Analysis of Fine-Line Screen and Stencil-Printed Metal Contacts for Silicon Wafer Solar Cells

Impact of the phosphorus emitter doping profile on metal contact recombination of silicon wafer solar cells

“…Typical Ag front grid metallisation of a Si solar cell covers about 6-10% of the total area using an Hpattern with busbars and perpendicular fingers. One approach to reduce the metallisation induced recombination losses is to use a dual print approach for the front side metallisation, wherein the busbars are printed using a less aggressive paste (which causes minimum damage to the SiN x coating) and also by printing finer fingers using stencils [8] or using advanced printing methods.…”

Influence of non‐uniform fine lines in silicon solar cell front metal grid design

“…At each node, the current density is given by J = J sc ( x , y ) − J R ( V ( x , y )), with J R ( V ( x , y )) = J 01 [exp( qV ( x , y )/ kT ) − 1] + J 02 [exp( qV ( x , y )/2 kT ) − 1], where J 01 and J 02 are the saturation current densities in the two‐diode model. Following Reference , J sc = 40.2 mA/cm 2 in the regions not covered by the front grid, and J 01 = 730 fA/cm 2 , and J 02 = 30 nA/cm 2 throughout the cell plane. The nodes are interconnected by resistors with values as appropriate for the sheet resistance of the region that the triangular element resides.…”

“…The Pareto distribution and the fitted parameters constitute the most detailed description of non‐uniform metal line conductance to date. To recapitulate from Reference , the Pareto distribution is given by where σ short, i is the line conductance (in S‐cm) of the short unit segments of 250 µm segment length, with σ short, io being the mean and α a power law scaling factor. The smaller the scaling factor, the larger is the spread in the data.…”

“…These imperfections add resistance to the metal front grid, causing the voltage to swell locally, rerouting completely or changing the division of current between finger segments where they occur. A recent study based on the surveying of screen printed 3D microscope line profiles, line segment resistance measurements, electroluminescence (EL) imaging checks for line breaks, and current–voltage (I‐V) measurements, revealed that even when the screen wire diameter and pitch are in accordance with design rules for the finger opening width in the screen, the printed lines (single print) are nevertheless severely non‐uniform when the opening width becomes less than 45 µm, to the extent of causing line breaks to be prevalent throughout the front grid with an accompanying fill factor of 71.1% at an opening width of 30 µm . This finding explains the trend that the industry is adopting double printing as a solution for using screen printing technology to print fine lines, where additional silver paste is printed on top of a first finger print to increase the overall line aspect ratio and uniformity in conductance .…”

“…Unlike screens with wire mesh, a stencil is designed to have cleanly opened apertures, which leads to an excellent paste transfer and printed line height uniformity. The same study has found that stencils with finger opening width of 30 µm results in sufficiently uniform lines to yield cells with average fill factor of 78.8% and front grid metal area fraction of 6.2% . In evaluating different printing technologies and accounting for print quality effects, it would be useful to develop a simulation tool that can capture the difference between front grids of varying degrees of uniformity, in their ability to distribute current and influence the cell I‐V curve.…”