Understanding the Sintering of Digital Inkjet Printed (DIP) Contacts to Achieve Low-contact Resistance on Silicon Solar Cells

Abstract

Digital inkjet printing (DIP) is a precise and promising technology to create fine gridlines for silicon solar cell. It is based on drop on demand (DoD) technology, which drops only when it is properly aligned to avoid wide gridlines.  The gridline spreading is controlled by heating the chuck (wafer holder) to ≥200oC at the time of the ink droplets. This eliminates drying step that is common to the screen-printing technology and eradicate gridline spreading. The screen-printed technology (SPT) on the other hand cannot exclude the drying step but relies on the silver loading and organics to balance the paste viscosity and rheology, which are used to control the gridline spreading.  The difference between the inkjet and screen-printed inks is the particle size of the paste or ink constituents. While the DIP inks are in the nano-particle range, the SPT counterparts are in the micro-regime. Therefore, understanding the sintering of the nano-particle sizes is needed to achieve low contact resistance and hence high fill factor (FF), which is influenced by the total series resistance of the device. In this paper the microstructural analysis of the inkjet contact system was used to optimize the peak firing temperature for DIP gridlines. The optimized temperature profile found was similar to the SPT. This led to FF of ~79.2% for mono crystalline cell and efficiency of ~19.3% and ~17.4% for multi-crystalline with FF of ~78.5%. It was also found that the series resistance of these cells was not dominated by contact but emitter resistance.