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News Article | November 24, 2015
Site: www.cemag.us

Stacking two solar cells one over the other has advantages: because the energy is “harvested” in two stages, and overall the sunlight can be converted to electricity more efficiently. Empa researchers have come up with a procedure that makes it possible to produce thin film tandem solar cells in which a thin perovskite layer is used. The processing of perovskite takes place at just 50 degrees Celsius and such a process is potentially applicable for low cost roll-to-roll production in the future. What is true for double-blade razors is also true for solar cells: two work steps are more thorough than one.  Stacking two solar cells one on top of the other, where top cell is semi-transparent, which efficiently converts large energy photons into electricity, while the bottom cell converts the remaining or transmitted low energy photons in an optimum manner. This allows a larger portion of the light energy to be converted to electricity. Up to now, the sophisticated technology needed for the procedure was mainly confined to the realm of space or concentrated photovoltaics (CPV). These “tandem cells” grown on very expensive single crystal wafers are considered not attractive for mass production and low cost solar electricity. The research team working under Stephan Buecheler and Ayodhya N. Tiwari from the Laboratory for Thin Films and Photovoltaics at Empa-Swiss Federal Laboratories for Material Science and Technology has now succeeded in making tandem solar cells that are based on polycrystalline thin films, and the methods are suitable for large area low cost processing, Flexible plastic or metal foils could also be used as substrate in future. This marks a major milestone on the path to mass production of high-efficiency solar cells with low cost processes. The secret behind the new process is that the researchers create the top solar cell perovskite film with a low-temperature procedure at just 50 C. This promises an energy-saving and cost-saving production stage for future manufacturing processes. The tandem solar cell yielded an efficiency rate of 20.5 percent when converting light to electricity. Already with this first attempt Empa researchers have emphasized that it has lots more potential to offer for better conversion of solar spectrum into electricity. The key to this double success was the development of a 14.2 percent efficient semi-transparent solar cell, with 72 percent average transparency, made from methylammonium lead iodide deposited in the form of tiny perovskite crystals. The perovskite is grown on a thin interlayer made of the substance abbreviated as PCBM (phenyl-C61-butyric acid methyl ester) is used. Each PCBM molecule contains 61 carbon atoms interconnected in the shape of a soccer ball. The perovskite film is prepared by a combination of vapor deposition and spin coating onto this layer, which has tiny football like structure, followed by an annealing at a “lukewarm” temperature. This magic perovskite crystal absorbs blue and yellow spectrum of visible light and converts these into electricity. By contrast, red light and infrared radiation simply pass through the crystal. As a result, the researchers can attach a further solar cell underneath the semi-transparent perovskite cell in order to convert the remaining light into electricity. For the lower layer of the tandem solar cell, the Empa researchers use a CIGS cell (copper indium gallium diselenide), a technique that the team has been researching for years. Based on the CIGS cells, small-scale production is already under way for flexible solar cells. The advantage of tandem solar cells is that they exploit sunlight better. A solar cell can only convert radiation with an energy level higher than the bandgap of the semiconductor used. If the radiation is weaker, no electricity is generated. If the radiation is higher in energy, the excess radiated energy is converted to heat and is lost. A double-layer solar cell like Empa’s perovskite CIGS cell can combine substances with differing bandgaps and thus convert a larger share of the irradiated solar energy to electricity. While very good single-layer polycrystalline solar cell may practically convert a maximum of 25 percent of the solar energy to electricity, tandem solar cells could increase this figure to beyond 30 percent. That’s according to Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory. He does say, however, that a lot of research work is needed before that will be possible. “What we have achieved now is just the beginning. We will have to overcome many obstacles before reaching this ambitious goal. To do this, we will need lots of interdisciplinary experience and a large number of combinatorial experiments until we have found a semi-transparent high-performance cell together with the right base cell, and technologies for electrical interconnections of these solar cells.” Stephan Bücheler, who coordinates the lab research in Tiwari’s team, reminds us that the race for efficiency in solar cell research is certainly not just an academic show. “When producing solar-powered electricity, only half of the costs are down to the solar module itself. The other half are incurred for the infrastructure: inverters, cables, carriers for the cells, engineering costs and installation. These ancillary costs are reduced when the solar cells become more efficient and can be built in smaller sizes as a result. This means that efficient solar cells are the key to low-cost renewable electricity.” Release Date: November 23, 2015 Source: Empa


