Researchers develop new manufacturing processes for ultra-thin and ultra-efficient solar cells

Rajesh Uppal February 23, 2021 Energy & Propulsion, Manufacturing, Material Comments Off on Researchers develop new manufacturing processes for ultra-thin and ultra-efficient solar cells 70 Views

Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These disks act as energy sources for a wide variety of uses, including: calculators and other small devices; telecommunications; rooftop panels on individual houses; and for lighting, pumping, and medical refrigeration for villages in developing countries. Solar cells in the form of large arrays are used to power satellites and, in rare cases, to provide electricity for power plants.

There is a need for cheaper, more efficient solar cells than the traditional silicon solar cells so that more people may have access to this technology. The prices for solar panels have plummeted over the past few years, so continuing to focus on making them less expensive would have little impact on the overall cost of a solar power system; expenses related to things like wiring, land, permitting, and labor now make up the vast majority of that cost. Making modules more efficient would mean that fewer panels would be needed to produce the same amount of power, so the costs of hardware and installation could be greatly reduced.

The basic component of a solar cell is pure silicon, which is not pure in its natural state. Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or crushed quartz. Currently, 90 percent of the world’s solar panels are made from crystalline silicon, and the industry continues to grow at a rate of about 30 percent per year, the researchers say.

The resulting pure silicon is then doped (treated with) with phosphorous and boron to produce an excess of electrons and a deficiency of electrons respectively to make a semiconductor capable of conducting electricity. The silicon disks are shiny and require an anti-reflective coating, usually titanium dioxide.

The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame. The protective material consists of an encapsulant of transparent silicon rubber or butyryl plastic (commonly used in automobile windshields) bonded around the cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight plastic cover on satellite arrays. The electronic parts are standard and consist mostly of copper. The frame is either steel or aluminum. Silicon is used as the cement to put it all together.

Reserachers have been developing new architectures , techniques, materials including nanomaterials, stacking multiple layers , developing hybrid technologies and manufacturing processes to enhance the efficnecy and also reduce the cost of solar cells.

For cheaper solar cells, thinner really is better

Costs of solar panels have plummeted over the last several years, leading to rates of solar installations far greater than most analysts had expected. But with most of the potential areas for cost savings already pushed to the extreme, further cost reductions are becoming more challenging to find. Now, researchers at MIT and at the National Renewable Energy Laboratory (NREL) have outlined a pathway to slashing costs further, this time by slimming down the silicon cells themselves.

Thinner silicon cells have been explored before, especially around a dozen years ago when the cost of silicon peaked because of supply shortages. But this approach suffered from some difficulties: The thin silicon wafers were too brittle and fragile, leading to unacceptable levels of losses during the manufacturing process, and they had lower efficiency. The researchers say there are now ways to begin addressing these challenges through the use of better handling equipment and some recent developments in solar cell architecture.

The new findings are detailed in a paper in the journal Energy and Environmental Science, co-authored by MIT postdoc Zhe Liu, professor of mechanical engineering Tonio Buonassisi, and five others at MIT and NREL. The researchers describe their approach as “technoeconomic,” stressing that at this point economic considerations are as crucial as the technological ones in achieving further improvements in affordability of solar panels.

Currently, 90 percent of the world’s solar panels are made from crystalline silicon, and the industry continues to grow at a rate of about 30 percent per year, the researchers say. Today’s silicon photovoltaic cells, the heart of these solar panels, are made from wafers of silicon that are 160 micrometers thick, but with improved handling methods, the researchers propose this could be shaved down to 100 micrometers — and eventually as little as 40 micrometers or less, which would only require one-fourth as much silicon for a given size of panel.

That could not only reduce the cost of the individual panels, they say, but even more importantly it could allow for rapid expansion of solar panel manufacturing capacity. That’s because the expansion can be constrained by limits on how fast new plants can be built to produce the silicon crystal ingots that are then sliced like salami to make the wafers. These plants, which are generally separate from the solar cell manufacturing plants themselves, tend to be capital-intensive and time-consuming to build, which could lead to a bottleneck in the rate of expansion of solar panel production. Reducing wafer thickness could potentially alleviate that problem, the researchers say.

The study looked at the efficiency levels of four variations of solar cell architecture, including PERC (passivated emitter and rear contact) cells and other advanced high-efficiency technologies, comparing their outputs at different thickness levels. The team found there was in fact little decline in performance down to thicknesses as low as 40 micrometers, using today’s improved manufacturing processes. “We see that there’s this area (of the graphs of efficiency versus thickness) where the efficiency is flat,” Liu says, “and so that’s the region where you could potentially save some money.” Because of these advances in cell architecture, he says, “we really started to see that it was time to revisit the cost benefits.”

