Influence of Recombination Velocity and Doping on the Photovoltaic Properties of Epitaxial Silicon Solar Cells

This work evaluates the variations of photovoltaic properties of epitaxial silicon solar cells, fabricated on UMG-Si substrates, as a function of the epilayer thickness for different recombination velocities and doping concentrations. Device simulation and optimization software, developed in a previous paper, has been extended and adapted to n(+)pp(+) type epitaxial solar cells. The variations of photocurrent density J(sc), and efficiency eta, as a function of the epilayer thickness d, shows that solar cells, with BSF (low recombination velocities), are more effective than cells with ohmic contact (high recombination velocities) on their back surface. Regarding the effect of the doping concentration N-A in the region p, J(sc) is not significantly influenced by this parameter, except for the cells with small grains fabricated on thin epilayer. In the rest of the cases J(sc) increases when the doping increases from 10(15) cm(-3) to 10(17) cm(-3) and the calculated solar cell efficiencies vary from 11% to 15% for cells with thin or thick epilayer respectively (independently of the grain size) while the optimum epilayer thickness shifts to lower values for cells with a thinner epilayer and small grains. The calculated results of maximizing eta, show that the highest efficiencies correspond to cells fabricated on thick epilayers (d > 50 mu m which is lower compared to the experimental) and a doping N-A = 10(17) cm(-3) independently of the grains size. When an epilayer thickness is lesser than this chosen, a minimal reduction in J(sc) and. is observed, but a significant gain in the material cost is achieved. The evaluation between the simulated curves of quantum efficiency QE, under 1000W/m(2) lighting, shows that, this is higher for the cells realized on thick epilayer. However, near the blue part of the solar spectrum, QE calculated is significantly higher for the cell with thin epilayer and small grains, compared to the ones with medium grains.