News Article | November 13, 2015
Site: phys.org

The key to success: pyrite (fool's gold) as a cathode material. Credit: JJ Harrison/commons.wikimedia.org High-performance lithium ion batteries face a major problem: Lithium will eventually start to run out as batteries are deployed in electric cars and stationary storage units. Researchers from Empa and ETH Zurich have now discovered an alternative: the "fool's gold battery". It consists of iron, sulfur, sodium and magnesium – all elements that are in plentiful supply. This means that giant storage batteries could be built on the cheap and used stationary in buildings or next to power plants, for instance. There is an urgent need to search for low-priced batteries to store electricity. Intermittency of green electricity is affecting the power grids, calling for stationary storage units to be connected into a smart grid., Electric cars are of increasing popularity, but are still to explensive. Efficient lithium ion batteries we know are not suitable for large-scale stationary storage of electricity; they are just too expensive precious lithium is too scarce. A cheap alternative is called for – a battery made of inexpensive ingredients that are available in abundance. But electrochemistry is a tricky business: Not everything that's cheap can be used to make a battery. Maksym Kovalenko, Marc Walter and their colleagues at Empa's Laboratory for Thin Films and Photovoltaics have now managed to pull off the unthinkable: by combining a magnesium anode with an electrolyte made of magnesium and sodium ions. Nanocrystals made of pyrite – more commonly known as fool's gold – serve as the cathode. Pyrite is crystalline iron sulfide. The sodium ions from the electrolyte migrate to the cathode during discharging. When the battery is recharged, the pyrite re-releases the sodium ions. This so-called sodium-magnesium hybrid battery already works in the lab and has several advantages: The magnesium as the anode is far safer than highly flammable lithium. And the test battery in the lab already withstood 40 charging and discharging cycles without compromising its performance, calling for further optimization. The biggest advantage, however, is the fact that all the ingredients for this kind of battery are easily affordable and in plentiful supply: Iron sulfide nanocrystals, for instance, can be produced by grinding dry metallic iron with sulfur in conventional ball-mills. Iron, magnesium, sodium, and sulfur are amongst hold 4th, 6th, 7th and 15th place by the abundance in the Earth's crust(by mass). One kilogram of magnesium costs at most four Swiss francs, which makes it 15 times cheaper than lithium. There are also savings to be made when it comes to constructing the cheap batteries: Lithium ion batteries require relatively expensive copper foil to collect and conduct away the electricity. For the fool's gold battery, however, inexpensive aluminum foil is perfectly sufficient. Potential for storing the electricity produced annually at Leibstadt power station The researchers primarily see potential in their development for large network storage batteries. The fool's gold battery is not suitable for electric cars – its output is too low. But wherever it boils down to costs, safety and environmental friendliness, the technology is a plus. In their paper recently published in the journal Chemistry of Materials, the Empa researchers propose batteries with extremely high storage capacity. Such a battery might be used to temporarily store the annual production from the Swiss nuclear power station in Leibstadt, for instance. "The battery's full potential has not been exhausted yet," says Kovalenko, who teaches as a professor at ETH Zurich's Department of Chemistry and Applied Biosciences alongside his research at Empa. "If we refine the electrolytes, we're bound to be able to increase the electric voltage of the sodium-magnesium hybrid cell even further and to extend its cycling life." He adds: "We also look for investors willing to support research into such post-Li-ion technologies and bring them to the market". Explore further: New electrolyte for the construction of magnesium-sulfur batteries More information: Marc Walter et al. Efficient and Inexpensive Sodium–Magnesium Hybrid Battery, Chemistry of Materials (2015). DOI: 10.1021/acs.chemmater.5b03531


Perrenoud J.,Laboratory for Thin Films and Photovoltaics | Buecheler S.,Laboratory for Thin Films and Photovoltaics | Kranz L.,Laboratory for Thin Films and Photovoltaics | Fella C.,Laboratory for Thin Films and Photovoltaics | And 2 more authors.
Conference Record of the IEEE Photovoltaic Specialists Conference | Year: 2010

