Organic field-effect transistors (OFETs) are considered in technological applications for which low cost or mechanical flexibility are crucial factors. The environmental stability of the organic semiconductors used in OFETs has improved to a level that is now sufficient for commercialization. However, serious problems remain with the stability of OFETs under operation. The causes for this have remained elusive for many years. Surface potentiometry together with theoretical modeling provide new insights into the mechanisms limiting the operational stability. These indicate that redox reactions involving water are involved in an exchange of mobile charges in the semiconductor with protons in the gate dielectric. This mechanism elucidates the established key role of water and leads in a natural way to a universal "stress function", describing the stretched exponential-like time dependence ubiquitously observed. Further study is needed to determine the generality of the mechanism and the role of other mechanisms.

Recently the commercialization of the first reflective organic displays employing organic field-effect transistors (OFETs) used for on-off switching of pixels was announced. These new electrophoretic displays are bistable and have low power consumption. Future, emissive, organic light-emitting displays (OLEDs), however, will operate at high powers and then reliability and excellent long-term stability OFETs will be crucial for stable operation. Unfortunately, OFETs commonly suffer significantly from gate-bias stress under ambient conditions that, over time, causes a detrimental shift in the voltage that the device requires to switch. This threshold-voltage shift limits the exploitation of the full potential of organic semiconductors and OFETs in low-cost, large-area, flexible applications. As an example, using current technologies OLEDs would require up to four driving OFETs per pixel to compensate for the threshold voltage shift in the transistors.[1] This demonstrates the urgency for a proper understanding of the stressing mechanism. In practice, OFETs are hybrid material devices that consist of metallic contacts, an inorganic gate dielectric, and an organic semiconductor. Silicon dioxide is commonly used as the gate dielectric. Although the top layer of SiO 2 is known to contain trap sites for charge carriers, [2] it is presently not clear if these traps are related to the bias-stress effect, and, if so, how this is affected by environmental conditions such as humidity. [3][4][5][6][7][8][9] In this Communication, the dynamics of trapping and detrapping of charges on bare SiO 2 are visualized in real time and space using scanning Kelvin probe microscopy (SKPM) and compared to the bias-stressing dynamics of an organic field-effect transistor using a SiO 2 dielectric. The results clearly show that the generally observed gate-bias stress effect in OFETs is due to water-related charge trapping at the SiO 2 surface, rather than to trapping in the organic semiconductor itself. This insight rationalizes previous results [3][4][5][6][7][8][9] and gives credence to the argument that the surface of the inorganic gate dielectric determines the reliability of organic transistors. We further explain why passivating the SiO 2 surface or decreasing the ambient humidity results in a significant reduction in the bias stress. For our studies, we used a polytriarylamine (PTAA) (Merck, UK) that yields reproducible transistors with a hole mobility of 10 -3 -10 -2 cm 2 V -1 s -1 .[10] The chemical structure of PTAA together with a schematic picture of the cross-section of a transistor is depicted in the inset of Figure 1a. Transistors were fabricated using heavily doped p-type silicon wafers as the common gate electrode with a 200 nm thermally oxidized SiO 2 layer as the gate dielectric. Gold source and drain electrodes were defined by photolithography with a channel width and length of 2500 and 10 lm, respectively. Before depositing gold, a 10 nm titanium adhesion layer was evaporated. The substrates were exposed to a UV-ozone tre...

During prolonged application of a gate bias, organic field-effect transistors show an instability involving a gradual shift of the threshold voltage toward the applied gate bias voltage. We propose a model for this instability in p-type transistors with a silicon-dioxide gate dielectric, based on hole-assisted production of protons in the accumulation layer and their subsequent migration into the gate dielectric. This model explains the much debated role of water and several other hitherto unexplained aspects of the instability of these transistors.

Application of a gate bias to an organic field-effect transistor leads to accumulation of charges in the organic semiconductor within a thin region near the gate dielectric. An important question is whether the charge transport in this region can be considered two-dimensional, or whether the possibility of charge motion in the third dimension, perpendicular to the accumulation layer, plays a crucial role. In order to answer this question we have performed Monte Carlo simulations of charge transport in organic field-effect transistor structures with varying thickness of the organic layer, taking into account all effects of energetic disorder and Coulomb interactions. We show that with increasing thickness of the semiconductor layer the source-drain current monotonically increases for weak disorder, whereas for strong disorder the current first increases and then decreases. Similarly, for a fixed layer thickness the mobility may either increase or decrease with increasing gate bias. We explain these results by the enhanced effect of state filling on the current for strong disorder, which competes with the effects of Coulomb interactions and charge motion in the third dimension. Our conclusion is that apart from the situation of a single monolayer, charge transport in an organic semiconductor layer should be considered three-dimensional, even at high gate bias.

. (2010). Proton migration mechanism for operational instabilities in organic field-effect transistors. Physical Review B, 82(7), 075322-1/11. [075322]. DOI: 10.1103/PhysRevB.82.075322 DOI:10.1103/PhysRevB.82.075322 Document status and date:Published: 01/01/2010 Document Version:Publisher's PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement: Organic field-effect transistors exhibit operational instabilities involving a shift of the threshold gate voltage when a gate bias is applied. For a constant gate bias the threshold voltage shifts toward the applied gate bias voltage, an effect known as the bias-stress effect. Here, we report on a detailed experimental and theoretical study of operational instabilities in p-type transistors with silicon-dioxide gate dielectric both for a constant as well as for a dynamic gate bias. We associate the instabilities with a reversible reaction in the organic semiconductor in which holes are converted into protons in the presence of water and a reversible migration of these protons into the gate dielectric. We show how redistribution of charge between holes in the semiconductor and protons in the gate dielectric can consistently explain the experimental observations. Furthermore, we show how a shorter period of application of a gate bias leads to a faster backward shift of the threshold voltage when the gate bias is removed. The proposed mechanism is consistent with the observed acceleration of the bias-stress effect with increasing humidity, increasing temperature, and increasing energy of the highest molecular orbital of the org...