A combination of theoretical modelling, gettering and passivation, and cell fabrication is presented in this paper to provide guidelines for improving efficiency of polycrystalline solar cells. Theoretical modelling was performed to show that grain boundary barrier height decreases and carrier diffusion length increases with illumination level (≤50 suns) in those polycrystalline materials where grain boundary dominates the recombination. Model calculations show that the efficiency spread due to grain boundary defect density (Nst) in the range of 1011–1012cm−2in 1 mm grain size polysilicon cells at one sun can be virtually eliminated at 50 suns due to higher relative enhancement in diffusion length in the cells with highNst. Thus higher illumination level behaves similar to grain boundary passivation. Phosphorous and aluminum treatments were investigated and optimized for gettering and passivation. A deep phosphorous diffusion on the front, followed by a partial etch back, and 850 °C aluminum treatment on the back gave record high 17.7% efficient one sun solar cells on Osaka Titanium Corporation (OTC) polycrystalline silicon.