of individual polymorphs formed in the samples strongly depends on the thermal history of the sample.