A new metallization technology for solar cells application

This Ph.D. thesis is focused on the development and optimization of front and rear side metallization of industrial silicon solar cells. The commonly adopted screen-printed silver metallization has several well-known issues, such as low contact resistance, moderate bulk conductivity and high cost. The approach of this work allows complete silver replacement, both on the front and the rear sides. The development of such a new technology is divided into several parts, each resulting in appropriate feedback in terms of solar cell operation parameters. A detailed investigation of the aluminum-silicon interdiffusion that occurs during the firing process of screen-printed aluminum layer usually deposited onto the rear of solar cells is reported. This process is very important because it affects solar cell operation and performance through back-surface field passivation. In this study different screen-printing aluminum pastes, differing one from each other in aluminum particle dimensions and glass frit composition, are evaluated in terms of their bulk resistivity, contact resistance to silicon, back surface field depth and solar cell performance. Finally, this study allowed to reveal certain dependences between pastes parameters and their effect on solar cells and to develop useful recommendations for better solar cell performance. In this work, a new metallization technology is based on an electroplating technique, which for a real industrial application, however, has some critical issues as throughput, floor space, quantity of liquid to manage and the necessity to use some masking technique, such as photolithography. These issues are strongly influencing the metallization technology cost, making it not economically convenient respect silver screen-printing technology. For this purpose, the proposed metallization technique is based on a novel dynamic liquid drop/meniscus (DLD/DLM) technique able to solve both issues. In this work DLD/DLM technique is studied for possible application in a new rear side metallization technology for solar cells, allowing localized formation of solder pads without any use of photolithography, limiting the cost of the process mainly to the cost of materials, such as nickel and tin, which are significantly cheaper than a silver counterpart that is currently adopted by the industry. The cost reduction is not a single advantage of the proposed technology. An efficiency improvement of up to 0.5 %abs is obtained due to a better back-surface field conditions. The development of a new front side metallization is based on a new approach which introduces a layer of mesoporous silicon helpful for further creation of nickel-copper electrical contacts to the emitter region of a solar cell. Process conditions of mesoporous silicon formation and further electroplating steps are studied and optimized in terms of contact resistance and adhesion of such a contacts, in order to guarantee a beneficial influence for solar cells fabricated with the new metallization approach. As for combination of both front and rear side metallization technologies, together, they result in complete silver removal from a metallization technology of a solar cell with a feasible efficiency enhancement of up to 1 %abs.