Richard H. Bube

Optical and electrical properties of ZnO films prepared by spray pyrolysis for solar cell applications

Zinc oxide films were produced by spray pyrolysis starting with aqueous solutions of ZnCl2, ZnCl2 plus H2O2, and Zn acetate, and structural, optical, electrical, and thermoelectrical properties of the deposited films were investigated. Highly transparent films with resistivity as low as 10−3 Ω cm can be produced by suitable control of deposition procedures and by postdeposition annealing in hydrogen.

Photovoltaic properties of ZnO/CdTe heterojunctions prepared by spray pyrolysis

An extended investigation has been made of the electrical and photovoltaic properties of heterojunctions prepared by spray‐pyrolysis deposition of thin ZnO films on single‐crystal p‐type CdTe. The principal experimental variables were the substrate temperature and the postdeposition temperature for annealing in H2. Under actual sunlight the optimum cell showed an open‐circuit voltage of 0.54 V, a short‐circuit current of 19.5 mA/cm2, and a solar efficiency (referred to the active area) of 8.8%, the highest value obtained to date for an authentic heterojunction on CdTe. The nature of the forward transport mechanism has been investigated, and a tunneling model in which bulk and interface deep traps control the forward characteristics is shown to provide good correlation with the experimental data.

Thermoelectric and photothermoelectric effects in semiconductors: Cadmium sulfide films

Techniques of thermoelectric and photothermoelectric analysis have been applied to investigate the electrical transport properties of both solution‐sprayed and evaporated polycrystalline CdS films. Dark electron densities are independent of temperature between 200 and 330°K because of shallow donors; dark electron mobilities are thermally activated with energies between 0.09 and 0.23 eV. Photoconductivity of solution‐sprayed films is caused primarily by an increase in electron mobility. Other effects investigated are the adsorption and desorption of oxygen, the diffusion of Cu acceptors, and field‐assisted drift of Cu impurities in the film at a slightly elevated temperature (100°C). The structure and characteristics of intergrain boundaries dominate the transport processes in these polycrystalline films.

Reinterpretation of degradation kinetics of amorphous silicon

Generation of light‐induced metastable defects in amorphous Si:H(a‐Si:H) is shown to follow the same stretched exponential (SE) that describes relaxation of thermally induced metastability at room temperature for a simple case. Apparent power laws derived from the central part of the SE are (time)0.3 and (intensity)0.6, agreeing well with the (time)1/3 and (intensity)2/3 dependences often reported in the mid range of defect density, thus providing an alternative description of defect generation. The SE link between light‐induced and thermally induced instabilities suggests that the thermal effects are also due to defect processes, and offers an alternative defect‐based explanation to a macroscopic ‘‘structural relaxation’’ or ‘‘glass transition.’’

Trap Density Determination by Space‐Charge‐Limited Currents

Trap depths and densities can be determined from the analysis of the dependence of space‐charge‐limited currents on applied electric field in insulators. Values obtained from space‐charge‐limited current measurements are compared with those obtained from thermally stimulated currents and from photocurrent decay on the same crystal of cadmium sulfide. The results indicate that gross errors in trap depth and density can be made if it is simply assumed that the rapid rise in current toward the trap‐free curve with increasing applied voltage always occurs when the traps are filled. Although this may sometimes be true, it is also possible for such a rapid rise to result from field ionization of the traps, or even in certain cases from the onset of double injection. Even if the latter effects occur, however, proper analysis of the space‐charge‐limited current data permits an evaluation of trap depth and density which is completely consistent with independent measurements of thermally stimulated current and phot...

Effects of heat treatment on the optical and electrical properties of indium–tin oxide films

The effect of heat treatment in various environments on the electrical and optical properties of indium–tin oxide (ITO) sputtered films has been investigated. As the resistivity is decreased by heat treatment in H2 from 8.3×10−3 to 4.3×10−4 Ω cm, the optical band gap increases from 3.05 to 3.42 eV consistent with a Burstein shift and intrinsic band gap of 2.98 eV, and for resistivities less than 8.3×10−4 Ω cm there is a rapid decrease in transmission at longer wavelengths due to free‐carrier absorption and formation of dendritic precipitates. Careful control over all sputtering variables is essential to obtain reproducible properties.

Determination of Electron Trapping Parameters

A detailed investigation of different methods for determining electron trap parameters has been made on crystals of CdS‐CdSe. The principal techniques involved are decay of photoconductivity and thermally stimulated conductivity (TSC). Direct evidence of a quasicontinuous trap distribution with total density of 5×1015 cm−3, trap depth range of 0.1–0.7 eV, and capture cross sections of the order of 10−16 cm2 is obtained, for which correct values of the parameters can be calculated from Fermi‐level analysis of either decay or TSC data. In the same crystals a discrete trap level with density of 2×1014 cm−3, depth of 0.73 eV, and apparent cross section of 10−14 cm2 is also found. In spite of the large value of cross section derived from the freeing of trapped electrons, these traps exactly obey monomolecular kinetics. A temperature threshold at 180°K is found, below which it is not possible to fill these traps. Examination of a number of possibilities favors the proposal that these traps are characterized by ...

Mechanism of Photoconductivity in Chemically Deposited Lead Sulfide Layers

Many of the classical problems involved in the mechanism of photoconductivity in chemically deposited PbS layers have been resolved by a combination of (a) materials control, (b) measurements of conductivity, photoconductivity, Hall effect, thermoelectric effect, and oxygen sorption, and (c) analysis of an inhomogeneous model. The dark conductivity process, characterized by a small thermally activated mobility, is the same for samples without added oxidant in the deposition and for samples with added oxidant, and is dominated by intergrain barriers to free‐hole transport. Carrier densities derived from Hall measurements are equal to those derived from thermoelectric power measurement and are therefore interpreted to be equal to the carrier density in the grains of the PbS layer. Layers prepared without added oxidant are only very weakly photosensitive, corresponding to sensitizing centers lying 0.13 eV below the conduction band, whereas layers prepared with added oxidant are strongly photosensitive, corre...

Analysis of photoconductivity applied to cadmium-sulfide-type photoconductors

Abstract The model of a photoconductor including two different types of recombination centers, filled in the dark, and with trapping centers, empty in the dark, is able to adequately describe such photoconductivity phenomena in CdS-type photoconductors as supralinearity, temperature dependence, infra-red quenching, and variations in speed of response. Thus the conceptual description of R ose , which has been previously shown to have semiquantitative validity, is put on a more detailed quantitative basis. The application of the theoretical calculations to experimental data for CdSe gives self-consistent values for the ratio (8 × 10 5 ) of the capture cross-section of the “sensitizing” centers for holes to that for electrons, and for the energy difference (0.64 eV) between the level corresponding to this type of center and the top of the valence band. On the basis of the model, the relatively fast response of CdSe is associated with the specific location of these levels, which location carries with it the occurrence of supralinearity and large temperature dependence of photosensitivity.

Properties of CdS Films Prepared by Spray Pyrolysis

A detailed investigation has been made of the properties of films deposited on amorphous glass substrates by the technique of spray pyrolysis. The variation of the electrical transport properties of these films as a function of substrate temperature has been correlated with variations in orientation, cubic/hexagonal phase ratio, and morphology. Additional effects of cooling rate, spraying rate, heat‐treatment in hydrogen, and variations in substrate are also considered. By heat‐treatment in hydrogen it is possible to produce films on amorphous glass substrates with an electron density in excess of 1018cm−3 and an electron mobility of 90 cm2/V‐sec at 300°K.