Changing over the huge panel-manufacturing plants to adapt to the thinner wafers will be a time-consuming and expensive process, but the analysis shows the benefits can far outweigh the costs, Liu says. It will take time to develop the necessary equipment and procedures to allow for the thinner material, but with existing technology, he says, “it should be relatively simple to go down to 100 micrometers,” which would already provide some significant savings. Further improvements in technology such as better detection of microcracks before they grow could help reduce thicknesses further. In the future, the thickness could potentially be reduced to as little as 15 micrometers, he says. New technologies that grow thin wafers of silicon crystal directly rather than slicing them from a larger cylinder could help enable such further thinning, he says.

Development of thin silicon has received little attention in recent years because the price of silicon has declined from its earlier peak. But, because of cost reductions that have already taken place in solar cell efficiency and other parts of the solar panel manufacturing process and supply chain, the cost of the silicon is once again a factor that can make a difference, he says. “Efficiency can only go up by a few percent. So if you want to get further improvements, thickness is the way to go,” Buonassisi says. But the conversion will require large capital investments for full-scale deployment.

The purpose of this study, he says, is to provide a roadmap for those who may be planning expansion in solar manufacturing technologies. By making the path “concrete and tangible,” he says, it may help companies incorporate this in their planning. “There is a path,” he says. “It’s not easy, but there is a path. And for the first movers, the advantage is significant.” What may be required, he says, is for the different key players in the industry to get together and lay out a specific set of steps forward and agreed-upon standards, as the integrated circuit industry did early on to enable the explosive growth of that industry. “That would be truly transformative,” he says.

Andre Augusto, an associate research scientist at Arizona State University who was not connected with this research, says “refining silicon and wafer manufacturing is the most capital-expense (capex) demanding part of the process of manufacturing solar panels. So in a scenario of fast expansion, the wafer supply can become an issue. Going thin solves this problem in part as you can manufacture more wafers per machine without increasing significantly the capex.” He adds that “thinner wafers may deliver performance advantages in certain climates,” performing better in warmer conditions.

Renewable energy analyst Gregory Wilson of Gregory Wilson Consulting, who was not associated with this work, says “The impact of reducing the amount of silicon used in mainstream cells would be very significant, as the paper points out. The most obvious gain is in the total amount of capital required to scale the PV industry to the multi-terawatt scale required by the climate change problem. Another benefit is in the amount of energy required to produce silicon PV panels. This is because the polysilicon production and ingot growth processes that are required for the production of high efficiency cells are very energy intensive.” Wilson adds “Major PV cell and module manufacturers need to hear from credible groups like Prof. Buonassisi’s at MIT, since they will make this shift when they can clearly see the economic benefits.”

Russian scientists unveil new manufacturing process for III-V solar cells

In feb 2020 it was reported that Researchers have integrated A3B5 semiconductors on a silicon substrate in a prototype solar cell and claim the technique could enable the production of III-V solar cells with conversion efficiencies of around 40%

Researchers at Russia’s ITMO University, in St Petersburg, are testing compound A3B5 semiconductors in the manufacture of multi-junction, III-V solar cells. A3B5 materials are a family of semiconductors including gallium arsenide (GaAs), indium arsenide (InAs), gallium phosphide (GaP), indium phosphide (InP), gallium antimonide (GaSb) and indium antimonide (InSb) which are used as basic materials for electronic and optoelectronic applications.

The ITMO University team say they fabricated the top layer of a small, laboratory prototype solar cell which featured A3B5 materials integrated on a silicon-substrate for the first time. They claim the innovation could lead to highly efficient solar cells at considerably lower costs, as the silicon substrate used in their device was much less expensive than materials used in IIV-V solar cells – so named after the groups of the periodic table the elements concerned occupy.

The researchers said the epitaxial-synthesis-on-silicon-substrate is a difficult manufacturing process as the deposited semiconductor must have the same crystal lattice parameter as silicon. “Roughly speaking, the atoms of this material should be at the same distance from each other as are the silicon atoms,” they said.

GaP is one semiconductor meeting those requirements but the researchers said its light trapping properties are limited. The gallium phosphide compound, however, when combined with nitrogen, shows direct-band property and strong light-trapping properties as well as being suitable for integration on the silicon substrate. “At the same time, silicon doesn’t just serve as the building material for the photovoltaic layers – it itself can act as one of the photo-active layers of a solar cell, absorbing light in the infrared range,” the ITMO University team said.

The scientists said the efficiency of their solar cell rises as extra photo-active layers are added, and they claim the A3B5 semiconductor can also be used for intermediate layers. They believe the potential efficiency of such solar cells could top 40% if used with concentrating PV technology. The findings were presented in the paper GaNP-based photovoltaic device integrated on Si substrate, published in Solar Energy Materials and Solar Cells, and on the ScienceDirect website.

Gallium arsenide and other III-V materials are among the best known in terms of efficiency potential for solar cells but cost has thus far limited them to niche applications such as powering satellites and drones.

Chinese researchers said in October 2018 they had developed a new technique to make solar cells that could allow them to avoid high-temperature processes, thus making those solar cells lower-cost and more efficient.

Silver becomes key component for ultra-efficient solar cells

Researchers at Ruhr-Universität Bochum and the University of Wuppertal in Germany have come up with a new fabrication process for transparent ultra-thin silver films. The new process produces a material capable of increasing the efficiency of solar cells and light-emitting diodes.

