Photoconductivity of Pentacene Thin Film Transistors
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PHOTOCONDUCTIVITY OF PENTACENE THIN FILM TRANSISTORS D. KNIPPa, D.K. MURTIb, B. KRUSORa, R. APTEa, L. JIANGb, J.P. LUa, B.S. ONGb, and R.A. STREETa a) Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA94304 b) Xerox Research Center of Canada, Mississauga, ON, Canada L5K 2L1 ABSTRACT A very large enhancement of the photoconductivity in pentacene transistors at negative gate voltages is observed. The enhancement is attributed to the separation of electron-hole pairs by the gate field and the consequent slow recombination. The ratio of photoconductivity to dark conductivity is approximately independent of mobility, for samples with a wide range of microstructure. The pentacene films were thermally deposited at different deposition rates and temperatures on silicon thermal oxide. The structure and the morphology of the films were studied by x-ray diffraction measurements and atomic force microscopy, and the influence of the deposition temperature on the morphology and structural properties is discussed. The size of the crystals is correlated with the crystalline bulk phase of the material, which increases with the deposition temperature and the film thickness. The mobility of the transistors increases with the size of the crystallites. INTRODUCTION The electronic transport and the structural properties of small molecules organic materials have been investigated for more than 40 years [1] and the first pure organic single crystals were grown more than one decade ago [2]. Nevertheless, organic electronics research has attracted more attention in recent years. The emerging interest in organic electronic devices can be attributed to further improvements of the electronic material quality [3] and the progress in advanced processing technologies such as printing [4] or thermal evaporation [5], which facilitates large area and low cost electronic applications. Up too now amorphous and polycrystalline silicon technology has dominated this type of applications [6]. Pentacene (C22H14), which is a material out of the family of benzene ring materials, has demonstrated the highest hole and electron mobility so far. The material exhibits a strong tendency of form highly ordered films, which can be poly or single crystalline depending on the deposition and substrate conditions. In the case of single crystal material, mobility of 3.2cm2/Vs and 2.0 cm2/Vs for holes and electrons have been realized at room temperature (rt) using a metalinsulator-semiconductor (SIM) transistor structures [3]. A more promising process to realize large area electronics is the thermal evaporation of the organic material, which yields a poly crystalline thin film with grain size usually in the range of a few microns [5]. The mobility of optimized polycrystalline thin film transistors (TFT) at room temperature is only slightly less than the single crystalline material. Despite much progress in the realization of TFTs with good properties, the transport mechanism of polycrystalline films is not completely understood, particularly with regard
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