Improving the stability of organic field-effect transistors

With the promise of low-temperature solution-processing, organic field-effect transistors (OFETs) have evolved from a convenient tool for semiconductor parameter analysis to a technology slowly entering the market as circuit components in flexible electronics. Since the first devices in the 1980s, OFET mobilities have increased by over six orders of magnitude and surpassed the benchmark of amorphous silicon. The bottleneck for OFETs to outperform their inorganic counterpart is their environmental, thermal, and operational stability issues. This dissertation focuses on the operational stability of OFETs under a prolonged bias-stress condition. While much effort has been made to explain ON-state bias-stress instabilities in p-type materials (also known as negative bias stress) and to minimise their effects, not much has been done to address the OFF-state bias-stress instabilities (positive bias stress in p-type materials). In fact, OFF-state bias-stress stability is arguably a more important parameter since, in most OFET applications, they stay in the OFF state for longer. We chose the high-performance donor-acceptor conjugated polymer indacenodithiophene-co-benzothiadiazole (IDT-BT) as our model system due to its low energetic disorder and high reported mobilities exceeding 1 cm²V⁻¹s⁻¹. Since stability studies require an exceptional level of reproducibility, we investigated the influence of the fabrication conditions and polymer batch-to-batch variation on the device performance. We uncovered the detrimental effects of glovebox atmosphere on the OFET characteristics and observed significant differences in various batches of the polymer, which we attributed to material contamination. We hope this work will serve as a solid starting point for understanding the intricacies of OFET fabrication. Further, we developed two methods to address the OFF-state bias-stress instabilities in OFETs, the origin of which we attributed to the trapping of electrons. The first approach involves the treatment of organic semiconductor (OSC) films with an orthogonal solvent that induces local aggregation of the polymer and results in an overall increased crystallinity. This leads to a decreased probability of electron trapping in the transport-sensitive regions in the film, and threshold-voltage shifts of less than 1 V upon application of 50 V gate-bias stress for over 10 hours. The second approach involves blending an insulating polymer matrix into the OSC layer in relatively high ratios. In addition to significant suppression of OFF-state bias-stress instabilities, we observe an increased degree of IDT-BT crystallinity, a typical lateral separation between the two polymers, and, interestingly, a higher level of charge accumulation in the insulating polymer. Our work offers simple processing strategies for achieving the reliability required for applications in flexible electronics.