Polymer electrolyte-gated organic field-effect transistors

Contemporary interest in organic semiconductors is driven both by questions regarding the fundamentals of charge transport in these materials and by their potential for flexible, low-cost electronic applications. The key device utilized in these endeavors is the organic field-effect transistor (OFET). Attaining large charge carrier densities in OFETs is desirable for two main reasons. First, because the conductivity in an OFET is proportional to the product of carrier mobility and charge density, increasing charge density levels can boost transistor currents significantly and facilitate low-voltage operation. Additionally, the achievement of carrier densities approaching the twodimensional (2D) molecular density (∼5 x 1014 cm-2) in an organic semiconductor monolayer can enable a variety of fundamental transport experiments. The results summarized in this thesis illustrate that charge densities exceeding 1014 charges/cm2 can be attained in a variety of organic semiconductors by using a solid polymer electrolyte as an OFET dielectric.; Polymer electrolytes can provide specific capacitances exceeding 10 muF/cm 2, resulting from the migration of ions within a polymer matrix. By measuring the transient gate displacement current caused by ionic motion in a polymer electrolyte-gated organic field-effect transistor (PEG-FET), large electrostatically-injected charge density values can be calculated; these are typically above 1014 charges/cm2 at gate voltages under 3 V. Negative transconductance at large carrier densities is observed in oligomeric, polymeric, and organic single-crystal semiconductors. This phenomenon is ascribed to charge correlations or a nearly complete filling of the semiconductor transport band with carriers.; Polymer semiconductors exhibited the highest performance among PEG-FETs with a top gate architecture. Nearly metallic conductivities (∼1000 S/cm), weak ON current temperature dependences, and large linear mobility values (∼3 cm2/V·s) were observed for PEG-FETs using regioregular poly(3-hexylthiophene) semiconductor films with a solution-processed polyethylene oxide/lithium perchlorate dielectric. Substitution of a larger electrolyte anion, bis(trifluoromethanesulfonyl)imide, yielded qualitatively similar results and improved current saturation behavior in the transistor output characteristics. Preliminary bias stress measurements demonstrated only a small decrease in PEG-FET conductance after 10 minutes. These results highlight the promise of PEG-FETs as low voltage/high current devices and as testbeds for examining charge transport at large 2D carrier densities in organic semiconductors.