Quick Determination of Copper-Metallization Long-Term Impact on Silicon Solar Cells

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...

“…The annealing was to stress the performance of the cell in an effort to evaluate the long-term reliability of the solar cell with metallization. 27 A moderately high temperature of 200…”

Section: Resultsmentioning

confidence: 99%

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

Huang¹,

Rao²,

Fisher³

2015

“…This depth difference increased to 65 nm after 5 minutes of annealing. While the top 70 nm was a ternary silicide alloy, Ni 45 Co 35 Si 20 in average, the bottom 65 nm layer had an average composition of Ni 60 Si 40 , or Ni 3 Si 2 . However, the double layered structure was not obvious in the cross sectional SEM in Figure 2b.…”

Section: Resultsmentioning

confidence: 99%

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

Huang¹

2015

Metallization of silicon solar cells using electroplating is of interest recently as a cheaper replacement of silver screen printing. Light assisted electroplating of Nickel-Cobalt (NiCo) alloys were studied on crystalline silicon (Si) solar cells in this paper. The effect of illumination was investigated in conjunction with the light absorption properties of the plating solution, emission spectrum of the light source as well as the plating current. A solution dependent threshold light intensity was observed, beyond which the solar cells were under reverse bias during electroplating. The detailed compositional depth profiles of the silicide layer were studied along the silicidation reaction using microscopic energy dispersive X-ray spectrum (EDS). A significant improvement in the performance degradation at high temperature thermal stress was observed for solar cells metalized with the ternary silicide. The cost of solar energy compared with fossil fuel is one of the key determining parameters in its adoption by the market. Therefore, ways of saving the manufacturing cost of silicon solar cells are of great interest. The current manufacturing process involves a screenprinting process to metalize the front side with silver paste. While the screen-printing process has been integrated into the process flow with standard tooling, it typically suffers from conversion loss due to the low conductivity of the paste and low line height-width aspect ratio. In addition, the high price of silver potentially limits the cost reduction of the solar cell.Replacing the screen-printed Ag with electroplated high aspect ratio Cu grids has been proposed decades ago.1,2 This method has recently gained more attention due to the high price of Ag and the easy integration of Cu electroplating with laser patterning. The selective emitter formed by the in-situ doping during laser patterning 3-5 further eases the process control for metallization and has the potential to improve the conversion efficiency of the solar cell.Different Cu metallization schemes involving electroplating have been reported. [6][7][8][9][10][11][12][13][14][15][16][17] Most of these schemes involve the formation of Ni silicide either from electroless or electrolytic deposited Ni. [8][9][10][11][12][15][16][17] 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 18 and thus improves the solar cell performance. Ni and Co silicidation has been extensively studied for the application in complementary metal oxide semiconductor (CMOS) contacts.18-23 The Ni silicidation starts at as low as 300• C for metal rich phases, followed by monosilicide phases around 400• C and silicon rich phases above 800• C. 18 Co silicidation follows a similar phase evolution but at a higher temperature. 24 While the low resistive Co disilicide was originally used in CMOS it suffers from poor nucleation rate and interface roughne...

“…The experimental procedure was similar to the recent report by Bartsch et al 21 except that annealing at much lower temperatures for much longer time is included here. Light I-V measurements were conducted periodically to monitor the performance change of the cells.…”

Section: Figures 4a and 4b Show The 200mentioning

confidence: 99%

“…Since the Ag screen printed Si solar cells have been manufactured for decades, the standard tests for quality control have been well established, including UV irradiation, thermal cycling, damp heat test, and many others. 19,20 On the other hand, to our best knowledge, studies on the reliability of the Cu plated cells were not reported except for a recent report by Bartsch et al 21 In that study, they accelerated the cell degradation at elevated temperatures between 180 and 300…”

mentioning

confidence: 99%

“…22 In this paper, a systematic study on the impacts of thermal stress on the Cu plated cell performance is presented. The thermal stress method is adapted from and similar to the method reported by Bartsch et al 21 Microscopic analysis was used to understand the function of each layer as well as the structural change along the performance degradation. By performing these analyses on a series of cells stressed at different temperatures, we attempted to understand the different degradation mechanisms in different temperature regimes.…”

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confidence: 99%

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

Huang¹,

Reuter²,

Zhu³

et al. 2015

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...