Advanced front side metallization for crystalline silicon solar cells with electrochemical techniques
The present work is an evaluation of the possibilities that electrochemical processes offer for the metallization of crystalline silicon solar cells. Based on the established metallization process for high-efficiency laboratory solar cells, metal plating processes have been investigated as part of the "seed & plate" two step metallization concept. This approach was expected to feature advantageous properties compared to the current industrial standard process, screen printing. General electrochemical interrelations of plating processes on solar cells are clarified. A novel method that allows the evaluation of individual electrode processes for light-induced plating is developed and implemented. Challenging process aspects, such as current density evaluation and control, mass transport and deposition homogeneity are addressed. Suitable process parameters are identified and successfully applied to crystalline silicon solar cells, comparing the results with the state of the art. It is demonstrated that electrochemical techniques are suitable to create a superior front side metallization for crystalline silicon solar cells. Contacts with decreased width and increased conductivity compared to screen printed ones can be created, leading to a reduction in shading (resulting in an increased solar cell current) at comparable or improved series resistance. A first, relatively simple contact architecture with a fine-line printed seed layer augmented by plated silver is already close to industrial introduction. A second approach with a plated nickel seed layer and plated silver or copper as conducting layer is even more promising, but requires further investigation. As the long-term stability of copper-containing contacts is a key aspect, a novel method that allows a quick estimation of this parameter is developed. Optimized process sequences are demonstrated to allow sufficient long-term stability for these copper-containing contact systems. Simultaneously, these approaches led to a contact system performance similar to the most advanced evaporated TiPdAg-system, which was successfully reproduced with industrially relevant electrochemical processes. Record efficiencies of 21.4% were achieved. Both approaches are estimated to be economically competitive with the state of the art.
Zugl.: Freiburg/Brsg., Univ., Diss., 2011