J. Hiie

Powder materials and technologies for solar cells

The regularities of recrystallisation of initial powders of cadmium and zinc chalcogenides and chalcopyrites in different molten fluxes are studied. It is shown that the capillary phenomena in the solidliquid phase boundary, wetting of the solid phase with the liquid flux phase are the processes that determine the mechanism of recrystallisation. Results indicate to the possibility of manufacturing of powders of complicate semiconductor materials in monograin form and with qualities acceptable for Monograin Layer (MGL) design of solar cells. Influence of several technological processes to parameters of monograin powders and MGL solar cells are studied.

Monograin A2B6 powders

This paper discusses the recrystallization mechanism, kinetics and morphology of the crystals of different A2B6 powders versus preparation conditions. It is shown that these parameters are mainly determined by the chemical nature and concentration of a flux.

The performance of thin film solar cells employing photovoltaic ZnSe/CdTe, CdS/CdTe and ZnTe/CdTe heterojunctions

This paper focuses on the photovoltaic parameters of ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe thin film heterojunction solar cells. ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe thin film heterojunction solar cells were fabricated by Close Space Sublimation (CSS) on TCO-coated glass substrates. All types of solar cells were fabricated in a superstrate configuration. The thickness of ZnSe and ZnTe layers was varied in order to adjust the solar cell performance. A similar cadmium chloride solution for the treatment of a CdTe layer with an elevated temperature air annealing of the completed devices before the back contact deposition was applied to ZnSe/CdTe and CdS/CdTe thin film heterojunctions solar cells with exception of ZnTe/CdTe. All cells were characterized through light and dark current density-voltage (J-V) measurements and quantum efficiency (QE) measurements. The saturation current, ideality factor and photovoltaic parameters for all thin film heterojunction solar cells are presented. The investigation at the room temperature under illumination of 100 mW/cm2 through the wide gap components of ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe heterojunctions showed a value of conversion efficiency (η) of solar energy to electric energy about 4.7%, 9.9%, and 1.3%, respectively. The incorporation of Zn at the ZnSe and CdTe interface doubles the short circuit current density and improves the performance of ZnSe/CdTe thin film heterojunction solar cells.

Scintillators for high efficiency and high spatial resolution in x-ray imaging applications

A new high efficiency, low-bandgap phosphor, ZnSe:Cu,Ce,Cl is described which exhibits a significantly higher quantum gain than conventional x-ray phosphors and more closely matches the spectral sensitivity of silicon sensors. For many imaging applications this phosphor thus promises significantly superior performance compared to conventional phosphors.

Monograin layers and membranes for photovoltaics

The possibilities of producing monograin powders of different A2B6 and chalcopyrite materials; different technologies of manufacturing monograin layers and cleaning of open surfaces of grains in the monograin layer by mechanical, chemical and electrochemical methods were studied. It was shown that up to now too low efficiency of solar cells by the monograin layer is associated with the insufficient cleaning of surfaces of crystals in the monograin layer before depositing the active contacts.

Growth of CdTe monograin powders

The preparation of CdTe single-crystalline monodisperse powders in cadmium chloride flux modified by adding elemental tellurium was studied for production different particle sizes (30-300 μm). The growth of monograin powders was carried out in isothermal conditions in closed evacuated ampoules. It was found that at lower temperatures (500-650 °C) after maintaining the equilibrium conditions primary crystals grow mainly by deposition of material from saturated solution onto smooth surfaces of crystals. In higher temperature region (700-800 °C) there occurs remarkable sintering of particles and grains loose their round shape. The activation energy for volume growth ΔEvol = 0.9 eV was found. Te in flux was found to prevent the formation of aggregates through sintering and to increase the crystal growth velocity.

Annealing effect on CdS films: transition from glass to ITO

Structural and optical properties of CdS films deposited on glass and ITO substrates and annealed in H2, N2 or H2+N2 atmospheres at 250 °C and 400 °C were compared. The optical thickness 405±10 nm of the CdS films was found not to be influenced by any annealing conditions or substrate. The as deposited films on both substrates had similar diffraction patterns with one narrow peak corresponding to the (111) cubic plane at 2θ 26.70° and 26.75°, respectively. Higher density of nucleation centres on the ITO surface generated denser CdS films. In the annealing process of CdS/ITO a slower shift of both (111) peak and lattice constant was observed in the direction of pure zinc blende modification. Similar values of band gap and transmittance were determined for as-deposited CdS while annealing at 400 °C decreased transmittance by 5% and 16% for CdS on glass and ITO, respectively. Transmittance dropped because of the destruction of hydroxide group in the CdS lattice, the formation of cadmium excess and the reduction of SnO2 to black SnO in the ITO structure.

CdTe Films on Mo/Glass Substrates: Preparation and Properties

Short-bandgap group II-VI compound cadmium telluride is widely used for the infrared optics, radiation detectors, and solar cells where p-type CdTe is needed. p-type conductivity of CdTe is mainly caused by the chlorine-based A-centers, and in part, by the less stable copperoxygen complexes. As a rule, CdTe films are recrystallized by the help of a cadmium chloride flux that saturates CdTe with chlorine. In chlorine-saturated CdTe A-centers are converted to isoelectronic complexes that cause resistivity increasement of CdTe up to 9 orders of magnitude. Excess copper and oxygen or group I elements as sodium also deteriorate the p-type conductivity of CdTe like excess chlorine. p-type conductivity of CdTe can be restored e.g. by the vacuum annealing which removes excess chlorine from the film. Unfortunately, treatment that betters ptype conductivity of the CdTe film degrades the junction of the superstrate configuration cells. In this work we investigate possibilities to prepare p-type CdTe films on the molybdenum coated glass substrates. Samples were prepared by the vacuum evaporation and dynamic recrystallization of 6N purity CdTe on the top of Mo-coated glass substrates. Then samples were recrystallized with cadmium chloride flux under tellurium vapour pressure. Results of the test studies on the structure and electronic parameters of samples are presented and discussed.

Monograin layers as optoelectronic devices

The possibility is demonstrated to manufacture by the recrystallization of initial powders in different molten fluxes different A2B6 powders with the qualities acceptable for monograin layer construction. Several technologies of formation of monograin layers and optoelectronic devices were developed and studied. It is shown, that insufficient electronic parameters of semiconductor sensors and solar cells designed as monograin layers are connected with the insufficient cleaning of surfaces of crystals in the monograin layer.

Influence of technological conditions on the properties of CBD CdS layers

This work evaluates the efficacy of the chemical bath deposition (CBD) and technological conditions for preparation of homogeneous CdS thin films with high transparency and low resistivity. The effects of various technological parameters (magnetic agitation of solution, pre-treatment in vacuum, annealing in hydrogen atmosphere, and concentration of chlorine as dopant in the deposition bath) on morphology, transmittance and electrical properties were studied. Together, these results show that CBD is an efficient technique, helpful for the scaling-up of the manufacturing process of suitable and reproducible window n-type CdS layers for further solar cell application.