Solar cell manufacturing: Where to innovate in the value chain
In order to meet future energy demands, companies are racing to develop renewable, efficient, and low-cost energy sources. Currently, silicon solar cells produce electricity at a cost of approximately 50 cents per kilowatt hour, which is forecast to decrease as new technologies are developed and implemented. One way to improve the cost effectiveness of silicon solar cells is to make them more efficient at absorbing light. Add-on components can be used to improve the efficiency of solar cells by installing an additional element onto a solar cell. In this paper, we discuss the merits and flaws of various types of add-on components that have recently emerged, including multijunction solar cells, perovskite solar cells, microtextured solar cells, solar concentrators, and 3D printed optics layers. We conclude that 3D printed optics components represent the most promising candidate for widespread implementation, due to ease and flexibility of the technology in terms of design, manufacturing, and installation. Additionally, this type of add-on component could be used to increase the efficiency of solar cells that are already in use.
EXCEEDING THE LIMIT
The cost of producing electric power from silicon solar cells has decreased significantly, and is now about 50 cents per kilowatt hour. The cost of generating electricity from solar cells will continue to fall as new technologies are implemented. While there has been much interest in new types of solar cell materials that are more efficient at converting light into electrical energy than silicon, these materials are expensive and will not be able to match the cost effectiveness of silicon-based solutions. This is primarily due to the high cost of the initial materials used and the need for highly-specialised fabrication methods. Increasing the efficiency of silicon solar cells by enhancing them with add-on components represents a more practical way of reducing the cost of solar energy generation. This approach involves using add-on components that improve the absorption properties of a solar cell, and thereby increase its ability to produce energy. While the theoretical efficiency of a planar silicon solar cell is estimated to be limited to around 30%, most commercially available silicon cells have efficiencies between 18–23%. With the help of add-on components produced using additive manufacturing technology, these limits can be reached, and even exceeded. Add-on components can be installed at three stages during manufacturing:
after the silicon is doped, or at the silicon doping stage
during module processing, but prior to encapsulation, or at the module processing stage
after encapsulation, or at the finished module stage
The effort and cost of installing add-on components varies greatly, depending on the type of add-on component used and on the stage of the manufacturing process when the component is added.
ENHANCEMENTS INSTALLED AT THE SILICON DOPING STAGE
At the silicon doping stage, additional, highly efficient layers can be grown directly onto the silicon wafer. In doing so, it is crucial to avoid contaminating the doped silicon, which would reduce the device’s performance. To prevent this from happening, specially designed equipment and a clean room environment are required. Therefore, introducing an add-on component at this stage is disruptive to the solar cell manufacturing process. Modifying cells in this way shows great potential, however, as multijunction solar cells, or cells that are made of two different types of light-absorbing materials, can capture much more light. Solar cells using III-V compounds—materials made from group III and group V of the periodic table—have been demonstrated to be able to achieve energy conversion efficiencies of 28%. While this type of multijunction solar cells have been successfully commercialised, they remain prohibitively expensive due to the specialized manufacturing equipment required and the high costs of the III-V compounds used in the process. Consequently, companies that have developed multijunction solar cells focus on very niche, specialized markets, such as aerospace and defence.
ENHANCEMENTS INSTALLED AT THE MODULE FABRICATION STAGE
Add-on components can also be installed during the module fabrication process—the step when electrical contacts and anti-reflective coatings are added to the silicon chip. While adding components at this stage is also disruptive to the manufacturing process, these components are more easily installed than enhancements applied during the silicon doping process, as the required equipment is more widely available. Several companies have demonstrated microstructured silicon cells, which have a carefully designed surface texture to increase the cell’s light absorption capabilities. Such surface textures are created using various etching techniques (i.e. electrochemical, chemical, or laser etching), which remove some of the silicon in a controlled manner. Currently, commercially available microstructured silicon solar cells have energy conversion efficiencies of around 24%.
