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| Fig. 1: California Solar Farm. (Source: Wikimedia Commons) |
Solar energy provides a large percentage of renewable energy in California and the world, and accounts for an ever-increasing percentage of the overall energy supply. However, fundamental limitations on the physics of solar cells limit the possible efficiency of currently used commercial solar panels. This could severely inhibit the potential for solar energy to become a viable substitute for non-renewable energy sources, such as coal and natural gas. Tandem, or double junction, solar cells are a potential improvement to current commercial solar cell technology that could help push past fundamental limits and help solar grow into a viable worldwide energy source.
California is often viewed as setting an example in the United States for renewable energy usage. Earlier this year, California set the lofty goal of generating 50% of energy via renewable sources by the year 2030 and 100% by 2045. [1] Solar energy makes up a significant proportion of energy generation in California, either through large scale production for utility purposes (Fig. 1), or small scale residential applications, such as rooftop solar panels. With the cost of producing solar panels dropping every year, it has the potential to become an even greater source of energy in the near future. [2]
Solar is also growing rapidly worldwide. In the five years preceding 2014, solar capacity increased by more than ten times. [3] China is leading the way in recent solar development, accounting for over 21% of total photovoltaic (PV) capacity in 2015. [2] With the looming specters of global warming and lack of fossil fuels always present, renewable energy and solar will seek to grow even faster in coming years.
The current solar cell market is absolutely dominated by single junction silicon solar cells. In 2017, crystalline silicon (c-Si) panels made up 90% of the market, mostly due to the reduction in cost of solar grade silicon in the past 10 years. [4] Silicon solar cells have a myriad of advantages, the first of which being the cost. The cost of a silicon solar module has dropped from over 5$/W to less than 0.6$/W in 2013. [4] This makes silicon extremely attractive as a competitor to other cheap energy sources. Another advantage of silicon is that it is perhaps the best understood material in the world. Many fields, such as computer electronics, telecommunications, and optics rely on properties of silicon to channel electricity or to absorb or emit light. With the wealth of knowledge and experience available regarding this material, solar cells using crystalline silicon can be engineered to near perfection.
However, there are some fundamental limitations to single junction silicon solar cells. In their famous 1961 paper, Shockley and Queisser derived that for any single junction solar cell, regardless of material, the maximum possible efficiency it could reach is ~29%. [5] This calculation involved comparing the solar spectrum to the absorption spectrum of materials with varying bandgaps, and calculating the energy generated if one were to ignore completely non-radiative recombination and reflection. Despite the calculation making several critical assumptions, what has become known as the Shockley-Queisser limit has become a generally accepted benchmark in the photovoltaic community, and while silicon solar cells have been engineered to the point where they are approaching this limit, fundamental physical constraints prevent these types of cells from advancing much further in terms of efficiency.
A tandem, or double junction, solar cell is one that creates two different areas for light to be absorbed by having materials with different bandgaps superimposed on one another. This is contrasted with a single junction solar cell, which only has one absorbing region, and a single bandgap. The benefit of the tandem configuration can be best explained by the following analogy.
Visualize the solar cell as a cashier at a concession stand, where incoming photons are represented by customers and energy is represented by money. This concession stand, being terrible, only sells soda. A single junction cell is like the cashier who can only sell one size of soda. If he decides to sell a small size, all of his customers will be able to afford a soda, but the richer customers will have money left over. This corresponds to a low bandgap solar cell, where all photons get absorbed, but the higher energy photons lose much of their energy to thermalization due to the low bandgap. The cashier could alternatively decide to sell large sodas. This would have the benefit of extracting more money from the richer customers, who all gladly pay more for the larger size. However, it has the drawback of outpricing the poorer customers, who are sadly forced to pass by with no refreshment. This is analogous to a wide- bandgap solar cell, which extracts much more energy from the high energy photons, but lets lower energy photons pass straight through. A tandem solar cell, on the other hand, is like a cashier who has the option to sell two sizes, letting the richer patrons pay for the larger size while still allowing the poorer ones to afford a drink. Clearly, the cashier can now make more money with the same number of incoming customers. Similarly, given the same solar spectrum, a tandem solar cell can generate a higher conversion efficiency than its single junction counterpart.
