Principle of Light Absorption of Silicon-Based Solar Texturing Surface
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Solar energy, as a green, sustainable, and clean resource, can be converted into heat or electricity and is one of the most important alternatives to traditional energy sources. Polycrystalline silicon solar cells are widely used in photovoltaic power generation due to their low cost. Reducing the surface reflectivity of polycrystalline silicon wafers is one of the key methods to improve the efficiency of polycrystalline silicon solar cells.
Introduction
According to the BP World Energy Outlook (2020 Edition) by British Petroleum, renewable energy is expected to experience the fastest demand growth over the next 30 years. By 2050, the proportion of renewable energy in primary energy is projected to increase from 5% in 2018 to 60% in the net zero scenario, 45% in the rapid transformation scenario, and 20% in the business-as-usual (BAU) scenario. Among renewable energy sources, solar energy can directly or indirectly convert into other forms of energy, such as heat or electricity, making it one of the most important alternatives to traditional energy. Compared to traditional energy, solar energy is clean, pollution-free, requires minimal maintenance, and can be considered a renewable and nearly inexhaustible resource throughout the expected duration of solar activity. Silicon solar cells are devices that directly convert light energy into electrical energy through the photoelectric or photochemical effect. When exposed to sunlight, they output voltage and current through conversion and have been widely used in production and daily life. The photoelectric conversion efficiency of solar cells is a key technical parameter that measures the utilization rate of solar energy. Improving the absorption efficiency of solar cells is crucial to their production.
This paper first analyzes the absorption mechanism and common texturing methods used to improve the absorption efficiency of silicon-based solar cells, while classifying and explaining the current mainstream laser texturing techniques. Next, the control methods and texturing effects of key process parameters in laser texturing, as discussed in various studies, are organized, and the influence of surface morphology on the absorption effect of laser-textured microstructures is explained. The research progress on chemical post-treatment methods for improving the light absorption effect and photoelectric conversion efficiency of laser-textured polycrystalline silicon surfaces is summarized.
This paper first analyzes the absorption mechanism and common texturing methods used to improve the absorption efficiency of silicon-based solar cells, while classifying and explaining the current mainstream laser texturing techniques. Next, the control methods and texturing effects of key process parameters in laser texturing, as discussed in various studies, are organized, and the influence of surface morphology on the absorption effect of laser-textured microstructures is explained. The research progress on chemical post-treatment methods for improving the light absorption effect and photoelectric conversion efficiency of laser-textured polycrystalline silicon surfaces is summarized.
Surface Absorption Principle of Silicon-Based Solar Texturing
A significant portion of solar cell efficiency loss is due to optical energy loss, which reduces the photocurrent of the cell. The primary factor contributing to optical energy loss is the reflection of light energy from the cell surface. Reducing surface-reflected light energy loss and increasing light absorption are key methods to improve solar cell efficiency. The more light that is reflected, the more it hinders the improvement of solar cell efficiency; conversely, the more light absorbed, the easier it is to enhance cell efficiency. As shown in Figure 1, when the energy (1) of incident light reaches the front surface of the material in Figure 1a, it is divided into absorbed energy (2) and reflected energy (3). To reduce light reflection, the surface of the silicon wafer can be micro-textured to form a pyramid structure, as shown in Figure 1b. In this structure, the energy (1) of incident light is divided into absorbed energy (2) and reflected energy (3). However, due to the inclined surface, the reflected energy (3) has the opportunity to bounce back and reach the next surface, forming a second reflected energy (5) and a second absorbed energy (4) that penetrates the material, thus reducing the silicon wafer's reflectivity and improving the solar cell's photoelectric conversion efficiency.
Figure 1 The influence of surface texture on incident light absorption
Currently, most solar cells are made from monocrystalline silicon or polycrystalline silicon. Monocrystalline silicon solar cells have higher photoelectric conversion efficiency, while polycrystalline silicon solar cells require lower raw material purity and have a broader supply, making them much more cost-effective than monocrystalline cells, as shown in Table 1. From an economic perspective, polycrystalline silicon solar cells offer greater future market share and development potential.
