Mortimer M. Labes

Organic Semiconductors. II. The Electrical Resistivity of Organic Molecular Complexes

Thirty molecular complexes involving quinones, halogens, polynitroaromatics, or tetracyanoethylene as electron acceptors and aromatic hydrocarbons or substituted aromatics as electron donors have been prepared and the room temperature electrical resistivities measured on compressed microcrystalline material. A lowering of electrical resistivity is generally associated with complex formation. The results are discussed in terms of crystal structure and charge transfer complex theory.

Organic Semiconductors. V. Comparison of Measurements on Single‐Crystal and Compressed Microcrystalline Molecular Complexes

Utilizing specialized micromanipulative techniques, measurements of conductivity, activation energy of conduction, and Seebeck coefficient have been performed on charge‐transfer complex single crystals. The results are compared with previous measurements on compressed microcrystalline materials.

Comparison of the Purity and Perfection of Vapor‐ and Melt‐Grown Anthracene Crystals

The purity and perfection of anthracene crystals have been examined for melt‐grown and thick ( > 1 mm) vapor‐grown crystals. The physical properties studied were the charge carrier lifetime and mobility of holes, dislocation density, and the bulk self‐diffusion coefficient. Chemical purity was assessed by gas chromatography. Melt‐grown crystals had hole lifetimes > 1 msec compared with 220 μsec for the best vapor‐grown crystal, although vapor‐grown crystals had lower dislocation densities and fewersubboundaries. Vapor‐grown crystals had lower measurable impurity concentrations, ∼2 ppm compared with ∼10 ppm for melt‐grown crystals, but these impurities appear to have no effect on charge carrier trapping. Both types of crystals gave the same self‐diffusion coefficient at 190°.

Diffusion in Organic Crystals. I. Self‐Diffusion in Anthracene

Self‐diffusion in high‐purity anthracene single crystals has been measured by means of a microtome sectioning technique with anthracene‐9‐14C as a tracer, in the temperature range from 140° to 196°. For diffusion perpendicular to the ab and ac planes, the data were respectively fitted as D(⊥ab)=3 exp (−22 kcal/RT) and D(⊥ac)=16 exp (−24 kcal/RT). The possible mechanism, the anisotropic nature of the diffusion process, and the results as compared with a previous investigation are discussed in detail.

Composition of Some Conducting Complexes of 1,6‐Diaminopyrene

The absorption spectra of solid complexes of 1,6‐diaminopyrene with quinones exhibiting high electrical conductivity show them to be mostly radical‐ion salts. The mole fractions of ionic component for complexes with bromanil, chloranil, and 2,3‐dichloro‐5,6‐dicyano‐benzoquinone are 0.65, 0.90, and 0.95, respectively. This is the order of the estimated electron affinities of the acceptors but not of the conductivity of the complexes. The iodanil complex, with much higher resistivity, shows little ionic component.

Diffusion in Organic Crystals. II. Lattice and Subgrain‐Boundary Diffusion

Self‐diffusion studies at extended diffusion anneal times and impurity (phenanthrene) diffusion studies in melt‐grown anthracene crystals have shown evidence for a fast subgrain‐boundary‐diffusion process in addition to a much slower lattice diffusion. Subgrain boundary diffusion is faster by a factor of 106−107, the activation energy being 25% to 66% the value for lattice diffusion.Phenanthrene diffusion is very similar to self‐diffusion in terms of diffusivity magnitude and activation energy. Diffusion anisotropy is more marked for phenanthrene diffusion, diffusivity perpendicular to the ac plane being greater than that perpendicular to the ab plane.

Effect of Gases on the Conductivity of Organic Solids. IV. Site of Carrier Injection

Abstract -Evidence suggesting the involvement of the electrode-crystal interface in ambient vapor induced charge carrier injection phenomena in organic crystals is presented. Space-charge-limited currents are not observed until high voltages are applied when superlinear current-voltage character-istics indicating such effects are also found in the unexposed crystal. Drift mobility measurements on anthracene crystals exposed to iodine vapor confirm that the ambient vapor induced bulk conductivity effects are phenomena in volving the production of charge carriers.

Photoeletric Characterization of X-Ray Damage in Anthrancene Crystals

Abstract Most physical effects of moderate levels of ionizing radiation on aromatic organic crystals are not readily observable. Thus anthracene, with a total G-yield for X-rays of less than 1, can be used as a scintillation detector with no decrease of fluorescence efficiency except after extreme exposure to ionizing radiation. The electrical properties of anthracene crystals, however, are markedly affected by thermal neutron bombardment at exposures which are too weak to lower the fluorescence efficiency.1,2 The photocurrent excitation spectrum is altered and shifted to longer wavelengths, photocurrent gain is reduced, and the peaks in thermal-conduction “glow curves” are intensified.1