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Electrostatic Injection of 1015 Charges Per Square Centimeter in Organic Semiconductors

Matthew J. Panzer and C. Daniel Frisbie. Chemical Engineering & Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Ave S.E., Minneapolis, MN 55455

The achievement of very high charge carrier densities (above 1015 charges/cm2) in organic field effect transistors (OFETs) can reveal intriguing transport phenomena at low operating voltages. Since many promising organic semiconductor materials (e.g., pentacene, poly(3-hexylthiophene), rubrene) pack with two-dimensional molecular densities on the order of ~5 1014 molecules/cm2, this value can be used as a benchmark against which charge carrier density levels are compared. Although rarely emphasized, maximum charge carrier densities reached during typical OFET operation are only ~1012 charges/cm2, a value that is quite low in comparison to 5 1014 cm-2. In fact, the common choice of SiO2 for the OFET dielectric material intrinsically limits the maximum amount of charge that one can induce via the field effect (near SiO2 breakdown conditions) to only ~1013 charges/cm2. We have shown that by using a solid polymer electrolyte as an OFET dielectric, two-dimensional charge densities exceeding 1015 charges/cm2 can be attained in a variety of organic semiconductors. Polymer electrolytes such as polyethylene oxide (PEO)/LiClO4 can provide specific capacitances on the order of 100 μF/cm2, resulting from the migration of ions within the polymer matrix. Measurement of the transient gate displacement current allowed us to determine the electrostatically-injected charge density values, which were typically above 1015 charges/cm2 at gate voltages less than 3 V. Conductivity maxima at carrier densities near 1 charge/molecule were observed in oligomeric, polymeric, and single-crystal organic semiconductors alike. This phenomenon is presumably related to carrier correlations or a complete emptying of the semiconductor transport band at such high charge densities. In addition, metallic conductivity (~1000 S/cm) and a nearly temperature-independent resistance ratio have been attained in spin-coated poly(3-hexylthiophene) films using a PEO/LiClO4-gated OFET.