© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3053 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Adv. Mater. 2012, 24, 3053–3058 Liqiang Li, Karin Meise-Gresch, Lin Jiang, Chuan Du, Wenchong Wang, Harald Fuchs, and Lifeng Chi* The Electrode’s Effect on the Stability of Organic Transistors and Circuits Dr. L. Li, Dr. L. Jiang, C. Du, Dr. W. Wang, Prof. H. Fuchs, Prof. L. Chi Physikalisches Institut and Center for Nanotechnology (CeNTech) Universität Münster Münster, 48149, Germany E-mail: chi@uni-muenster.de Dr. K. Meise-Gresch Physikalische Chemie Institut Universität Münster Münster, 48149, Germany DOI: 10.1002/adma.201200792 Electronic circuits based on organic transistors are envisaged to be pervasive in future-generation electronic products that have some unconventional characteristics, such as low cost, light weight, mechanical flexibility, transparency, and large-area coverage. [1–10] However, the device stability is one of the main obstacles overshadowing and hindering the practical applica- tions of organic circuits. [11–17] Generally, the device degrada- tion has been attributed to the chemical decomposition of the organic semiconductor molecules and/or a physical change of the organic films on the dielectric surface. [11–24] Nevertheless, no special attention has heretofore been paid to investigate whether the electrodes themselves have an influence on the device sta- bility. Here we present a new understanding for the stability of organic transistors and circuits and provide a promising solu- tion for increasing device stability. We demonstrate here that the electrodes play an unexpectedly important role on the stability of p-/n-type organic transistors and complementary circuits, as explicitly proved by electrical, spectral, and morphology meas- urements. Au electrodes are found to be detrimental for device stability. By using conducting polypyrrole (PPY) as the elec- trodes, the overall device stability (meaning the stability of all of the important electrical parameters) of p-/n-type transistors and complementary inverter circuits is improved significantly. Monitoring the stability of devices with different electrodes is a straightforward way of testing the effect of the electrodes on the device stability. We fabricated a series of p-type penta- cene and n-type N, N′-bis(n-octyl)-dicyanoperylene-3,4:9,10- bis(dicarboximide) (PDI-8CN2) transistors with Au and polypyr- role (PPY) electrodes ( Figure 1), and further conducted tests both for continuous operational stability ( ≈16 h, 38 000 cycles) ( Figure 2a) and shelf-life (60–120 d) stability (Figure 2b,c) on these transistors in ambient condition. In both tests, PPY- contacted transistors with a 20 μm channel length showed a sig- nificantly improved stability compared with Au-contacted devices, which routinely underwent dramatic degradation, as reported previously. [14,17–22] The electrical characteristics and field-effect parameters of the pentacene and PDI-8CN2 transistors before, during, and after cycle tests (Figure 2a and Table S1, Supporting Information) reveal clearly that continuous operation does not lead to an obvious degradation of the PPY-contacted transis- tors, apart from a slight threshold-voltage shift that can recover after measurement, while Au-top-contact devices undergo a dramatic degradation, especially in mobility, on/off ratio, and subthreshold swing for pentacene transistors, and in mobility and threshold voltage for PDI-8CN2 transistors. The shelf- life stability was respectively monitored for the pentacene and PDI-8CN2 transistors over 50–120 d. The evolution of the field- effect parameters (Figure 2b,c and Table S2) versus storage time of the representative transistors reveals that there were slight fluctuations for the PPY-contacted devices, but no deg- radation effects were visible for all of the key field-effect para- meters. In contrast, the Au-contacted pentacene and PDI-8CN2 devices underwent considerable degradation or instability. The stability data of other devices with PPY electrodes are shown in Figure S1. Furthermore, during the shelf-life test period, con- tinuous cycle tests, which did not influence the shelf-life sta- bility, were carried out on some PPY-contacted devices. To the best of our knowledge, such excellent operational and shelf-life stability has rarely been reported previously for pentacene and PDI-8CN2 transistors. The above measurements directly reflect the significantly improved device stability with PPY electrodes, which not only implies the paramount significance of PPY elec- trodes in organic transistors, but may also stimulate consider- able research interest to reconsider the stability issue of organic devices. It is known that many organic devices with Au electrodes undergo a dramatic performance degradation upon operation or during storage, which is generally thought to be caused by the chemical decomposition of the semiconductor molecules and/or physical instabilities such as phase transitions and a morphology evolution of the semiconductor films on the dielec- tric surfaces. [11–24] Until now, no systematic investigation has been conducted on the relationship between the device deg- radation and the electrodes, because it is empirically accepted that: i) the excellent chemical stability of commonly used Au electrodes should have no influence on the device stability; and ii) charge transport mainly occurs in the channel area, so device degradation should also be strongly related to the channel area rather than the electrode/semiconductor interface. How- ever, our experiments demonstrate that PPY electrodes indeed provide good stability in the well-known device model (semicon- ductor film on an octadecyltrichlorosilane (OTS)-modified insu- lator), which has been frequently reported to be quite unstable with Au electrodes [14,17–22] (our experiments also confirm this point). These comparative results on pentacene and