C. Q. Wu

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...

Solution‐Grown Cadmium Sulfide Films for Photovoltaic Devices

This paper reports on thin films of cadmium sulfide (CdS) which have major applications in optoelectronic devices. Several techniques have been developed for the deposition of CdS films. Among these, growth of CdS films from an aqueous solution is the low-cost technique suitable for many applications. In this work, the deposition of device quality CdS films on glass and SnO[sub 2]:F/glass substrates from an aqueous solution containing cadmium acetate, ammonia, ammonium acetate, and thiourea has been investigated. The structural and electrical properties of CdS films have been characterized. The doping of CdS films with boron and the properties of boron-doped CdS films have also been studied. High efficiency thin film CdS/CdTe solar cells have been prepared from solution-grown CdS films.

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...

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 (...

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 ...

Cadmium telluride films by metalorganic chemical vapor depositiona)

Polycrystalline films of cadmium telluride (CdTe) have been deposited on glass and SnO2:F/glass substrates at 350–400 °C by the reaction of dimethylcadmium (DMCd) and diisopropyltellurium (DIPTe) in a hydrogen atmosphere. The DMCd/DIPTe molar ratio in the reaction mixture is an important factor affecting the deposition rate, conductivity type, and photoluminescence of deposited films. Photoluminescence measurements indicate that CdTe films deposited at 400 °C are of better quality than films deposited at 350 °C. The films deposited at 400 °C are p‐type at DMCd/DIPTe molar ratios of 0.51 or less and are n‐type at higher ratios. The structural, optical, and electrical properties of CdTe films have been characterized.

Thin films of II–VI compounds and alloys

II–VI semiconductors and their alloys are promising thin film photovoltaic materials. Polycrystalline films of cadmium telluride (CdTe), zinc telluride (ZnTe), cadmium zinc telluride (CdxZn1−xTe), and mercury zinc telluride (HgxZn1−xTe) were deposited onto glass and transparent-conducting-semiconductor (TCS) coated glass substrates by metal-organic chemical vapor deposition. Emphasis was directed to the doping of CdTe films, ohmic contacts to p-CdTe, and thin film CdTe homojunctions. CdTe films may be doped intrinsically or extrinsically; gallium and arsenic were used as the extrinsic n and p dopant respectively. p+-ZnTe films deposited in situ were used as an ohmic contact to p-CdTe films. Thin film CdTe homojunctions were prepared by the successive in situ deposition of n-CdTe, p-CdTe, and p+-ZnTe films on SnO2-coated glass substrates, and their properties were investigated. The properties of CdxZn1−xTe and HgxZn1−xTe films with band gap energy in the range 1.65–1.75 eV deposited onto glass and TCS-coated glass substrates were studied.

High efficiency thin film cadmium telluride solar cells

Cadmium sulfide (CdS), grown from an aqueous solution, and zinc oxide (ZnO), cadmium zinc sulfide (Cd1−xZnxS), and zinc selenide (ZnSe), deposited by metalorganic chemical vapor deposition (MOCVD), have been used as the window for thin film cadmium telluride (CdTe) solar cells. Thin film solar cells were prepared by the successive deposition of the window and p‐CdTe (by MOCVD and close‐spaced sublimation, CSS) on SnO2:F/glass substrates. CdS/CdTe(CSS) solar cells show considerably better characteristics than CdS/CdTe(MOCVD) solar cells because of the better microstructure of CSS CdTe films. Total area conversion efficiency of 14.6%, verified by the National Renewable Energy Laboratory, has been achieved for solar cells of about 1 cm2 area. Solar cell prepared by using ZnO, ZnSe, or Cd1−xZnxS as window have significantly lower photovoltage than CdS/CdTe solar cells.

Zinc telluride films by photoenhanced metalorganic chemical vapor deposition

Polycrystalline films of zinc telluride (ZnTe) have been deposited on glass and conducting semiconductor coated glass substrates at 270°-350° C by photoenhanced metalorganic chemical vapor deposition (PECVD) using the reaction of dimethylzinc (DMZn) or diethylzinc (DEZn) and diisopropyltellurium (DIPTe) in hydrogen under atmospheric pressure. The deposited films are always ofp-type conductivity. Their properties are affected by the DMZn/DIPTe or DEZn/DIPTe molar ratio in the reaction mixture. The optimum DMZn/DIPTe ratio has been found to be approximately 0.9 on the basis of the open-circuit voltage of ZnTe/CdS heterojunctions and photoconductivity measurements. Without intentional doping, the deposited films are of high resistivity (>107 ohm-cm) at room temperature, and the resistivity of these films has been controlled by using arsine as a dopant. The structural, optical, and electrical properties of ZnTe films have been characterized.