Thin Multifunctional Dielectric Layers for Surface Passivation and Masking Against Wet Chemistry and Dopant Diffusion

Abstract

In order to achieve cost competitiveness for energy production, the solar PV technology is dynamically shifting towards thinner wafers. Due to this cost driven reduction of solar wafer thickness, there is more influence of surface recombination in limiting the overall solar cell efficiency. Therefore a high surface passivation quality, using low cost process techniques is crucial for the concept of high-efficiency silicon solar cells with low cost per Watt peak. A new generation of silicon solar cell type is the passivated emitter and rear solar cell (PERC). In this technology, the cell is passivated on both emitter and rear sides to reduce recombination at the surface. During large scale development of PERC solar cells, there are certain drawbacks confronting current PERC cell development. Therefore, industries sought after process optimization techniques for developing high efficiency PERC cells through effective use of materials and low cost production methods. Hence within the framework of this Master thesis, further investigation to reduce the total cost of PERC-type solar cells is carried out. The thesis work is dedicated to investigating an optimized PERC concept to enhance the method of solar cell production, by suggesting an alternative process flow for the manufacture of PERC cells. This can be achieved through a re-arrangement of inline process steps involved in the standard fabrication mechanism and also through the omission of the rear polishing step. After a thorough study of different passivation layers, it was evident that the stack of Al2O3 rear passivation layer with SiNx capping layer proves to be a very suitable option for the proposed new concept of PERC cells fabrication. At Fraunhofer ISE a Sputter and PECVD deposition process for Al2O3 and SiNx layers has been successfully established. Therefore, experiments are performed with the main focus of investigating the stability of these Al2O3/SiNx rear passivation stacks as a barrier against the texturing and diffusion process and also to investigate the quality of surface passivation after the high temperature diffusion process. After a detailed analysis of the results of the texturing and diffusion experiments, three potential Al2O3/SiNx stack solutions are proposed as rear passivation stacks for the new PERC concept, which are deposited using the MW PECVD, ICP PECVD and Sputter techniques. Also, an economic analysis of the proposed solutions is performed, to validate the implementation of the new PERC concept