At this stage, it is also possible to fabricate a multi-layered solar cell using perovskites, a type of crystal. Perovskites absorb light, produce electricity very efficiently, and crucially, they can be made into an ink and printed directly onto an existing silicon solar cell. Multi-layered solar cells created this way are conceptually very similar to the multijunction III-V silicon solar cells described above. However, making a perovskite-silicon solar cell is far simpler, due to the ease of printing the perovskite ink. The major drawbacks of perovskites include that they degrade quickly when in contact with air or water, and that they contain lead. However, lead-free alternatives are being researched, and with suitable encapsulation to protect the crystals from degradation, perovskite lifetimes can be extended. Multi-layered solar cells of this type have achieved efficiencies of up to 42% in laboratory conditions.
ENHANCEMENTS INSTALLED AFTER THE ENCAPSULATION STAGE
Once a solar cell is encapsulated, implementing an add-on component becomes a less involved process, as there is no risk of contaminating the device. Furthermore, modifying a completed solar cell with an add-on component at this stage has two great advantages: It does not disrupt the silicon manufacturing process, and the add-on components can be added to any type of finished module.
Solar concentrators have been on the market for several years, and boost solar cell efficiency by increasing the amount of light reaching the cell. Initial attempts to create solar concentrators resulted in products that were too bulky and expensive to be added to domestic installations. The next generation of concentrators were fabricated using novel materials and optics, however, which decreased the cost and size of the devices, and resulted in more widespread adoption. The use of solar concentrators does pose some risk, as the additional light the concentrator focuses on a solar cell also increases the amount of heat that build ups within the cell. This heat can seriously damage solar cells if it is not effectively dissipated. State-of-the-art concentrators are often sold as ‘smart solutions’, in which the solar cell’s excess heat is absorbed by water that can then be used by a building’s occupants. Commercially available solar concentrators have been shown to increase the efficiency of silicon cells by 5%.
Recent improvements of 3D printing have led to exciting new developments in the field of solar concentrators. 3D printing technology is now advanced enough to produce an optics layer that can act as a solar concentrator. Such a layer consists of a sheet of tiny lenses, which are coated with silver ink to make them more reflective. These lens sheets must be fabricated to extremely precise specifications to mediate enhanced light absorption effects. Traditionally, these lens sheets had to be manufactured using labour-intensive laboratory techniques such as lithography, but similar results can now be achieved through 3D printing. Using a printed optics layer instead of a conventional solar concentrator results in great reductions in terms of the add-on component’s size and weight, and can increase the energy conversion efficiency of silicon solar cells to 34%. While this new method for fabricating 3D printed optics layers has not yet been commercialised, it has enormous potential as it makes use of inexpensive, readily available plastics common in 3D printing, and allows for the design of lens sheets that can be easily customised to fit any type of solar cell.
THE COMMERCIAL POTENTIAL OF SOLAR CELL ENHANCEMENTS
This insight discussed various approaches to increase the efficiency of silicon solar cells through add-on components. Multijunction solar cells made with III-V materials can achieve higher levels of performance, but they are expensive to produce and therefore suitable only for niche applications. Perovskite inks are another promising candidate for enhancing silicon solar cell performance, provided their operating lifetimes can be extended and their toxicity reduced. Microstructured silicon designs can also boost the efficiency of the solar cell with clever cell surface designs, and have been successfully demonstrated in commercially available solar cell products.
Traditional solar concentrators have been improved to form systems which produce more electricity and also heat water, making them an attractive add-on for larger commercial installations. While all of the methods described above are viable approaches for increasing the efficiency of silicon solar cells, we believe that 3D printed solar concentrators have shown the greatest potential for future commercial applications. 3D printed lens sheets avoid many of the traditional drawbacks of solar concentrator technology in terms of size and weight, and can significantly increase the efficiency of solar cells. As a result, 3D printed solar concentrators appear to be the most promising add-on component that could easily and inexpensively provide dramatic improvements to the energy conversion performance of silicon solar cells.