(585d) Electrostatic Injection of 1015 Charges Per Square Centimeter in Organic Semiconductors

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.