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Chalcopyrite Quantum dots for Intermediate band Solar cells

Final Report Summary - CHALQD (Chalcopyrite Quantum dots for Intermediate band Solar cells)

Solar cells based on today’s technology based on a single pn-junction are limited to power conversion efficiencies around 30%. To achieve mass deployment of photovoltaics, there is the need to lower the production costs per power generated. One way of doing so is to produce solar cells with higher values of power conversion efficiency. Intermediate band solar cells are, in theory, limited to efficiencies as high as 60% and they can be prepared by incorporating quantum dots in a matrix material. This proposal gave an important step in the preparation of chalcopyrite quantum dots using molecular beam epitaxy and in identifying a suitable matrix material. Chalcopyrites were chosen because they are known to have good optoelectronic and material properties as demonstrated by their performance when used in thin film solar cells. Solar cells based in chalcopyrites have the highest performance of all thin film solar cells.This project started by preparing a molecular beam epitaxy tool to evaporate Cu, In and Se and by setting-up preparation methods for cleaning substrates. The approach that was followed to grow the quantum dots/nano crystals was to evaporate at the same time Cu and In in an excess of Se. These are the thermodynamic conditions where the desired tetragonal CuInSe2 phase appears. The growth of the nanostructures was tested in different crystalline and non-crystalline substrates: clean and passivated Si(111) and Si(100) as well as wafers still with the native oxide and a common solar cell substrate that comprises of soda-lime glass coated with molybdenum. It was observed that for the Si wafers with the native oxide, CuInSe2 grown with very slow rates produce the desired nano-structures, as shown in figure a), b) and c). For the clean substrates, we observed that a higher temperature would be required to achieve a complete dot nucleation and that such high temperatures dramatically change the thermodynamic growth conditions of the semiconductor. Complete non-crystalline substrates were also tested and nano-crystals that showed evidences of Cu, In and Se, were prepared as depicted in figures d), e) and f).
To evaluate if the nano-structures could be implemented in a matrix material, we started by doing a literature review and a theoretical comparison of several materials that could be grown in the molecular beam evaporation chamber. Gallium Selenide was the chosen material due to its optical band gap energy, coefficient of thermal expansion, crystalline properties and electrical properties. A gallium evaporation source was installed in the molecular beam epitaxy tool and preliminary tests were started.
Dr. Salomé suggested the use of a high-bandgap dielectric material that should be used in these type of solar cell as passivation layers. This approach was one of the predicted risk assignment tasks, and to be tested, electron-beam lithography was used to pattern a substrate. In a collaboration with the University of Uppsala, conventional solar cells were performed and the results show that the combination of this nano-patterning technique with the dielectric material is a suitable one since it creates a passivation effect. Such effect is required in intermediate band solar cells and so an important first step was given for the definition of the architecture of the solar cell stack. A patent on this topic was filed. In collaboration with the University of Aveiro, Pedro studied some opto-electronic properties of the chalcogenide materials by several techniques, since this step is needed for a correct understanding of the electrical behaviour of these materials at the nano-scale.
Pedro Salomé has also been involved in the co-supervision of a Ph.D. student, in the organization of several workshops and meetings, in the proposal writing of several European projects, in contacting industry to establish collaborations, and participated in a 5-day training course by the Porto Business School to strengthen his soft skills.
Doctor Salomé presented his work in a student´s seminar at the University of Coimbra, acting so as a Marie Curie ambassador. Pedro also received several student visits to INL where he presented the institute, the laboratories and his own work. In two occasions, Pedro did a similar introduction to high-school teachers.
Figure/Plate captions: a) Scanning electron image of CuInSe2 nanostructures grown on Si b) an atomic force microscopy of the same nanostructures grown on Si and c) a plot showing the dimensions of the CuInSe2 nano-crystal depicted in c). d) is a Scanning electron image of CuInSe2 nanostructures grown on a non crystalline substrate (i.e. Mo) e) is the atomic force microscopy image of the same nanostructures and f) an energy-dispersive X-ray spectroscopy plot showing the presence of Cu, In and Se.