Semiconducting Properties of Organic Polyiodides

Abstract 1) The electrical conductivities of ten organic polyiodides have been measured: tetramethylammonium triiodide, tetramethylammonium pentaiodide, trimethylphenylammonium triiodide, trimethylphenylammonium pentaiodide, trimethylphenylammonium heptaiodide, 1-methyl-2-picolinium triiodide, 1-methyl-2-picolinium pentaiodide, 1-methyl-2-picolinium heptaiodide, 1-methylquinolinium triiodide and 1-methylquinolinium pentaiodide. At the room temperature specific resistivities are 107∼1010Ωcm. and the band-gap energies, as calculated from ρ=ρ0exp(ε⁄2kT), are 1.3∼2.6 eV. 2) Both ε and ρ20 decrease with an increase in the number of iodine atoms in the polyiodide anion, not so much difference in these values being observed with different kinds of cations. The iodine framework in the crystal structure is of primary importance in determining the electrical conductivity. 3) For the trimethylphenylammonium triiodide single crystal, an anisotropy of ε values was observed between the normal direction and the direction parallel to the cleavage plane along the growth axis. 4) Preliminary thermoelectric power measurements show that the majority carrier is the hole. 5) The solid absorption spectra of polyiodides have been measured. The band-gap energy, ε,does not agree with the absorption maximum, being rather near the very beginning of the first absorption. 6) All the polyiodides investigated show a photoconductivity response.