Analysis of solar cell silicon

Importance of the Topic

The purity of solar-grade silicon directly impacts the efficiency of photovoltaic cells. Rapid, reliable trace impurity analysis in bulk silicon is essential for production control, quality assurance, and minimizing performance losses in solar modules.

Objectives and Study Overview

This study evaluates the Thermo Scientific Element GD Plus glow discharge mass spectrometer (GD-MS) for direct, ultra-trace quantification of impurities in bulk crystalline silicon. Key goals are to achieve sub-ppb detection limits, rapid turnaround (<20 minutes), and robust operation suitable for multi-shift manufacturing environments.

Methodology and Instrumentation

Sample Preparation:

• Mechanical shaping and diamond grinding to expose a clean, flat surface (20–60 mm diameter).
• Sequential surface cleaning: dilute HNO₃, deionized water, 20% HF, and isopropanol rinses.
• Drying under argon; residual contaminants removed by a 12-minute presputter step in the GD source.

Instrument and Settings:

• Thermo Scientific Element GD Plus GD-MS with high-purity graphite consumables kit.
• Discharge in continuous DC mode at 65 mA and ~1 000 V with 400 mL/min Ar gas.
• Medium resolution (R ≈ 4 000) to resolve polyatomic interferences (e.g., ^28Si^36Ar⁺ vs. ^64Zn⁺, ^28Si₂⁺ vs. ^56Fe⁺).
• Optimum measurement: 12 min presputter + 7 min data acquisition per spot (20 min total).

Main Results and Discussion

Using medium resolution, all major interferences from Si-Ar and Si₂ species were fully separated from target analyte peaks. Analysis at five sample locations yielded semiquantitative concentrations with detection limits typically below 1 ppb for most elements. Theoretical detection limits approach 20 ppt based on signal-to-noise considerations. Elements with low isotopic abundance (e.g., ^44Ca, ^82Se) exhibited higher limits but remained in the sub-ppb range. Precision is governed by counting statistics (≈14% RSD at 1 ppb for a 5 s count time) and can be improved by extending acquisition time on critical isotopes.

Benefits and Practical Applications of the Method

• Direct solid analysis eliminates lengthy wet chemical digestion, reducing acid and water consumption and contamination risk.
• Fast, reproducible results support real-time process control in solar cell manufacturing.
• Broad elemental coverage enables comprehensive impurity profiling for performance optimization and yield improvement.

Future Trends and Potential Applications

Integration of GD-MS with complementary techniques (FT-IR, Raman, XPS) will provide multi-dimensional silicon characterization. Advances in standard reference materials and RSF calibration will improve absolute quantitation. Automated, in-line GD-MS analysis and miniaturized sources are emerging for real-time monitoring of novel silicon formulations, including multicrystalline and doped materials.

Conclusion

The Thermo Scientific Element GD Plus GD-MS delivers rapid, robust, semiquantitative multi-element analysis of solar-grade silicon at sub-ppb levels. Its minimal sample preparation, high sensitivity, and interference-free measurements make it a valuable tool for impurity control and process optimization in photovoltaic manufacturing.

References

Hinrichs J, Rottmann L. Analysis of solar cell silicon. Thermo Fisher Scientific Application Note No. 30164, 2019.