A Study on the Long-Term Degradation of Crystalline Silicon Solar Cells Metallized with Cu Electroplating

Abstract

While using Cu electroplating to metallize silicon cells is of great interest, the long term reliability of such cells have not been well understood. In this paper silicon solar cells metallized with electroplated Cu were thermally stressed. The cell performance was characterized in conjunction with microscopic structure analysis to understand the long-term degradation mechanism and the effect of each different metal layer. The Ni silicide layer had little impact on the performance degradation at 200 • C. Increasing the thickness of a second Ni layer between the silicide and Cu significantly delayed this degradation, acting as a diffusion barrier. A series of cells annealed at different temperatures were used to extrapolate the degradation kinetics. Two different mechanisms were observed. Further silicidation of the second Ni layer was observed and was believed to cause the degradation at a higher temperature of 250 • C. However, this silicidation reaction rate dropped quickly as the temperature decreased and Cu diffusion became the dominant mechanism for cell degradation at temperatures below 200 • C. These two different mechanisms cross over at about 200 • C in this study, where both silicidation and Cu diffusion were observed. An improvement in the cell lifetime by replacing pure Ni with NiCo alloy was studied. In the standard manufacturing process flow of silicon solar cells, the front grids are formed with Ag screen printing. Replacing the screen-printed Ag with electroplated high aspect ratio Cu line has been proposed decades ago.1,2 This method has been of great interest primarily because of the lower conductivity and higher cost of Ag paste. In addition, the plated Cu potentially allows a higher aspect ratio or a smaller shadowing of the front grid and thus enables higher cell efficiencies. More importantly, the most recent development in laser patterning of the front grid allows an easy integration of the electroplating on the selective emitter formed by laser doping. [3][4][5] Different Cu metallization schemes have been reported before. 6,7 The schemes included electroless deposition of Ni and silicidation, [8][9][10] Cu electrodeposition on printed thin seed layer, 11 and electrodeposition of Ni and Cu.12-14 Among them, most of the schemes involve the formation of Ni silicide by annealing either electroless or electrolytic deposited Ni. The main purpose of the silicide is to lower the contact resistance between the metal and emitter and thus to improve the cell performance.Ni silicidation has been extensively studied for the application in complementary metal oxide semiconductor (CMOS) contacts. [15][16][17] Ni 2 Si silicide formation starts at as low as about 300• C, followed by monosilicide phases at around 400• C and silicon rich phases at above 800• C. However, small amount of Ni-richer phases such as Ni 3 Si and Ni 5 Si 2 could be observed at as low as 200• C. 15 All the studies for CMOS applications were performed on extremely thin Ni films fabricated using vacuum deposition. The silicide forme...

Degradation of copper‐plated silicon solar cells with damp heat stress

“…Currently, for Cu‐plated contacts, less is known. There are a number of studies that utilize a 175° C–275° C thermal stress test where the Cu‐contacts caused cell degradation, but these failures were on unencapsulated cells, where the mechanism is assumed to be thermally activated diffusion of Cu through the barrier and seed layers and into the c‐Si 40‐43 . Cu diffusion to the c‐Si results in degradation of pFF, and there is evidence that diffusing Cu forms recombination‐active or shunting precipitates or silicides in the space‐charge region of the emitter 44,45 .…”

A Study on the Electrodeposition and Silicidation of Nickel-Cobalt Alloys for Silicon Photovoltaic Cell Metallization

“…Detailed studies were carried out on the thermal degradation of different cells up to a few years and the results will be published separately. 46 While the degradation mechanism will be discussed in detail there, the improvement in the thermal stability of solar cells with the ternary silicide is believed to relate to the higher temperature needed for further silicidation.…”