One way to reduce the cost of silicon solar cells is to minimize the amount of high-quality silicon material used, such as in thin-film solar cells. However, the efficiency of these solar cells is only around 11-12%. Researchers are seeking ways to improve their efficiency. Recent breakthroughs include optimizing the structure of the upper surface through dry texturing and inserting a porous silicon mirror at the epitaxial layer/substrate interface. These two methods can increase the efficiency of solar cells to approximately 14%.
1. Two technologies to improve efficiency
Epitaxial thin-film solar cells are relatively inexpensive compared to bulk silicon-based solar cells. However, their main drawback is their relatively low efficiency. Two techniques have been shown to improve the efficiency of thin-film solar cells. One involves optimizing the upper surface structure using halogen atom plasma processing, and the other involves introducing an intermediate reflector at the epitaxial layer/substrate interface. The optimized upper surface structure combines the advantages of satisfying the requirements of uniform light scattering (Lambertian refraction) and reducing reflection by minimizing silicon removal (because the epitaxial silicon layer is already quite thin). Introducing an intermediate reflector (multiple Bragg mirror) extends the path length of low-energy photons by at least 7 times, ultimately significantly improving the efficiency of the solar cell.
2. Low-cost solar cells
Silicon solar cells based on monocrystalline or polycrystalline silicon substrates constitute the mainstay of the photovoltaic market. However, producing these solar cells entirely from high-purity silicon is extremely energy-intensive and expensive. To further promote the development of the photovoltaic industry, the production cost of solar cells should be significantly reduced by lowering material costs.
Epitaxial thin-film silicon solar cells have the potential to become a low-cost alternative to bulk silicon solar cells. Compared to current bulk silicon solar cells (200 μm), these screen-printed solar cells utilize cheaper substrates and have thinner active silicon layers (20 μm). These low-cost substrates consist of highly doped crystalline silicon wafers (pure silicon processed from metallurgical-grade silicon or waste). A thin epitaxial active silicon layer is deposited on this substrate using chemical vapor deposition (CVD).
3. Industrial Competitiveness
The manufacturing process of epitaxial thin-film silicon solar cells is very similar to that of traditional bulk silicon solar cells. Therefore, compared with other thin-film technologies, it is relatively easy to realize the production of epitaxial thin-film silicon solar cells in existing production lines. However, the main disadvantage of the epitaxial thin-film silicon solar cell industry in terms of competitiveness lies in the lower efficiency of thin-film silicon solar cells compared to traditional bulk silicon solar cells: these cells can achieve open-circuit voltage and fill factor levels similar to those of bulk silicon solar cells, but due to the presence of an optically active thin layer (the active layer thickness of thin-film silicon is only 20 μm, compared to 200 μm for bulk silicon), light is lost due to poor substrate quality when light is transmitted from the epitaxial layer to the substrate, resulting in light loss and short-circuit current loss, which can be as high as 7 mA/cm2.
The challenge lies in achieving a perfect balance between efficiency and cost, while also considering large-scale industrial production. This paper introduces two techniques that can extend the optical path length and thus improve the efficiency of epitaxial thin-film silicon solar cells: plasma texturing and inserting porous silicon mirrors at the interface between a low-cost silicon substrate and the active layer. Results show that these measures can improve the efficiency of epitaxial thin-film silicon solar cells to approximately 14%.
4. Plasma textured surface on the top surface
By manipulating the upper surface of the active layer of a solar cell, surface light scattering changes, thus affecting the cell's performance. The goal is to create an ideal upper surface with 100% diffuse reflection (Lambertian refraction, exhibiting total scattering). At this point, photons pass through the active layer at an average angle of 60°, doubling the propagation path length. In other words, the optical performance of a 20μm thick active layer is equivalent to that of a 40μm thick layer.
Using fluorine-based plasma treatment, only a very small amount of silicon (only 1.75 μm) is removed, yet an ideal upper surface exhibiting Lambertian refraction can be obtained. This is extremely important for epitaxial thin-film silicon solar cells, as the active layer of this type of solar cell is quite thin (20 μm). In addition to optimizing scattering and improving cell efficiency, plasma treatment also reduces reflection, enables tilted optical coupling, and reduces contact resistance. This reduces short-circuit current by 1.0 to 1.5 mA/cm², further improving cell efficiency by 0.5 to 1.0%.
5. Silicon mirror
Another way to improve the efficiency of epitaxial thin-film silicon solar cells is to insert a porous silicon mirror at the interface between the active layer and the low-cost substrate. This mirror can reduce the amount of long-wavelength light propagating into the substrate.
In practice, a porous silicon stack is formed by electrochemically alternating growth of porous and few-porous thin layers (a type of multiple Bragg reflector) to create a reflector, with the thickness of the alternating layers defined by the quarter-wavelength law. During epitaxial growth of the active layer, the porous silicon in the stack contains voids of varying sizes, which recombine into thin layers while maintaining the original layout. This structure has proven to be an efficient reflective structure. This reflector reflects photons reaching the interface through the Bragg effect (a conventional incident reflector) or total internal reflection (light is incident on the reflector at an angle greater than the critical angle). As a result, these photons pass through the active layer again. Photons reflected outside the escape angle (most of the reflected photons, as the light has been scattered) reach the upper surface of the active layer and are reflected again. This extends the optical path length and improves the efficiency of the solar cell. Results show that, with a perfect Lambertian surface achieved on the upper surface, a 15-layer porous silicon reflector can increase the light propagation path length by 14 times, meaning that an epitaxial thin-film silicon solar cell with a 15 μm active layer will have the same performance as a 210 μm thick bulk silicon solar cell.
Introducing porous silicon mirrors can achieve an internal reflectivity of 80-84%, with 25% attributed to the Bragg effect itself. Optimized mirror designs can further improve the Bragg effect, allowing the thickness of both porous and low-pore layers to vary with depth (flexible porous silicon stacking), thus significantly increasing the mirror's bandwidth. Utilizing this flexible and unique structure, the path length for low-energy photons can be increased by up to seven times. Solar cells fabricated on low-cost silicon substrates using this reflective layer and screen-printed contacts achieved a high efficiency of 13.9%, with a Jsc of 29.6 mA/cm².
