Comparison of the passivation quality of boron and aluminum BSF for wafers of varying thickness

Abstract

The aim of the work is the comparison of large area Cz-Si solar cells with boron back surface field (B-BSF) to those with full area aluminum (Al) BSF. The investigations are carried out with respect to the influence of the wafer thickness on the cell performance. Boron doped Cz-Si wafers of varying thickness (115-240 μm) are used as bulk material. The B-BSF is formed by BBr3-diffusion. Metallization is applied via screen printing a grid on the front and rear side. Full area aluminum BSF solar cells are used as reference. The solar cells show high open-circuit voltages (VOC) up to 635.4 mV and short-circuit current densities (JSC) up to 36.5 mA/cm2. Neither VOC nor JSC decrease with decreasing wafer thickness. The full area Al-BSF reference solar cells show decreasing fill factors and therefore reduced cell efficiencies with decreasing wafer thickness. The cells with B-BSF reveal internal quantum efficiencies (IQEs) of 60% at 1100 nm. Going to thinner wafers, the IQE is not reduced in the long wavelength regime. The effective back surface recombination velocities (SRVs) extracted from a physical model fitted to spectral response measurements are in the range of 80-200 cm/s for the boron BSF cells. Effective back SRVs of the aluminum BSF cells are determined to 260-700 cm/s.