Refractive index of PECVD-SiNy rear passivation films and its effect on assisted passivation of PERCs

Abstract

This paper presents a study on the influence of the refractive index, n, of silicon nitride (SiNy) rear passivation films on the performance of passivated emitter and rear cells (PERCs). Symmetrical sample structures were fabricated in order to further investigate the associated electrical and interfacial properties. When the refractive index of the SiNy films, n, which can be varied by controlling the concentrations of silicon, hydrogen, and nitrogen, increases up to n = 2.3, the implied open-circuit voltage (iVoc) decreases significantly by 20 mV compared to that of the samples with refractive indexes n<2.3. The SiNyfilms with low refractive indexes have relatively high concentrations of nitrogen (N-rich films), while the films with low concentrations of nitrogen exhibit amorphous Si-like properties with a higher refractive index (Si-rich films). For the fabricated PERCs, the Voc values and the short-circuit current density (Jsc) gradually decrease with increasing refractive index n. The decrease in Voc for refractive index n > 2.3 is attributed to poor field-effect passivation and chemical passivation. For field-effect passivation, the effect is observed clearly through the reduction of the negative fixed charge density (Qf) near the interface due to the high silicon content in the films; here, a silicon atom back-bonded to three nitrogen atoms, known as the so-called K-centers, can be positively charged, thereby reducing Qf. For chemical passivation, an increase in the interface trap density (Dit) and increasing effective surface recombination velocity (Seff, max) are observed. These results suggest that controlling the conditions of SiNy deposition is crucial for assisting the surface passivation process that determines the final output of PERCs.