Overview
An MIT review of solar energy technology concludes that today’s widely utilized crystallized silicon technology is highly efficient and reliable and can be used on the massive size needed to slow the climate’s effects in the mid-century. However, developing novel photovoltaic (PV) technologies with specially-designed nanomaterials could provide substantial advantages. They might be less complicated, more affordable to produce, and be made into ultra-thin, light, versatile solar cells that will be simple to carry and install. They could also have unique characteristics like transparency, which could lead to new applications, such as integrating fabrics or windows. Since no single technology–established or emerging–offers benefits on all fronts, the researchers recommend rapidly scaling up current silicon-based systems while continuing to work on other technologies to increase efficiency, decrease materials use, and reduce manufacturing complexity and cost.
Solar is one of the only renewable and low-carbon energy sources that could be scaled to meet the global demand for electricity. Silicon solar cells can do an excellent job of transforming sunlight’s energy into electricity; however, will they be sufficient shortly as a massive solar energy deployment is required to counteract the effects of climate change? What role could be played by other PV technologies being developed by research labs across the globe?
The answer to these concerns was the purpose of a new, comprehensive review of Vladimir Bulovic, who is one of Fariborz Maseeh’s (1990) Professors of Emerging Technology and MIT’s associate dean for innovation, Tonio Buonassisi, an assistant professor in mechanical engineering. Robert Jaffe is the Jane and Otto Morningstar Professor of Physics, and Graduate student Joel Jean of electrical engineering and Computer Science and Patrick Brown of Physics.
The solar energy resource
The researchers’ first goal was to analyze their primary energy source–sunlight. No one was surprised when the study revealed it was widely available and equally distributed worldwide. The distribution is only three times in densely populated regions but is not strongly dependent on economic prosperity. Contrary to this, carbon-based fuels and resources like uranium and hydropower-producing sites are incredibly concentrated, causing tensions between the wealthy and the not-so-happy. “Solar is a much more democratic resource,” Jean says. Jean.
The world is now beginning to benefit from it. Over 1% of worldwide electricity supply is currently provided by solar. In the United States, solar deployment is expanding at a rate far over the estimates of experts only a few years back. As of 2014, solar was responsible for more than a third of the newly added US generation capacity. As you can see in the graphic below, commercial, residential, and (especially) utility-scale PV installations have all exploded in the past few years.
Annual PV capacity enhancements within the United States by system type
The installed capacity of PV in the world is more significant than 200 gigawatts (GW) and accounts for over 1% of world electricity production. The chart below shows the annual growth in solar power capacity across the United States from 2008 to 2014. The expansion of commercial, utility, and residential capacity increased significantly every year, with the most significant increase taking place in the area of utilities. From 2008 to 2014, the total US solar power grid-connected capacity rose from 0.8 billion to 18.3 GW. To put that number into perspective, the solar-generated ability added in 2014 is comparable to the degree of several power plants.
Around 90% of solar PV installations are built on silicon crystal cells. This technology has been in use for decades and is constantly developing. This reliable, efficient technology can achieve large-scale deployment with no major technological breakthroughs, according to Bulovic.
However, it isn’t easy to reduce the cost. For the PV solar market, the costs are split into two groups that include the price of the solar module, which is the panel made up of numerous solar cells, wiring glass, encapsulation materials, and frame, and what’s known as the “balance of system” (BOS) comprising equipment like inverters, wiring, installation labor permitting grid interconnection, checks financing, and similar. From 2008 onwards, the price at which the modules are sold has fallen by 85 percent. However, the BOS cost has mostly stayed the same. Currently, the solar module accounts for only one-fifth of the overall cost for a typical residential installation and one-third of the cost for a utility-scale project across the United States. The remaining cost is to the BOS.
The reduction of BOS costs is a challenging task using silicon. Silicon isn’t great at absorbing sunlight, so a highly thick and fragile layer is required to accomplish the mission and prevent it from cracking. is a matter of mounting it on a large glass piece. Silicon PV modules are thus heavy and rigid, which can increase the BOS cost. “What we need is a cell that performs just as well but is thinner, flexible, lightweight, and easier to transport and install,” Bulovic says. Bulovic.
