Kotte Ramakrishna Reddy

Chemical bath deposition of zinc sulfide based buffer layers using low toxicity materials

ZnS thin films that are used as buffer layers in thin film solar cells are usually deposited by chemical bath deposition using hydrazine hydrate as a complementary complexing agent. This material is, however, highly hazardous and its replacement by a less-hazardous material is desirable. In this work thin films of zinc sulfide have been deposited using sodium citrate as a novel complementary complexing agent and experiments performed to determine the optimum conditions for producing conformal films, free from pinholes. The layers deposited were found to consist of Zn and S but with a small concentration of O present. Data relating to how the deposition conditions affected the physical properties of the deposited layers are reported.

Structural and optical properties of Cd1−xZnxTe thin films

Abstract Polycrystalline thin films of Cd 1− x Zn x Te ( x = 0.0 to 0.5) deposited by co-evaporation of CdTe and ZnTe in a vacuum better than 5 × 10 −6 Torr on Corning 7059 glass substrates maintained at 300 °C and subsequently annealed in vacuum at 400 °C for 1 h were found to be single-phase and nearly stoichiometric. The structural, electrical and optical properties of these films were studied. The lattice parameter and the optical energy gap in relation to the composition were estimated and the results are discussed in detail.

Characterization of two-source evaporated cadmium zinc telluride thin films

Abstract Thin films of Cd 1− x Zn x Te (0 ≤ x ≤ 1) have been deposited by two-source vacuum evaporation technique onto Corning 7059 glass substrates maintained at 573 K. The structural and optical properties of these films were studied. The composition dependence of the optical band gap for the films was found to be curvi-linear. The wavelength dependence of the refractive index and extinction coefficientof the films was studied. Reflectance spectra of the films exhibited the prominent E 1 . E 1 + Δ 1 and E 2 peak energies of the interband transitions. These peak energies were found to vary linearly with composition of the films.

Investigations on n-CdO/p-CdTe thin film heterojunctions

Abstract n-CdO films were deposited by activated reactive evaporation on Corning 7059 glass substrates maintained at 473 K. p-CdTe films were electron beam evaporated onto n-CdO films at 548 K. A typical Corning 7059 glass/n-CdO/p-CdTe/Cu—Au heterojunction was fabricated and its current-voltage, capacitance-voltage characteristics and spectral response were studied. An electrical conversion efficiency of about 7.7%, with an open-circuit voltage of 695 mV and short-circuit current density of 17.3 mA cm−2, was observed for the cell of an active area of 1 cm2 under a solar input of 85 mW cm−2.

Preparation and properties of CdO/CdTe thin film solar cells

Abstract Some n-CdO films were deposited by activated reactive evaporation on Corning 7059 glass substrates maintained at different substrate temperatures in the range 30–300°C. The films formed at a temperature of 200°C showed a low resistivity and high carrier concentration. A typical Corning 7059 glass/n-CdO/p-CdTe/CuAu heterojunction was fabricated and its current—voltage characteristics were studied. A maximum conversion efficiency of about 7.7% was observed for a cell with an active area of 1 cm2.

Characterization of inorganic materials with photoacoustic spectrophotometry

Abstract A photoacoustic spectrometer is described which has been constructed to measure the spectra of photovoltaic or other solid materials in the ultraviolet (UV), visible and near infrared regions. The range of this photoacoustic spectrometer is from 0.3 to 1.8 μm. The spectrometer was used to characterize the optical properties of polycrystalline and single phase p-CuInSe2 thin films which were grown by selenization of sputtered Cu/In multilayer at a temperature of 500°C. The films showed a three-fold optical structure with three high energy transitions, which is the characteristic behavior of the material. The photoacoustic spectrum is reported and discussed.

Properties of Tin Monosulphide Films Grown by Chemical Bath Deposition

Tin monosulphide (SnS) films have been successfully grown by a simple and low-cost wet chemical process, chemical bath deposition (CBD), using tin chloride and thioacetamide as precursors and tartaric acid as complexing agent. The layers were grown on glass substrate at different bath temperatures () that varied in the range 50–70°C. The energy dispersive X-ray analysis (EDAX) studies showed that all the grown films were nearly stoichiometric. The X-ray diffraction analysis indicated that the films had an intense peak at 31.6° that corresponds to the (111) plane of SnS and exhibited orthorhombic crystal structure. The intensity of (111) plane increases with the increase in bath temperature and became sharp at °C, where the other crystal planes got suppressed, this indicates better crystallinity of the layers grown at this temperature. No other secondary phases of tin sulphide were observed. The structural parameters such as lattice constants and crystallite size were also calculated. The optical studies revealed that the layers had high optical absorption coefficient (>104 cm−1). The energy band gap was found to be allowed and direct and varied between 1.30 eV and 1.35 eV. The band gap decreased with the rise in bath temperature. The refractive index and the extinction coefficient were also evaluated. The details of these results will be presented and discussed.

Crystalline behaviour of SnS layers produced by sulfurization of Sn films using H2S

Tin sulfide (SnS) is receiving increasing interest for its potential application as an absorber layer in thin film solar cells. In this work, a novel method for the formation of SnS layers on soda-lime glass substrates was investigated. The layers were formed by first sputtering tin onto glass followed by annealing in a 5% H2S and Ar gas environment over the temperature range of 300-450°C for 2 hours. The structural properties of the layers synthesized, including the crystal structure, phases present, crystallite size, strain and dislocation density are reported.

Surface characterization of sprayed CuGaxIn1-xSe2 layers

Small-area polycrystalline CuGaxIn1−xSe2-based solar cells produced in the laboratory have been reported with efficiencies up to 18.8%. This success at achieving high-efficiency devices has stimulated a parallel research effort to produce large-area modules based on the use of CuGaxIn1−xSe2 with reduced cost of production. One method that has potential to achieve these objectives is chemical spray pyrolysis. Preliminary studies have resulted in devices with efficiencies of 4–5%. Further improvements toward implementing higher-efficiency devices are expected to result from a better understanding of the physical and chemical properties of the chalcopyrite layers produced. In this work we have investigated, for the first time, the properties of these layers using X-ray photoelectron spectroscopy (XPS) to identify the secondary phases and residual impurities present. The layers studied were for In/Ga mole ratios of x=0, 0.2, 0.4, 0.6, 0.8, and 1.0 in CuGaxIn1−xSe2. The layers were also profiled by sputter etching away the surface of the layers and repeating the XPS measurements. The effects of annealing the layers in air, vacuum, and selenium vapor were also investigated.

Photoluminescence study of chemical bath deposited ZnIn2Se4 thin films

Thin films of ZnIn2Se4 (ZIS) have been prepared by chemical bath deposition (CBD) using a novel and water soluble Na2SeO3 as a source of selenium ion for the first time. The deposition of the films was carried out at various pH values in the range of 6–11, keeping all other bath parameters at optimised values. EDAX analysis revealed that all the grown layers were selenium deficient with trace amounts of oxygen and chlorine incorporation. FTIR spectral measurements revealed the associated symmetric and asymmetric stretching modes of bridged oxygen atoms on the surface of the films. The room temperature photoluminescence properties of these samples at different pH values are studied in detail. Further, Gaussian curve fitting was employed to deconvolute the PL spectra and the change in intensities of these peaks with respective to pH values was addressed and correlated to the role of native defects that were incorporated while deposition. The results indicate that the variation of solution pH had a noticeable effect on the photoluminescence ZIS films.