Solar Cell Analysis - Scanning Electron Microscopy (SEM) Applications

August 16, 2022

Recently, global oil prices have risen sharply and the renewable energy industry represented by solar photovoltaic (PV) power generation has received widespread attention. As the core component of PV power generation, the development prospects and market values of solar PV cells are the focus of attention. In the global battery market, PV cells account for about 27%[1]. The scanning electron microscope plays a great role in enhancing the production process and related research of PV cells.

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PV cell is a thin sheet of optoelectronic semiconductor that converts solar energy directly into electrical energy. The current commercial mass-produced PV cells are mainly silicon cells, which are divided into monocrystalline silicon cells, polycrystalline silicon cells and amorphous silicon cells.

Surface Texturing Methods for Solar Cell Efficiency Enhancement

In the actual production process of photovoltaic cells, in order to further improve the energy conversion efficiency, a special textured structure is usually made on the surface of the cell, and such cells are called "non-reflective" cells. Specifically, the textured structure on the surface of these solar cells improves the absorption of light by increasing the number of reflections of irradiated light on the surface of the silicon wafer, which not only reduces the reflectivity of the surface, but also creates light traps inside the cell, thus significantly increasing the conversion efficiency of solar cells, which is important for improving the efficiency and reducing the cost of existing silicon PV cells[2].

Comparison of Flat Surface and Pyramid Structure Surface

Compared to a flat surface, a silicon wafer with a pyramidal structure has a higher probability that the reflected light from the incident light will act again on the surface of the wafer rather than reflecting directly back into the air, thus increasing the number of light scattered and reflected on the surface of the structure, allowing more photons to be absorbed and providing more electron-hole pairs.

solar-cell-analysis-sem-applications-light-path

Light Paths for Different Incident Angles of Light Striking the Pyramidal Structure

The commonly used methods for surface texturing include chemical etching, reactive ion etching, photolithography, and mechanical grooving. Among them, the chemical etching method is widely used in the industry because of its low cost, high productivity, and simple method[3]. For monocrystalline silicon PV cells, the anisotropic etching produced by alkaline solution on different crystal layers of crystalline silicon is usually used to form a structure similar to the "pyramid" formation is the result of anisotropy of alkaline solution on different crystal layers of crystalline silicon. The formation of the pyramid structure is caused by the anisotropic reaction of alkali with silicon[4]. In a certain concentration of alkali solution, the reaction rate of OH- with the surface of Si(100) is several times or even a dozen times higher than that of the surface of Si(111), and it is this difference in reaction rate that leads to the formation of the pyramid structure.

Scanning Electron Microscopes Help Solar Cell Quality Improvement

In the process of chemical etching, the concentration of etching solution, temperature, reaction time and other factors will affect the preparation of silicon crystal cell fleece surface, resulting in different reflectivity. Using CIQTEK SEM3100 tungsten filament scanning electron microscope can effectively observe the size of the etched area and the surface pyramidal structure during the fabrication process.

Thanks to the advantages of the CIQTEK SEM3100 electron microscope's large capacity sample compartment, users can put in samples up to 370mm in diameter without cutting, and the five-axis fully automated sample stage on the electron microscope can be tilted from -10° to 75°, enabling multi-angle observation of different positions of the sample.

Sample Table Tilted at 45°

Sample Table Tilted at 45°

Sample Table Tilted at 30°

Sample Table Tilted at 30°

Sample Placed Horizontally

The lower acceleration voltage of 3~5kV is used to observe the surface pyramidal structure of PV cells in SEM3100 electron microscope, which can reduce the penetration depth of electron beam on the sample surface and make the observed surface details richer, and better characterize the surface defects and structure shape, thus helping users to compare and analyze the different velvet production processes.

According to GIR (Global Info Research) research, global solar cell (PV) equipment revenue will be approximately $44.7 billion in 2021 and is expected to reach a size of $55.57 billion in 2028. Among the product types, monocrystalline silicon will continue to occupy an important position. As a powerful tool for microscopic analysis, the CIQTEK SEM3100 will be a powerful tool for enhancing the production process of PV cells and related research.

References:

[1]Wu Jiejie, et al. Battery industry research and outlook[J]. Modern Chemical, 2017, 37(9):5.

[2]Li Jiayuan. Study of solar cell fleece surface [D]. Dalian University of Technology, 2009.

[3] Li H-L, Zhao L, Diao H-W, et al. Analysis of the factors affecting the pyramid structure in monocrystalline silicon flux production[J]. Journal of Artificial Crystals, 2010, 39(4):5.

[4]Nishimoto Y, Namba K. Investigation of Texturization for Crystalline Silicon Solar Cells with Sodium Carbonate Solutions[J]. Solar Energy Material & Solar Cells, 2000, 61(4):393-402.

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