Researchers across the globe are working towards the creation of this kind of PV cell. They’re not beginning with silicon–a straightforward structural material but with a range of more complex nanomaterials which can be explicitly designed to harness sunlight and transform it into electricity.
Contrasting and discussing the different technologies
Assessing the various PV technologies currently being developed and tested is challenging because they’re very different. Simply put, they utilize various active materials to absorb light and capture the electric charge. They generally are classified into three general categories. Wafer-based cells are comprised of traditional silicon and alternatives like gallium arsenide. Commercial thin-film cells are made up of amorphous (non-crystalline) silicon as well as cadmium telluride, and copper indium (di)selenide (CIGS), as well as the latest thin-film technologies, including organic, perovskite as well as quantum dots (QD) solar cells.
The comparison of those strengths with the weaknesses each, as well as other alternatives, requires a method to arrange the options. The traditional classification system, established in 2001–divides solar technology into three “generations” based on efficiency and costs. However, that classification system “may not adequately describe the modern PV technology landscape,” Bulovic says. Bulovic, as a lot of the technology–both old and new fall outside their assigned categories. Furthermore, such chronological classifications treat older technologies in a negative light. “Third generation” will always be better than “first generation.” But silicon is a first-generation technology with its advantages and is the dominant majority of the market for solar cells.
To guide the thinking of today, the MIT team has come up with a brand new framework to show the way forward. It is based on the complex nature of the light-absorbing material. This concept is roughly defined as the number of atoms that make up the crystal unit or molecule that forms the foundation of the material. The elements that make up contemporary PV technologies range in complexity, ranging from single silicon atoms to more complex materials and nanomaterials, from cadmium-telluride through organics and perovskites to then on to QDs (see the image below). In the modern classification method, most technologies are at a single scale; they don’t shift across time, and a place isn’t superior to another. Furthermore, according to Jean, “We find that there’s some correlation between complexity and the performance measures that we’re interested in.”
PV technology classification is based on the material’s complexity.
The figure below shows the researchers’ suggested scheme to classify PV technologies based on their material complexity, roughly defined as the number of atoms that make up the structure of a molecule or repeating crystal units. The “building blocks” are highlighted above to highlight their respective degree of complexity. Technologies based on wafers near the top comprise small or single-atom building blocks. The thin-film technologies are placed in an order with increasing complexity and range from amorphous elements like amorphous silicon through polycrystalline thin films, such as cadmium telluride, to more complex nanomaterials like quantum dots that contain thousands of sulfur and lead atoms.
One of these measures is the complexity of manufacturing and the cost. While silicon is a straightforward structural material but transforming it into wafers or solar cells can be complicated and costly, primarily due to the necessity for high purity (>99.9999 percent) as well as high temperature (>1400degC). Processing more complex-looking nanomaterials is usually simpler, cost-effective, more affordable, and less energy-intensive. For instance, chemical reactions at low temperatures could transform the starting substances into organic molecules, or QDs. These complex building blocks could be later placed on the surface at low temperatures using either solution or vapor processing. They could be suitable for a range of substrates and high-speed production processes like roll-to-roll printing.
Another crucial aspect of PV technologies is power conversion efficiency, the proportion of solar energy converted into electrical power. Crystalline silicon is the most efficient technology, with cell efficiency records that can reach 26 percent. The latest nanomaterial-based technologies are within the range of 10% to 20. But, complex nanomaterials can be designed to maximize light absorption and take in the same light intensity as silicon but with an order of magnitude lower amount of material. “So while the typical silicon solar cell is more than 100 microns thick, the typical nanostructured solar cell–one that uses QDs or perovskites–can be less than 1 micron thick,” Bulovic says. Bulovic. The active layer can be applied to flexible substrates like paper and plastic without supporting it mechanically with a bulky chunk of glass.