CdTe solar cells with ZnS window layer deposited by ultrasonic spray pyrolysis (USP) were grown. The current density of such solar cells reached 24.7 mA/cm2 without anti reflection coating (Voc 594 mV, FF 64.2%, η 9.4%). For CdTe solar cells with CdS and ZnS window layers we quantified the Jsc loss mechanisms in detail. In order to tune the conduction band alignment, and increase the Voc, ZnO1-xSx grown by atomic layer deposition (ALD) was used. The band alignment in these devices was estimated and interpreted with the help of 1D simulation. According to the estimated band alignment the optimum sulfur content of the ZnO1-xSx compound seems to be around x=0.6 which was confirmed experimentally. The minimum absorber bandgap in final devices was found to be reduced in ZnO 1-xSx/CdTe devices, which indicates interdiffusion of elements from the window layer into the absorber. To reduce the impact of thereby possibly formed pinholes a high resistive transparent layer was introduced between TCO and window layer. This layer could improve the performance of the device with the thinnest ZnO1-xSx but decreased the performance of the others. In combination with ALD grown ZnO 1-xSx the TCO material Fluorine doped tin oxide lead to higher efficiency than aluminum doped zinc oxide. © 2010 IEEE.


Reinhard P.,Laboratory for Thin Films and Photovoltaics | Bissig B.,Laboratory for Thin Films and Photovoltaics | Pianezzi F.,Laboratory for Thin Films and Photovoltaics | Avancini E.,Laboratory for Thin Films and Photovoltaics | And 10 more authors.
Chemistry of Materials | Year: 2015

The introduction of a KF postdeposition treatment (KF PDT) of Cu(In,Ga)Se2 (CIGS) thin films has led to the achievement of several consecutive new world record efficiencies up to 21.7% for the CIGS solar cell technology. The beneficial effect of the KF PDT on the photovoltaic parameters was observed by several groups in spite of differing growth methods of the CIGS layer. For CIGS evaporated at lower temperature on alkali-free, flexible plastic substrates, a postdeposition treatment to add Na was already successfully applied. However, with the introduction of additional KF under comparable conditions, distinctly different influences on the final absorber alkali content as well as surface properties are observed. In this work we discuss in more detail the intrinsically different role of both alkali-treatments by combining several microstructural and compositional analysis methods. The ion exchange of Na by K in the bulk of the absorber is carefully analyzed, and further evidence for the formation of a K-containing layer on the CIGS surface with increased surface reactivity is given. These results shall serve as a basis for the further understanding of the effects of alkali PDT on CIGS and help identify research needs to achieve even higher efficiencies. (Figure Presented). © 2015 American Chemical Society.


