Thin-film yield in the chemical bath deposition technique is studied as a function of separation between substrates in batch production. Based on a mathematical model, it is proposed and experimentally verified in the case of CdS thin films that the film thickness reaches an asymptotic maximum with increase in substrate separation. It is shown that at a separation less than 1 mm between substrates the yield, i.e. percentage in moles of a soluble cadmium salt deposited as a thin film of CdS, can exceed 50%. This behaviour is explained on the basis of the existence of a critical layer of solution near the substrate, within which the relevant ionic species have a higher probability of interacting with the thin-film layer than of contributing to precipitate formation. The critical layer depends on the solution composition and the temperature of the bath as well as the duration of deposition. An effective value for the critical layer thickness has been defined as half the substrate separation at which 90% of the maximum film thickness for the particular bath composition, bath temperature and duration of deposition is obtained. In the case of CdS thin films studied as an example, the critical layer is found to extend from 0.5 to 2.5 mm from the substrate surface, depending on the deposition conditions.
A thin layer of CdO is formed on a chemically deposited CdS thin film through reaction with atmospheric oxygen during heating in air for 5-240 min at 370-500 • C. The sheet resistance of the film drops from about 10 13 -3.5 k (370 • C) and 470 (500 • C). From optical transmittance and reflectance spectra, the thickness of the CdO layer was found to be about 10 nm; the crystalline grain size is 12-25 nm, depending on the temperature and duration of heating; and the estimated electrical resistivity is 10 −4 -10 −3 cm. The optical bandgap of the CdS-CdO layer is effectively that of the underlying CdS thin film, about 2.45 eV in the annealed film. The CdS, which remains under the conductive CdO top layer, is photosensitive, with a photo-to-dark current ratio of 10 3 and crystalline grain diameter of about 10 nm in the case of a film heated at 500 • C for 5 min. These results are discussed in the context of window layers in solar cells.
We report the formation of a thin layer of CdO on chemically deposited CdS thin films during air anneal at 370°C to 500°C for 5 min to 120 min. During a 5 min anneal, the sheet resistance of the CdS thin films drops from about 1013 □/9 to 3.5 k□/□(370°C) and 470 □/□(500°C). X-ray diffraction studies showed that this is associated with the formation of a thin layer of CdO layer, which occurs at temperatures above 370°C. The CdS, which remains under the conductive CdO top layer, is photosensitive - with photo-to-dark current ratio of 103 - 104. Essentially the air annealing converts the highly resistive and highly photosensitive intrinsic (i) CdS thin film into a (i)CdS-( n +)CdO layer. The technique offers prospects to convert the top part of a chemically deposited CdS thin film window layer of high photosensitivity, deposited on an absorber layer, to a conductive layer. This is of interest in thin film solar cell technology.
Studies of the properties of CBD-CdS thin films grown under variable conditions have been carried out. In particular,the influence of temperature, concentration of the reactants, application of a magnetic DC field and thermal annealing on the optical properties of the CDB-CdS has been studied. In this work, we are focusing our interest in the influence of the combination of the last two conditions, i.e., magnetic field application and annealing at 65, 75 and 85°C and a fixed c(thio)/c( Cd 2+) (ratio=1) on the band gap energy of CBD-CdS thin films for its use in solar cell as window material. The results about the influence of the substrate temperature and c(thio)/c( Cd 2+) ratios in the final thickness of the layers is also presented.
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