Solder Paste Printing and Optimizations for Interconnecting Back Contact Cells


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Introduction
The interdigitated back contact (IBC) is one of the methods to achieve rear contact solar cell interconnection. The contact and interconnection via rear side theoretically achieve higher efficiency by moving all the front contact grids to the rear side of the device. This results in all interconnection structures being located behind the cells, which brings two main advantages. First, there is no frontside shading of the cell by the interconnection ribbons, thus eliminating the need for trading off series resistance, losses for shading losses when using larger interconnection ribbons. Second, a more homogeneous looking frontside of the solar module enhances the aesthetics. This combined increased power yield and improved aesthetics make back-contact modules particularly suited for special applications such as vehicle and building integration. 

Out of many ways of interconnecting the IBC cells, busbar stringing, which is similar to conventional tabbing and stringing of two-side contacted cells, is the most common method2. In this technique, the metallization design of the cell contains multiple parallel-printed busbars distributed over the cell, allowing shorter finger length, and ribbon on busbar soldering. This reduces the resistance losses in the metallization compared to the edge stringing. With the advent in multibar bar (MBB) technology, the width of these busbars is also reducing and is down to around 300 to 500 microns. The interconnection can be carried out by either ECA (electrically conductive adhesives) or by using direct ribbon/wire. Both materials have created some challenges. The poor peel strength is often the major issue. The uniform IMC (intermetallic layer), which is characteristic of reliable bond strength, is absent with ECA. The metallization paste used in IBC is low-temperature-curing silver paste. The paste is fired at lower temperature around 500°C or less and deposited on silicon cell. In addition to that, the height of the paste is only about 8-12 micron. Because of this, silver leaching during interconnection is commonly observed. The metallization just comes off during interconnection at high temperature. If used, solder wire, cold solder joints, and solder diffusion through the cell are the major issues. Poor adhesion between cell and ECA, and interconnecting wire, high contact resistance are other common problems. Accordingly, when a circuit or conductive layer or interconnection is formed on a substrate using such conventional pastes, damage to the substrate or failure in reliability of the device may occur. Further, when ECA is used, silver in the ECA is expensive and appears on various restricted chemicals lists due its short supply. 

 To read this entire paper, which appeared in the July 2022 issue of SMT007 Magazine, click here.

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