There are applications of solar cells under illumination conditions for lower than "one sun". Solar cell efficiency decreases with decreasing irradiance. Several factors affecting conversion efficiency at low irradiances (down to ~0.01 sun) are studied. Experiments were performed with bifacial n+-p-p+ Si solar cells, fabricated using an ion implantation procedure for BSF formation. Optimal resistivity of the starting Si is evaluated taking into consideration recombination in the ion induced defect layer. Use of high resistivity Si eliminates the influence of this defect layer on recombination in the base region. Lifetime dependence on injection level can cause current non linearity and therefore an additional efficiency drop at low irradiance. Light induced defects can be responsible for this effect. The quality of the p-n junction as expressed by the ideality factors of the two-diode model is one of the most important factors affecting solar cell conversion efficiency at low light intensities. Keywords: Silicon Solar Cell, Low Irradiance, Bifacial, Ion Implantation. For the full article>>