S. Oktik

Transmission electron microscopy of CdTeCdS based solar cells

Abstract An investigation of the effects of annealing thin film CdTe/CdS/ITO/glass photovoltaic cells in air after prior exposure to CdCl 2 has been made. The influence of this on the efficiency, current transport mechanism, grain size and microstructure of the films has been studied. A CdCl 2 treatment reduces the influence of interfacial states on the current transport mechanism, causes grain growth in the CdS and CdTe and also decreases the density of stacking faults and dislocations in the CdTe. However, it introduces Cl-rich precipitates into the lattice. Cross-section and plan-view TEM evidence is presented and the usefulness of moire fringe analysis is highlighted.

Transmission electron microscopy of CdTe/CdS based solar cells

An investigation of the effects of annealing thin film CdTe/CdS/ITO/glass photovoltaic cells in air after prior exposure to CdCl 2 has been made. The influence of this on the efficiency, current transport mechanism, grain size and microstructure of the films has been studied. A CdCl 2 treatment reduces the influence of interfacial states on the current transport mechanism, causes grain growth in the CdS and CdTe and also decreases the density of stacking faults and dislocations in the CdTe. However, it introduces Cl-rich precipitates into the lattice. Cross-section and plan-view TEM evidence is presented and the usefulness of moire fringe analysis is highlighted.

Mechanically induced phase transformations in CdS, CdSe and ZnS

The chalcogenides of zinc and cadmium usually crystallize in either the sphalerite cubic or wurtzite hexagonal modification, except in rare cases where very high pressure leads to the rocksalt structure. Most of the compounds in question have a phase transition from hexagonal to cubic at some characteristic temperature, and which phase a particular compound assumes, depends on such factors as the temperature of crystallization and the nature of the bonding [1]. The incorporation of impurity ions can also be important [2]. It has long been known that the luminescence efficiency of hexagonal ZnS phosphor powders decreases on grinding and ball milling, and at t he same time the proportion of the cubic phase detected in X-ray powder photographs increases [3 -5 ] . However, as far as we are aware there is no published account of the structure of mechanically polished layers on large single crystals of II-VI compounds, although such layers can have profound effects on the apparent electrical and optical properties of a material. For example polished layers may introduce difficulties in experiments in which measurements of carrier diffusion lengths are made on angle lapped devices [6]. For some time past we have been carrying out a programme of work concerned with the preparation of photovoltaic cells and light emitting diodes on single crystals of hexagonal II-VI compounds. Devices are usually fabricated on dice of millimetre dimensions cut from large single crystals of CdS, CdSe and ZnS grown in this laboratory. Photovoltaic devices are prepared in CdS [7] and CdSe, and light emitting diodes on ZnS [8]. Sawing with the diamond wheel leaves dice with severely work-damaged surfaces. If attempts are made at this stage to remove the work-damaged layer by chemical etching, the result is a rough surface with numerous large hillocks, steps and kink sites. Devices prepared on such surfaces invariably have poor photosensitivity or low luminescence efficiency. In order to prepare more efficient devices with greater reproducibility, it is necessary to polish the sawn faces mechanically to produce physically flat surfaces before the chemical etch is administered. The purpose of the etch is to remove the damaged layer completely so that its effects are eliminated. Recently, however, we have made a brief study of the properties of the work damaged layers. The results are reported here. When a crystal of CdSe with the wurtzite structure was polished by hand, or on a lapping wheel, using alumina powder with a particle size of 1/am or less, the resulting surface, regardless of the orientation of the underlying crystal, gave rise to the reflection electron diffraction (RED) pattern shown in Fig. 1. The diffraction rings in this pattern indicate that the surface layer on this sample was entirely polycrystalline. The radii of the first three rings increase in the sequence X/3, x/8, x/l l, and are indexed as the (1 11), (220) and (311) reflections of the cubic sphalerite phase of CdSe. Similar cubic polycrystatline surface layers were produced on single crystals of CdS

Single crystal ZnxCd1-xS/Cu2S photovoltaic cells

Abstract Zn x Cd 1- x S/Cu 2 S photovoltaic cells have been prepared on oriented single crystal substrates of Zn x Cd 1- x S for compos itions with 0 x x Cd 1- x S crystals, (2) the surface preparation prior t o the formation of the copper sulphide layer, (3) the phase of that layer, and (4) the properties of the heterojunction devices. The crystals were grown from the vapour phase. Their crystal structure, lattice parameters and band gaps have been determined. The copper sulphide layers were formed by the chemiplating process, and it is demonstrated that the preparation of the substrate surface strongly affects the ultimate behaviour of cells. Examination in the scanning electron microscope using the secondary emission and electron beam induced current modes associates localized variations in efficiency with topographical features. The open circuit voltages obtained under AM1 illumination increased from 500 mV for a CdS based cell to 720 mV for a Zn 0.4 Cd 0.6 S based one, while simultaneously there was a considerable decrease in short circuit current. In this and in many other respects the characteristics of devices made on these single crystals were found to be closely related to those of thin film material.

