XPS depth profiling of advanced solar cells with femtosecond laser ablation

XPS depth profiling of perovskite solar cells using femtosecond laser ablation — Summary

Significance of the topic

Perovskite absorbers offer high power-conversion efficiency and reduced weight compared with conventional silicon, making them attractive for terrestrial and aerospace photovoltaics. Accurate compositional and chemical-state information through the device depth is essential to establish undisturbed baselines prior to environmental or radiation testing. Conventional ion-beam sputter depth profiling frequently alters the chemistry of soft, hybrid materials (preferential loss of light elements, chemical reduction), producing artifacts that obscure true material behavior. Applying a minimally damaging removal technique such as femtosecond laser ablation (fs-LA) for XPS depth profiling can preserve oxidation states and stoichiometry and therefore improve reliability of analytic baselines for sensitive devices.

Objectives and study overview

• Compare XPS depth-profiling outcomes for a halide-perovskite solar cell using monatomic Ar+ sputtering, Ar cluster-ion sputtering, and femtosecond laser ablation.
• Assess which approach preserves the original elemental ratios and oxidation states across the perovskite layer and multilayer stack.
• Demonstrate the suitability of fs-LA to provide a chemically undisturbed baseline for subsequent environmental simulations relevant to aerospace applications.

Methodology and experimental approach

• Device architecture: a multilayer stack comprising glass substrate, ITO (∼110 nm), SnO2 (∼30 nm), and a mixed-cation halide perovskite ((FAPbI3)0.95(MAPbBr3)0.05) with an absorber thickness near 500 nm.
• Comparative depth profiling: (a) 500 eV monatomic Ar+ sputtering; (b) 8 keV Ar150+ cluster-ion sputtering; (c) femtosecond laser ablation using a 1,030 nm laser with 160 fs pulses and pulse energy ramped from ~42 µJ up to ~167 µJ when entering the substrate region.
• XPS analysis: carried out on a Thermo Scientific Hypulse Surface Analysis System in SnapShot acquisition mode with X-ray analysis spot sizes between 30 and 200 µm depending on profiling method; a flood gun was used for charge compensation during profiling.

Instrumention used

• MAGCIS ion gun capable of generating both 500 eV monatomic Ar+ and 8 keV Ar150+ cluster beams.
• Femtosecond laser source: 1,030 nm wavelength, 160 fs pulse duration; adjustable pulse energy (42–167 µJ reported).
• Thermo Scientific Hypulse Surface Analysis System for XPS data collection in SnapShot mode.
• Flood gun for charge neutralization during depth profiling.

Main results and discussion

• Monatomic Ar+ sputtering caused strong preferential sputtering: carbon and nitrogen levels were substantially reduced and the I/Pb ratio dropped from the theoretical ~3:1 to about ~1.7:1. A metallic lead (Pb0) component appeared in Pb 4f spectra within the perovskite layer, indicating beam-induced chemical reduction.
• Cluster Ar150+ profiling, while generally preferred for organics because of lower energy per atom, still produced significant damage in this system. Preferential loss of light elements was more pronounced than for monatomic sputtering and the Pb0 signal was even stronger, suggesting thermal or energy-density effects from cluster impact accelerate degradation of the perovskite.
• Femtosecond laser ablation provided depth profiles that closely matched expected stoichiometry and preserved chemical states across the perovskite layer. No Pb0 signal was detected in core-level spectra obtained after fs-LA, and organic cations (FA+, MA+) along with halides (I, Br) were retained at expected relative intensities. A minor phosphorus increase at the near-surface region was attributed to surface contamination rather than profiling artifacts.
• Overall, fs-LA avoided the preferential removal and reduction artifacts evident for ion-beam methods, enabling a more faithful representation of the true starting composition of the thin-film stack.

Benefits and practical applications of the method

• Fs-LA XPS depth profiling produces chemically undisturbed compositional profiles for perovskite solar cells, crucial for establishing baselines prior to environmental or radiation testing, especially in aerospace contexts where small compositional changes may affect performance or degradation pathways.
• Preservation of oxidation states and organic/halide content improves the reliability of failure analysis, lifetime studies, and process optimization for device fabrication.
• Method reduces false positives for beam-induced degradation (e.g., apparent Pb0 formation), improving confidence in linking observed changes to experimental conditions rather than analytical artifacts.

Future trends and potential applications

• Broader adoption of fs-LA for depth profiling of soft, hybrid, and organic-rich materials beyond perovskites (e.g., organic electronics, multilayer coatings) where ion-based sputtering introduces artifacts.
• Optimization and standardization of fs-LA parameters (wavelength, pulse energy, spot size, raster strategies) tailored to different material classes to maximize material removal control while minimizing thermal effects.
• Integration with complementary in situ or correlative techniques (e.g., time-of-flight secondary ion mass spectrometry, cross-sectional TEM) to link composition, chemistry, and microstructure through depth.
• Development of standardized reference materials and protocols to qualify fs-LA XPS for routine QC and certification in industrial settings, including aerospace qualification workflows.
• Automation and data-analysis pipelines that account for laser-material interaction effects to extract quantitative, reproducible depth profiles for multi-layer devices.

Conclusion

Femtosecond laser ablation combined with XPS depth profiling offers a robust alternative to conventional ion-beam sputtering for perovskite solar cells. In this comparative study fs-LA maintained expected stoichiometry and oxidation states across the absorber layer and avoided artifacts such as preferential loss of light elements and beam-induced Pb2+ reduction to Pb0. These advantages make fs-LA particularly valuable for establishing chemically accurate baselines before environmental and radiation testing, improving the quality of degradation studies and supporting device optimization for aerospace and other demanding applications.

References

1. Hoffmann V, et al. Accessing Elemental Distributions in Thin Films for Solar Cells. In: Advanced Characterization Techniques for Thin Film Solar Cells, Second Edition, 2016, p. 523–567. doi: 10.1002/9783527699025.ch19
2. Chandler CW, Baker MA, Yun JS. Femtosecond Laser Ablation (fs-LA) XPS Depth Profiling of Lead Halide Perovskite Thin Film Solar Cells. Surface and Interface Analysis. 2025;57(3). doi: 10.1002/sia.7374