News Article | November 15, 2015
Site: cleantechnica.com

The energy storage field could be in for a transformation if a team of researchers from Switzerland is on the right track. They’re hot on the trail of a battery deploying the humble crystalline material fool’s gold — aka iron pyrite — with the aim of pulling off a twofer. The new pyrite battery would enable low-cost, large-scale energy storage at power plants and other facilities, thus freeing up global supplies of lithium for small-scale use, including electric vehicle batteries. Lithium supply is a key issue for the growing EV and stationary energy storage markets. Though lithium-ion battery technology is a proven and efficient solution, as these markets grow, the cost of lithium could surge, so the hunt is on for cheaper, more abundant materials to take over. Iron pyrite is a common sulfide material easily mistaken for gold by the uninitiated due to its yellowish appearance. Pyrite has many industrial applications, and the US battery company Energizer is apparently the first to use pyrite in a single-use consumer battery in tandem with lithium. Deploying pyrite in rechargeable batteries presents a different set of challenges, and that’s the task undertaken by Switzerland’s Laboratory for Thin Films and Photovoltaics at the leading research institution Empa (Empa is the German acronym for Swiss Federal Laboratories for Materials Science and Technology). Dubbed “unthinkable” by Empa’s press office, the new energy storage solution consists of a magnesium anode and a cathode made of pyrite nanocrystals, with an electrolyte of magnesium and sodium ions. The sodium ions from the electrolyte migrate to the cathode during discharging. When the battery is recharged, the pyrite re-releases the sodium ions. This so-called sodium-magnesium hybrid battery already works in the lab and has several advantages: The magnesium as the anode is far safer than highly flammable lithium. And the test battery in the lab already withstood 40 charging and discharging cycles without compromising its performance, calling for further optimization. As Empa notes, all of the materials used in the new battery — iron, magnesium, sodium, and sulfur — are among the most abundant on Earth’s crust by mass, ranging from 4th to 15th place. In contrast, lithium clocks in around number 33. That doesn’t sound too bad, but consider that fully half of the world’s known lithium reserves are concentrated in Bolivia and the supply chain begins to look a little dicey. Here in the US, the Obama Administration has been eyeballing just such supply chain issues related to renewable energy and energy storage. In 2013, the Administration launched the Critical Materials Institute under the Energy Department to address supply chain bottlenecks, including the development of abundant, low-cost substitutes for lithium (that includes using iron pyrite for solar cells, btw). Manufacturing costs are another consideration, and the Empa team has that angle covered, too. The idea would be to manufacture iron sulfide nanocrystals by grinding iron and sulfur using existing ball-mill technology. In terms of energy density, pyrite is a weak player, so to be clear, the Empa team is not proposing that a pyrite battery that would compete in the EV market, or in the emerging small-scale residential or commercial energy storage market. They’re talking about energy storage on the scale of terrawatts. Of course, a terrawatt-scale solution is not particularly useful or cost-effective across the board. However, it could be enormously useful in countries like Switzerland, where a good deal of the country’s existing hydropower potential is untapped due to lack of storage. Historically, the country has relied on nuclear energy, and the Empa team anticipates that a low-cost terrawatt-scale energy storage solution could replace the entire annual production of one if its nuclear plants. The next step for the team is to increase the voltage of the battery by refining the electrolytes. The next step also involves finding a private sector angel to help push things along, so if you’re sitting on a pile of dough and you want to invest in the post-lithium economy, give them a ring over there at the Laboratory for Thin Films and Photovoltaics. While the Empa team is focusing on large-scale pyrite-enabled systems, researchers over at Vanderbilt University in the US are working at the opposite end of the scale, adding pyrite to small “button-type” lithium batteries used in watches, LED flashlights, and other smaller-than-a-smartphone devices with pyrite. The aim is to come up with a rechargeable energy storage formula that improves on the Energizer disposable pyrite/lithium battery. The key obstacle to recharge-ability is the tendency of nanoscale pyrite crystals to interact chemically with the electrolytes. To prevent the unwanted reaction, you can use larger or “bulk” iron pyrite, but then the energy storage results are less than optimal because the iron is farther from the surface of the crystal. In contrast, using nanoscale or “quantum dot” pyrite would move the iron right up to the surface, allowing the sulfurs to react with lithium (or sodium, as the case may be). The Vanderbilt team tinkered with a number of different combinations until they hit a sweet spot by deploying nanocrystals of about 4.5 nanometers on button batteries. The pyrite assist involves a kind of shape-shifting process in the material itself, which does not take place in conventional lithium batteries. Here’s how one researcher describes it: Storing lithium or sodium in conventional battery materials is like pushing chocolate chips into the cake and then pulling the intact chips back out. With the interesting materials we’re studying, you put chocolate chips into vanilla cake and it changes into a chocolate cake with vanilla chips. Based on their results, the team envisions a mobile energy storage future based on engineered nanomaterials, in which charging takes a matter of seconds and discharge stretches over days, with a lifespan extending over tens of thousands of cycles. That could be somewhere far off in the sparkling green future, but on the other hand, look how far EV battery technology has come in the last ten years or so. Follow me on Twitter and Google. Images/Screenshots: Pyrite battery schematic and pyrite crystal (cropped) via Empa, bottom schematic via Vanderbilt University.    Get CleanTechnica’s 1st (completely free) electric car report → “Electric Cars: What Early Adopters & First Followers Want.”   Come attend CleanTechnica’s 1st “Cleantech Revolution Tour” event → in Berlin, Germany, April 9–10.   Keep up to date with all the hottest cleantech news by subscribing to our (free) cleantech newsletter, or keep an eye on sector-specific news by getting our (also free) solar energy newsletter, electric vehicle newsletter, or wind energy newsletter.  


