In a solar cell: stainless steel/SnS/CdS/ZnO/ZnO:Al, we report conversion efficiency of 1.28%, open-circuit voltage (V oc ) of 0.470 V, and short-circuit current density (J sc ) of 6.2 mA cm À2 , measured on cells of area 1 cm 2 under standard conditions. The thin film of SnS absorber of 550 nm in thickness used in this cell was deposited from a chemical bath. Average crystalline diameter of the material is 24 nm, and its X-ray diffraction pattern fits a cubic unit cell with cube edge of 1.159 nm. The optical band gap of the material is 1.74 eV and its electrical conductivity is 10À6 Ω À1 cm À1 .The mobility-lifetime product of the film was determined as 2 Â 10 À7 cm 2 V À1 from photoconductivity measurement.To build the solar cell, a CdS thin film of 50 nm in thickness was deposited from a chemical bath on the SnS thin film prepared on the stainless steel substrate. Subsequently, a ZnO film of 180 nm and ZnO:Al film of 450 nm in thickness were deposited on this CdS defining a solar cell area of 1 cm 2 . This solar cell is stable under concentrated sunlight of 2-16 suns, attaining V oc of 0.6 V and J sc of 35 mA cm À2 under 16 suns.

Metal sulphide thin film (MSTF) photography based on photo-accelerated chemical deposition (PACD) of PbS thin films is described. Here an intensity distribution over the surface of a growing PbS thin film produces a thickness variation (0.06-0.15 pm) of the film which, when viewed under daylight, yields a specularly reflective image. Under 800 W m-' of solar radiation a bluish MSTF photographic image (0.1 5 pm film thickness) on a coppery-bronze background (0.08 am) is obtained in the PACD of PbS at the end of 25 min deposition when a high-contrast photographic negative is used as the object. The best contrast of 0.46 in the PbS MSTF photography in the reflection mode is obtained under the above condition of exposure when the optical transmission in a photographic negative in the image area is -30% and that in the background is -1%. The contrast available in the transmission mode in the MSTF

Kapton polyimide is known for its high thermal stability, >400 • C. Copper sulfide thin films of 75 and 100 nm thickness were coated on DuPont Kapton HN polyimide foils of 25 µm thickness by floating them on a chemical bath containing copper complexes and thiourea. The coated foils were annealed at 150-400 • C in nitrogen, converting the coating from CuS to Cu 1.8 S. The sheet resistance of the annealed coatings (100 nm) is 10-50 / and electrical conductivity, 2-10×10 3 −1 cm −1 , which remain nearly constant even after the foils are immersed in 0.1-1 M HCl for 30-120 min. The coated polyimide has a transmittance (25-35%) peak located in the wavelength region 550-600 nm, with transmittance dropping to near zero below 450 nm and below 10% in the near-infrared spectral region. These characteristics are relevant in solar radiation control applications. The coated foils might also be used as conductive substrates for electrolytic deposition of metals and semiconductors and for optoelectronic device structures.

A technique to improve thin-film yield in chemical bath deposition of semiconductor thin films is presented. This involves the use of very small substrate separation, 0.1 mm, to eliminate the passive layer of the bath, which contributes solely to precipitation. At small substrate separation, a thin layer of the bath mixture is held by surface tension between pairs of substrates. The thin-film yield, which is the percentage of the metal ions in the bath utilized towards the film formation, obtained in this experimental set-up is considered to be near 100% for CuS, Cu 2−x Se, CdS and CdSe thin films. The final thickness estimated for the films is about 40-50 nm. The optical and electrical properties of these films are presented to illustrate that at such film thickness they fulfil the requirements for certain applications.

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.