The high-efficiency promises of these novel thin-film PV technology have been realized only with laboratory samples less than a fingernail. Moreover, the long-term stability of these PV systems is a concern. However, with further work, technology based on complicated materials may offer a variety of beneficial properties. These technologies could be developed into light, flexible, solar solid modules, which could help reduce BOS costs for systems connected to the grid. They could provide power to portable electronic devices ranging from mobile phones to small water purification systems. They can be carried and put in place in remote locations, and they might be suitable for those with low-power lighting or communications needs in developing countries. Additionally, they may have peculiar properties that make them ideal for novel applications. For instance, specific nanomaterials can suck up infrared and ultraviolet light while passing through visible light, which can be incorporated into skylights, windows, and building facades.
Materials are available
The possibility of increasing the solar power generation of today–perhaps by a factor of 100, raises another issue: the availability of materials. Can massive solar power deployment be limited due to the availability of essential materials required to make solar cells? What are the various technologies that accomplish this?
To determine this, researchers determined the requirements for materials of each technology. They calculated the materials required for the technology to meet the 5% and 50% needs, which is 100% or 5% of the global energy demand by 2050. (Using an estimate from the International Energy Agency of deficiency for 2050, these percentages correspond to the capacity of PV installed, which is 1,250, 12,500, 25,000, and gigawatts of power, all of which are far more than the current existing capacity for installed PV of around 200GW.) Then, they analyzed how much production is currently being produced worldwide for each material and calculated the number of additional hours, days, or even years of production required to reach the targets for deployment with the different technologies.
The following figure summarises their research findings. The ability to meet 100% of 2050’s global demand for electricity using crystal silicon solar cells would take six years of silicon production. The possibility of doubling silicon production in 2050 seems feasible. Therefore, material constraints are relatively minor for silicon.
The requirements of PV materials for the production technology
The availability of crucial materials could hinder a massive solar capacity expansion using specific PV technology. This chart shows the additional time required at the current production rate for the supply of critical materials to satisfy three levels of energy demand – 5%, 50%, and 100% using selected PV technology. Materials availability allows the expansion of use for today’s silicon-based cells and the newest PV technologies. However, using commercial thin-film technologies such as cadmium Telluride to provide the majority of expected electricity demand would need hundreds of years to produce the essential materials at the current rate. The required expansion in annual production of these materials between now and 2050 is well outside the historical norm.
This isn’t true about today’s commercial thin-film techniques. Consider cadmium telluride. Tellurium is approximately half as plentiful as gold and is produced mainly as a byproduct of refining copper. Making the tellurium needed to be required for cadmium telluride cells to satisfy all 2050 demands would take the amount of 1400 years of mining at current rates. Gallium, indium, and selenium also are manufactured as byproducts from significant metals. Using solar cells made of CIGS to supply all of the electricity requirements in 2050 would require more than 100 years of production for the three. “That isn’t to say these technologies don’t have a future–they could still generate hundreds of gigawatts of power,” Brown says. Brown. “But materials constraints make it seem unlikely that they will be the dominant solar technology.”
However, the newest thin-film technology utilizes plentiful primary metals that can be produced in large amounts. For instance, supplying the entire demand for QD solar cells would require just 22 days of lead production and six hours of sulfur production across the globe. Perovskites will need at least three years of production of their elements.
The most important thing is the bottom line.
The research team concludes that work must continue to be done on all technology, with a focus on improving the efficiency of conversion, reducing the amount of material used, and reducing manufacturing complexity and costs. Currently, no technology is guaranteed to perform most effectively in all three areas, as predicting how each will develop over time is challenging. For instance, if new technologies are employed on mobile devices, curtains, or windows, meeting this demand may aid manufacturers in tackling production problems and allow more efficient production at a lower cost shortly.
The researchers also highlight the length of time needed to bring the new technology created and put into launch into the market. “Today’s emerging technologies are improving far faster than currently deployed technologies improved in their early years,” Bulovic adds. Bulovic. “But the road to market and large-scale deployment is invariably long.” Furthermore, unpredictable technical, economic, or political elements could limit or affect PV deployment. According to Brown, due to the urgent nature of the global climate issue, “We need to deploy and improve our current technology while simultaneously laying the stage for new technologies that might be discovered at the laboratory. It’s crucial to push forward on both sides.”