From Structure to Performance: The Critical Role of DNTT Morphology in Organic TFTs

The electrical performance of organic thin-film transistors (OTFTs) based on DNTT as the semiconductor active layer (DNTT, which stands for dinaphtho [2,3-b:2',3'-<i>f</i>] thieno [3,2-<i>b</i>] thiophene) is investigated and related to the structural properties of the organic films grown on SiO₂ and Cytop substrates. Conventional current-voltage measurements and high-sensitivity low-frequency measurements show a lower mobility and correspondingly higher defect density for DNTT/SiO₂ devices. Morphological and structural characterizations of DNTT films grown on the two dielectrics were performed using atomic force microscopy (AFM) and X-ray diffraction (XRD), revealing a highly ordered crystalline structure. Consistent with DFT simulation results, morphological analysis shows that the semiconductor films are layered, with DNTT molecules arranged with their longest axis perpendicular to the substrate. However, in only DNTT/SiO₂ films, some molecules were found to be ordered and arranged parallel to the substrate. This "horizontal" orientation causes differences in charge transport properties in the semiconductor films grown on SiO₂, reducing the field-effect mobility. TCAD simulations indicate that this horizontal molecular orientation can be modeled as highly defective regions at semiconductor grain boundaries, consistent with low-frequency noise measurement results.