M. Makala, M. Barłóg, D. Dremann, S. Attar, E. G. Fernández, M. Al-Hashimi, O. D. Jurchescu
Wake Forest University,
United States
Keywords: organic field-effect transistors, bias stress, polymers, traps
Summary:
Organic Field-Effect Transistors (OFETs) represent the fundamental building block for the next generation of low-cost, flexible devices and large-area circuits. However, the imbalances in the performance and stability between p-channel and n-channel materials remains still a critical bottleneck. While p-type devices have advanced significantly, their n-type counterparts continue to lag behind; under continuous use, these devices frequently suffer from a severe signal drift, rendering them unsuitable for signal precision applications. In this work, we address this challenge by investigating a series of donor-acceptor copolymers which we integrated into a device architecture featuring a double-layer polymer dielectric and chemically modified electrodes. We subjected these devices to an aggressive bias stress protocol for an extended period of time, which simulates the demanding “always-on” environment of active layer in device far better than standard pulsed-testing. Our findings reveal an intriguing trade-off between high performance and operational stability, governed not only by device architecture, but also by the geometry of the polymer side chains. We observed that polymers with linear, optimized side chains, resulted in high electron mobilities exceeding 1 cm²/Vs. However, under electrical stress, these same high-performance polymers exhibited a high threshold voltage shifts of 1.2 V. On the contrary, the branched polymer variants exhibited a slightly lower mobility caused by steric hindrance, but demonstrated exceptional bias stress stability, restricting shifts in threshold voltage to just 0.5 V. To understand these results and generate guidelines for material design, we performed Trap Density of States (trap-DOS) analysis and found that trap-DOS generated in the branched polymer variant during the stress is one order of magnitude lower than in other polymer variants. In summary, our results provide a framework for better understanding and mitigating bias stress effects in OFETs, a crucial step for the development of stable and reliable organic electronic devices.