Surface passivation of boron emitters on n-type c-Si solar cells using silicon dioxide and a PECVD silicon oxynitride stack

Abstract

Recombination of charge carriers is a significant loss mechanism in solar cells. To achieve high efficiency, recombination losses should be minimized. The surface passivation technique is used to minimize recombination losses. Also, surface passivation is essential to achieve high conversion efficiency, especially for thin crystalline-Si (c-Si) wafers. We have investigated the passivation properties of a silicon oxynitride (SiO_xN_y)/silicon dioxide (SiO₂) stack for boron doped emitters. SiO_xN_y single layer properties were optimized using various gases (N₂O, NH₃ and SiH₄) and gas flow ratios. Optimized SiO_xN_y films resulted in low refractive indices ranging from 1.49 to 1.64. FTIR spectroscopy was used to analyze the chemical composition of the SiO_xN_y films. As the gas flow ratio increases, the absorbance peaks shift towards higher wave numbers due to an increase in oxygen concentration in the film with a decrease in refractive index. After optimization, SiO_xN_y film was capped over with a 10 nm thermal SiO₂ layer. The effective lifetime of the SiO_xN_y/SiO₂ stack was found to be 0.690 ms. Light I–V results showed an efficiency of 19.58% with V_oc, J_sc and fill factor of 644 mV, 37 mA cm⁻² and 82.3%, respectively. The role of SiO_xN_y/SiO₂ stack passivation for the improvement of solar cell efficiency is discussed.