Light Assisted Electrodeposition and Silicidation of Ni for Silicon Photovoltaic Cell Metallization

Abstract

Light assisted electrodeposition of Ni films was studied on crystalline Si photovoltaic cells. The nucleation, film uniformity and silicidation behaviour were studied at different locations across the busbar and were found to strongly depend on the plating solution, plating current and annealing conditions. The photovoltaic diode can be operated at forward bias or reverse bias during the light assisted electroplating depending on the plating current and light intensity. Suns-Voc of the solar cells was examined at different stages along the process to evaluate the impacts of silicidation on the cell performance. Detailed microscopic analysis was conducted for the metallization stack. The post-silicide degradation in Suns-Voc was correlated with local shunting due to non-optimized silicidation condition. © The Author Solar energy, one of the main types of renewable energy, is critical to help meet the ever-increasing demand for energy from our human society. Among the different solar cells, crystalline silicon solar cells dominate the global solar market. The manufacturing process currently used involves screen-printing for the front metallization using Ag paste. While the screen-printing process has been integrated into the process flow with standard tooling, it typically causes higher wafer breakage and suffers conversion loss due to the lower conductivity, the lower line height-width aspect ratio and the large printing spot size. In addition, the high price of Ag metal drives up the cost of the solar cell and thus delays the break-even point.Replacing the screen-printed Ag with electroplated high aspect ratio Cu lines has been proposed decades ago.1,2 This method has recently gained more and more attention due to the increasing price of Ag and its potentially easy integration with the laser patterning. The selective emitter formed by the in-situ doping during laser patterning [3][4][5] further improves the conversion efficiency of the solar cell.Different Cu metallization schemes involving electroplating have been reported, 6,7 such as electroless deposition of Ni and silicidation, [8][9][10] Cu electrodeposition on printed thin seed layer, 11,12 and electrodeposition of Ni and Cu. 13,14 Among them, most of the schemes involve the formation of Ni silicide either from electroless or electrolytic deposited Ni. The electrolytic deposition process in these reports typically involves light, so called light induced or light assisted electrodeposition. The formation of silicide lowers the contact resistance between the metal and Si and thus improves the solar cell performance.Ni silicidation has been extensively studied for the application in CMOS (complementary metal oxide semiconductor) contacts. 15-17The Ni silicidation starts at as low as about 300• C for metal rich phases, followed by monosilicide phases at around 400• C and silicon rich phases at above 800• C. 15 As far as the authors are aware of, all the studies for CMOS applications were performed on extremely thin Ni or Co films fabricated using vacuum depos...

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

“…A light intensity of 17000 lux was used during plating except for the study on the light intensity effect. This light intensity was measured in an empty plating setup but 25 The total metal concentration, NiSO 4 and CoSO 4 , is therefore kept constant at 0.2 M. As shown in Table I, the ratio between the two metal species varies for different solutions in this study. A rapid thermal annealing (RTA) furnace with a thermal couple embedded in the carrier was used to anneal the NiCo plated solar cells to form silicide.…”

“…We previously reported a study on the light assisted electroplating of Ni and the silicidation on silicon solar cells. 25 The uniformity of the Ni silicide formed from the electrodeposited Ni and the performance of the solar cell highly depended on the plating solution, light illumination, and the plating current density as well as the silicidation conditions. In this paper, light assisted electrodeposition of NiCo alloy was studied in conjunction with the light illumination intensity and the energy spectrum.…”

Electrochemical Investigation of Si of Various Dopant Concentrations at Negative Overpotentials in Aqueous Electrolyte

“…Electrochemical processes at positive overpotentials (dissolution, etching, passivation, machining, and polishing) of Si, relevant to semiconductor technology, are widely reported in the literature. − At the same time, processes at negative overpotentials have been rarely reported . These processes are of great importance to photoelectrochemical cells (PECs), Si-based electrochemical energy storage in lithium-ion batteries, and photoelectrochemical plating. − Research activities on the cathodic reactions on Si have significantly increased in recent years. This is due to the industry’s initiative to employ cheaper materials for contacts in microdevices and photovoltaic applications. − Since hydride formation and hydrogen evolution reaction (HER) occur at relatively low overpotentials in aqueous electrolytes, these reactions interfere with most of the cathodic processes on Si and often become detrimental to the final outcome. − HER is also a process of enormous technological significance to produce pure hydrogen by electrolysis.…”

“…These processes are of great importance to photoelectrochemical cells (PECs), Si-based electrochemical energy storage in lithium-ion batteries, and photoelectrochemical plating. − Research activities on the cathodic reactions on Si have significantly increased in recent years. This is due to the industry’s initiative to employ cheaper materials for contacts in microdevices and photovoltaic applications. − Since hydride formation and hydrogen evolution reaction (HER) occur at relatively low overpotentials in aqueous electrolytes, these reactions interfere with most of the cathodic processes on Si and often become detrimental to the final outcome. − HER is also a process of enormous technological significance to produce pure hydrogen by electrolysis. This process of producing hydrogen is considered to be an important option to store renewable energy. − …”

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

“…14,18 In addition, the interface between the silicide and metal typically are not well controlled. A recent study showed that an additional Ni layer freshly plated after the removal of the excessive un-reacted Ni from the silicidation improves the interface and adhesion between the Ni silicide and Cu grid.…”