Measuring the Film Thickness of ITO and Perovskite Layers in Perovskite Solar Cells

Significance of the Topic

Perovskite solar cells require tight control of thin functional films (transparent electrodes and absorber layers) to optimize optical transmission, electrical conductivity and device performance. Indium tin oxide (ITO) thickness influences visible transmittance and sheet resistance, while perovskite absorber thickness controls light absorption and carrier generation. Rapid, nondestructive and spatially flexible thickness measurement techniques are therefore essential for R&D and quality control of perovskite photovoltaics.

Objectives and Study Overview

This application study evaluated the capability of energy dispersive X-ray fluorescence (EDXRF) to measure nanometer-scale film thicknesses for:

• ITO layers deposited on glass
• Perovskite (formamidinium lead iodide, FAPbI3) layers deposited on ITO/glass

The work compared EDXRF results with optical laser-scanning microscopy (OLS) for the ITO-only sample to assess quantitative accuracy and demonstrated quantification of multiple stacked layers by specifying compositional and density information for each layer.

Methodology

EDXRF thickness quantification used the film FP method, which reports the surface mass (deposition quantity, µg/cm2). Film thickness (nm) is obtained by dividing deposition quantity by layer density (g/cm3) and applying the unit conversion factor to nanometers.

Key experimental approaches:

• Specify elemental composition and density of each layer in the measurement model. When an element occurs in both target layer and substrate, select a different element unique to the target layer for quantification or supply the thickness, composition and density of the interfering layers so the software can separate contributions.
• Measure at multiple positions to evaluate lateral uniformity; EDXRF reports an average within the instrument collimator area.
• Use OLS laser microscopy to measure step heights where lithographically defined patterns expose a cross-section for independent thickness validation and to assess surface roughness.

Used Instrumentation

The analyses were performed with Shimadzu instrumentation and standard EDXRF configurations as follows:

• EDX-7200 energy dispersive X-ray fluorescence spectrometer
• Detector: silicon drift detector (SDD)
• X-ray tube target: Rhodium
• Atmosphere: air
• Film FP analysis group (software model for thin-film quantification)

Representative measurement conditions for the ITO-only sample:

• Quantitative element: In (In Kα line)
• Tube voltage: 50 kV; tube current: auto
• Integration time: 60 s; dead time limited to 30%
• Collimator: 3 mm diameter; no primary filter

For perovskite-on-ITO samples:

• Quantitative elements: Pb (Pb Lβ1 line) for FAPbI3; In (In Kα) for ITO
• Primary filter: #4; integration time: 60 s; collimator: 10 mm diameter
• FAPbI3 density specified as 4.00 g/cm3 in measurement conditions

OLS validation instrument:

• OLS5500 laser microscope used to measure step height in lithographically defined pattern to obtain direct thickness and surface topography (roughness)

Main Results and Discussion

ITO-only sample (lithographically patterned):

• EDXRF film FP results at three positions: 212 nm, 213 nm, and 210 nm (measured with 3 mm collimator, 60 s acquisition).
• OLS5500 laser microscope measured a step height of 214 nm in the patterned area. The close agreement indicates EDXRF can produce thickness values comparable to OLS for continuous ITO films.
• EDXRF provides an areal average within the collimator footprint; repeated positions showed negligible spatial variation for this sample.
• Surface roughness observed by OLS confirms that topography can be assessed with OLS, while EDXRF reports the average mass-thickness and is less sensitive to localized topography.

Perovskite (FAPbI3) on ITO samples:

• Two samples with different perovskite thicknesses were measured simultaneously for both FAPbI3 and underlying ITO by specifying layer compositions and densities.
• Quantitative results: Sample A — FAPbI3 = 957 nm, ITO = 161 nm; Sample B — FAPbI3 = 325 nm, ITO = 128 nm (results from film FP analysis, 60 s acquisitions).
• The ability to deconvolute signals from stacked layers was achieved by using Pb as the quantitative element for the perovskite and providing explicit layer information for the model.

Key practical considerations discussed:

• When an element in the target layer is also present in substrate or other layers (e.g., Sn present in both ITO and glass), quantification requires either selecting a different element unique to the target layer (In for ITO) or providing precise information on interfering layers.
• EDXRF measures mass-thickness nondestructively and without special sample pretreatment; it can be applied to intact devices where cross-sections are not available.

Benefits and Practical Applications

EDXRF film FP analysis offers several practical advantages for perovskite solar cell development and production:

• Nondestructive, no cross-sectioning or coating removal required — useful for monitoring production substrates and finished devices.
• Rapid measurement (typical 60 s integration) with spatial averaging over the chosen collimator area — suitable for spot-checks and homogeneity assessments.
• Capability to quantify multiple stacked layers when layer compositions and densities are specified in the measurement model.
• Comparable quantitative accuracy to high-resolution profilometry/OLS for continuous thin films, enabling complementary use in R&D and QC workflows.

Future Trends and Potential Uses

EDXRF thickness mapping and modelling are likely to become more integrated into perovskite research and manufacturing for several reasons:

• Increasing demand for in-line, nondestructive thickness control in roll-to-roll and larger-area perovskite modules.
• Development of improved software models and multilayer fitting routines to handle complex stacks and gradient compositions.
• Integration with automated sampling stages for spatial mapping and statistical process control across substrates.
• Combination with complementary metrology (e.g., OLS, ellipsometry, electrical sheet-resistance mapping) to correlate mass-thickness with optical and electrical performance metrics.

Conclusion

This application demonstrates that EDXRF using the film FP method can measure nanometer-to-micrometer scale film thicknesses of ITO and perovskite layers nondestructively and rapidly. By specifying layer compositions and densities, thicknesses of multiple stacked layers (FAPbI3 on ITO) can be quantified. Results for ITO agreed closely with independent OLS step-height measurements, supporting the method's utility for R&D and quality-control tasks in perovskite photovoltaic development.

References

• Tamura Y., Futamata Y., Fujii T. Energy Dispersive X-ray Fluorescence Spectrometer — Measuring the Film Thickness of ITO and Perovskite Layers in Perovskite Solar Cells. Shimadzu Application News, First Edition Oct. 2026.
• Shimadzu Corporation. EDX-7200 Energy Dispersive X-ray Fluorespectrometer product documentation (application note content).