C.S. Ferekides

13.4% efficient thin‐film CdS/CdTe solar cells

Cadmium telluride is a promising thin‐film photovoltaic material as shown by the more than 10% efficient CdS/CdTe heterojunction solar cells. In this work, thin‐film CdS/CdTe solar cells have been prepared using CdS films grown from an aqueous solution and p‐CdTe films deposited by close‐spaced sublimation (CSS). The properties of CdS films deposited from an ammonical solution of a Cd‐salt, an ammonium salt, and thiourea have been controlled by optimizing the temperature and composition of the solution. The solution‐grown CdS films have a high photoconductivity ratio, and its optical transmission is superior to that of vacuum evaporated CdS films. The properties of p‐CdTe films deposited by CSS have been optimized by controlling the temperature and composition of the source material, and the substrate temperature. The properties of CdS/CdTe heterojunctions have been studied; junction photovoltage spectroscopy is used for the qualitative comparison of junction characteristics. Solar cells of 1‐cm2 area wit...

14.6% efficient thin-film cadmium telluride heterojunction solar cells

Thin-film CdS/CdTe heterojunction solar cells have been prepared by the successive deposition of thin films of fluorine-doped SnO/sub 2/, CdS, p-CdTe, and an ohmic contact on glass substrates, followed by the deposition of an antireflection coating. The properties of CdS/CdTe heterojunctions have been studied. Under global AM1.5 conditions, a solar cell of about 1 cm/sup 2/ area has an open-circuit voltage of 805+or-5 mV, a short-circuit current density of 24.4+or-0.3 mA/cm/sup 2/, and a fill factor of 70.5+or-0.5%, corresponding to a total area conversion efficiency of 14.6+or-0.3%, verified by the National Renewable Energy Laboratory.<<ETX>>

Cadmium zinc sulfide films and heterojunctions

Cadmium sulfide (CdS) and zinc sulfide (ZnS), direct gap semiconductors with room temperature band‐gap energy of 2.42 and 3.66 eV, respectively, form a continuous series of solid solutions (Cd1−xZnxS). The band‐gap energy of Cd1−xZnxS can be tailored in the range of the binary band gaps. In this work, polycrystalline films of Cd1−xZnxS have been deposited on glass, SnO2:F/glass, and ZnO:F/glass substrates by the reaction of dimethylcadmium (DMCd), diethlyzinc (DEZn), and propyl mercaptan (PM) in a hydrogen atmosphere. The deposition rate and properties of Cd1−xZnxS films depend on the substrate temperature and the composition and flow rate of the reaction mixture. The deposition rate of Cd1−xZnxS films has been measured at 375 and 425 °C as a function of the DMCd/DEZn molar ratio in the reaction mixture. Without intentional doping, the deposited films are of high lateral resistivity, and the resistivity increases with increasing ZnS concentration. The electrical resistivity of the deposited films can be r...

Films and junctions of cadmium zinc telluride

Cadmium telluride (CdTe) and zinc telluride (ZnTe), direct gap semiconductors with room‐temperature band gap energies of 1.45 and 2.25 eV, respectively, form a continuous series of solid solutions (Cd1−xZnxTe). The band gap energy of Cd1−xZnxTe can be tailored in the 1.45–2.25 eV range. Cd1−xZnxTe with band gap energy of 1.65–1.75 eV is suitable as the upper member of a two‐cell tandem structure for the photovoltaic conversion of solar energy. In this work, polycrystalline films of Cd1−xZnxTe have been deposited on glass, CdS/SnO2:F/glass, and Cd1−xZnxS/SnO2:F/glass substrates at 400 °C by the reaction of dimethylcadmium (DMCd), diethlyzinc (DEZn), and diisopropyltellurium (DIPTe) in a hydrogen atmosphere. The composition of Cd1−xZnxTe films determined by wavelength dispersive spectroscopy and x‐ray diffraction has been correlated with the band gap energy deduced from the junction photovoltage spectroscopy and optical transmission. The structural and electrical properties of Cd0.7Zn0.3Te (band gap energy ...

