Filament stretching during micro-extrusion of silver pastes enables an improved fine-line silicon solar cell metallization

Abstract

The metallization of heterojunction solar cells requires a further reduction of silver consumption to lower production costs and save resources. This article presents how filament stretching of polymer-based low-temperature curing Ag pastes during micro-extrusion enables this reduction while at the same time offering a high production throughput potential. In a series of experiments the relationship between the printing velocity and the filament stretching, thus the reduction of Ag-electrode widths and Ag laydown is evaluated. Furthermore, an existing filament stretching model for the parallel dispensing process is advanced further and utilized to calculate the elongational viscosity. The stretching effect enables a reduction of the Ag-electrode width by down to Δwf = − 40%rel. depending on the nozzle diameter and paste type. The Ag laydown has been reduced from mAg,cal. = 0.84 mg per printed line to only mAg,cal. = 0.54 mg per printed Ag-electrode when 30 µm nozzle openings are used, demonstrating the promising potential of parallel dispensing technology for the metallization of silicon heterojunction solar cells.

Paste Development for an Optimized Filament Stretching Effect during Parallel Dispensing on Transparent Conductive Oxide Layers for Solar Cell Metallization

“…That means that the shear viscosity η s decreases when the shear rate γ : increases. The rheological properties of the reference paste Ref have been investigated in the literature, [23,25] so-called paste A. Such shear-thinning behavior is well known for printing pastes applied for solar cell metallization.…”

“…Complexity of dispensing processes. This overview illustrates different influencing factors on printed electrodes and their functionality [9,10,14,16,64,65] classified into six categories -I) printing pastes and their compositions, [13,23,[25][26][27]33,34,37,49,50,[66][67][68][69][70][71][72][73][74][75][76] II) substrates, III) print heads, [18][19][20][21][22][23]77] IV) nozzles, [15,18] V) printing machines, and VI) process parameters. [23,25,78] The list of influencing factors is created based on some articles and the authors' hands-on laboratory experiences and may be incomplete.…”

“…This effect shows great potential to enable narrower electrodes due to paste properties without decreasing the nozzle diameter as smaller nozzles may have increased risks for process instabilities and nozzle clogging. [23,25] Figure 2 depicts an overview of the parallel dispensing process and the effect of filament stretching. Low-temperature curing pastes may exhibit the effect of filament stretching during microextrusion, showing a process velocity range of category II.…”

“…The classification of different dispensed electrode shapes are illustrated in the ref. [23,25]. The phenomenon of filament stretching is not restricted to silver-containing low-temperature curing pastes.…”

Thermal Stable High‐Efficiency Copper Screen Printed Back Contact Solar Cells

“…The use of dispensing equipment has also been developed for the metallization of solar cells, [11] with 0.54 mg per Ag finger being reported. [12] The third way is to replace or reduce the use of Ag is to deploy other metals such as copper (Cu). As Cu is an abundant commodity, replacing Ag with Cu will not have an effect on competing technologies.…”