CuO and Cu 2 O Nanoparticles for Thin Film Photovoltaics

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CuO and Cu2O Nanoparticles for Thin Film Photovoltaics Jan Flohre1, Maurice Nuys1, Christine Leidinger1, Florian Köhler1 and Reinhard Carius1 1 Institute of Energy and Climate Research 5 -Photovoltaics-, Forschungszentrum Jülich GmbH, D-52425 Jülich Germany ABSTRACT Laser annealing experiments on commercially available phase pure tenorite (CuO) nanoparticles (NPs) were performed in air and nitrogen atmosphere to improve the structural and electronic properties, with respect to their suitability for photovoltaic applications. The particles exhibit size variations from about 30 nm to 100 nm. The influence of the thermal treatment is investigated by photoluminescence (PL) and Raman spectroscopy. Annealing of the particles in air by a laser treatment improved the material quality by defect reduction. Additional laser annealing in N2 atmosphere leads to a phase transition of the NPs from tenorite to cuprite (Cu2O). Due to the low partial oxygen pressure, the transition is initiated at about 1/3 of the maximum laser power used for the series in air, which is indicated by a drastic increase of the band edge emission from Cu2O. However, annealing with higher laser power leads to strong defect luminescence, which originates from copper and oxygen vacancies. A weak remaining tenorite band edge emission shows an incomplete phase transition. INTRODUCTION Cost effective solar cells with high efficiency based on abundant, non-toxic materials is the long term target of present research and development. Multijunction, material saving, thin film solar cells, including nanostructures or NPs, is considered as an important option for future solar cell technologies. Tenorite and cuprite fulfill many of these requirements, e.g. the suitable band gap energies of about 1.3 eV to 1.5 eV and 2.1 eV [1, 2]. In previous studies we showed that thermal annealing in an oven in nitrogen atmosphere improves the material quality of the tenorite NPs up to a temperature of 700°C and leads to a phase transition to cuprite after annealing at 800°C [3]. In this work, commercially available tenorite nanoparticles are investigated and modified by laser annealing in order to improve their electronic and structural properties for use as active absorber material in solar cells. Laser annealing has the advantage that the heat penetration into the material is locally, so that a thermal treatment can be performed on non-heat resistant substrates, which is important for the use in thin film technology. The impact of the laser annealing on the material properties is investigated by Raman and PL spectroscopy. Since a correlation between cell efficiency and PL at room temperature is given by the theory of detailed balance [4], PL is a powerful method to investigate absorber materials for photovoltaic devices. EXPERIMENT Commercially available tenorite NPs (IoLiTec GmbH; Germany) were studied. The nominal diameter of the particles is 40 nm to 80 nm. Preliminary TEM investigations showed that the particles exhibit size variations from about 30 nm to 150 nm [3]. The NP

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