Institute of Solid State Physics
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Organic Thin Film Transistor Based on the Molecule OEG-BTBT-OEG Organic semiconductors (OSCs) are promising for thin-film transistor applications as they potentially offer distinctive advantages over their inorganic counterparts, particularly in terms of their properties, processing techniques, and cost-effectiveness. Small molecules with extended aromatic cores and solubilizing long chains are budding candidates for solution-processed organic semiconductors. Among the various OSCs, small molecules with [1]benzothieno [3,2-b]benzo thiophene (BTBT) cores are identified as the best p-type semiconductor. Here we investigated the crystal structure solution, film-forming properties, charge transport properties, and polymorphism of OEG BTBT, a BTBT derivative with oligo ethylene glycol side chains. [RR1] Good quality single crystals were grown on a Si substrate by slow evaporation of a high boiling point solvent cyclohexanone at room temperature. Single crystal X-ray analysis revealed a monoclinic phase (P21/c) with unit cell parameters a = 18.635 Å, b = 7.6670 Å, and c = 8.2930 Å. Here, the BTBT layers are arranged along the crystallographic a-axis, and the herringbone arrangement in the bc plane of the BTBT core facilitates 2D charge transport in this direction. Thin film-forming properties were investigated on mono to bulk layer samples prepared by spin coating and physical vapor deposition. The major drawback with the solution processing of OEG-BTBT was its poor solubility in almost all of the organic solvents, as this hinders the formation of homogeneous and thicker layers. Nevertheless, with physical vapor deposition, we could overcome these hurdles, and so, as the next step, we proceeded with the investigations of structure-property relationships between molecular packing and charge transport properties. Unfortunately, OEG-BTBT exhibits only a poor FET mobility close to the measurement limit under most thin film growth conditions. Our best results were obtained in carefully optimized thin films deposited by vacuum evaporation, for which FET devices exhibited mobilities of 6×10-4 cm2 ⋅V-1 ⋅s-1, which is significantly lower than that of unsubstituted BTBT and BTBT derivatives with non-polar alkyl side chains. Atomic force microscopy investigations revealed the unfavourable morphology of the OEG-BTBT films, as the first monolayer is never fully closed. Therefore, the poor performance of OEG-BTBT-based devices could be attributed to the unfavourable morphology of OEG-BTBT films. Polymorph screening of OEG-BTBT done in bulk gave 3 phases: Form I, Form II, and Form III. Moreover, extending the screening investigation onto a substrate surface unveiled 4 additional thin-film phases. |
