K.T. Ramakrishna Reddy

Growth of high-quality CuInSe2 films by selenising sputtered Cu–In bilayers using a closed graphite box

High-quality CuInSe2 thin films have been prepared using a two-stage process. Multiple bilayers of Cu and In were deposited onto Mo-coated glass substrates by magnetron sputtering to produce a predominant Cu11In9 phase, and the layers were selenised using elemental selenium in a closed graphite box at different temperatures in the range of 250–550°C. The films were characterised to evaluate their physical behaviour by X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray analysis. Polycrystalline and single-phase CuInSe2 films with a strong (112) preferred orientation were obtained at an annealing temperature of about 500°C. These layers were uniform, and the crystals were densely packed with a grain size of about 2.0 μm. At lower annealing temperatures, the presence of different binary phases of Cu–In, Cu–Se and In–Se were observed in the films. It was found that these binary phases react with each other, resulting in the formation of the ternary CuInSe2 phase.

Determination of the minority carrier diffusion length of SnS using electro-optical measurements

The minority carrier diffusion length of the “absorber layer” in a solar cell is generally accepted to be one of the most important parameters that govern the performance of a solar cell device. In this work, thin films of SnS have been thermally evaporated onto cadmium sulphide/indium tin oxide/glass substrates, to fabricate heterojunction solar cell devices. The minority carrier diffusion length was determined for the first time for SnS layers using spectral response measurements in conjunction with optical absorption coefficient versus wavelength measurements. The minority carrier diffusion length was determined to be in the range 0.18–0.23 µm for the SnS/CdS devices investigated in this work.

Structural, optical, and electro-optical properties of thermally evaporated tin sulphide layers

Thin films of tin sulphide (SnS) were deposited onto glass substrates using the thermal evaporation method. The substrate temperature, Ts was varied in the range, 280–360 °C, keeping other growth parameters constant and the effects on the chemical and physical properties of the layers deposited were investigated. The layers were observed to consist of densely packed grains, up to 9.5 µm in diameter, and X-ray diffraction studies showed the layers had a strong preferred (040) orientation. The energy band gap, determined from optical studies was found to be in the range 1.30–1.34 eV, while the optical absorption coefficient was found to be >104 cm-1 for photons with energies greater than the energy bandgap. The electrical resistivity of the films decreased with an increase of substrate temperature. Heterojunction devices were made using chemical bath deposited cadmium sulphide (CdS) as the window layer and the minority carrier diffusion length in the SnS measured to be 0.23 µm using spectral response studies.

R.f. sputtering of high-quality Cu/In precursor layers and conversion to CuInS2 using elemental sulfidization processes

Thin films of CuInS2 have been produced by a two-stage process, the formation of a Cu/In precursor layer using r.f. magnetron sputtering of alternate layers of the elements followed by the conversion into the compound by either (i) annealing the precursor layers in a closed chamber in the presence of sulfur or (ii) annealing the precursor layers in sulfur that was transported over the layers using argon as a carrier gas. These out-of-line-of-sight methods have potential for large-scale “batch processing” of the absorber layers. The physical and chemical properties of the precursor layers and the CuInS2 formed were investigated using a range of methods including energy dispersive X-ray analysis, Rutherford backscattering spectrometry, and synchrotron X-ray diffraction. The data confirms that the uniform layers produced using the former method have potential for use in both substrate and superstrate configuration devices. For the latter method there was significant indium loss during the conversion process and this problem needs to be overcome before the layers can be used in solar-cell devices.

Detection of crystalline phases in CuInSe2 films grown by selenisation process

CuInSe 2 thin films have been prepared using a two-step process. The process involved the deposition of alternate layers of Cu and In onto molybdenum coated glass substrates using magnetron sputtering resulting in a Cu 11 In 9 precursor phase, free from inhomogeneous secondary phases, followed by annealing in an excess selenium environment at various temperatures ranging from 250 to 450 °C to synthesis the compound. This method results in densely packed, randomly oriented polycrystalline CuInSe 2 films with the chalcopyrite crystal structure with grain size larger than 2.0 μm for annealing temperatures of 500 °C. The layers annealed at 500 °C showed nearly stoichiometric composition with a direct energy band gap of 1.01 eV.

Type conversion of p-SnS to n-SnS using a SnCl 4 /CH 3 OH heat treatment

Thin films of tin sulphide were prepared at varying thicknesses with a deposition rate of 2 nm/s using the thermal evaporation method. Some films were then dipped in a solution containing tin (IV) chloride in methanol (SnCl4/CH3OH) at varying concentrations and then annealed in air at annealing temperatures ≥ 400°C and annealing times ≥ 60 min. The effects of the treatments on the structural, optical and electrical properties of the layers were investigated. The structural, optical and electrical properties of the as-deposited layers exhibited a thickness dependent behaviour. It was found that the SnCl4/CH3OH heat treatments convert the SnS layers from p-type to n-type in a controllable way. This process could be used to produce SnS homojunction solar cells or to passivate the grain boundaries in p-type SnS layers.

On the Structural and Optical Properties of SnS Films Grown by Thermal Evaporation Method

Tin sulphide (SnS) has a direct energy band gap of 1.35 eV and it consists of abundant, non-toxic elements. It is therefore of interest for use as an absorber layer material in thin film photovoltaic solar cells. In this work, SnS layers with thicknesses in the range 2-3.6μm, were thermally evaporated onto glass substrates using substrate temperatures in the range 280°C to 360°C, and the way the structural and optical properties of the layers varied with the deposition conditions investigated. X-ray diffraction spectra showed a strong (040) reflection as the most prominent peak for films formed between 320°C to 360°C. The peak intensity ratio, crystallite size and grain size were observed to increase with increasing substrate temperature whilst the strain decreased. All the layers were highly light absorbing with the optical absorption coefficient, α > 104 cm−1. The optical energy band gap was found to be in the range, 1.30-1.34eV, it not changing substantially with substrate temperature. Other optical parameters such as the refractive index and optical conductivity were also evaluated for the layers grown at different substrate temperatures.