Photovoltaic

“If you assume that humanity as a whole will opt for the cheapest alternative, then you’ll reach the conclusion that photo­voltaics are going to become the most important energy source,” says Dr. Jan Christoph Goldschmidt, who has worked at Fraunhofer ISE for many years and re­cently became professor of experimental physics at the University of Marburg. “In the long term, around half the electric­ity for the entire world must come from the sun.” That means by 2050, we must have installed a photovoltaic capacity of 20 to 80 terawatts, and by 2100, we will need 80 to 170 terawatts.” By way of comparison, a nuclear power plant has a capacity of a little more than a gigawatt, which is 0.001 terawatts, so the resources required for this expansion of worldwide photovoltaic capacity will be significant. Fraunhofer ISE and the Potsdam Insti­tute for Climate Impact Research have conducted a new study to highlight this shortcoming, but they found that this rap­id increase in photovoltaic capacity is in fact possible. However, it will require more efficient manufacturing technologies, as well as the infrastructure for recycling the old photovoltaic systems. “To reach this target, we must work at full tilt to install solar cells in their current form, while simultaneously driving innovation forward,” stressed Dr. Goldschmidt. The development of solar cells has involved a number of learning curves, with not only costs but also energy requirements for manufacturing and silver consumption continuously dropping.

However, by 2100, it’s possible that pho­tovoltaic manufacturing will require more glass than is currently produced world­wide. The supply conditions for metals like silver could be critical. The best case scenario would be for total consumption to remain around the current level of 2860 tons per year − provided the rate of innovation remains the same. The study also offered some encouragement in the context of energy requirements for man­ufacturing. The energy consumed in pro­ducing photovoltaic systems is expected to level out at around 4 percent of the solar cells’ power output, which is about the same proportion as the energy consumed in producing power from fossil fuels.

However, this will also require some innovative developments. If many photo­voltaic modules are manufactured using power generated from coal, this would use up a significant amount of the CO₂ budget. In fact, not all photovoltaic systems are made equal when it comes to their “eco­logical rucksacks.” “Photovoltaic modules produced in the EU represent a 40 percent saving in CO₂

The basis for the study came in the form of a cost calculation tool developed by Fraunhofer ISE. The tool covers each in­dividual step in the manufacturing process, from raw silicon through wafer production right up to manufacturing the solar cells and modules. “We were able to clearly determine how much energy is required to manufacture a specific module and how big the ‘ecological rucksack’ is in various different countries,” explains Dr. Neuhaus. “The main thing that makes a difference here is the energy mix that the respective country uses. While China generates a large portion of its energy by burning coal, Germany now obtains more than 50 per­cent of the necessary power from renewable sources.” By contrast, the process of trans­porting the cells from China to Germany results in an emissions increase of only 3 percent. The carbon footprint will also vary depending on the type of solar cell. For example, manufacturing frameless glass-glass modules emits 7.5 to 12.5 percent less greenhouse gas than manu­facturing photovoltaic modules with backsheet films. If the much longer service life of the glass-glass modules is taken into account, then the reduction in CO₂

However, rather than contenting itself with theoretical studies, Fraunhofer ISE is also developing the technologies needed to maintain the rate of innovation. For ex­ample, by making the contacts in the cells thinner, the researchers have reduced silver consumption by around 20 percent and increased efficiency by 1 percent. The Fraunhofer spin-off HighLine is con­tinuing to advance this technology, while the ISE spin-off PV2plus has replaced the silver contacts entirely using copper con­tacts. With this approach, the amount of copper recycled in Germany alone would be sufficient to cover the future global de­mand for solar cell production. NexWafe, another company built on Fraunhofer ISE technology, is working on energy-efficient manufacturing of photovoltaic modules. Using an innovative production process, it has succeeded in manufacturing silicon wafers − the heart of every photovoltaic cell − far more efficiently than was pre­viously possible.

As if on cue, an innovative technology that could reduce the “ecological ruck­sack” of solar cells manufactured in Europe even further has appeared on the scene – printed solar cells made from perovskite, a double salt that consists of one organic and one metallic salt and can be crystallized from a solution at room temperature. “The CO₂ from the manufacturing of the glass substrate is the only emission produced when man­ufacturing perovskite solar cells – that’s only an eighth of what’s required for even the latest silicon technology,” reports Dr. Lukas Wagner, a scientist at the Fraunhofer Institute for Solar Energy Systems ISE. Perovskite solar cells also come with lower costs. A module factory in Germany would be 80 percent cheaper than a con­ventional silicon photovoltaics factory and the solar cells themselves would be 50 percent cheaper.

Although the suitability of perovskite for use in photovoltaics was only discovered by accident in 2009, the metal-organic compound is already competing with the top-tier materials. “Today, perovskite solar cells have already reached a higher level of efficiency than most established tech­nologies – only monocrystalline silicon and GaAs (gallium arsenide) remain a bit more efficient,” enthuses Dr. Wagner. So what is it that makes perovskite so special? It’s produced from a solution that is very easy to handle and forms a very thin crys­tal film – 300 to 500 nanometers to be exact. By comparison, silicon is 180 mi­crometers, i.e. 180,000 nanometers thick. As such, the perovskite solar cells are very material-efficient to manufacture. “At Fraunhofer ISE, we’re developing printable solar cells that can be applied to a glass plate using a screen or ink-jet printer – roll-to-roll procedures can also be used,” Dr. Wagner explains. “Our vision is that we will use established processes from the vehicular glass industry for manufactur­ing perovskite solar cells, and so make them fit for mass production. This will allow the German glass industry to move into pho­tovoltaic manufacturing,” says Dr. Wagner.

