Degradation of organic field-effect transistors made of pentacene
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This article reports degradation experiments on organic thin film transistors using the small organic molecule pentacene as the semiconducting material. Starting with degradation inert p-type silicon wafers as the substrate and SiO2 as the gate dielectric, we show the influence of temperature and exposure to ambient air on the charge carrier field-effect mobility, on-off-ratio, and threshold-voltage. The devices were found to have unambiguously degraded over 3 orders of magnitude in maximum on-current and charge carrier field-effect mobility, but they still operated after a period of 9 months in ambient air conditions. A thermal treatment was carried out in vacuum conditions and revealed a degradation of the charge carrier field-effect mobility, maximum on-current, and threshold voltage.
I. INTRODUCTION
Organic semiconductors have increasingly attracted researchers’ interest in the past few years. Much progress was made on the electrical properties of organic thin film transistors (OTFTs) based on the hydrocarbon pentacene. As a result, the electrical characteristics, such as the charge carrier field-effect mobility and OTFT’s on-off ratio and switching speed, now exceed these of amorphous silicon-based transistors.1 This opens up the way to low cost, low performance electronics for the use in, for example, wireless identification tags for single use purposes. The great advantages of the organic semiconductors (flexible substrates and cost-efficiency) contrast with the poor stability in environmental ambient. Although OTFTs are intended for short-term use, the stability should reside in the range of months to ensure a secure function of the circuits even under critical conditions. In general, an influence to the OTFT’s performance is attributed to ultraviolet (UV) light, temperature, water damp, and oxygen. The exact mechanism of each disturbance to the electrical key parameters of the die is not well known yet. An influence of water damp2,3 and oxygen4,5 is predicted on the charge carrier field-effect mobility and the threshold voltage. Generally, these effects are thought to be reversible, heating up the OTFTs in a dry atmosphere up to 100–250 °C.2,3,6
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0267 J. Mater. Res., Vol. 19, No. 7, Jul 2004
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We fabricated OTFTs on p-type Si substrates with a thermally grown gate dielectric layer of 110-nm SiO2 to exclude parasitic degradation effects by an e.g., polymeric substrate. Thermally evaporated pentacene up to a thickness of nearly 60 nm formed the organic semiconductor layer. To estimate the stability and the lifetime of OTFTs in the environmental ambient, the wafer was kept unprotected in dark laboratory conditions (air, ambient humidity, 20 °C) and was measured in intervals of 3 months. After 9 months, this device was found to have considerably degraded but it still operate with decreased charge carrier field-effect mobility and on-current, a
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