ENFORCE · ENhancing the power conversion eFficiency of mOnocRystalline nitrogen-doped silicon solar CElls

Silicon solar cells are the most commercialized photovoltaic devices due to their high-power conversion efficiencies (PCE). Over the different types of silicon substrates for solar cells, monocrystalline silicon is the one with the highest PCE reported. Monocrystalline silicon is commonly grown by the Czochralski method, a process in which a small seed crystal is dipped into a melt in a crucible, pulling the seed upwards to obtain a single crystal. Nonetheless, by the same process, two types of intrinsic defects can be incorporated: additional atoms (interstitials) or missing atoms (vacancies); additionally, the crucible used is generally silica, so the result is an oxygen contaminated ingot. Oxygen tends to react with vacancies, seriously affecting the PCE of the synthesized solar cells.Nitrogen has long been known to simultaneously suppress interstitial and vacancy related defects, the higher the nitrogen concentration, the lower the defect size, which is highly favourable for defect annealing; besides, strongly enhances oxygen precipitation. Unfortunately, quantitative data on the chemical and physical properties of nitrogen in silicon are rare, so the mechanism through which it reacts with intrinsic defects and oxygen is still relatively unknown. In consequence, it is not possible to know what variables should be modified to improve the quality of the crystal.The main idea of this project is to investigate the effect mechanism of nitrogen on grown-in oxygen precipitates. A complete understanding would lead us to find the ideal conditions to dope silicon with nitrogen, in order to reduce defect sizes and the oxygen amount to a minimum, so as to reach the maximum PCE in a monocrystalline silicon solar cell.