Improvements in CdTe- and CIGS-based thin-film solar cells and investigation on new materials for photovoltaic applications.

Currently, thin-film solar cells are one of the most promising technologies for low-cost renewable energy production. CdTe- and CuInGaSe2-based cells, which achieved record efficiencies of 22.1% and 22.6% respectively, are the most attractive among thin-film solar cells. These high efficiencies have had a huge influence in making them highly competitive in the photovoltaic market, with an estimated final cost per module lower than US 0.50 per peak-watt. At the Thin Film Laboratory of the University of Parma, a fully scalable process to produce high-efficiency CdTe- and CIGS-based solar cells has been developed. The only deposition techniques the production process requires are: close-spaced sublimation and sputtering for CdTe and sputtering and selenisation for CIGS. Our choice of sputtering as a core deposition technique is justified by its great flexibility: it can be used to deposit a wide variety of materials, such as insulators, semiconductors, conductors, alloys and composites. At the same time, we have chosen close-spaced sublimation for making CdTe films because it is one of the simplest approaches to physical vapour deposition and it offers high deposition rates. In the wider context of implementing these technologies, my PhD research project has privileged three main aspects: 1. Implementation of high-performance CdTe-based devices fabricated by using close-spaced sublimation and sputtering in an attempt to improve both grain boundary passivation and back-contact ohmicity. 2. Implementation of high-efficiency (> 16%) CIGS-based devices with the aim of exporting the fabrication process developed for soda-lime glass and ceramic substrates to flexible substrates, such as thin metal foils. 3. Investigation of sputter-deposited copper-based delafossites as novel transparent conducting oxides for photovoltaic and electronic applications. At the end of my PhD project, I have been able to achieve several of my initial goals, which will be described in detail in my PhD thesis. In regard to CdTe-based solar cells, I will illustrate the development of an innovative process, which reduces the thickness of the CdTe absorber layer and incorporates a barrier-free back contact, suitable for the production of high-efficiency solar cells. With respect to CIGS-based solar cells, I will report on the identification of the main process requirements that are needed for preparing high-efficiency CIGS devices, making use of scalable techniques such as sputtering and selenisation. These results have been used to extend this process, which is an established one for soda-lime glass and ceramic substrates, to flexible thin metal foil substrates. I will also address the new technological challenges introduced by this type of substrate. These include the problem of film adhesion and the necessity of a barrier layer between the metallic substrates and the active layer to prevent the diffusion of impurities and to provide electrical insulation between the metal substrate and the monolithically interconnected solar cells. Finally, I will show the successful sputtering-based preparation of CuGaO2 and CuBO2 delafossite thin films suitable to use as transparent back contacts in bifacial CIGS- and CdTe-based solar cells.