Rear surface passivation for high efficiency silicon solar cells

Abstract

In order to adapt laser grooved buried contact (LGBC) solar cells to a thinner silicon substrate than usually used, we have investigated the reduction of charge carrier loss at the rear surface of p-type silicon wafers by plasma-enhanced chemical vapour deposition (PECVD) of a-Si:H and SiNx films. The efficiency of these passivating films has been measured via the surface recombination velocity (SRV) which is wanted as low as possible. The SRV values of our samples have been compared with the expected theoretical values given by the Shockley-Read Hall (SRH) recombination model. SRH theory is a description of the electron-hole recombination via defects inducing energy levels in the forbidden bandgap. This way of recombination is the predominant mode for semiconductors with indirect bandgaps like silicon. Two influential factors in regard to SRV can be understood from this theory. These two factors are the electron to hole capture cross section ratio and the fixed charge density Qf at the silicon substrate / film interface. These two factors induce an asymmetry between the electron and the hole recombination and are responsible for the excess charge carrier concentration dependence of the SRV. In other words, the SRV depends on the illumination intensity. In this work, the SRV has been measured for an excess charge carrier injection level in the 1.1013-1.1016 cm-3 range and then it has been compared with the theoretical value given by the SRH theory in order to determine the fixed charge density and have an estimation of the defect characteristics including its density. Simulations of LGBC cells under one sun illumination have then been performed using the PC1D5 software. The measured SRV value corresponding to one sun has been integrated in the simulation and the expected efficiency has been extracted as a function of the wafer thickness. It results from this study that 150μm LGBC solar cells can theoretically have an efficiency of 19% by the integration of passivating SiNx films. A second aspect of this work is an effort to understand the relation between the passivating film quality, structure, and the PECVD parameters during deposition. The films have been characterized principally by ellipsometry and also by XPS. All of our SiNx films are located in the Si-rich region (x<1.1) and the passivating quality increases with x.