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3D Surface Measurement: Characterization of Solar Cells

Combined confocal and interferometry technology enables non-contact, high-precision analysis of the surface texture

Solar energy is becoming more and more important all around the globe. Not only is it available in unlimited supply, it also offers key advantages for protecting the climate and the environment. Every year, many thousands of solar cells are produced worldwide for new photovoltaic plants. An important criterion for quality control is 3D characterization of the light-absorbing surface. In the past, this required time-consuming SEM analysis. A Dual-Core 3D Measuring Microscope that combines confocal and interferometry technology offers non-contact, high-precision analysis of the surface texture of solar cells in a matter of seconds.


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Monocrystalline and polycrystalline silicon

The most frequently used basic material for solar cells is silicon. In large-scale solar power generation, thick-film cells are most common, with a differentiation being made between  monocrystalline and polycrystalline. Monocrystalline cells are made of monocrystalline silicon wafers like the ones used for semiconductor fabrication. Polycrystalline cells consist of wafers with varying crystal orientation. For one thing, they can be manufactured by casting and cost less than monocrystalline cells.

The efficiency of the solar cell depends on the silicon dopant, the light intensity and the wavelength range used, the optical thickness and surface texture. Currently, the energy efficiency of a solar cell is about 20 per cent. Applying specific but extremely expensive surface texturing processes, the solar cell can absorb more light, increasing its efficiency by up to 50 per cent. The application that demands the greatest cell efficiency is space travel.

Surface texture enhances energy efficiency

Numerous techniques have been tested for increasing cell efficiency, for instance light focusing with Fresnel lenses, solar concentrators or anti-reflection coatings. The most effective way to increase light absorption is by increasing the effective optical thickness of the silicon surface. This method, called surface texturing, depends to a large extent on the type of silicon. Monocrystalline silicon texturing is accomplished by a wet anisotropic etching process based on sodium hydroxide solution. The cystallographic silicon plane {111} is slowly etched, and the result is squared pyramids grown randomly with equal angled surfaces. The quality of the surface and the amount of pyramids depend on the temperature and the composition of the etching solution. The light absorption of this surface texture is extremely effective by increasing the number of internal light reflections. 

In comparison, polycrystalline silicon texturing is not quite as effective due to the fact that most of the grains have incorrect orientation. Different grains etch at different rates, causing the formation of steps at grain boundaries, which may lead to problems with soldering zones and contacting structure in the subsequent metal screening process.

Fig. 2 (right): The height distribution statistics of the segmentation are used for surface structure characterization.

3D surface measurement in a few seconds

Solar cell quality control is done at the end of the production chain, testing each individual cell for efficiency. The optical surface metrology system Leica DCM 3D combines confocal and interferometry technology and offers the possibility to check silicon surface texture, roughness, pyramid statistical characterization and metal contact in a few seconds.

Unlike the time-consuming scanning electron microscope method, the wafer is simply placed under the Leica DCM 3D and a 3D measurement taken in less than 10 seconds. The high local slope of the pyramid faces demands the use of objectives with a high numerical aperture, which are only available in confocal technology.

Figure 4 shows a 3D measurement of a monocrystalline silicon wafer after pyramid etching. For a 3D measurement of this kind, a 150x objective with a numerical aperture of 0.95 was used. As a result, the size of the visual field was reduced to a few tens of microns, which is roughly equivalent to the visual field of an SEM. The surface is scanned a few microns along the focus position of the objective, collecting the confocal images plane by plane. The result is a high-resolution image similar to that generated by an SEM with infinite focus and precise 3D information on the height of the pyramids.