Off-State Bias Stress Stability in Polymer Transistors—An Often Overlooked Prerequisite

During the last decades, the performance and environmental stability of organic transistors have been improved significantly. However, a remaining challenge for the employment of organic transistors in industrial applications such as flexible displays is their operational stability.<br/>A widely studied topic is the on-state bias stress stability of (organic) transistors: During operation, trapped charges screen the applied gate field and lead to undesired threshold voltage shifts. In displays for example, this instability in transistor performance would lead to varying brightness of the respective pixels. One unanimously reported cause of such operational instabilities are water-induced traps that affect the performance of various devices, such as organic transistors and diodes. [1-3]<br/>Such water-induced charge traps in polymer devices can be passivated by the addition of doping [4] and non-doping[5] molecular additives, leading to significant improvements in the device performance and the on-state bias stress stability of polymer TFTs (thin-film transistors).<br/>However, water and oxygen do not only impact the on-state, but also the off-state bias stress stability (non-conducting stress, positive bias stress for p-type transistors), as e.g. reported for oxide TFTs. While the on-state stress is widely studied, the off-state bias stress is mostly overlooked in organic TFTs, even though equally important for the stable operation of displays as well as logic circuits.<br/>In this work, we close the gap and focus on the off-state bias stress stability of polymer transistors, show that threshold voltage shifts can be remarkably reduced and discuss the underlying mechanisms with the aid of temperature-depend bias stress measurements.<br/><br/>[1] M. Kettner, R. T. Weitz et al. ACS Appl. Mater. Interfaces, 2018, 10, 35449;<br/>[2] P. A. Bobbert, D.M. de Leeuw et al., Adv. Mater, 2012, 24, 1146<br/>[3] H. F. Iqbal, O. Jurchescu et al. , <i>Nature Commun.</i> 2021, 12, 2352<br/>[4] M. P. Hein, K. Leo et al. , <i>Appl. Phys. Lett.</i> 2014, 104, 013507<br/>[5] M. Nikolka, H. Sirringhaus et al. Nature Mater., 2016, 16, 356