Physicists at the University of Warwick published new research in the Journal Science that could literally squeeze more power out of solar cells by physically deforming each of the crystals in the semiconductors used by photovoltaic cells. The paper entitled the “Flexo-Photovoltaic Effect” was written by Professor Marin Alexe, Ming-Min Yang, and Dong Jik Kim who are all based in the University of Warwick’s Department of Physics.
The limits of a solar cell
The Warwick researchers looked at the physical constraints on the current design of most commercial solar cells which place an absolute limit on their efficiency. Most commercial solar cells are formed of two layers creating at their boundary a junction between two kinds of semiconductors, p-type with positive charge carriers (holes which can be filled by electrons) and n-type with negative charge carriers (electrons). When light is absorbed, the junction of the two semiconductors sustains an internal field splitting the photo-excited carriers in opposite directions, generating a current and voltage across the junction. Without such junctions the energy cannot be harvested and the photo-exited carriers will simply quickly recombine eliminating any electrical charge.
That junction between the two semiconductors is fundamental to getting power out of such a solar cell but it comes with an efficiency limit. This Shockley-Queisser Limit means that of all the power contained in sunlight falling on an ideal solar cell in ideal conditions only a maximum of 33.7% can ever be turned into electricity.
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The new approach
Image of the crystal deformation.
There is however another way that some materials can collect charges produced by the photons of the sun or from elsewhere. The bulk photovoltaic effect occurs in certain semiconductors and insulators where their lack of perfect symmetry around their central point (their non-centrosymmetric structure) allows generation of voltage that can be actually larger than the band gap of that material (the band gap being the gap between the valence band highest range of electron energies in which electrons are normally present at absolute zero temperature and the conduction band where electricity can flow). Unfortunately the materials that are known to exhibit the anomalous photovoltaic effect have very low power generation efficiencies, and are never used in practical power-generation systems.
The Warwick team wondered if it was possible to take the semiconductors that are effective in commercial solar cells and manipulate or push them in some way so that they too could be forced into a non-centrosymmetric structure and possibly therefore also benefit from the bulk photovoltaic effect. For this paper they decided to try literally pushing such semiconductors into shape using conductive tips from atomic force microscopy devices to a “nano-indenter” which they then used to squeeze and deform individual crystals of Strontium Titanate (SrTiO3), Titanium Dioxide (TiO2), and Silicon (Si). They found that all three could be deformed in this way to also give them a non-centrosymmetric structure and that they were indeed then able to give the bulk photovoltaic effect.
Professor Marin Alexe from the University of Warwick said:
“Extending the range of materials that can benefit from the bulk photovoltaic effect has several advantages: it is not necessary to form any kind of junction; any semiconductor with better light absorption can be selected for solar cells, and finally, the ultimate thermodynamic limit of the power conversion efficiency, so-called Shockley-Queisser Limit, can be overcome. There are engineering challenges but it should be possible to create solar cells where a field of simple glass based tips (a hundred million per cm2) could be held in tension to sufficiently de-form each semiconductor crystal. If such future engineering could add even a single percentage point of efficiency it would be of immense commercial value to solar cell manufacturers and power suppliers.”
— Solar Builder magazine
One huge drawback of going solar is the fact that it only makes sense in areas which get abundant sunshine year-round. However, a team of scientists from China has now come up with a new solar cell, which can harvest energy even when it’s raining.
This solar cell is made using graphene, which has proven to be a very promising material for use in the production of solar cells in the past. One of these properties of graphene is its conductivity, which is such that it allows electrons to flow freely across its surface. So when this material is put into an aqueous solution, the so-called Lewis acid-base reaction occurs, namely that pairs of positively charged ions bind with the material’s negatively charged electrons. Studying this property of graphene, the team developed a solar cell, which can generate power from raindrops.
Raindrops are comprised of various salts, which have positively and negatively charged ions. So when rainwater hits graphene the positive ions bind with the negative ions on its surface. Where the rainwater and graphene come into contact, they form a double-layer of electrons and positively-charged ions, which creates a so-called pseudocapacitor. The two layers thus have a difference in potential, which is sufficient to generate a voltage and current.
The scientists have produced a prototype dye-sensitized solar cell and applied a thin film of graphene to it. They tested this cell in a lab, using salty water made to closely resemble rain. The cell they tested successfully generated hundreds of microvolts and had the solar to electricity conversion efficiency of 6.5 percent. Their next step will be to further refine the cell, and they are confident that they will succeed in creating a market-ready all-weather solar cell soon.
As you’re probably already aware, perovskite solar cells have the greatest potential of being the most prominent source of solar energy in the near future. They’re cheap to make and flexible enough to be applied to most any surface.
And now a team of researchers from the University of New South Wales (UNSW) in Sydney, Australia has made a breakthrough by creating the biggest perovskite solar cell so far, and setting a new efficiency record with it.
