Optimisation of Screen-Printed Metallisation for Industrial High-Efficiency Silicon Wafer Solar Cells

In the photovoltaic industry, screen printing accounts for majority of the metallisation processes for silicon wafer solar cells. Contact formation by co-firing of front and rear screen printed metal pastes for mainstream p-type standard solar cells is a well-established process. It is of utmost importance to use front and rear metallisation pastes that are co-firing compatible. In this paper, we describe a methodology for front and rear paste selection and process optimisation. We explore the usage of HF dip (20 seconds) on finished cells to differentiate between over-fired and under-fired contacts. Two commercially available front Ag pastes (paste „A‟ & paste „B‟) with fine line printing capability were tested along with two commercially available rear Al pastes (paste „C‟ & paste „D‟). Firing optimisation was conducted to achieve a one-Sun fill factor of 80.7% and a series resistance at maximum power point (MPP) of 0.4 Ωcm2 at the optimised firing profile. The Al paste „C‟, which resulted in a better and more uniform back surface field at the optimised profile, was selected to study the effect of post -metallisation HF etching. By choosing the correct combination of metal pastes, at the optimised firing profile, efficiencies of 18.5% were realised for standard screen-printed 156 mm x 156 mm p-type pseudo-square monocrystalline solar cells.