This letter describes the fabrication and characteristics of high-efficiency thin-film CdS/CdTe heterojunction solar cells. CdS films have been prepared by chemical bath deposition and p-CdTe films have been deposited by close-spaced sublimation. A CdS/CdTe solar cell of greater than 1 cm2 area with an AM1.5 efficiency of 15.8% is reported.
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 with an AM 1.5 efficiency of 13.4% are reported.
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 reduced by using octyl chloride or trimethylaluminum as a dopant. The effects of DMCd/DEZn and (DMCd+DEZn)/PM molar ratios on the optical and electrical properties of Cd1−xZnxTe films have been investigated. Thin film heterojunctions have been prepared by the successive in situ metal organic chemical vapor deposition of Cd0.7Zn0.3S (Eg∼2.8 eV), an absorber, and the ohmic contact on a ZnO:F/glass substrate, and their electrical and photovoltaic properties characterized.
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