Fuchs P.,Laboratory for Thin Films and Photovoltaics | Hagendorfer H.,Laboratory for Thin Films and Photovoltaics | Romanyuk Y.E.,Laboratory for Thin Films and Photovoltaics | Tiwari A.N.,Laboratory for Thin Films and Photovoltaics
Physica Status Solidi (A) Applications and Materials Science | Year: 2014

Aluminum-doped zinc oxide (AZO) thin films are prepared by a low temperature (100°C) aqueous solution deposition method with a subsequent UV post-deposition treatment at 140°C. Film growth is governed by the retrograde solubility of zinc-ammine complexes at basic conditions (pH 11.4). Aluminum was introduced into the film as a dopant by co-precipitation, either using an aluminum metal foil or aluminum nitrate as a precursor. As the presence of Al ions in the solution influences the film morphology as well as the opto-electronic properties, different profiles of the dopant supply were examined and optimized with respect to the optical and electrical properties of the AZO layer. It was found that independent of the chosen aluminum precursor a linearly increasing dopant concentration in the solution is favorable for achieving dense, highly transparent (<8% absorption between 400 and 900nm) AZO thin films with a lowest resistivity of 3.4×10-3Ωcm. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


News Article | November 23, 2015
Site: phys.org

What is true for double-blade razors is also true for solar cells: two work steps are more thorough than one. Stacking two solar cells one on top of the other, where top cell is semi-transparent, which efficiently converts large energy photons into electricity, while the bottom cell converts the remaining or transmitted low energy photons in an optimum manner. This allows a larger portion of the light energy to be converted to electricity. Up to now, the sophisticated technology needed for the procedure was mainly confined to the realm of Space or Concentrated Photovoltaics (CPV). These "tandem cells" grown on very expensive single crystal wafers are considered not attractive for mass production and low cost solar electricity. The research team working under Stephan Buecheler and Ayodhya N. Tiwari from the Laboratory for Thin Films and Photovoltaics at Empa-Swiss Federal Laboratories for Material Science and Technology has now succeeded in making tandem solar cells that are based on polycrystalline thin films, and the methods are suitable for large area low cost processing, Flexible plastic or metal foils could also be used as substrate in future. This marks a major milestone on the path to mass production of high-efficiency solar cells with low cost processes. The secret behind the new process is that the researchers create the top solar cell perovskite film with a low-temperature procedure at just 50 degrees Celsius. This promises an energy-saving and cost-saving production stage for future manufacturing processes. The tandem solar cell yielded an efficiency rate of 20.5% when converting light to electricity. Already with this first attempt Empa researchers have emphasized that it has lots more potential to offer for better conversion of solar spectrum into electricity. Molecular soccer balls as a substrate for the magic crystal The key to this double success was the development of a 14.2% efficient semi-transparent solar cell, with 72% average transparency, made from methylammonium lead iodide deposited in the form of tiny perovskite crystals. The perovskite is grown on a thin interlayer made of the substance abbreviated as PCBM (phenyl-C61-butyric acid methyl ester) is used . Each PCBM molecule contains 61 carbon atoms interconnected in the shape of a soccer ball. The perovskite film is prepared by a combination of vapour deposition and spin coating onto this layer, which has tiny football like structure, followed by an annealing at a "lukewarm" temperature. This magic perovskite crystal absorbs blue and yellow spectrum of visible light and converts these into electricity. By contrast, red light and infrared radiation simply pass through the crystal. As a result, the researchers can attach a further solar cell underneath the semi-transparent perovskite cell in order to convert the remaining light into electricity. Advantage of the double-layer cell: better use of the spectrum of sunlight For the lower layer of the tandem solar cell, the Empa researchers use a CIGS cell (copper indium gallium diselenide), a technique that the team has been researching for years. Based on the CIGS cells, small-scale production is already under way for flexible solar cells. The advantage of tandem solar cells is that they exploit sunlight better. A solar cell can only convert radiation with an energy level higher than the bandgap of the semiconductor used. If the radiation is weaker, no electricity is generated. If the radiation is higher in energy, the excess radiated energy is converted to heat and is lost. A double-layer solar cell like Empa's perovskite CIGS cell can combine substances with differing bandgaps and thus convert a larger share of the irradiated solar energy to electricity. More than 30% efficiency is possible While very good single-layer polycrystalline solar cell may practically convert a maximum of 25% of the solar energy to electricity, tandem solar cells could increase this figure to beyond 30%. That's according to Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory. He does say, however, that a lot of research work is needed before that will be possible. "What we have achieved now is just the beginning. We will have to overcome many obstacles before reaching this ambitious goal. To do this, we will need lots of interdisciplinary experience and a large number of combinatorial experiments until we have found a semi-transparent high-performance cell together with the right base cell, and technologies for electrical interconnections of these solar cells." Stephan Bücheler, who coordinates the lab research in Tiwari's team, reminds us that the race for efficiency in solar cell research is certainly not just an academic show. "When producing solar-powered electricity, only half of the costs are down to the solar module itself. The other half are incurred for the infrastructure: inverters, cables, carriers for the cells, engineering costs and installation. These ancillary costs are reduced when the solar cells become more efficient and can be built in smaller sizes as a result. This means that efficient solar cells are the key to low-cost renewable electricity." Explore further: PolyU develops novel efficient and low-cost semitransparent perovskite solar cells with graphene electrodes More information: Fan Fu et al. Low-temperature-processed efficient semi-transparent planar perovskite solar cells for bifacial and tandem applications, Nature Communications (2015). DOI: 10.1038/ncomms9932


