Andrew R. Melnyk

Hole emission defect states in amorphous As2Se3

Abstract The influence of composition (30 to 40% As and 0 to 0.5% I), deposition parameters, and subsequent annealing on charge emission centers in vacuum deposited As 2 Se 3 films was studied. The thermal emission of holes from deep gap states was measured by a time resolved charge depletion technique. The observed states appear to be a tail of a distribution that peaks above 1.1 eV. The emission rate decreased with reduction of arsenic and increased with the addition of iodine. The rate was sensitive to sample fabrication; decreasing with substrate temperature in the vicinity of the glass temperature T g . Freshly made films had a high initial rate which relaxed with a time constant of about 100 days at room temperature. Annealing by heating the samples to T g and cooling slowly relaxed the emission rate further. The results are discussed in terms of the recent charged defect models.

Organic photoreceptors: an overview

When Chester Carison invented xerography, he employed sulfur and anthracene as photoconductors. Although the initial commercialization of his idea relied on inorganic photoconductors, the current trend is towards use of organic photoconductors because of their material variety, economy and flexibility. High speed copying and printing machines use belts coated with organic photoreceptors, while personal copiers and printers use aluminum drums dip-coated with organic photoreceptors. Multilayered, organic photoreceptors are now routinely mass produced by the millions with both visible sensitivity for copiers and infrared sensitivity for printers. This paper presents a brief overview of key photoreceptor properties and follow with a survey of electronic organic materials of current interest. The photodischarge characteristic is determined mainly by three factors: the photogeneration, the injection, and the transport of charge carriers. These functions can be accomplished by separate electronic material layers; photogeneration by organic pigments and charge transport by aromatic-amine electron-donor molecules. The photogeneration layers are usually fabricated by solvent coating a dispersion of a pigment in a polymeric binder while the charge transport layers are solvent coated to form a solid solution of the aromatic amine in a polymeric binder. Examples and characteristics of organic pigments and charge transport molecules of current interest are discussed.

The effect of disorder on excited state dynamics in organic molecular films

We have investigated the dynamics of highly excited electron states in thin films of N,N′-diphenethyl-3,4,9,10-perylenetetracarboxylic-diimide (DPEP), a well-known organic photoconductor, with time-resolved photoemission spectroscopy. We observe an extremely fast relaxation process of photoexcitations with a typical lifetime of 30 fs in DPEP films and attribute it to a rapid internal conversion process from S2 and S1 levels to the vibrational manifolds of S1 and S0 states. Interestingly, the relaxation rate is almost twice as fast at low excitation energies in polycrystalline DPEP films as it is in less-ordered DPEP films. We explain this difference by fast transitions within the manifold of extended states that are shown to form in ordered DPEP films.

MORPHOLOGY AND EXCITED STATE DYNAMICS IN THIN ORGANIC MOLECULAR FILMS PROBED BY FEMTOSECOND TIME-RESOLVED PHOTOEMISSION SPECTROSCOPY

We have studied the lifetimes of the excited electron states of thin N,N′bis(phenethyl)perylene-3,4:9,10-bis(dicarboximide) (DPEP) films, prepared in situ, using femtosecond time-resolved photoemission spectroscopy. DPEP is an organic compound similar to photoreceptor materials widely used in many imaging applications. By controlling the evaporation conditions, we have been able to grow films of different morphologies, and found that the relaxation dynamics depends on the morphology. We have investigated two distinct films characterized by very different absorption spectra. We have found that for the film with absorption maximum at 500 nm, a typical lifetime is 55 fs at 2.1 eV above the highest occupied molecular orbital level. For the other film with absorption maximum at 630 nm, the relaxation rate is almost twice as fast, resulting in a lifetime of 30 fs at the same energy. We attribute the extremely short lifetimes to a rapid internal conversion process from S2 and S1 levels to the vibrational manifol...

Observation of field and thickness independent hole mobility in amorphous As2Se3

Abstract Hole transport in amorphous As 2 Se 3 was studied by xerographic discharge measurements. Films of 20 μm to 100 μm are corona charged to electric fields of 0.5 to 16 v/μm and immediately discharged with a flash of intense surface-absorbed light, sufficient to inject all the surface charge. Two time regimes are examined; the initial and the final 10% of the voltage discharge. The initial discharge rate yields an electric field and thickness independent hole mobility which is activated with an energy of 0.65 eV. The last 10% of the discharge yields a mobility that has an activation energy of 1.00 eV. These results are consistent with the multiple trap-limited models of mobility.