Properties of ZnO layers deposited by “photo-assisted” spray pyrolysis

Abstract Layers of ZnO have been deposited onto glass substrates by “photo-assisted” spray pyrolysis using zinc chloride and zinc acetate as precursors in aqueous and non-aqueous solutions. The structure of these films has been shown by reflection high energy electron diffraction (RHEED) to be hexagonal, the crystallites exhibiting a pronounced preferred orientation with their (0002) planes lying parallel to the substrate. The transmission properties of layers deposited using an aqueous solution of zinc acetate were found to be superior. In their as-deposited form the films repeatedly exhibited carrier concentration, Hall mobility and resistivity values of ∼(1–2) × 1017cm−3, ∼ 0.1–0.2 cm2V−1s−1 and ∼ 150–600 Ω · cm respectively. However, following annealing at 400°C for 15 min in forming gas (H2 + N2), these parameters improved to ∼ 5 × 1019cm−3, ∼ 29 cm2V−1s−1 and ∼ 1.5 × 10 −2 Ω · cm respectively. Annealing did not change the transmittance but there was a noticeable improvement in the degree of preferred orientation. The considerable improvement in the electrical properties after annealing can be largely attributed to the increases in carrier concentration and mobility due to the change in stoichiometry and possibly a reduction in inter-granular potential barriers respectively.

The correlation between current transport mechanisms and etch features in Au-CdS single-crystal Schottky diodes

The effects of surface features on chemically etched (0001) planes of single-crystal CdS on the dark current transport mechanisms in Schottky diodes prepared on them by vacuum evaporation of gold are investigated. By combining scanning electron microscope (SEM) observations of these surface features with measurements of current-voltage (I-V) characteristics, the diode behaviour of the Schottky barriers is demonstrated to be strongly influenced by the surface topography. The temperature dependence of I-V capacitance-voltage (C-V) and photo-electric measurements are also studied in an attempt to correlate the junction parameters with surface features.

Single crystal Cu2S/CdS photovoltaic devices with optimum performance before a post barrier air bake

Abstract The separate stages in the preparation of Cu 2 S/CdS photovoltaic devices on single crystal substrates have been investigated carefully using the techniques of reflecttion electron diffraction and scanning electron microscopy. Mechanical polishing of hexagonal single crystal CdS produces a polycrystalline surface layer with cubic structure which, for efficient cells, has to be removed by chemical etching before the topotaxial Cu 2 S layer is formed. Different types of etched surfaces have been produced and found to be closely related to the polishing procedure. The nature of the etched surface has a strong influence on the cell characteristics. Devices made on surfaces with many ledges and kink sites on the sides of the etch hillocks have poor diode characteristics and photovoltaic behaviour in the as-made condition, whereas cells formed on surfaces with faceted hillocks have good rectifying and photovoltaic properties at this stage. While a post barrier air-bake is essential to improve cells which have poor characteristics in the as-prepared condition, it has a deleterious effect on devices with good characteristics in the as-made state. The highest efficiencies are obtained with as-plated cells fabricated on surfaces etched to produce hillocks with well developed facets on substrates with optimum resistivity.

Single-crystal ZnxCd1−xS/Cu2S photovoltaic cells

Photovoltaic cells were prepared on oriented single-crystal substrates of ZnxCd1−xS for compositions with 0 ⩽ x ⩽ 0.4. The copper sulphide layers were produced by dipping the substrates into a solution containing cuprous ions. The phases of these layers were determined using reflection electron diffraction. The open-circuit voltage under air mass one conditions of illumination increased with increasing zinc content, reaching a value of 0.72 V for x = 0.4. At the same time the short-circuit current density and the fill factor decreased, so that there was no improvement in efficiency over the CdS/Cu2S cell. Measurements of spectral response and of the effects of heat treatment at 200 °C are reported. These are interpreted to suggest that the thickness and the chalcocite content of the copper sulphide layers decreased as the zinc content of the substrates was increased.

Effects of Surface Topography on Dark Current Conduction Processes in Single Crystal Cu2S/Cds Solar Cells

The results of an investigation into the correlation between the surface topography of etched \(\left( {000\overline 1 } \right)\) CdS single crystal substrates and the mechanisms of dark current conduction processes in Cu2S/CdS heterojunctions formed on them are reported. By working with single crystals and using scanning electron microscopy (SEM) in conjunction with dark current-voltage (J-V) measurements, it is shown that on a microscopic scale, the Cu2S grown topotaxially on different surface features of etched \(\left( {000\overline 1 } \right)\) CdS substrates results in devices for which the junction parameters vary dramatically with the local topography of the surface. It is demonstrated that the relative proportions of the specific types of surface feature play a key role in determining the overall behaviour of these heterojunctions.

An Ebic Study of the CdS-Ci2S Photovoltaic Cell

Layers of copper sulphide have been grown topotaxially on the (0001) sulphur faces of single crystals of cadmium sulphide using a plating solution containing cuprous ions. This particular plane was chosen because it corresponds to the preferred orientation of the surfaces of grains in the evaporated films used in large area devices. With single crystals, the complexities associated with grain boundaries can be eliminated, and it becomes possible to study more closely the effects of other crystallographic features on cell performance. This has been done here using a scanning electron microscope (SEM) in the electron beam induced current (EBIC) mode. The main surface features produced on the (0001) plane of CdS by etching in HCl were etch hillocks. The EBIC signal, which is a measure of the localised microscopic photovoltaic sensitivity, varied substantially over the surfaces of the hillocks in a manner which can be explained in terms of a variation in the density of interface states and hence of recombination. By operating the SEM in the stationary spot mode, current-voltage characteristics were measured with the beam incident on different regions of the hillocks. In this way it has been possible to study the behaviour of a cell at a microscopical level, and measurements made before and after a short 200°C bake in air demonstrate a direct similarity between optical and electron beam excitation. The most important conclusion is that the density of recombination centres was greater at the apexes of the hillocks than at their sloping sides, with the result that the PV sensitivity was reduced by a factor of three at the apexes.