How Advanced Metrology Improves Solar Cell Efficiency

Research, development, and production of solar photovoltaics are advancing rapidly, with innovations in texturing and metallization driving higher efficiency and more cost-effective solar cells.

KLA Instruments^™, a division of KLA Corporation, has developed an array of cutting-edge metrology solutions tailored to the needs of solar cell production, such as the Zeta^™-20HR and Zeta^™-Solar optical profilers, the P-7 stylus profiler, and the R54 sheet resistance mapper.

Image Credit: KLA Instruments^™

Pyramid Texture Monitoring: Correlation of Zeta-20HR to SEM

Measuring the size and height of textured light-absorbing surfaces is essential for optimizing electrical generation performance from a solar cell, as topographic characteristics directly influence sunlight absorption.

In response to the solar industry’s roadmap request, KLA has developed the Zeta-20HR 3D Optical Profiler with enhanced resolution. Zeta-20HR is engineered to clearly visualize and measure surface textures with smaller feature sizes than earlier, less efficient solar cells.

Zeta-20HR delivers a complete solution for the solar industry by leveraging high optical resolution to rapidly measure the height and the lateral dimensions of solar cell textures. These measurements can be performed across the solar cell’s entire area, allowing manufacturers to preserve the uniformity of the cross-wafer quality of the solar cell surface.

KLA Instruments has also engineered the Zeta-20HR stage mechanics for flexibility. The Zeta-20HR features an integrated 230 mm x 230 mm XY stage that supports the latest large-sized cells and multiple generations of legacy smaller solar cell formats still in production.

The stage’s large sample size enables measurement of texture uniformity across the solar cell surface, which is crucial for ensuring production quality.

Scanning Electron Microscopy (SEM) has been proposed as a technique for measuring solar cell texture. However, while SEM effectively measures lateral texture size, it does not offer a full solution like the Zeta-20HR. It is slower, more expensive (particularly for those able to accommodate full wafer sizes), more complex, and time-consuming for measuring the height of solar cell textures.

The solar industry requires both height and lateral size metrology, and the Zeta-20HR integrates all necessary advanced texture measurements to meet these needs, in contrast to SEM.

To demonstrate Zeta-20HR’s optical measurement quality, different pyramid sizes were compared using both SEM and Zeta-20HR.

The results revealed a strong linear size correlation, with a correlation coefficient of R² = 0.95 for a set of multiple generation solar cells with textures ranging from 0.6-1.9 µm in lateral size.

The Zeta-20HR provides a rapid and precise method for characterizing solar textures' lateral and height dimensions.

The Zeta-20HR optical profiler measurements of lateral texture size correlate well with SEM images [images are 10 µm x 10 µm]. Image Credit: KLA Instruments^™

Solar texture lateral size measurements by SEM and Zeta-20HR show a correlation of R² = 0.948. Image Credit: KLA Instruments^™

Metallization Measurement Solution with the Zeta-Solar 3D Optical Profiler

KLA recently introduced the Zeta-Solar 3D optical profiler, which was designed to measure electrically conductive metallization properties in solar cells. The back of a solar cell has a variety of metallic parts known as fingers, busbars, and pads that conduct the solar-generated electrical energy away from the device.

Metallization in advanced solar cells is evolving rapidly, with novel device designs being introduced for solar cell production to both (1) reduce material expenses and (2) increase the surface area available for light capture. These improvements have been found to lower solar cell fabrication costs.

To further reduce material costs, electrically conducting metal lines have been made thinner than previous lines, and the busbar (the intersection region where electrical signals from the fingers are combined) is being replaced with a compact silver pad, reducing the quantity of the materials required.

The Zeta-Solar optical profiler has been engineered for measurement characterization and quality control of Ag and Ag/Cu metallization lines, as well as bond and contact pads in advanced solar cell technologies.

The Zeta-Solar optical profiler utilizes high dynamic range (HDR) capability to automatically detect and measure highly reflective metallic lines on the very low reflectivity textured surface. Statistical and Pass/Fail data are automatically generated. Image Credit: KLA Instruments^™

Silver Pad Solution with the Zeta-Solar Optical Profiler

An example of an advanced pad feature is the 1 mm²-sized silver pad depicted below. Zeta-Solar automatically assesses multiple region measurements to analyze the entire structure. These novel solar cells feature significantly reduced reflective texture regions surrounding the metallization. Zeta-Solar uses High Dynamic Range (HDR) to measure these narrower metal lines against the very low reflectivity textured surface.

Silver pad measured by the Zeta-Solar optical profiler. The Zeta-Solar automatically measures pad and line dimensions to enable process monitoring. Image Credit: KLA Instruments^™

Additional Solar Metrology Solutions from KLA Instruments

The Filmetrics R54 sheet resistance mapper measures sheet resistance in silicon solar cells. KLA’s leading sheet resistance technology provides crucial data for solar applications and monitors diffusion, oxidation, and annealing processes.

The R54 four-point probe (4PP) technology is a semiconductor industry standard. It delivers outstanding short-term repeatability and long-term stability.

Combining silica and thin film cell processes to maximize efficiency is an emerging trend in solar technology. For instance, a perovskite solar and Tandem (heterojunction technology [HJT] + perovskite) structure substantially enhanced efficiency.

As these technologies mature, they will replace traditional process steps and require additional thin-film step height monitoring. The Tencor^® P-7 stylus profiler provides precise, repeatable results for monitoring nm- and Å-level film thickness in perovskite and Tandem solar cells.

Summary and Outlook

TOPCon cells dominated in 2023, but HJT and tandem technologies are now on the rise and set to continue to grow in popularity through 2025.

Critical measurement points for novel developments in TOPCon solar cells include texture pyramid control with the Zeta-20HR optical profiler, post-diffusion tracking with R-series sheet resistance mappers, and metallization monitoring with Zeta-Solar.

Zeta-20HR can be used for texture monitoring in HJT cells, while Zeta-Solar supports electrode printing. For Tandem technology devices, thin film thickness is typically monitored using the P-series stylus profiler.

• The Zeta^™-20HR optical profiler distinguishes the size and height of pyramid structures in the 0.6-2.0 µm range.
• The Zeta^™-Solar optical profiler is optimized for solar PV metallization metrology and metallization quality control to less than 1 % SD.
• The Filmetrics^® R54 4-Point Probe measures 0.1 % σ, tracking several process quality steps even after more than 150,000 measurements.
• The Tencor^® stylus profiler can measure nm-scale film thickness on perovskite solar cells with Å-level Z resolution.

The KLA Instruments portfolio of solar cell metrology tools fulfills industry requirements and drives innovation in both current and next-generation fabrication processes. Additional details regarding stylus profiler and sheet resistance measurement applications in solar cell manufacturing are available directly from KLA Instruments.

This information has been sourced, reviewed, and adapted from materials provided by KLA Instruments^™.

For more information on this source, please visit KLA Instruments^™.