Double, or even triple or quadruple junction solar cells (think 3 or 4 sizes of soda) are not a new phenomenon. In fact, the current world record holder for most efficient solar cell is four-junction solar cell with a power conversion efficiency (PCE) of 46%. [2] This is equivalent to the cashier above collecting 46% of all the money that passed by. Note that this exceeds the Shockley Queisser limit (29% PCE) by almost 20%! This is because the limit was derived for a single junction cell. Tandem cells are not bound by the limit derived for single junction cells.
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| Fig. 2: Perovskite-Silicon Tandem Solar Cell Schematic. [7] (Source: S. Swifter) |
So why don't tandem solar cells dominate the market, given that they have better efficiency? The main answer is cost. Gallium Arsenide cells, the most common high efficiency solar cells, can cost significantly more than even high efficiency silicon cells, making them only applicable for specific applications such as space solar and concentrated PV. [6] This is primarily due to the fact that these particular multi-junction cells are fabricated using a process known as Molecular Beam Epitaxy, which creates nearly perfect atomic layers, but is very expensive and slow. Another barrier to the breakout of tandem solar cells is that current multi-junction cells are generally made from materials other than silicon. Since silicon currently has a stranglehold on the PV market, to gain a foothold in the global PV market, a new material tandem solar cell technology would not only have to be commercially competitive with current silicon technology, it would have to be significantly better to be able to garner a significant percentage of the market.
However, new tandem solar cell technologies might be able to solve to dual issues of cost and silicon monopoly. The technology is based on combining already engineered silicon solar cells with novel solar cell materials to create highly efficient silicon tandem solar cells. The potential benefits of his technology are immense. The cost to produce these types of cells would not be that much more than pure silicon cells, as these cells use many of the same materials and processes as single junction silicon cell manufacturers. Furthermore, a company utilizing this technology would not have to compete head to head with the global powerhouse that is silicon solar cells. Instead, they could work with existing silicon solar cell companies to create a new, highly efficient product that breaks through the fundamental limitations of single junction cells.
Many variants of this technology are currently popular research topics at universities and laboratories worldwide. One material that has garnered the most attention as a potential combination with silicon is perovskites. The term perovskite refers to any material with the specific ABX3 crystal structure. Perovskites have become increasingly popular over the past decade because they are cheap to produce, have easily tunable bandgaps, and have relatively good absorption. Recently, a group out of Stanford demonstrated perovskite-silicon tandem cells with a PCE of 23.6% (see Fig. 2). [7] This is already competitive with commercially available silicon solar cells, the best of which are around 22%, and has much room for improvement.
Renewable energy, and solar in particular, is of increasing importance both in the state of California and the rest of the world. While remarkable advances have been made in the global production of solar energy, the overall efficiency of commercially available solar cells is limited by fundamental physical constraints. New multi-junction or tandem solar cell technology could increase the efficiency ceiling for commercial solar cells while still being economically viable, allowing for ever more efficient generation of renewable energy.
© Simon Swifter. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] L. Dillon, California to Rely on 100% Clear Electricity by 2045 Under Bill Signed by Gov. Jerry Brown," Los Angeles Times, 10 Sep 18.
[2] P. G. V. Sampaio and M. O. A. González, "Photovoltaic Solar Energy: Conceptual Framework," Renew. Sust. Energ. Rev. 74, 590 (2017).
[3] O. Ellabban, H. Abu-Rub, and F. Blaabjerg, "Renewable Energy Resources: Current Status, Future Prospects and Their Enabling Technology," Renew. Sust. Energ. Rev. 39, 748 (2014).
[4] K. Sopian et al., "An Overview of Crystalline Silicon Solar Cell Technology: Past, Present, and Future," AIP Conf. Proc. 1877, 020004 (2017).
[5] W. Shockley and H. J. Queisser, "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells," J. Appl. Phys. 32, 510 (1961).
[6] R. W. Miles, K. M. Hynes, and I. Forbes, "Photovoltaic Solar Cells: An Overview of State-of-the-Art Cell Development and Environmental Issues," Prog. Cryst. Growth 51, 1 (2005).
[7] K. A. Bush et al., "23.6%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability," Nat. Energy 2, 17009 (2017).
