Defect tolerance in as-deposited selenium-alloyed cadmium telluride solar cells

The efficiency of cadmium telluride (CdTe) solar cells is limited primarily by voltage, which is known to depend on the carrier concentration and carrier lifetimes within the absorber layer of the cell. Here, cathodoluminescence measurements are made on an as-deposited CdSeTe/CdTe solar cell that show that selenium alloyed CdTe material luminesces much more strongly than non-alloyed CdTe. This reduction in non-radiative recombination in the CdSeTe suggests that the selenium gives it a certain defect tolerance. This has implications for carrier lifetimes and voltages in cadmium telluride solar cells

Radiation resistance of thin-film solar cells for space photovoltaic power

Copper indium diselenide, cadmium telluride, and amorphous silicon alloy solar cells have achieved noteworthy performance and are currently being studied for space power applications. Cadmium sulfide cells had been the subject of much effort but are no longer considered for space applications. A review is presented of what is known about the radiation degradation of thin film solar cells in space. Experimental cadmium telluride and amorphous silicon alloy cells are reviewed. Damage mechanisms and radiation induced defect generation and passivation in the amorphous silicon alloy cell are discussed in detail due to the greater amount of experimental data available

ALTERNATIVE ARCHITECTURE FOR SOLUTION PROCESSED THIN FILM SOLAR MATERIALS

A variety of competing semiconductor materials are used to create p-n solar cells. Currently silicon solar cells dominate more than 90% of the solar market. The major competitors to silicon solar cells are inorganic thin-film solar cells, mainly cadmium telluride and copper indium gallium sulfide. These materials evolved from initial devices based off of cadmium sulfide. All of the non-silicon commercial cells still use cadmium sulfide in their production, which is of concern due to the toxicity of cadmium and limits their application, especially in Europe due to the E.U. Restriction of Hazardous Substances directive. If these devices are to remain market competitive, increased cost efficiency and the removal of cadmium are necessary

The effect of a post-activation annealing treatment on thin film CdTe device performance

The cadmium chloride activation treatment of cadmium telluride solar cells is essential for producing high efficiency devices. The treatment has many effects but the most significant is the complete removal of stacking faults in the cadmium telluride grains and the diffusion of Chlorine along the grain boundaries of the device. Chlorine decorates all cadmium telluride and cadmium sulphide grain boundaries and also builds up along the CdTe/CdS junction. . This paper reveals that by annealing devices to temperatures of 400ºC to 480 ºC for times ranging from 30 to 600 seconds in moderate vacuum results in the re-appearance of stacking faults and the removal of Choline from the grain boundaries. STEM analysis confirms the re-appearance of the stacking faults and SIMS and EDX confirm the removal of chlorine from the grain boundaries. This directly corresponds to a lowering in cell efficiency. The study provides further evidence that CdCl2 diffusion and certain microstructural defects directly affect the performance of cadmium telluride photovoltaic devices

Investigation of Photoelectric Converters with a Base Cadmium Telluride Layer with a Decrease in Its Thickness for Tandem and Two-sided Sensitive Instrument Structures

Photovoltaic cells with a base layer of cadmium telluride with a decrease in its thickness are studied. It is known that the widespread use of photovoltaic converters is constrained by their high price in the case of highly efficient instrument structures, or low efficiency. The creation of tandem and two-sided sensitive photoelectric converters will reduce their cost while increasing their efficiency. However, to create tandem and two-sided sensitive photoelectric converters, the necessary conditions are the use of transparent contacts and a decrease in the thickness of the base layer for efficient absorption of incident radiation by the converter, which is lower. In the research process, it was found that reducing the thickness of the base layer to 1 μm allows to increase the efficiency of the photoelectric transducer during irradiation from the back. An increase in the efficiency of the photoelectric converter occurs due to a decrease in the distance from the generation region of nonequilibrium charge carriers in the region of separation. If the thickness of the base layer is less than 1 μm, then regardless of which side of the irradiation is carried out, a decrease in the efficiency of the instrument structure is observed. Increase in the efficiency of photoconverters is associated with an increase in the negative influence of recombination processes on the back contact, a decrease in the number of charge carriers generated due to incomplete absorption of incident radiation, and a decrease in the volume of the built-in field of the separating barrier when it overlaps with the depletion region of the back contact. ITO/CdS/CdTe/Cu/ITO SCs with a base layer thickness of 1 μm demonstrates degradation stability. The highest value of efficiency in the case of illumination from the front side 8.1 % and with illumination from the back side 3.8 % received after a year of operation of the photovoltaic converter

Advances in thin-film solar cells for lightweight space photovoltaic power

The present stature and current research directions of photovoltaic arrays as primary power systems for space are reviewed. There have recently been great advances in the technology of thin-film solar cells for terrestrial applications. In a thin-film solar cell the thickness of the active element is only a few microns; transfer of this technology to space arrays could result in ultralow-weight solar arrays with potentially large gains in specific power. Recent advances in thin-film solar cells are reviewed, including polycrystalline copper-indium selenide (CuInSe2) and related I-III-VI2 compounds, polycrystalline cadmium telluride and related II-VI compounds, and amorphous silicon:hydrogen and alloys. The best experimental efficiency on thin-film solar cells to date is 12 percent AMO for CuIn Se2. This efficiency is likely to be increased in the next few years. The radiation tolerance of thin-film materials is far greater than that of single-crystal materials. CuIn Se2 shows no degradation when exposed to 1 MeV electrons. Experimental evidence also suggests that most of all of the radiation damage on thin-films can be removed by a low temperature anneal. The possibility of thin-film multibandgap cascade solar cells is discussed, including the tradeoffs between monolithic and mechanically stacked cells. The best current efficiency for a cascade is 12.5 percent AMO for an amorphous silicon on CuInSe2 multibandgap combination. Higher efficiencies are expected in the future. For several missions, including solar-electric propulsion, a manned Mars mission, and lunar exploration and manufacturing, thin-film photovolatic arrays may be a mission-enabling technology

