CZTS-based thin film solar cells

Growing public awareness on global warming and climate change as well as rising oil prices led to a tremendous increase of interest into renewable energy sources. Solar energy is a sustainable and `inexhaustible` solution. The earth receives in one hour enough solar energy to meet the world`s energy demands for one year But even though the photovoltaics (PV) industry showed an explosive growth over the last decade, PV module production levels of about a hundred times the actual value (~ 3 gigawatt peak (GWp) per year) are required to measurably contribute to global electricity production. To achieve this goal, the production costs of PV modules have to be reduced to well below 1 /Wp by the majority of all present and possible future technologies, but this appears hard to achieve with the current PV technology, which is still based on various forms of crystalline silicon offering only low cost- reduction potential. In this context, thin film PV has a great potential to reduce the module costs and obtain the targeted values, due to among other things a greatly reduced semiconductor material consumption, or the possible implementation of cheap large-area substrates. The solar cell absorber material should offer a band gap between 1.1 and 1.7 eV, as well as a direct band structure, the consistency of readily available, non-toxic materials, the possibility of an implementable cost-effective large- area deposition, a high photovoltaic conversion efficiency, and long-term stability. The copper indium di-selenide (CIS)-based material system constitutes the basis of one of today`s most promising thin film solar cell technologies, exhibiting confirmed laboratory efficiencies of up to 20%. Moreover, CIS-based PV modules are fabricated in the substrate configuration, offering the possibility to use flexible cost-effective substrates. Commercial production of CIS-based thin film modules mainly arose during the last few years, but with the shift to mass production the supply of the rare metal indium will be problematic and costly in the near future. Additionally, the materials selenium and cadmium (which is present in the buffer (or emitter) layer cadmium-sulphide in most present technologies) are toxic and therefore not an ideal solution. Cu2ZnSnS4 (CZTS) is a relatively new material investigated as an absorber layer in thin film solar cells that constitutes the abundant and non-toxic materials sulphur and zincin instead of selenium and indium, respectively. Here, the deposition technology and the related control of the composition are of highest importance and still under emerging investigation. Additionally, recent publications frequently point out the lack of studies addressing the basic properties of CZTS thin films. These facts strongly motivate the proposal of this project and clearly show that the scientific community is awaiting reports on this topic eagerly. Hence, scientific results on this topic may have a huge impact on the future development of CZTS based thin film solar cells, as well as on thin film solar cells in common. The main objective of this project is to fabricate CZTS-based thin film solar cells by reactive co-sputtering and sulphurisation, and to characterize the related structural, optical, and electrical properties. Since these properties are strongly dependent on various deposition parameters, as well as on the films itself, the development of a well- controlled and reproducible process scheme on glass substrates will be a first milestone. To achieve this goal, the applicant will implement and improve deposition processes addressing bottom electrodes, buffer layers, transparent window layers, and contact grids, which in turn shall steadily increase the obtained cell efficiencies. As a final milestone - and a novel ambition - substrate foil-based CZTS thin film solar cells are to be fabricated and evaluated regarding their structural, optical, and electrical properties. This project is to be carried out at Prof. Ferekides` group at the University of South Florida. Beside the knowledge and the experience of Prof. Ferekides` group, which attracts worldwide attention in the field of thin film solar cells, the applicant will benefit from a Department of Energy (DOE) program addressing next generation substrate foil-based solar cells. By means of a comprehensive project plan it is aimed to strongly and sustainably contribute to the scientific development of highly-efficient, cost-effective, and long-term stable thin film solar cells from inexpensive, abundant and non-toxic materials.