Complete Suppression of Bias-Induced Threshold Voltage Shift below 273 K in Solution-Processed High-Performance Organic Transistors.

Abstract

Despite their importance for future applications, the operational electrical stability of organic thin-film transistors is far from being understood. Even in the most stable organic field-effect transistors (OFETs) operated under vacuum, a hitherto unknown source leads to bias stress. Here, we investigate the electrical characteristics and operational stability of a high-performance diketopyrrolopyrrole- alt-terthiophene organic semiconductor. Even though the OFETs are characterized by a high mobility of 3 cm2 V-1 s-1 and trap-free transport, the threshold voltage shift in all stress modes remains sensitive to the presence of water even when operating devices in high vacuum. Exponential fitting from current bias-stress measurement up to 500 000 s showed a bias-voltage shift of <1 V, which corresponds to the density of the bias-induced trap states at infinite time NT∞ = 7.6 × 1010 cm-2. We have surprisingly found that electrical stress could be completely suppressed when devices are cooled to below 273 K. We present evidence that H3O+ and OH- stemming from the autoionization of liquid water is the hitherto unidentified universal trap (i.e., an extrinsic trap not stemming from the semiconductor itself) causing threshold voltage shift even in the otherwise stable devices. This interpretation would also clarify why in the literature similar NT have been reported in various semiconductors, suggesting that this number is independent of the organic semiconductor, processing and measurement environment but only dependent on residual contaminants-most notably water.