Nano - structural materials for thin film solar cells

The aim of the project was to examine the possibility of improving the efficiency of thin film solar cells based on Si by using nano-crystalline and amorphous phases for active and passive elements of device. The project is based on using quantum size effects resulting from varying electron and phonon density of states by the manipulation of nano-crystalline sizes and theirs volume contribution in thin films. In particularly, the optical gap for Si can be changed from 1.1 eV for crystals larger than several microns to more than 2 eV for crystals of a few nm, which can be utilized in multilayer structures for high efficient solar cells. It was important to show that this kind of nano-structures can be formed on large scale (m2) using conventional deposition techniques as it is plasma enhanced chemical vapor deposition from gas (PECVD). In the frame of this part of the project it was demonstrated that amorphous-nano-crystalline thin film with 30-40% crystalline phase could be used for multilayer multigap solar cells. It was demonstrated that this kind of layers can be produced using industrial production line for amorphous Si solar cells and standard plasma enhanced chemical vapor deposition technique. The test series of tandem solar cells produced in Solar cells, Split, show higher efficiency than single p-i-n cells and much higher stability.

Using the same concept, the transparency of the first electrode and hence the solar cell efficiency could be improved by using nano-crystalline form of the first electrode. Such possibility was tested in Ruđer Bošković Institute by formation of nano-crystalline ZnO using magnetron sputtering on specially designed laboratory deposition system built as a part of this project. The obtained increase in optical transparency was between 10 and 15% comparing with standard SnO coating.

Computer modeling of solar cell with nano-crystalline Si and nano-structured ZnO could have the efficiency up to 15% which is substantial improvement comparing with industrial value of 7% and laboratory successes somewhat above 10%.

In order to ensure the relevance of the project for industrial application, samples were produced primary in Solar Cells, Split, on industrial plant in close collaboration with Ruđer Bošković Institute. The deposition conditions were correlated with films properties which enables testing of improvements and/or new concepts on large scale.

The nano-structures were studied primary by X-Ray Diffraction (XRD) by analyzing angle dependent diffraction line broadening. The results were compared with the measurements carried out by electron microscopy and the measurements of vibrational, optical and electrical properties. The modeling of optical properties was done in the frame of effective medium approximation, on the basis of structural measurements and compared with experimental values. The analysis of results was performed in close collaboration between Rudjer Boskovic Institute and Department of Physics and Astronomy at the University of Denver which enabled the transfer of knowledge to the Institute and as the next step providing multiple benefits to the Croatian industry.

The correlation between crystal size and other properties was subject of scientific publications (15 conference contributions and 5 CC referred articles) and part of doctoral thesis of PhD students involved in the project while design and way of deposition of high efficient solar cells could be applied in photovoltaic industry assuming the protection of intellectual properties which needs some more time than project time.

Project assumes an enhancement of capacity in S&T in Croatia, in several ways. As a first, 3 PhD were engaged in the project working in the same time on PhD thesis and one of them has got the PhD degree during the project time. During the project activities, the collaboration between University of Denver and Ruđer Bošković Institute is fostered and as one of results, 1 student will continue his career as post-doctoral student in Denver. The second student will continue his education in England.

The second benefit comes from new equipment. In the frame of the project, the new system for thin film deposition by using magnetron sputtering was built and used in fulfillment of project goals. The new system enables better quality of further scientific work and opens new possibilities in domestic and international collaboration and new projects.

The collaboration between Ruđer Bošković Institute, Institute Jozef Stefan in Ljubljana, Denver University, Denver and Solar Cells factory, Split, as a final users of project goals, foster innovative approach, enable better technology and knowledge transfer and built internationalization of research.