News Article | November 23, 2015
Site: www.rdmag.com

What is true for double-blade razors is also true for solar cells: two work steps are more thorough than one. Stacking two solar cells one on top of the other, where top cell is semi-transparent, which efficiently converts large energy photons into electricity, while the bottom cell converts the remaining or transmitted low energy photons in an optimum manner. This allows a larger portion of the light energy to be converted to electricity. Up to now, the sophisticated technology needed for the procedure was mainly confined to the realm of Space or Concentrated Photovoltaics (CPV). These "tandem cells" grown on very expensive single crystal wafers are considered not attractive for mass production and low cost solar electricity. The research team working under Stephan Buecheler and Ayodhya N. Tiwari from the Laboratory for Thin Films and Photovoltaics at Empa-Swiss Federal Laboratories for Material Science and Technology has now succeeded in making tandem solar cells that are based on polycrystalline thin films, and the methods are suitable for large area low cost processing, Flexible plastic or metal foils could also be used as substrate in future. This marks a major milestone on the path to mass production of high-efficiency solar cells with low cost processes. The secret behind the new process is that the researchers create the top solar cell perovskite film with a low-temperature procedure at just 50 C. This promises an energy-saving and cost-saving production stage for future manufacturing processes. The tandem solar cell yielded an efficiency rate of 20.5% when converting light to electricity. Already with this first attempt Empa researchers have emphasized that it has lots more potential to offer for better conversion of solar spectrum into electricity. Molecular soccer balls as a substrate for the magic crystal The key to this double success was the development of a 14.2% efficient semi-transparent solar cell, with 72% average transparency, made from methylammonium lead iodide deposited in the form of tiny perovskite crystals. The perovskite is grown on a thin interlayer made of the substance abbreviated as PCBM (phenyl-C61-butyric acid methyl ester) is used. Each PCBM molecule contains 61 carbon atoms interconnected in the shape of a soccer ball. The perovskite film is prepared by a combination of vapour deposition and spin coating onto this layer, which has tiny football like structure, followed by an annealing at a "lukewarm" temperature. This magic perovskite crystal absorbs blue and yellow spectrum of visible light and converts these into electricity. By contrast, red light and infrared radiation simply pass through the crystal. As a result, the researchers can attach a further solar cell underneath the semi-transparent perovskite cell in order to convert the remaining light into electricity. Advantage of the double-layer cell: Better use of the spectrum of sunlight For the lower layer of the tandem solar cell, the Empa researchers use a CIGS cell (copper indium gallium diselenide), a technique that the team has been researching for years. Based on the CIGS cells, small-scale production is already under way for flexible solar cells. The advantage of tandem solar cells is that they exploit sunlight better. A solar cell can only convert radiation with an energy level higher than the bandgap of the semiconductor used. If the radiation energy is lower, no electricity is generated. If the radiation is higher in energy, the excess radiated energy is converted to heat and is lost. A double-layer solar cell like Empa's perovskite CIGS cell can combine substances with differing bandgaps and thus efficiently convert a larger share of the incident solar energy to electricity. More than 30% efficiency is possible While very good single-layer polycrystalline solar cell may practically convert a maximum of 25% of the solar energy to electricity, tandem solar cells could increase this figure to beyond 30%. That's according to Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory. He does say, however, that a lot of research work is needed before that will be possible. "What we have achieved now is just the beginning. We will have to overcome many obstacles before reaching this ambitious goal. To do this, we will need lots of interdisciplinary experience and a large number of combinatorial experiments until we have found a semi-transparent high-performance cell together with the right base cell, and technologies for electrical interconnections of these solar cells." Stephan Bücheler, who coordinates the lab research in Tiwari's team, reminds us that the race for efficiency in solar cell research is certainly not just an academic show. "When producing solar-powered electricity, only half of the costs are down to the solar module itself. The other half are incurred for the infrastructure: inverters, cables, carriers for the cells, engineering costs and installation. These ancillary costs are reduced when the solar cells become more efficient and can be built in smaller sizes as a result. This means that efficient solar cells are the key to low-cost renewable electricity."


