Thiophene Oligomer and NTCDI Semiconductors with High Field-effect Transistor On/off Ratios
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Title: Thiophene Oligomer and NTCDI Semiconductors with High Field-effect Transistor On/off Ratios Authors: Howard E. Katz*, Andrew J. Lovinger, X. Michael Hong, Jerainne Johnson, Bo-Cheng Wang, and Krishnan Raghavachari
Introduction Organic semiconductors are of continued interest for low-cost display drivers and logic elements. Field-effect transistors (FETs) with organic semiconductor channels have been fabricated in arrays to drive electrophoretic display pixels[1] and polymer dispersed liquid crystals (http://www.research.philips.com/pressmedia/releases/000901a.html). Complementary logic elements and shift registers containing hundreds of organic-based FETs have been produced[2], and high-speed organic circuits have been fabricated on polyester substrates.[3] The source and drain electrodes of individual FETs have been patterned using microcontact printing and inkjet methods[4] to give extraordinary aspect ratios. Inorganic[5] and hybrid[6] materials have been deposited as FET semiconductors using the methods of “organic electronics”. Organic FET channels have been harnessed to demonstrate ambipolar transport[7], chemical sensitivity[8], superconductivity[9], and electrically pumped lasers[10].
We have recently focused on semiconductors with high on/off ratio that can potentially be deposited from solution, which would be required for reel-to-reel fabrication of transistor-based circuits and circuit elements such as pixel drivers, inverters, and ring oscillators. In this work, we survey a variety of thiophene co-oligomers and naphthalenetetracarboxylic diimide (NTCDI) compounds for their behavior as FET semiconductors. The HOMO energies of the oligomers were modulated by the introduction of phenylene and thiazolidene subunits. NTCDIs were obtained with a variety of N-substituents. Many of these display high on/off ratios, in some cases above 10,000.
Experimental Oligomers were generally synthesized by coupling organometalloid derivatives of the terminal rings with the dibromide of a central subunit. The dibromo compounds were obtained by direct bromination with N-bromosuccinimide in dimethylformamide. NTCDIs were obtained by the condensation of amines with naphthalenetetracarboxylic dianhydride (NTCDA) above 200 deg C in quinoline with Zn(OAc)2 catalyst. All compounds were purified after isolation as crude solids by vacuum sublimation. Soluble compounds were examined by NMR, and the new compounds were assayed by elemental analysis. Structures for the oligomers are given in Figure 1.
C7.6.1
S
R
S
R
n
n PTTP, n = 2 PTTTP, n = 3 PTTTTP, n = 4
TTTTT, n = 5, R = H TTTTTT, n = 6, R = H 2-dH-TTTTT, n=5, R=C6H13
S
S S
S S
N TTZTT
S S N
S
PTZTP
S
S
PTPTP
R’ R
S S
S S
R’
R
TTPTT, R = R’ = H 2-dH-TTPTT, R = C6H13 3-dH-TTPTT, R’ = C6H13
Figure 1 Oligomers synthesized for this study.
C7.6.2
FETs were fabricated and evaluated as previously described (except for liquid phase deposition), using 500 Angstrom-thick unpatterned semiconductor films and top gold contacts (width/length was 18) on 3000
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