According to them, they have managed to achieve a 12.1 percent energy conversion efficiency rating for a 6.3 sq in (16 sq cm) perovskite solar cell. This cell is also about 10 times larger than any existing high-efficiency perovskite cell. The team also managed to achieve 18 percent efficiency for a 0.5 sq in (1.2 sq cm) single perovskite cell, as well as 11.5 percent for a 6.3 sq in (16 sq cm) four-cell perovskite mini-module. They are also confident that they can achieve a 24 percent efficiency within a year or so.
These cells get their name from the crystals they are made of, which are grown into a structure called perovskite. Due to their special characteristic, such as the smooth layers of perovskite with large crystal grain sizes, these cells can absorb more light than solar cells made of silicon. They are also much cheaper to produce.
Perovskite cells can also be created in different colors, or be transparent due to their chemical composition. This means that they can be used to cover virtually any surface, such as the sides or roofs of buildings, gadgets, cars and even windows.
One of the major downsides of perovskite solar cells is the fact that they are not very durable. However, the team believes that they can also improve their durability as they strive for even higher levels of efficiency.
Yingli Green Energy Holding Co. says it has produced the first interdigitated back contact (IBC) n-type solar cells based on 6-inch wafers at an industrial pilot line within just three months by adopting the new production process co-developed by Dutch research center ECN and equipment manufacturer Tempress.
The production process is based on Yingli’s commercialized PANDA process for the low-cost production of conventional n-type solar cells (n-PERT). The process was adapted for IBC-type cells using the screen printing technology for patterning and metallization. The production of IBC cells in Yingli’s industrial pilot line proves that the commercial production of efficient IBC cells is feasible on short term.
ECN is focusing its research on n-type silicon solar cells as these are potentially more efficient than p-type cells and are less sensitive to degradation and impurities. Additionally, the back contact design of solar cells offers several advantages. They exhibit a higher voltage and deliver a higher current, because of reduced losses via recombination and a larger surface on the sunny side.
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By producing the cells at Yingli, the consortium has now tackled the complexity of producing such cells. The consortium aims to produce cells that have an efficiency of 22 percent by the end of 2017. The development and production of commercial modules is expected for 2018. Moreover, the inherent bifacial character of the IBC cells will also allow pursuing the route of truly bifacial module technology.
“This result shows once again the synergy of our long lasting and fruitful cooperation with Yingli and ECN, who did the majority of work to achieve this. For Tempress this is an important opportunity to adapt and develop equipment and process that can be used in the production process of these next generation cells. A partner like Yingli combined with ECN, puts us in a position where developments can go really fast, which I think is best demonstrated by achievements like this. We are very thankful to have such valuable partners.” commented Albert Hasper, general manager of Dutch Solar Equipment company Tempress.
The cooperation with Yingli is very important for this development, says ECN researcher Dr. Ilkay Cesar. “Yingli has the facilities to produce high-quality solar cells on a large scale at low cost. This greatly enhances our opportunities for research and development on the cell process and module integration in a new way for our program. We are happy to partner with Yingli to continue the development of commercial processes for n-type solar cells. The pilot line now provides IBC cells in sufficient quantities to enable efficient back-contact module development which will boost the Dutch and EU PV tool and material supplier industry. The ECN Industry Research Program (IRP) aims to bring our simplified IBC technology to 23% within 3 years. IRP partners can start pilot production in less than 3 months as already demonstrated by Yingli.”
“It is our honour to cooperate with ECN and Tempress in producing IBC cells and we appreciate their highly industrial focus, which will enhance the chances to bring this product to the market in the short term. This cooperation and the pilot production of IBC cells are consistent with our long-term commitments to making solar electricity affordable and accessible for all through continued technology innovation,” said Dr Dengyuan Song, Chief Technology Officer of Yingli.
— Solar Builder magazine
Breakthroughs big and small are important in the quest towards greater reliance on renewable power, of which solar power is at the top of the list. And one such breakthrough was recently achieved by a team of scientists from the Karlsruhe Institute of Technology (KIT) and the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) in Germany. Inspired by the rose’s ability to convert sunlight into energy they designed a film that greatly increases the efficiency of solar cells.
Basically, they lifted an imprint off the petals of a rose and used it to create a film that can be attached to existing solar cells. They started the process by first studying the epidermal layer of cells from many different plants, since this outer layer actually has the ability to absorb, as opposed to reflect, light. They chose rose petals since their epidermal cells were best at this task. This is due to the fact that the epidermis of rose petals is made up of a disorganized arrangement of densely packed microstructures, and there are also additional ribs, which are created by randomly positioned nanostructures. Because of these characteristics, rose petals are able to absorb more light.
This unique surface was duplicated by making an imprint of it using a silicon-based polymer, which produced a mold. Next, they poured clear optical glue into the mold and dried it using a UV light. The result was a transparent copy of the epidermal layer of the rose petal, which was then applied to the solar cell.
Compared to normal solar cells, they found that cells with the film attached had a 12 percent boost in efficiency when placed vertically, and an amazing 44 percent boost in efficiency when the cell is placed at an 80-degree angle.
They are currently working on further researching the role played by the disorganized surface (like that of the rose petal’s epidermis) in other photosensitive surfaces. The researchers also hope to find ways to even further improve the film they created so that it will yield an even greater energy efficiency improvement.