Patterning effects on poly (3-hexylthiophene) organic thin film transistors using photolithographic processes

We explore the effects of conventional photo lithographic patterning of the active layer of poly (3-hexylthiophene) (P3HT) organic thin film transistors (OTFT) on device performance. The performance of the devices was monitored in each step of the patterning process. We successfully developed a patterning process which is compatible with plastic substrates and P3HT as the organic semiconductor. In this process, parylene and atomic layer deposition (ALD) Al2O3 were used as capping layers. Al2O3 and parylene/P3HT were etched using Al etchant and O2 plasma reactive ion etching (RIE), respectively. The degradation occurred primarily during the ALD Al2O3 deposition and capping layer etching. There was a 30% degradation in mobility, a 1–2× reduction in drive current, and an increase in threshold voltage after the ALD Al2O3 deposition. In the capping layer etching, a near 50% degradation in mobility was observed. The patterned devices have a mobility of 0.02 cm2/V s, which is 1000× better than photo lithographically patterned P3HT OTFTs previously reported in the literature, and comparable to un-patterned P3HT devices.