Most of the present global energy production is accomplished by burning fossil fuels. However, the inherent problems associated with the use of fossil fuels such as their limited availability and the environmental issues force the mankind to look for new, more sustainable long-term energy solutions to provide the future energy supply.
One of the most powerful alternatives for future large scale electricity production is photovoltaics, i.e., the conversion of sunlight directly into electricity. Sunlight is available in most locations, and it provides such an enormous supply of renewable energy that if the whole global electricity demand would be covered exclusively by photovoltaics, the total land area needed for light collection would be only a few percent of the world’s desert area. [1, 2] Solar cells are easy to install and use, and their operational lifetimes are long, which eliminates the need for continuous maintenance. Since photovoltaic systems are modular, they are equally well suited for both centralized and non-centralized electricity production. Therefore their potential uses range from consumer electronics (pocket calculators, wristwatches etc.) to large power plants.
Due to its reliability and stability, solar energy is a good choice in applications where power outages or shortages cannot be tolerated, for example in hospitals and certain production plants.
Photovoltaic systems can be installed on rooftops and facades of buildings, and they can be combined with solar water heating systems. The power generated by rooftop solar cells can be used locally, and the surplus can be exported to the commercial grid if there is one in the region.
[2, 3] The possibility for local electricity production offers consumers more freedom by reducing their dependence on the availability and price of commercial electricity. This is a crucial feature especially in remote areas that lack the infrastructure of electrification. It is actually more cost-effective to install a photovoltaic system than to extend the grid if the power requirement lies more than about half a kilometer away from the electrical line [4]. Rooftop photovoltaic installations, both by public institutions and by individual citizens, are becoming more and more common worldwide. [3]
One of the main obstacles for photovoltaics to become more popular in the short term is the fact that the price of the electricity (cost per watt) produced by photovoltaics is in most cases not yet competitive with that produced by the conventional methods. Cost reduction can be achieved by either improving the efficiencies or reducing the production costs of photovoltaic modules.
Among the most promising absorber materials for solar cells are CuInSe2-based chalcopyrite materials (copper indium selenide, CIS). The material properties can be varied by replacing part of the indium by gallium and/or part of the selenium by sulfur to form Cu(In,Ga)(S,Se)2. High conversion efficiencies of almost 19 % [5] have been achieved using these materials. Moreover,
CIS-based solar cells are very stable, and thus their operational lifetimes are long. The favorable optical properties of these materials (direct energy band gap and high absorption coefficient) allow the use of thin films (few micrometers) of material instead of thick slices of bulk silicon, reducing the consumption of materials. CIS-based thin films can be prepared both from gas and liquid phases by a variety of methods.
Electrodeposition is a liquid phase deposition method that can be used for the preparation of metal, semiconductor and conducting oxide thin films. Its advantages include the feasibility of upscaling to large substrate areas and production volumes. Moreover, the deposition equipment is relatively simple and the deposition temperatures are considerably lower than in many other methods. These features make electrodeposition a low-cost deposition method. Thus the fact that the solar cell efficiencies achieved with electrodeposited films are generally somewhat lower than those achieved by the more expensive gas-phase methods is not necessarily a major drawback, since it is compensated by the lower process costs.
The purpose of this study was to develop and study electrodeposition processes for the preparation of thin films for CuInSe2 solar cells. Cu2-xSe, CuInSe2 and doped ZnO films were deposited from aqueous solutions. Cu2-xSe and CIS films were deposited by the induced co-deposition method [6] where the compound formation occurs underpotentially, that is, at less negative potentials than where at least one of its component ions would reduce into its elemental state. This positive potential shift is caused by the energy released in compound formation.
Reproducible film growth is achieved since the film composition is not sensitive to small variations in growth conditions such as precursor concentrations and deposition potential but is automatically directed toward being stoichiometric. The most well-known example of the utilization of the induced co-deposition mechanism is CdTe [7]. The mechanism had not been utilized for the deposition of ternary compounds prior to [I] where suitable conditions for induced co-deposition were achieved by complexing Cu+ ions with thiocyanate ions to form strong complexes, thereby shifting the deposition potential of metallic Cu to negative direction which enables the deposition of Se first.
The formation mechanisms of Cu2-xSe and CIS thin films were studied in detail by cyclic voltammetry and electrochemical quartz crystal microbalance measurements. The properties of the CIS films and the effects of post-deposition treatments were studied by cyclic photovoltammetry, photoelectrochemical measurements and capacitance-voltage measurements.
ZnO films doped with In and Al were prepared electrochemically for the first time. Finally, solar cell structures were prepared using the electrodeposited CIS and ZnO thin films. For comparison, for some devices the ZnO films were deposited by atomic layer deposition and sputtering.
The present thesis introduces first the concept and operation principle of thin film solar cells as well as the most important thin film solar cell materials. Next, the methods used for the preparation of thin films used in CuInSe2 solar cells are reviewed, with particular attention to electrodeposition of CuInSe2-based absorber materials. Last part of the literature survey deals with cyclic voltammetry and related methods used for the characterization of thin films and growth processes in this study. After the experimental details, the main results of this study will be presented and discussed.