News Article | December 4, 2015
Site: www.materialstoday.com

Stacking two solar cells on top of each other means that solar energy can be ‘harvested’ in two stages, allowing sunlight to be converted into electricity more efficiently. Researchers at Empa, the Swiss Federal Laboratories for Materials Science and Technology, have now come up with a thin-film process for producing such tandem solar cells, in which one of the films comprises a thin layer of perovskite. Because the perovskite layer is fabricated at temperatures of just 50°C, this process could be used for low cost roll-to-roll production of solar cells in the future. What is true for double-bladed razors is also true for solar cells: two layers are better than one. The top layer of a tandem solar cell is semi-transparent and efficiently converts large energy photons into electricity, while the bottom layer converts the remaining or reflected low energy photons. This allows a larger portion of the light energy to be converted into electricity. Up to now, tandem cells have been produced by growing the layers on very expensive single crystal wafers, but this is not considered an attractive process for mass production. The research team working under Stephan Buecheler and Ayodhya Tiwari from the Laboratory for Thin Films and Photovoltaics at Empa has now succeeded in making tandem solar cells based on polycrystalline thin films, using a method that is suitable for large area, low cost processing, Flexible plastic or metal foils could also be used as the substrate in future. This marks a major milestone on the path to mass production of high-efficiency solar cells using low cost processes. As the researchers report in Nature Communications, the secret to the new process is creating the top layer from a perovskite film produced with a low-temperature procedure at just 50°C, offering an energy- and cost-efficient production stage for future manufacturing. The resultant tandem solar cell boasted an efficiency rate of 20.5% when converting light to electricity, but the Empa researchers emphasize that this process has lots more potential to provide even better conversion of sunlight into electricity. This high conversion rate was achieved through the development of a 14.2% semi-transparent solar cell, with 72% average transparency, made from methylammonium lead iodide deposited in the form of tiny perovskite crystals. The perovskite crystals are grown on a thin interlayer made of PCBM (phenyl-C -butyric acid methyl ester); each PCBM molecule contains 61 carbon atoms interconnected in the shape of a soccer ball. The perovskite film is prepared by a combination of vapor deposition and spin coating onto the PCBM layer, followed by annealing at a ‘lukewarm’ temperature. The perovskite crystals absorb blue and yellow wavelengths of visible light and convert these into electricity. By contrast, red light and infrared radiation simply pass through the crystals. To capture this red light and infrared radiation and convert it into electricity, the researchers attach a further layer underneath the semi-transparent perovskite layer. This layer is made from copper indium gallium diselenide (CIGD), which the team has been researching for years. A standard solar cell can only convert radiation with an energy level higher than the bandgap of the semiconductor used. If the radiation energy is lower, no electricity is generated. If the radiation is higher in energy, the excess radiated energy is converted into heat and lost. By combining substances with differing bandgaps, a tandem solar cell like Empa’s perovskite CIGS cell can efficiently convert a larger share of the incident solar energy to electricity. While very good single-layer polycrystalline solar cell may practically convert a maximum of 25% of the solar energy to electricity, tandem solar cells could increase this figure to beyond 30%. That’s according to Tiwari, head of the Thin Film and Photovoltaics laboratory. He does say, however, that a lot of research work is needed before that will be possible. “What we have achieved now is just the beginning. We will have to overcome many obstacles before reaching this ambitious goal. To do this, we will need lots of interdisciplinary experience and a large number of combinatorial experiments until we have found a semi-transparent high-performance cell together with the right base cell, and technologies for electrical interconnections of these solar cells.” Stephan Bücheler, who coordinates the lab research in Tiwari’s team, asserts that the race for efficiency in solar cell research is certainly not just an academic pursuit. “When producing solar-powered electricity, only half of the costs are down to the solar module itself. The other half are incurred for the infrastructure: inverters, cables, carriers for the cells, engineering costs and installation. These ancillary costs are reduced when the solar cells become more efficient and can be built in smaller sizes as a result. This means that efficient solar cells are the key to low-cost renewable electricity.” This story is adapted from material from Empa, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.


