Mapping of Bulk Diffusion Length and Effective Back Surface Recombination Velocity in Silicon Solar Cells

Abstract

Mapping of diffusion length (L) in the bulk region and effective back surface recombination velocity (SRV) in AI-BSF and PERC Si solar cells has been carried out by utilizing the spectral response (SR) at desired wavelengths. Light beam induced current (LBIC) technique was used to generate the maps of SR and reflectivity (R) on cell area (6” x6”). MATLAB tool was used to convert the spatial maps of SR and R into L and SRV. We found that (i) the distribution of L in multi-crystalline cells varied from grain to grain in wide range (150-600 $\mu$m) while in mono-crystalline cells, it varied in rather narrow range (450-600$\mu$m); (ii) the values of SRV for PERC cells (120-250 cm/sec for mono-crystalline and 100-250 cm/sec for multi-crystalline) and AI-BSF cells (320-400 cm/sec for mono-crystalline and 250-350 cm/sec for multi-crystalline) differ by considerable magnitude due to passivation quality at back side. Three multi-crystalline AI-BSF Si solar cells of cell efficiencies 17.6%, 17.9% and 18.1% were investigated with the proposed methodology and demonstrated that the efficiency deficit is primarily due to defects present in bulk material and poor back surface passivation.