Table 1 Comparison of experimental efficiency and performance between monocrystalline silicon and polycrystalline silicon solar cells (data taken from the U.S. Renewable Energy Laboratory - NREL)
Figure 1 The influence of surface texture on incident light absorption
Currently, most solar cells are made from monocrystalline silicon or polycrystalline silicon. Monocrystalline silicon solar cells have higher photoelectric conversion efficiency, while polycrystalline silicon solar cells require lower raw material purity and have a broader supply, making them much more cost-effective than monocrystalline cells, as shown in Table 1. From an economic perspective, polycrystalline silicon solar cells offer greater future market share and development potential.
Table 1 Comparison of experimental efficiency and performance between monocrystalline silicon and polycrystalline silicon solar cells (data taken from the U.S. Renewable Energy Laboratory - NREL)
| Cell Type | Maximum Efficiency under Non-concentrated Conditions (%) | Maximum Efficiency under Concentrated Conditions (%) | Advantages | Disadvantages |
| Single Crystal Silicon | 25.0 | 27.6 | High efficiency, and long service life | High cost |
| Polycrystalline Silicon | 21.3 | - | Low cost, simple to manufacture | Low efficiency |
Note: "_" means this item is not indicated in the literature
The technology of micro-texturing the surface of polycrystalline silicon solar cells is commonly referred to as surface texturing technology. Commonly used texturing methods include organic grooving, chemical etching, reactive ion etching, plasma etching, and laser etching. These methods reduce the surface reflectivity of polycrystalline silicon wafers, but each has inherent drawbacks. For example, the mechanical grooving has a deep groove depth, making it unsuitable for processing thin substrate silicon wafers. The light-trapping microstructure created by chemical etching has a relatively small depth-to-diameter ratio and a limited number of light reflections, and it also causes environmental pollution. Reactive ion etching technology takes a long time and has high production costs.
Laser etching technology is a non-contact process characterized by flexibility, environmental friendliness, and being pollution-free. Laser etching is also referred to as laser surface texturing or laser surface texture, and in the field of polycrystalline silicon solar cells, it is commonly known as laser texturing. This article primarily uses the term laser texturing to describe it. Laser texturing technology involves directly or indirectly etching the surface of polycrystalline silicon wafers using a laser under optimal parameters to create a pyramid-shaped microstructure. Generally, the thickness of an untextured silicon wafer ranges between 400 and 1,000 nm. The average reflectivity is around 35% when exposed to light with wavelengths between 100 and 200 nm. After laser surface texturing, the reflectivity of the polysilicon surface decreases by 10% to 20%, depending on various factors, which influence the process both before and after texturing.
The technology of micro-texturing the surface of polycrystalline silicon solar cells is commonly referred to as surface texturing technology. Commonly used texturing methods include organic grooving, chemical etching, reactive ion etching, plasma etching, and laser etching. These methods reduce the surface reflectivity of polycrystalline silicon wafers, but each has inherent drawbacks. For example, the mechanical grooving has a deep groove depth, making it unsuitable for processing thin substrate silicon wafers. The light-trapping microstructure created by chemical etching has a relatively small depth-to-diameter ratio and a limited number of light reflections, and it also causes environmental pollution. Reactive ion etching technology takes a long time and has high production costs.
Laser etching technology is a non-contact process characterized by flexibility, environmental friendliness, and being pollution-free. Laser etching is also referred to as laser surface texturing or laser surface texture, and in the field of polycrystalline silicon solar cells, it is commonly known as laser texturing. This article primarily uses the term laser texturing to describe it. Laser texturing technology involves directly or indirectly etching the surface of polycrystalline silicon wafers using a laser under optimal parameters to create a pyramid-shaped microstructure. Generally, the thickness of an untextured silicon wafer ranges between 400 and 1,000 nm. The average reflectivity is around 35% when exposed to light with wavelengths between 100 and 200 nm. After laser surface texturing, the reflectivity of the polysilicon surface decreases by 10% to 20%, depending on various factors, which influence the process both before and after texturing.