High efficiency CdTe solar cells by close spaced sublimation

High efficiency CdTe/CdS solar cells have been prepared on glass substrates. The CdS films are deposited on SnO/sub 2/ coated 7059 glass substrates by the chemical bath deposition process (CBD) to a thickness of 500-1200 /spl Aring/. The CdTe films are deposited by the close-spaced sublimation (CSS) technique to a thickness of 4-8 /spl mu/m. Doped-graphite paste is used as the ohmic contact to the CdTe. The substrate temperature used for the deposition of the CdTe films appears to be an important parameter for the junction formation process. It is believed to be critical in the formation of an interface Cd/sub x/S/sub 1-x/Te mixed crystal layer. The heat treatment of the CdS films in hydrogen is also an important processing step, and it must be modified as the thickness of the CdS films is decreased in order to maintain high fill factors. High photocurrents in conjunction with high open-circuit voltages and fill factors are achieved by properly adjusting the annealing conditions of thin CdS films. The open-circuit voltage, short-circuit current, and fill factor of the best device were 843 mV, 25.1 mA/cm/sup 2/, and 74.5% respectively, corresponding to a conversion efficiency of 15.8% as measured under AM1.5 conditions at the National Renewable Energy Laboratory.<<ETX>>

Thin‐film junctions of cadmium telluride by metalorganic chemical vapor deposition

Polycrystalline films of cadmium telluride (CdTe) deposited by the metalorganic chemical vapor deposition (MOCVD) technique using the reaction of dimethylcadmium (DMCd) and di‐isopropyltellurium (DIPTe) can be p type or n type, depending on the DMCd/DIPTe molar ratio in the reaction mixture. Extrinsic CdTe films have been deposited by using group III and group V compounds as dopants during the MOCVD process. Gallium can be incorporated into CdTe films to yield a dark resistivity of about 1000 Ω cm and a carrier concentration of about 2×1017 cm−3; however, the incorporation of arsenic or antimony is considerably more difficult, and low resistivity p‐CdTe films cannot be obtained. Extrinsically doped CdTe films show significantly different photoluminescence spectra from intrinsic films of the same conducting type. Heterojunctions have been prepared by depositing p‐CdTe films on CdS‐coated SnO2:F/glass substrates. The junction properties and the post‐deposition treatments have been investigated. Large‐area (...

Electronically active layers and interfaces in polycrystalline devices: Cross-section mapping of CdS/CdTe solar cells

Electronic mapping of cross sections of a polycrystalline device, the n-CdS/p-CdTe solar cell, show that the photovoltaic and metallurgical junctions coincide to within experimental resolution (50 nm), which rules out both type conversion of CdS and buried homojunctions. Compositional analysis of the CdS supports this. Mapping was done using scanning capacitance, complemented by scanning Kelvin probe microscopy. Our results explain why a high-resistance transparent conducting oxide layer is needed as contact to the CdS for successful device operation. They define limits on inputs for modeling performance of these devices.

Reduction of recombination current in CdTe/CdS solar cells

A study of the current–voltage behavior of recombination current in CdTe/CdS solar cells has shown that the reverse saturation current, J0, and the diode quality factor, A, are correlated. Our better devices typically have low values of both A and J0. Spectral response data indicate that devices with a gradual decline in quantum efficiency beginning at about 600 nm show better performance than devices with a sharp drop at the CdS band gap of 510 nm, which is interpreted as an indication that mixing between CdS and CdTe during processing reduces the amount of recombination current at the interface.

The effect of the CdCl2 treatment on CdTe/CdS thin film solar cells studied using deep level transient spectroscopy

Abstract Thin film CdTe/CdS solar cells have been studied using deep level transient spectroscopy. The correlation-deep level transient spectroscopy (DLTS) technique was utilized as conventional analysis methods such as the boxcar-based approach were found to be inadequate under certain experimental conditions. The primary objective was to study the effect of a key processing step in the fabrication of thin film CdTe solar cells, namely the post-deposition heat treatment in the presence of CdCl2. The substrate temperature as well as the ambient used during this process were varied around predetermined conditions for optimum solar cell performance, in order to identify performance-limiting defects, and in general improve our understanding of thin film CdTe solar cells. Solar cells without the CdCl2 heat treatment were also fabricated. A series of electron and hole traps were found in the various devices studied, with electron traps being present primarily in solar cells with limited performance.

Zinc selenide films and heterojunctions

Polycrystalline films of zinc selenide (ZnSe) have been deposited on glass and ZnO:F/glass substrates at 400–500 °C by the reaction of diethylzinc (DEZn) and diethylselenium (DESe) in a hydrogen atmosphere. The DESe/DEZn molar ratio in the reaction mixture is an important factor affecting the deposition rate and dopant incorporation in deposited films. The deposited films have high lateral electrical resistivity and poor photoconductivity. The resistivity can be reduced and photoconductivity significantly improved by the incorporation of a group VI (Cl or Br) or a group III (Al) dopant, and the use of trimethylaluminum (TMAl) as a dopant is considerably more effective than the use of Cl or Br compounds. The structural, optical, and electrical properties of ZnSe films have been characterized. The use of ZnSe films as a heterojunction partner in II‐VI thin‐film solar cells has been explored. Zinc telluride and cadmium telluride films were deposited on ZnSe/ZnO:F/glass substrates, and the characteristics of ...