[Abstract]

Ketopyrrolopyrrole-thieno[3,2-b] thiophene (DPPT-TT) and indacenodithiophene-co-enzothiadiazole (IDT-BT) were employed for organic field-effect transistors (OFETs) to understand the effects of semiconductor channel thickness on electrical properties. The mobility was found nearly constant, whereas the threshold voltage, contact resistance, and subthreshold slope reached an optimum at a semiconductor layer thickness of about 40 nm. This value was related to the height of the source/drain electrodes. The device performance could be degraded when the thickness of semiconductor film is higher or lower than this critical value. Two different mechanisms are proposed to explain the experimental results.