News Article | November 24, 2015
Site: www.nanotech-now.com

Home > Press > Tandem solar cells are simply better: Higher efficiency thanks to perovskite magic crystal Abstract: What is true for double-blade razors is also true for solar cells: two work steps are more thorough than one. Stacking two solar cells one on top of the other, where top cell is semi-transparent, which efficiently converts large energy photons into electricity, while the bottom cell converts the remaining or transmitted low energy photons in an optimum manner. This allows a larger portion of the light energy to be converted to electricity. Up to now, the sophisticated technology needed for the procedure was mainly confined to the realm of Space or Concentrated Photovoltaics (CPV). These "tandem cells" grown on very expensive single crystal wafers are considered not attractive for mass production and low cost solar electricity. The research team working under Stephan Buecheler and Ayodhya N. Tiwari from the Laboratory for Thin Films and Photovoltaics at Empa-Swiss Federal Laboratories for Material Science and Technology has now succeeded in making tandem solar cells that are based on polycrystalline thin films, and the methods are suitable for large area low cost processing, Flexible plastic or metal foils could also be used as substrate in future. This marks a major milestone on the path to mass production of high-efficiency solar cells with low cost processes. The secret behind the new process is that the researchers create the top solar cell perovskite film with a low-temperature procedure at just 50 degrees Celsius. This promises an energy-saving and cost-saving production stage for future manufacturing processes. The tandem solar cell yielded an efficiency rate of 20.5% when converting light to electricity. Already with this first attempt Empa researchers have emphasized that it has lots more potential to offer for better conversion of solar spectrum into electricity. Molecular soccer balls as a substrate for the magic crystal The key to this double success was the development of a 14.2% efficient semi-transparent solar cell, with 72% average transparency, made from methylammonium lead iodide deposited in the form of tiny perovskite crystals. The perovskite is grown on a thin interlayer made of the substance abbreviated as PCBM (phenyl-C61-butyric acid methyl ester) is used . Each PCBM molecule contains 61 carbon atoms interconnected in the shape of a soccer ball. The perovskite film is prepared by a combination of vapour deposition and spin coating onto this layer, which has tiny football like structure, followed by an annealing at a "lukewarm" temperature. This magic perovskite crystal absorbs blue and yellow spectrum of visible light and converts these into electricity. By contrast, red light and infrared radiation simply pass through the crystal. As a result, the researchers can attach a further solar cell underneath the semi-transparent perovskite cell in order to convert the remaining light into electricity. Advantage of the double-layer cell: better use of the spectrum of sunlight For the lower layer of the tandem solar cell, the Empa researchers use a CIGS cell (copper indium gallium diselenide), a technique that the team has been researching for years. Based on the CIGS cells, small-scale production is already under way for flexible solar cells (see Empa News from 11 June 2015). The advantage of tandem solar cells is that they exploit sunlight better. A solar cell can only convert radiation with an energy level higher than the bandgap of the semiconductor used. If the radiation energy is lower, no electricity is generated. If the radiation is higher in energy, the excess radiated energy is converted to heat and is lost. A double-layer solar cell like Empa's perovskite CIGS cell can combine substances with differing bandgaps and thus efficiently convert a larger share of the incident solar energy to electricity. More than 30% efficiency is possible While very good single-layer polycrystalline solar cell may practically convert a maximum of 25% of the solar energy to electricity, tandem solar cells could increase this figure to beyond 30%. That's according to Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory. He does say, however, that a lot of research work is needed before that will be possible. "What we have achieved now is just the beginning. We will have to overcome many obstacles before reaching this ambitious goal. To do this, we will need lots of interdisciplinary experience and a large number of combinatorial experiments until we have found a semi-transparent high-performance cell together with the right base cell, and technologies for electrical interconnections of these solar cells." Stephan Bücheler, who coordinates the lab research in Tiwari's team, reminds us that the race for efficiency in solar cell research is certainly not just an academic show. "When producing solar-powered electricity, only half of the costs are down to the solar module itself. The other half are incurred for the infrastructure: inverters, cables, carriers for the cells, engineering costs and installation. These ancillary costs are reduced when the solar cells become more efficient and can be built in smaller sizes as a result. This means that efficient solar cells are the key to low-cost renewable electricity." For more information, please click If you have a comment, please us. 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