Methods to reduce the cost of silicon solar cells

One way to reduce the cost of silicon solar cells is to minimize the amount of high-quality silicon materials used, such as thin-film solar cells. However, the efficiency of such solar cells has only reached about 11-12%. Researchers are looking for ways to increase their efficiency. Recent breakthroughs include optimizing the structure of the upper surface through dry texturing and inserting an intermediate porous silicon mirror at the epitaxial layer/substrate interface. These two methods can increase the efficiency of solar cells to about 14%.

Two technologies to improve efficiency

Compared to bulk silicon-based solar cells, epitaxial thin-film solar cells are relatively cheap. But the main disadvantage of epitaxial thin-film solar cells today is their relatively low efficiency. Two technologies have been shown to improve the efficiency of thin-film solar cells. One is to optimize the upper surface structure using halogen atom plasma processing, and the other is to introduce an intermediate reflector at the epitaxial layer/substrate interface. The optimized upper surface structure has the advantages of both meeting the requirements of uniform light scattering (Lambertian refraction) and reducing reflection by slightly removing silicon (because the epitaxial silicon layer is already quite thin). The introduction of an intermediate reflector (multiple Bragg reflectors) extends the path length of low-energy photons by at least 7 times, which ultimately greatly improves the efficiency of the solar cell.

Low-cost solar cells

Silicon solar cells based on single-crystal or multi-crystalline silicon substrates are the mainstay of the photovoltaic market. However, if all are made of high-purity silicon, the production of such solar cells is very energy-intensive and relatively expensive. To further promote the development of the photovoltaic industry, the production cost of solar cells should be greatly reduced by reducing material costs. Epitaxial thin-film silicon solar cells have the potential to become a low-cost alternative to bulk silicon solar cells. Compared with current bulk silicon solar cells (200μm), this screen-printed solar cell uses a cheaper substrate and a thinner active silicon layer (20μm). This low-cost substrate consists of highly doped crystalline silicon wafers (pure silicon processed from metallurgical-grade silicon or waste). A thin layer of epitaxial active silicon is deposited on this substrate using chemical vapor deposition (CVD). Industry competitiveness

The production process of epitaxial thin-film silicon solar cells is very similar to that of conventional bulk silicon solar cells. Therefore, epitaxial thin-film silicon solar cell production is relatively easy to implement in existing production lines compared to other thin-film technologies. However, the main drawback of the competitiveness of the epitaxial thin-film silicon solar cell industry is the lower efficiency of thin-film silicon solar cells compared to conventional bulk silicon solar cells: the open-circuit voltage and fill factor of these cells can reach levels similar to those of bulk silicon solar cells, but due to the presence of the optically active thin layer (the active layer of thin-film silicon is only 20μm thick compared to 200μm thick for bulk silicon), the light is lost due to the poor quality of the substrate when the light is transmitted from the epitaxial layer to the substrate, and the short-circuit current loss can be as high as 7mA/cm2. The challenge is to achieve a perfect balance between efficiency and cost, and large-scale industrial production must also be considered. This article introduces two technologies that can extend the optical path length and therefore improve the efficiency of epitaxial thin-film silicon solar cells: plasma texturing and the insertion of porous silicon mirrors at the interface between the low-cost silicon substrate and the active layer. The results show that these measures can increase the efficiency of epitaxial thin-film silicon solar cells to around 14%.

Plasma velvet on the upper surface

By treating the upper surface of the solar cell's active layer, the surface light scattering changes, which affects the performance of the solar cell. The goal is to create an optimal upper surface with 100% diffuse reflection (Lambertian refraction, showing total scattering). At this time, photons pass through the active layer at an average angle of 60°, which doubles the propagation path length. In other words, an active layer that is only 20μm thick will optically appear to be 40μm thick.