Dynamic Infrared Electro-optic Response of Soluble Organic Semiconductors in Thin Film Transistors
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Dynamic Infrared Electro-optic Response of Soluble Organic Semiconductors in Thin Film Transistors Emily G. Bittle, Joseph W. Brill, Joseph P. Straley Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055 USA ABSTRACT We use a frequency-dependent electro-optic technique to measure the hole mobility in small molecule organic semiconductors, such as 6,13 bis(triisopropylsilylethynyl)-pentacene. Measurements are made on semiconductor films in bottom gate, bottom contact field-effect transistors (FETs.) Because of the buried metal layer effect the maximum response, due to absorption in the charge layer, will be for a dielectric film ~ 1/4 of a wavelength (in the dielectric) (e.g. ~ 1 micron thick in the infrared.) Results are presented for FETs prepared with both spin-cast polymer and alumina dielectrics prepared by atomic layer deposition. At low frequencies the results are fit to solutions to a non-linear differential equation describing the spatial dependence of flowing charge in the FET channel, which allows us to study multiple crystals forming across one set of drain-source contacts. FETs prepared on alumina dielectrics show interesting deviations from the model at high frequencies, possibly due to increased contact impedance. INTRODUCTION Reflecting the growing interest in developing devices using organic semiconductors and in understanding their properties, probes are being developed that can measure the spatial and dynamic dependence of semiconductor properties within the channel of thin-film field effect transistors, in which the mobile charge is induced by applied voltages. Optical probes have been used to measure the distribution and dynamics of charge in the channel [1-6] and are going beyond the usual “lumped impedance” approximation of the field-effect transistor (FET.) In this study, we use spatially resolved voltage modulated infrared reflection to measure the dynamic response of charge in field effect transistors. We investigate the difference in transport between two dielectrics and the variation of mobility within one transistor. Results are compared to a model of dynamic response. EXPERIMENTAL DETAILS Samples are made with 6,13 bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene), provided by John Anthony’s group [7], which is drop cast onto a transistor structure built on a glass slide. (Thermally oxidized silicon films have huge parasitic electro-optic responses, presumably due to absorbed species, so are inappropriate substrates for our measurements.) Drop casting the TIPS-pentacene results in large, thin crystals which can be oriented by tilting the sample during drying so that the long axis is parallel to the drain-source gap (L ≈ 350 microns.) Multiple crystals can form on one set of contacts.
The gate, drain and source electrodes are vapor deposited gold; the dielectric layer is either spin cast poly(4-vinyphenol) (PVP) cross-linked with polymelamine co-formaldehyde or alumina films, prepared by atomic layer deposition to avoid pinholes which typically oc
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