Influence of aluminum workfunction and capping dielectric thickness on the performance of local back surface field solar cell using numerical simulation

Abstract

High efficiency partial rear contact solar cells forms a Metal-Insulator-Semiconductor (MIS) stack in the rear side. Aluminum is used as a local back surface field (localized heavily doped pregion) and a hole contact. The workfunction difference between the aluminum contact and the c-Si(p) causes inversion in the silicon surface of the MIS structure. The surface inversion affects the quality of passivation and hence, the performance of the solar cell which requires further understanding. In this work, we have analysed the influence of aluminum on surface passivation in two passivation stacks namely (i)AlOx/SiNx and (ii) SiOx/SiNx using Sentaurus TCAD simulation.Our simulation result shows that in case of positive dielectric stack (SiOx/SiNx), aluminum workfunction enhances the passivation (effective surface recombination velocity <15cm/s) for the injection condition of 2×1013cm-3 which is common at the rear side of AlLBSF solar cell. But, in negative dielectric stack (AlOx/SiNx), aluminum workfunction influence is negligible since it is surpassed by high negative fixed charge density. In addition, we have varied the capping layer (SiNx) thickness in the dielectric stack to alter the aluminum influence on the crystalline silicon surface. It shows up to 1.0% absolute improvement in the efficiency for reduced SiNx thickness in the SiOx/SiNx stack for the fixed charge density of 2×1011cm-2, interface trap density of 5×1011cm-2, assumed electron and hole capture cross section of 10-14cm2 and 4×10-16cm2 respectively. This is solely attributed to the aluminum workfunction influence on the rear passivation.