I. Chen

On the thermal dissociation of hydrogen

The important role atomic hydrogen plays in the low‐temperature and pressure deposition of diamond has renewed interest in the dissociation processes of hydrogen. Following a method originally developed by Langmuir and co‐workers, the voltage‐current characteristics of refractory filaments in vacuum and reduced‐pressure gaseous environments are analyzed. Using hydrogen, deuterium, and helium, it is concluded that the difference in the power consumption by the filament in hydrogen and in vacuum is a good measure for the rate of hydrogen dissociation. This rate is shown by experiments and mathematical modeling to depend on the geometry of the refractory heater element. Relatively high dissociation rates, normalized per heater area, are obtained for small‐diameter wires, and it is argued that this is indicative of a nonequilibrium dissociation process.

Electronic carrier transport and photogeneration in buckminsterfullerene films

Information concerning electronic carrier transport and photogeneration in films of undoped C60/70 has been obtained from photoinduced xerographic discharge curves in sublimed films up to 40 μm in thickness. The results, using strongly absorbed light, unambiguously reveal that the electron range, μτ, where μ is the electron drift mobility and τ the deep trapping lifetime, is ∼10−7 cm2/V. By contrast, a barely detectable discharge is observed to be associated with holes. The photogeneration efficiency for electrons is field dependent with a value at 4000 A of 5×10−2 at a field of 2×105 V/cm. These results are discussed in a context where both charge photogeneration and transport in undoped C60/70 involve localized electronic states.

Effects of geminate recombination on the photovoltaic characteristics of a‐Si:H Schottky barrier solar cells

The photovoltaic characteristics of an a‐Si:H Schottky barrier cell have been analyzed theoretically taking into account geminate recombination and the localized state distribution. A comparison of the results with the experimental data in the literature leads to fundamentally different conclusions regarding the limitations of this type of photovoltaic cell. The limitation due to carrier lifetime is unimportant in a cell of typical thickness (∼0.5 μm), while geminate recombination can essentially account for observed recombination losses.

Charge-transfer processes in Buckminsterfullerene films

Abstract The spectral dependence of the photogeneration efficiency. η in sublimed undoped C60-C70 films has been determined from photoinduced xerographic discharge curves. The dependence of η on the exciting light wavelength from 7250 to 3500A and electric field are reported. At an applied field of 2–7 × 105 V cm−1 η is essentially wavelength independent with a value of about 5 × 10−3 from 7250 to 5500A. At shorter wavelengths, η is essentially proportional to the absorption coefficient, rising by more than an order of magnitude to about 6 × 10−2 at 3500A. The difference between the charge-transfer efficiencies of these two regions is discussed in terms of an Onsager surface-mediated dissociation of singlet excitons.

Field‐effect phenomena in hydrogenated amorphous silicon photoreceptors

The imaging properties of hydrogenated amorphous silicon (a‐Si:H) photoreceptors overcoated with insulating passivation layers are discussed. Field‐effect phenomena are shown to interfere with the electrophotographic imaging process in this device configuration. Since photoreceptors are majority carrier devices and a‐Si:H is an extrinsic material, the field‐effect phenomena can be counteracted by proper doping of the a‐Si:H‐insulator interface.

Calculation of capacitance‐voltage characteristics of hydrogenated amorphous silicon Schottky diodes

By adopting a new boundary condition appropriate to hydrogenated amorphous silicon diodes, and using the nonequilibrium Fermi–Dirac statistics, we predict a low‐frequency capacitance‐voltage relation which is quite different from that obtained by previous calculations in the literature, while in good agreement with more recent experimental results. The physical reasons for the differences are discussed.

Geminate recombination‐controlled photogeneration in amorphous solids

Experimental and theoretical results are presented which suggest a unified picture of photogeneration processes in amorphous selenium and hydrogenated amorphous silicon. The different wavelength dependent quantum efficiency of photogeneration in these technologically important materials can be explained by a geminate recombination model if account is taken of their different dielectric constants.

Space‐charge‐perturbed electrophoresis in nonpolar colloidal dispersions

This paper reports the observation of bipolar, space‐charge‐perturbed transport in colloidal dispersions using an experimental technique that time‐resolves electrophoresis in nonpolar colloidal systems. Unlike existing methods for studying electrophoresis, this technique can be applied to dispersions of diverse types and concentrations over a wide range of electric fields, including the space‐charge‐perturbed conditions often encountered in practical applications. The phenomenon is investigated as a special case of dielectric relaxation in a leaky capacitor connected in series to a perfect one. Using the first principle charge transport theory, such dielectric relaxation, occurring under the non‐Ohmic supply and space‐charge‐perturbed transport conditions, is shown to differ from that expected by the conventional equivalent‐circuit treatment. The combined theoretical analysis and the experimental technique provides a means for independently determining the densities and mobilities of charged species in su...

Theoretical analyses of amorphous semiconductor multijunctions

The transport of charge carriers in a multijunction, consisting of alternating thin p‐ and n‐type sublayers of amorphous semiconductor, has been analyzed theoretically. It is shown that the multijunction behaves in the dark like a pn‐junction diode, while it has an advantage in reducing range limitation of photogenerated carriers. These properties are realized when the sublayer thickness is of the order of 100 nm for realistic values of doping levels and localized state density.

On the field-effect in amorphous tetrahedral photoreceptors

Abstract The imaging properties of photoreceptor structures based on amorphous silicon are discussed. When the external surface of such structure is an electrically insulating film, it functions during the imaging process like the gate insulator in a thin film transistor. The resulting field effect phenomena are shown to decrease the attainable electrophotographic resolution. Methods are discussed to counteract the occurence of the field effect in photoreceptors made of tetrahedrally bonded amorphous materials.