Review of thin film solar cell technology and applications for ultra-light spacecraft solar arrays

Developments in thin-film amorphous and polycrystalline photovoltaic cells are reviewed and discussed with a view to potential applications in space. Two important figures of merit are discussed: efficiency (i.e., what fraction of the incident solar energy is converted to electricity), and specific power (power to weight ratio)

Cadmium telluride for solar cells

Cadmium telluride is an attractive material for solar cell applications because of its near optimum bandgap and high absorption coefficient. This thesis presents the results of a study into the use of CdTe for solar cells. Three types of cell have been investigated, namely; CdS/CdTe devices fabricated by the vacuum evaporation of CdS onto either: (a) single crystal p-CdTe substrates or (b) p-CdTe thin films, and (c) p-Cu(_2)Te/n-CdTe devices made by a chemiplating process onto single crystal n-CdTe. The effects of substrate polishing and preparation on the performance of CdS/CdTe bulk crystal cells have been Investigated together with the problems of doping and contacting to p-type CdTe. These studies have shown that the best results are obtained with devices that have been prepared on pad polished, phosphorus doped substrates using carbon contacts (efficiency = 7.2%). The influence of deposition conditions on the electrical and structural properties of thin CdS and CdTe layers, and their effect on CdS/CdTe device efficiency were also studied, and optimum growth conditions established. In the third group of CU(_2)Te/CdTe solar cells a number of structural and electrical aspects such as the phase of Cu(_2)Te, and the Influence of dopants, substrate resistivity and preparation and ageing on cell efficiency have been examined. As secondary objectives, an investigation Into the epitaxial growth of CdS on CdTe, and the characterisation of as-grown and doped CdTe have been carried out. It has been shown that epitaxy is possible on the {111} and {221} faces of CdTe. The characterisation of CdTe has revealed the presence of dominant levels at energies above the valence band of 0.50 eV in the as-grown crystals; 0.53, 0.71 and 0.84 eV in Te-annealed single crystals; and 0.35 eV in Cu doped CdTe thin films

Passivation studies on Cd0.6Zn0.4Te films using CdCl2, MgCl2 and ZnCl2 for top cell application in a multijunction solar cell

2018 Summer.Includes bibliographical references.Passivation treatment with the chloride compounds is an important step in the fabrication of II-VI solar cells for improving the device performance. In cadmium telluride solar cells, cadmium chloride passivation treatment incorporates chlorine along the grain boundaries and helps in recrystallization, grain growth, removal of stacking faults and doping grain boundaries as n-type. In cadmium zinc telluride solar cells, the retention of zinc after the cadmium chloride passivation treatment is one of the challenges incurred in fabricating the top cell in a multijunction solar cell. During the passivation treatment, the loss of zinc occurs in the form of volatile zinc chloride compound. The depletion or complete loss of zinc reduces the higher band gap ternary alloy into lower band gap binary compound of CdTe. This impedes the purpose of fabricating a high band gap top cell in a multijunction solar cell. The focus of this study is on passivating Cd0.6Zn0.4Te (CdZnTe) films using three different chloride compounds separately and understanding the effects by studying the material properties of the passivated films and electrical performance of the fabricated devices. In the preliminary experiments, CdZnTe films were deposited by RF sputtering from a single target. Initial characterization of CdZnTe films deposited on plain glass indicated that the films had a strong preferred orientation along {111} plane with a band gap of ~1.72eV. In the cadmium chloride passivation treatment, loss of zinc from the surface and no chlorine along the grain boundaries was observed from transmission electron microscope images and X ray diffraction measurements. No loss of zinc was observed after the magnesium chloride and zinc chloride passivation treatments. Increase in the grain size of the CdZnTe films after magnesium chloride treatment and decrease of the preferred orientation after zinc chloride treatment were the benefits of the individual passivation treatments. Modifying the test structure by adding a cadmium telluride film as a capping layer on the back of RF sputtered CdZnTe and then carrying out the cadmium chloride passivation treatment helped in retaining the zinc. Heavy diffusion of zinc into cadmium sulphide due to cadmium telluride deposition at high temperature and difficulty to isolate the photo current generated by cadmium telluride were few drawbacks of this test structure. Based on the insights gained from the preliminary experiments, two sets of experiments were conducted. In the first set, cadmium sulphide cap as a barrier was deposited on the back of RF sputtered CdZnTe and co-sublimated cadmium telluride and zinc films with a band gap of 1.72 eV. The bulk composition was maintained after the cadmium chloride passivation treatment in the films deposited by both the methods. However the device performance of co-sublimated films was better than the RF sputtered CdZnTe devices. The transmission electron image obtained from the cross section of co-sublimated film fitted with cadmium sulphide cap and then treated with cadmium chloride showed presence of chlorine along the grain boundaries. The zinc chloride passivation treatments with higher substrate temperature compared to the source were the second set of experiments. The zinc loss from RF sputtered CdZnTe films after the cadmium chloride treatment did not occur. The fabricated devices exhibited diode like behavior. The images under scanning electron microscopy showed that the grain size did not increase after the zinc chloride treatment