The EU project UNIQUE, which is led by Fraunhofer ISE and involves all the stakeholders in printable solar cells, is pushing the efficiency of these new solar cells to even higher levels. The long-term objective of the project is to reach excellent efficiency at the module level, with rates of more than 20 percent. “Effi­ciency records are mostly set using mate­rials that are far too expensive to go on the roofs of houses. By contrast, we’re using inexpensive materials and employing processes that would be suitable for capac­ities of a terawatt and more — such as graphite contacts, for example,” says Dr. Wagner, whose research took second place in the energy campus (“Energie-Campus”) idea competition by a German foundation for energy and climate protection (“Stiftung Energie und Klimaschutz”) three years ago. In addition to the efficiency, the research­ers are also working on the stability of the perovskite solar cells. “The hot-spot test had the perovskite community seriously worried for a long time. But now, for the first time, we’ve shown that it’s possible to pass it,” reveals Dr. Wagner. In this test, the light to one cell is blocked – as can happen in real-life conditions due to fall­en leaves, for example. This sounds harm­less, but the cell then operates in reverse voltage and all the power flows through it, which can cause damage.

With the highly stable contact layers used at Fraunhofer ISE, the printed solar cells were not only able to withstand the hot-spot test, but also to reach operation­al life spans of 10,000 hours – on a German roof, that would be the equivalent of ten years of continuous operation.

Efficiency is a decisive factor for solar cells, but individual silicon solar cells can’t go beyond 29 percent – it’s the theoretical upper limit for efficiency, this high and no farther. However, if you connect two solar cells to create a tandem cell, it would the­oretically be possible to reach up to 40 percent efficiency. With three cells, this could even go as high as 45 percent. If you were to stack an endless number of solar cells one on top of another, then the abso­lute maximum efficiency would reach more than 85 percent. The reason is that while individual solar cells collect light of all colors, the light is split for tandem cells. It’s as if the work were divided up based on the different wavelength ranges. The top cell only “sees” blue light, the one be­neath only green, and so on, with each solar cell being optimized for its range of the spectrum.

In the Fraunhofer lighthouse project MaNiTU, Fraunhofer ISE, the Fraunhofer Institute for Mechanics of Materials IWM, the Fraunhofer Institute for Silicate Re­search ISC, the Fraunhofer Institute for Microstructure of Materials and Systems IMWS and the Fraunhofer Research Insti­tution for Materials Recycling and Resource Strategies IWKS are developing sustainable materials for tandem cells based on silicon and perovskite. This involves applying a layer of perovskite to a silicon substrate. “We expect these tandem solar cells to achieve high efficiency levels at low cost and with low resource consumption,” con­firms Dr. Goldschmidt. The first step was to screen possible materials to see if they would work in theory. “We took sustain­ability factors into account right from the outset — so that meant ruling out toxic materials like lead and materials that are not available in sufficient quantities,” said Dr. Goldschmidt. Some possible materials have already been identified and various different materials have also already been synthesized. “Things are going well, and I expect us to set a new record in the course of this project,” Dr. Goldschmidt affirms. In the long term, also Dr. Wagner hopes to join his perovskite solar cells together to form tandem solar cells.

To date, silicon-based solar cells them­selves have been virtually unbeatable on an individual cell level when it comes to electricity generation costs. However, other solar cell types are becoming in­creasingly relevant. For example, flexible solar cells could be used to generate elec­tricity wherever silicon modules can’t be installed due to their rigid form, like the pillars on wind turbines, curved build­ing facades and — in the case of partially transparent cells — even on window panes, or as solar curtains that provide shade. This would become economical­ly viable once 10 percent efficiency is achieved. By comparison, silicon solar cells have currently reached around 25 percent efficiency. “If we succeed in transferring this efficiency of more than 10 percent from the lab to our roll-to-roll plant, industrial mass production would be within our grasp. The laboratory groundwork and the necessary modifica­tions to the plant were completed in 2021. We could revolutionize the production of flexible solar cells in 2022,” says Ludwig Pongratz, a scientist at the Fraunhofer In­stitute for Laser Technology ILT. He hopes that by 2030, the flexible solar cells could be on the market, making a significant addition to the energy mix in the form of greenhouses or solar awnings.

Researchers at the Fraunhofer Institute for Applied Polymer Research IAP also hope to use large glass facade surfaces for electricity generation in the future. To make this vision a reality, stakeholders all along the value chain, from material and window manufacturers right up to housing companies, have joined forces in CoSoWin, a project funded by the German Federal Ministry for Economic Affairs. Fraunhofer IAP uses nanoparticles to apply a coating to the window panes that collects light, conducts it to the front side of the glass pane and feeds it into an organic solar cell there. “The efficiency is only at 4 to 7 percent at the moment, which is very limited. However, we’re proceeding on the assumption that it will be possible to reach 10 percent,” says Dr. Armin Wedel, a department and research area head at Fraunhofer IAP.