Study On The Contact Mechanism Of Front Metallization Of Multi-Crystalline Silicon Solar Cells

In the production process of crystalline silicon solar cells, a thin layer of SiNx is deposited on the textured emitter in order to passivate the dangling bonds and reduce the light reflection. After the deposition step, screen-printing of silver paste, followed by a firing process, is widely used in the front electrode fabrication. In the firing process, silver particles in the paste grow together due to the driving force of the minimization of surface energy. At the same time, the SiNx layer is etched and an electrical contact (ohmic contact) is formed between the silver finger and the silicon emitter. As an important part of the series resistance, the contact resistance will directly affect the conversion efficiency of silicon solar cells. However, a complete and clear picture of the contact formation between silver finger and silicon emitter is still not available, especially in the textured multi-crystalline solar cells. In this paper, the electrical contact between the silver finger and the silicon emitter in the multi-crystalline silicon solar cells has been studied, using the common analysis methods in the production line and laboratory. The main results are shown as following.(1) The average conversion efficiency of the multi-crystalline Si solar cell has been improved 0.1% and the highest conversion efficiency is 18.58 by optimizing the preparation conditions.(2) According to the principle of the phosphorous diffusion, the phosphorous diffusions with different diffusion times have been simulated, using the model of constant source diffusion. The contact resistance in the multi-crystalline silicon solar cells has been studied by using a 2-point and 4-point probe method. The contact resistance of 1.2 mΩ is the optimal choice to match the experimental silver paste, corresponding to the sheet resistance of 87 Ω/□. Furthermore, the relation between phosphorous diffusion and contact resistance has been discussed.(3) In the experiment, the SiNx firing-through process, Ag/Si ohmic contact formation and the current transport mechanism have been studied. A three layer model, silver layer/glass layer/silicon layer, is proposed to explain the contact formation and the current transport. A large number of silver crystallites penetrate into the glass layer with the size from tens nanometers to hundreds nanometers. Some of the silver crystallites penetrate through the glass layer and contact with both the silver finger and silicon emitter. The current transports through these direct contacts. Using the model, the effects of the preparation conditions to the contact resistance could be explained easily. At the same time, it is proved that the formation of the silver crystallites is related to the existence of SiNx layer and the redox reaction between silver oxide and silicon nitride seems to be the most likely source of the silver crystallites.