Investigation of Reactive Silver Ink Formula for Reduced Silver Consumption in Silicon Heterojunction Metallization

Abstract

Silver is the most expensive non-silicon component in photovoltaic cells. This is particularly salient for silicon heterojunction (SHJ) cells, which rely on large quantities of low-temperature silver pastes (LT-SP). SHJ cells would benefit greatly from an industrially scalable metallization process that simultaneously offers low silver consumption, low finger resistivities (<20 μΩ·cm), and low processing temperatures. Printed reactive silver inks (RSI) are an innovative candidate to this end. This work furthers the research on RSI metallization by investigating the impact of ink formula on properties pertinent to SHJ cells, including electrical properties, line width, ink splatter, silver consumption, cell performance, and adhesion. We introduce a scalable, high-throughput flexible needle contact printing approach for metallization that solves many of the issues associated with drop-on-demand printing. The printed silver fingers are characterized using electrical measurements and top-down and cross-sectional microscopy. The best performing ink, consisting of silver acetate, ethylamine, and formic acid, achieved silver fingers with total resistivities of 3.1 μΩ·cm and contact resistivities of 3.2 mΩ·cm2 when printed at 61 °C. This ink metallized a full-sized 156 mm × 156 mm SHJ cell. This is the first reported data for RSI metallization of a full-sized SHJ cell and shows how an optimized RSI can achieve similar performances to LT-SP while consuming 80–90% less silver.

Importance of the Heat Treatment Scheme on Self-Reducing Reactive Silver Inks

“…This well-studied ink is the most cited reactive silver ink formula to date, and its widespread use ensures the broad applicability of our findings. Furthermore, there are many ink formulas based on Walker’s ink that have the same reduction mechanisms and, therefore, can utilize the conclusions from this study. ,,,,,,,,− Additionally, this ammonia-based ink is known to have a relatively poor low-temperature performance, often requiring high sintering temperatures or aggressive sintering techniques to achieve good electrical performance. As this work shows, the electrical performance of this ink can be improved by 56× to achieve resistances only 5.5× higher than that of bulk silver.…”

“…Complexing agent loss occurs after printing as the labile and high vapor pressure complexing agent molecules evaporate. Without the stabilizing ligands, the silver ions are reduced, producing metallic silver. , Since the complexing agent loss reduction mechanism relies on evaporation, it produces silver at the liquid/vapor interface . This silver formed at the liquid/vapor interface creates a self-supporting, skin-like structure, which results in high porosity once the ink dries.…”

“…This equates to a silver amount of 20.0 μg Ag cm –1 (see Supporting Information, Section 2, for silver amount calculation). Although printing 10 layers per line increased the experiment’s complexity, printing multiple layers is more representative of reactive inks for printed electronics. ,,,,, Each substrate contained 10 lines to provide statistically meaningful results.…”

“…Reactive inks are a second type of conductive ink that can produce highly conductive features at temperatures below 100 °C. ,− Instead of metal particles, reactive inks contain dissolved chemical precursors that when printed react to form the desired material. In addition to low-temperature processing, reactive inks are advantageous over particle-based inks because they avoid nozzle clogging issues, show higher ink stability, and are easier to synthesize .…”

“…Self-reducing reactive inks have demonstrated remarkable performance at low temperatures and have gained widespread adoption in the field. Some self-reducing reactive silver ink applications include transparent conducting electrodes, − solar cells, , flexible/stretchable interconnects, ,, RF inductors, fabric heaters, and electromagnetic interference (EMI) shielding . Furthermore, self-reducing reactive silver inks are under active commercialization…”