Organic Transistor Sensors and Memory Elements Fabricated VIA Solution Deposition
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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
D4.7.2
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)
D4.7.3
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 mobilities between 0.01 and 0.09 cm2/Vs as vacuum-evaporated films, and between 10-3 and 0.028 cm2/Vs as solution-cast films, except for 1, whose solution-cast films had marginal observed mobility. These data are summarized in Table 2. Pre-treatment of substrates with HMDS before vacuum evaporation has negligible effects on mobility (with the exception of dH-PTP, 1); however, the treatment improves on/off ratios for all compounds 1-6 by as much as one or two orders of magnitude. In all cases, FET characteristics for both evaporated and solution-cast films are highly reproducible.
Table 2. FET mobilities and on/off ratios for vacuum-evaporated and solution-deposited phenylene-thiophene semiconductor films on silicon/silicon oxide Solution-Cast Films Vacuum-Deposited Films a Semiconductor Mobility ION/IOFF Mobility ION/IOFF (cm2/Vs)a (cm2/Vs) 6PTP6 (1) 10-5 60 0.01 (0.01)b 2500 (3000)b 4 6PTTP6 (2) 0.02 2x10 0.07 (0.09) 1x105 (4x104) 0.004 100 0.054 (0.054) 7700 (6000) 6PT3P6 (3) 6PT4P6 (4) 0.028 1000 0.09 (0.07) 900 (1400) 6PTPTP6 (5) 0.01 1700 0.054 (0.028) 900(5x104) 6PPTPP6 (6) 0.001 600 0.001 (0.018) 1100 (104) a Values in parentheses were obtained on films deposited at substrate temperature of 50°C. bSilicon substrates treated with hexamethyldisilazane before deposition.
A “bit” of information can be stored in some organic FETs by applying a prolonged “writing” voltage to the gate, thus shifting the threshold voltage of the device and changing the state of the device at certain voltage s
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