Up till now, traditional chemical methods have not been able to produce ultra-thin and pure silver films, the scientists say in a research paper published in the journal Angewandte Chemie. This is because precursors for the fabrication of ultra-thin silver films are highly sensitive to air and light. They can be stabilised with fluorine, phosphorus or oxygen but these elements contaminate the thin films as well as the equipment used for the production.

However, with the newly developed synthesis, the team headed by Professor Anjana Devi and Nils Boysen were able to create a chemical silver precursor where the silver is surrounded by an amide and a carbene, which is even stable without fluorine, phosphorous or oxygen. Following this process, they demonstrated that a silver thin film can be applied to an electrode with the new precursor by atomic layer deposition. When this is done, the gaseous precursor is transported to the electrode and a silver film is deposited there as a layer with a thickness of merely a few atoms.

Because it is so thin, the silver film is transparent. It is also pure and electrically conductive. “As far as process technology is concerned, the successful synthesis of the new precursor paves the way for the development of ultra-thin silver films,” said Thomas Riedl, from the Chair of Electronic Devices in Wuppertal, in a media statement. “It constitutes a first step towards the production of novel electrodes for highly efficient solar cells and lights.” In the same brief, team lead Anjana Devi added that since the process can be operated under atmospheric pressure and at low temperatures, the conditions for industrial production are favourable.

Shin-Etsu Chemical has established mass production process of Solar Cell having 21% conversion efficiency

Shin-Etsu Chemical Co., Ltd. (TOKYO:4063), has established mass production process for 156 x 156mm advanced mono crystalline silicon solar cells that have a industry’s highest conversion efficiency of 21%. The manufacturing process can be easily implemented by introducing a few additional systems into the existing mass production process, that is based on screen-printing technology. Shin-Etsu Chemical possessesing 10 patents regarding this technology, and has started Licensing Its Solar Cell Manufacturing Technology.

The solar panel modules based on their technology exhibit high performance, even in areas where snow and sand fall and pile up on the module because of bifacial light-receiving solar cell design that generate greater out put power in a strong reflected-light environment.

A new manufacturing technique creates ultrathin solar cells that are so light and flexible that they can rest on the surface of a soap bubble.

Creating ultrathin solar panels has been a longstanding goal in the field of materials sciences, and will help countless technologies transition to wireless power sources for enhanced application, mobility, and freedom. In a recent study published in Advanced Materials Technologies in 2020, researchers Professor Derya Baran and her team from King Abdullah University of Science & Technology (KAUST) have developed solar panels that are so thin and flexible they can be embedded into the surface of a soap bubble.

“The tremendous developments in electronic skin for robots, sensors for flying devices, and biosensors to detect illness are all limited in terms of energy sources,” said Eloïse Bihar, a postdoc in Baran’s lab who led the research. “Rather than bulky batteries or a connection to an electrical grid, we thought of using lightweight, ultrathin organic solar cells to harvest energy from light, whether indoors or outdoors.” Organic materials have shown promise in this area and have been used to create the next generation of ultralight solar harvesting devices for small-scale applications, such as powering drones. Current manufacturing practices, however, limit the design freedom and capabilities of current devices.

So instead, the KAUST researchers turned to inkjet printing, a conventional technique used in manufacturing that has many advantages over other deposition techniques, such as high versatility, easy customization, and low cost. Although the printing technique itself is amenable to scale-up and the building of layered devices, such as solar panels, developing the appropriate inks is where the challenge lies. “We formulated functional inks for each the layer of the solar cell architecture,” said Daniel Corzo, a Ph.D. student in Baran’s team. “Inkjet printing is a science on its own. The intermolecular forces within the cartridge and the ink need to be overcome to eject very fine droplets from the very small nozzle. Solvents also play an important role once the ink is deposited because the drying behavior affects the film quality.”

To build their solar panels, the team used a highly conductive polymer called PEDOT:PSS to sandwich a light-capturing material in a thin film. The device was then sealed with a parylene coating, which is flexible and prevents degradation through moisture or chemical decomposition, and lends the device a degree of biocompatibility for use in medical settings. After optimizing the ink composition for each layer of the device, the solar cells were printed onto glass to test their performance. They achieved a power conversion efficiency (PCE) of 4.73 percent, beating the previous record of 4.1 percent for a fully printed cell. For the first time, the team also showed that they could print a cell onto an ultrathin flexible substrate, reaching a PCE of 3.6 percent.

“Our findings mark a stepping-stone for a new generation of versatile, ultralightweight printed solar cells that can be used as a power source or be integrated into skin-based or implantable medical devices,” Bihar said. Reference: Eloïse Bihar, et al., Fully Inkjet‐Printed, Ultrathin and Conformable Organic Photovoltaics as Power Source Based on Cross‐Linked PEDOT:PSS Electrodes, Advanced Materials Technologies (2020). DOI: 10.1002/admt.202000226