Effects of illumination level on diffusion length and efficiency of poly-Si cells

Theoretical modeling to quantify the effects of illumination level (⩽50 suns) on grain boundary barrier height (V_g), minority carrier diffusion length (L), and efficiency of p-type polycrystalline silicon solar cells is discussed. It is shown that V_g decreases and the diffusion length increases with illumination level in those polysilicon materials where grain boundary dominates the recombination. In addition, the diffusion length is not uniform under illumination, and increases significantly in the first 50-100 μm under high concentrations. Model calculations showed that the efficiency spread in 1 mm grain size polysilicon cells at one sun due to grain boundary defect density (N_ts) in the range of 10¹¹-10¹² cm^-2 is virtually eliminated at 50 suns due to higher relative enhancement in diffusion length in materials with high N_ts