Organic Transistor Sensors and Memory Elements Fabricated Via Solution Deposition
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ORGANIC TRANSISTOR SENSORS AND MEMORY ELEMENTS FABRICATED VIA SOLUTION DEPOSITION H.E. Katz, T. Someya, A. Gelperin, Bell Laboratories-Lucent Technologies, Murray Hill, NJ; M. Mushrush, A. Facchetti, T.J. Marks, Northwestern University, Evanston, IL.
INTRODUCTION Thiophene-phenylene oligomer semiconductors (structures below) have many favorable attributes for electronics on flexible and rounded substrates. They are synthesized in 2-3 steps from readily available starting materials and are chemically stable. Their mobilities are typically above 0.01 cm2/Vs and their threshold voltages can be tuned by altering the thiophene-phenylene ratio. Deposition from solution is facile, and cast films with usable mobilities are repeatedly obtained on a variety of surfaces. Higher-order function such as nonvolatile memory and chemical sensitivity have been demonstrated. In this presentation, we describe the characterization of 5,5’-bis(4-hexylphenyl)-2,2’bithiophene (2) and related compounds, illustrating their activity in organic field-effect transistors (FETs) and their potential use on plastic and fabric substrates. RESULTS The phenylene-thiophene oligomers prepared in this study all yield highly ordered films on Si/SiO2 and ITO/GR substrates, both by evaporation under high vacuum and by deposition from cast xylenes solutions, as assessed by X-ray diffraction. Other solvents led to rougher and less uniform films. Intense reflections observed and calculated interplanar d spacings for solution-cast and vacuum-evaporated films of oligomers 1-6 are summarized in Table 1. In all films, the first-order reflection is very strong, and many films exhibit well-resolved higher-order peaks as well, indicating well-ordered, layered microstructures. Table 1. X-ray diffraction data for vacuum-evaporated and solution-cast films of phenylene-thiophene oligomers: major reflections and derived interplanar distances Films Vacuum-Deposited (25°C Substrate Temperature) Compound d (Å) Other Observable d (Å) Other Observable Peaks Peaks (Order) (Order) dH-PTP (1) 25 None 25 None a b dH-PTTP (2) 30 (26) b3, 4, 5, 6 30 2, 3, 4, 5, 6 dH-PT3P (3) 31 2 (weak) 35 2 37 2 39 2 dH-PT4P (4) dH-PTPTP (5) 33 2 (weak) 35 2, 4 (weak), 6 (weak) dH-PPTPP(6) 35 None 38 2 a b Peaks corresponding to a second phase are observed. Scan did not extend higher than 6th order peaks. Solution-Cast Films
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C6H13
C6H13 S n n = 1; dH-PTP (1) n = 2; dH-PTTP (2) n = 3; dH-PT3P (3) n = 4; dH-PT4P (4)
C6H13 S
S
C6H13
dH-PTPTP (5) C6H13
C6H13
S
dH-PPTPP (6)
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Scanning electron microscopy (SEM) reveals very smooth morphologies for evaporated films of all phenylene-thiophene compounds 1-6 grown both at ambient temperature and at 50°C (the lower-melting 1 was not examined by SEM). Much greater variation in morphologies is observed for solution-cast films, with the larger oligomers 3-6 displaying overlapping crystallites with dimensions as large as several tens of microns. In p-channel operation, all six phenylene-thiophene compounds 1-6 display hole mobil
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