Fabrication and characterization of CuInSe₂/CdS/ZnO thin film solar cells

M.Sc.I-III-VI2 compound semiconductors are important photovoltaic (PV) materials with optical and electrical properties that can be tuned for optimum device performance. Recent studies indicated that the efficiencies (1) > 18%) of CuInSe2/CdS/ZnO thin film devices are in good agreement with that of standard silicon cells. In this study, CuInSe 2 absorber films with excellent material properties were produced by relatively simple and reproducible two-stage growth techniques. In these approaches, metallic precursors (Cu/InSe, InSe/Cu, Cu/InSe/Cu and InSe/Cu/InSe) were deposited by thermal evaporation from specially designed graphite heaters at temperatures around 200°C. In the second stage of the process, the alloys were exposed to elemental Se vapour or H2Se/Ar gas. A systematic study was conducted in order to determine optimum growth parameters for the different deposition processes. Optimum material properties (homogeneous and dense films with a high degree of compositional uniformity) were obtained when InSe/Cu/InSe precursors were selenized in elemental Se vapour or H2Se/Ar gas. Comparative studies also indicated that the reaction kinetics is enhanced when H2Se/Ar is used as chalcogen source. Fully selenized films were obtained at temperatures as low as 450°C in a H2Se/Ar atmosphere, compared to temperatures of 600°C in the case of Se vapour. The optical and electrical properties of the absorber layers were accurately controlled by small variations in the bulk composition of the films. A standard CdS/ZnO window layer technology was also developed in our laboratories and preliminary solar cell devices were fabricated and evaluated

Optimization of quaternary and pentenary chalcopyrite for applications in thin film solar cells

Ph.D.One of the solutions to the high cost of solar modules is the development of thin film solar cell technologies, which enable material saving, few processing steps, good stability in outdoor testing, high conversion efficiency and flexibility for large area coatings. Polycrystalline CuInSe2 (CIS) thin films and related quaternary and pentenary compounds such as Cu(In,Ga)Se2 (CIGS) and Cu(In,Ga)(Se,S)2 (CIGSS) are the most promising thin film candidates to fulfil the requirements of economically viable solar modules. Presently CIS, CIGS and CIGSS thin film solar cells are prepared mostly by two – stage deposition processes, where Cu-In-Ga alloys are deposited, followed by selenization and/or sulfurization using H2Se/Ar and/or H2S/Ar gases, Se and/or S vapours. Key problems related to this approach are (1) the widely reported compositional change and loss of material during the annealing and selenization stages, and (2) the formation of a graded film structure with most of the Ga residing at the back of the film, due to the difference in the reaction rates between the binary selenides. The present study aims to develop CIGS quaternary and CIGSS pentenary thin film absorbers which are substantially homogeneous and single phase. In order to achieve this aim different deposition processes were developed. This included thermal evaporation of pulverized compound materials from a single crucible with and without subsequent reaction of the precursors in Se vapour or H2Se/Ar atmosphere. Alternatively, controlled partial selenization/sulfurization of the Cu-In-Ga magnetron sputtered precursor films under controlled conditions of reaction time, temperature and gas phase concentration were applied to produce CIGSS films. The latter approach allowed homogeneous incorporation of Ga and S species into CIS compound material, and with that a corresponding increase of band gap of the material in the active region of the solar cell. CIGS quaternary and CIGSS pentenary based solar cells were completed by depositing a CdS buffer layer of around 50 nm thickness, high resistivity ZnO and low resistivity Al – doped ZnO with thicknesses of about 50 nm and 0.5 μm respectively. I-V measurements on fabricated solar cells, under standard A.M. 1.5 conditions, demonstrated good solar cell device quality with efficiencies of about 10 % and 15% respectively

A solar energy resources assessment in Mozambique

Just as with other Southern African Development Community (SADC) countries, Mozambique faces severe, interrelated problems of energy and environment linked, with massive consumption of fuel wood biomass. The conventional power grid provides less than 7% of the energy needs for the country's 17 million inhabitants, and about 83% of the energy consumed in the country comes from biomass. Renewable energy resources can play an important role in the process of development of the country. From the vast renewable energy resources available in the country, solar energy represents one of those with the highest potential. Thus, the evaluation of the potential of solar energy systems in small-scale applications suitable for villages is a strategically good starting point for promotion of sustainable rural development. One of the major impediments In carrying out such studies is the fact that the exact behaviour of solar energy resources throughout the country has not been well studied. In this paper a general characterisation of the global, diffuse and direct solar radiation fields in Mozambique Is presented. The study Is based on experimental data measured by the National Institute of Meteorology (INAM) In the period 1970-2000. For these analyses global, diffuse and direct solar radiation data from three stations along the coast line and three stations in the interior of the country have been used. The six stations were representative of the three main regions of the country, namely south, centre and north. Furthermore, sunshine hours data of one selected station was analysed

General characterisation of the solar radiation components in Mozambique

Just as with the other Southern African Development Community (SADC) countries, Mozambique faces severe and interrelated problems of energy and environment linked with the massive consumption of fuel wood biomass. The conventional power grid provides less than 7% of the energy needs for the country's 17 million inhabitants, and about 83% of the energy consumed in the country comes from biomass. Areas around the major urban centres and along the main development corridors are the most affected by energy shortages. This hinders the country's economic and social development as it is generally acknowledged that no development can be sustainable without linking it to energy planning and environmental management. Renewable energy resources can play an important role in the process of development of the country. From the vast renewable energy resources available in the country, solar energy represents one of those with the highest potential. Thus the evaluation of the potential of solar energy systems in small-scale applications suitable for villages is a strategically good starting point for promotion of a sustainable rural development. One of the major impediments in carrying out such studies is the fact that the exact behaviour of solar energy resources throughout the country has not been well studied yet. In this paper a general characterisation of the global, diffuse and direct solar radiation fields in Mozambique is presented. The study is based on experimental data measured by the National Institute of Meteorology (INAM) in the period 1970-2000. Data from 11 stations recording global solar radiation, from 6 stations measuring diffuse solar radiation, as well as data from 80 sunshine hours stations have been used for this work. For this purpose the country has been divided into four main climate zones. The northern and coastal regions, representing 60% of the total surface of the country, have a tropical rain savanna climate. The inland parts of the central and southern edimentary terrains, with a share of 28%, have a dry savanna climate. A small area around the border crossing of the Limpopo River, representing 2% of the country's surface, has a dry desert climate, whereas the upland areas, with a share of 10%, have a humid temperate climate. Results of the present work reveal that the country has substantial solar energy resources for a variety of solar energy technologies. Areas with the dry savanna and dry desert climates, representing in total around 30% of the country's surface are particularly appropriate for dense placement of solar energy technologies