The effects of the band gap and defects in silicon nitride on the carrier lifetime and the transmittance in c-Si solar cells

In this paper, silicon nitride thin films with different silane and ammonia gas ratios were deposited and characterized for the antireflection and passivation layer of high efficiency single crystalline silicon solar cells. An increase in the transmittance and a recombination decrease using an effective antireflection and passivation layer can be enhanced by an optimized SiN_x film in order to attain higher solar cell efficiencies. As the flow rate of the ammonia gas increased, the refractive index decreased and the band gap increased. Consequently, the transmittance increased due to the higher band gap and the decrease of the defect states, which existed for the 1.68 and 1.80 eV in the SiN_x films. The interface trap density found in silicon can be reduced down to 1.0×10¹⁰ cm⁻² eV⁻¹ for the SiN_x layer deposited under the optimized silane to ammonia gas ratio. Reduction in the carrier lifetime of the SiN_x films deposited using a higher NH₃/SiH₄ flow ratio was caused by the increase of the interface traps and the defect states in/on the interface between the SiN_x and the silicon wafer. Silicon and nitrogen rich films are not suitable for generating both higher carrier lifetimes and transmittance. An improvement in the single c-Si solar cell parameters was observed for the cells with an optimal SiN_x layer, as compared to those with non-optimal SiN_x layers. These results indicate that the band gap and the defect states of the SiN_x films should be carefully controlled in order to obtain the maximum efficiency for c-Si solar cells.