Can we get more power out of a solar cell? These UK physicists think so

warwick solar cell

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

solar cell

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

SunPower buys SolarWorld Americas to ramp up P-Series panel production

sunpower logo

In the latest episode of As the Solar Industry Turns, SunPower is acquiring 100 percent of SolarWorld Americas. You may recall SolarWorld Americas being one of the companies that hit a rough patch and filed a 201 trade petition that launched the #TrumpTariffs now in place. SunPower plans to inject fresh capital into the SolarWorld Americas facility and implement high-efficiency P-Series solar panel manufacturing technology.

“The time is right for SunPower to invest in U.S. manufacturing, and SolarWorld Americas provides a great platform for us to implement our advanced P-Series solar panel manufacturing technology right here in our home market,” said Tom Werner, SunPower CEO and chairman of the board. “P-Series technology was invented and perfected in Silicon Valley, and will now be built in SolarWorld Americas’ factory, helping to reshape solar manufacturing in America.”

SunPower plans to ramp SolarWorld Americas operations to capitalize on strong U.S. market demand. The company will invest in factory improvements and increased working capital, while retrofitting a portion of the facility to produce P-Series solar panels, in addition to continuing to produce and ship SolarWorld Americas’ legacy products. Like SolarWorld Americas, SunPower has spent decades perfecting its technology and manufacturing processes, and this announcement marks the company’s return to U.S. manufacturing. The agreement is subject to necessary U.S. and German regulatory approvals and other closing conditions. At closing, which is expected in the next several months, SunPower will become the largest U.S. solar panel manufacturer.

“This is a smart move for SunPower,” said Vikram Aggarwal, CEO and founder of EnergySage. “As our latest report shows, SunPower has typically been the most expensive panel brand offered to consumers, while SolarWorld has been among the lowest priced. This acquisition gives SunPower the ability to better serve both quality-driven and price-conscious consumer segments, particularly those looking for American-made products. It should also help the company minimize the impact of Trump’s solar tariff.”

“SunPower is the solar industry technology leader,” said Jürgen Stein, CEO of SolarWorld Americas. “We are delighted that SunPower has agreed to inject fresh capital and their industry leading P-Series technology into SolarWorld Americas operations here in Hillsboro. Our hundreds of long-time employees are excited to be part of this next chapter in SolarWorld Americas’ long history. We are thrilled about this acquisition as it means quite simply, that our company can look forward to redoubled strength as it continues to innovate and expand into the future. This outcome is ideal for SolarWorld Americas and its employees.”

 

— Solar Builder magazine

NREL update: The puzzle of scaling perovskite solar cells (and possible solutions)

perovskite solar cell

As perovskite solar cells set efficiency records and the nascent technology becomes more stable, another major challenge remains: the issue of scalability, according to researchers at the Department of Energy’s National Renewable Energy Laboratory (NREL).

“It is scalable,” said Kai Zhu, a materials science researcher at NREL. “We just need to demonstrate efficiency and yield at a large-scale to move the technology beyond the laboratory.”

Lead author of a new Nature Reviews Materials paper titled, “Scalable Fabrication of Perovskite Solar Cells,” Zhu and his colleagues at NREL reviewed efforts to move perovskites from the laboratory to the rooftop. Zhen Li, Talysa Klein, Dong Hoe Kim, Mengjin Yang, Joseph Berry, and Maikel van Hest are the co-authors.

Most solar panels on the market today are made of silicon, but perovskite solar cells have the potential to accelerate the growth of photovoltaic (PV) manufacturing in the United States because they’re much cheaper to make and have shown performance potential in the lab. Perovskites have achieved record efficiency levels faster than any other solar cell technology with the current record—certified last summer—now standing at 22.7 percent. But efficiency in a perovskite solar cell declines as the cell and module area increases. A combination of factors is attributed to the decline, including the non-uniform coating of chemicals in the cell. Also, when any type of solar cells are joined together to create modules, inactive zones form between cells where sunlight isn’t converted to electricity, leading to efficiency declines.

RELATED: Module Evolution: What big-time PV improvements will boost panel efficiency?

To make a perovskite solar cell in the laboratory, scientists deposit chemicals onto a substrate. The perovskite material forms as the chemicals crystallize. The most commonly used deposition method in the laboratory, called spin coating, produces devices with the highest efficiency, but the process wastes more than 90 percent of the chemicals used, the so-called perovskite ink. Spin coating also works best on cells smaller than four square inches, but there isn’t an easy way to enable this technology to be used on a larger surface.

The NREL researchers examined potential scalable deposition methods, including:

• Blade coating, which uses a blade to spread the chemical solution on substrates to form wet thin films. The process can be adapted for roll-to-roll manufacturing, with flexible substrates moving on a roller beneath a stationary blade similar to how newspapers are printed. Blade coating wastes less of the ink than spin coating.

• Slot-die coating, which relies on a reservoir to supply the precursor ink in order to apply ink over the substrate. The process hasn’t been as well explored as other methods and so far has demonstrated lower efficiency than blade coating. But the reproducibility of slot-die coating is better than blade coating when the ink is well-developed, so this is more applicable for roll-to-roll manufacturing.

• Ink-jet printing, which uses a small nozzle to disperse the precursor ink. The process has been used to make small-scale solar cells, but whether it is suitable for the high-volume, large-area production will depend on the printing speed and device structure.

Other methods exist, such as electro-deposition, but there haven’t been any reports of that being used to make direct deposition of halide perovskites in perovskite solar cells.

Despite numerous challenges, impressive progress is being made toward scaling up production of these solar cells, the NREL researchers noted in the paper. The new paper outlined research that needs to be addressed to scale-up the technology. One area in particular that needs more attention is the ideal architecture of a perovskite solar module.

Several studies have estimated perovskite solar cells could generate electricity at a lower cost than other photovoltaic technologies, although those figures are based on hypothetical research. But one conclusion that can be drawn from the studies is that the highest input costs for perovskite modules will come from substrates and electrode materials, which points to a range of opportunities for innovation in these areas.

— Solar Builder magazine

Solaria, Seraphim teaming up to develop, mass produce solar module technology

Solaria

Solaria Corporation, a global provider of solar module products and technologies, has entered into a strategic cooperation agreement with Jiangsu Seraphim Solar. Pursuant to the agreement, both companies will work together to develop advanced photovoltaic manufacturing technologies and implement these technologies for mass production.

“Key energy industry players recognize the technology advancements that Solaria has commercialized with its high performance PowerXT module series and its solar shingling products,” said Solaria CEO Suvi Sharma. “Seraphim is the ideal partner for the joint development of new solar technologies and helping us realize those technologies into mass production. Through economies of scale, we expect to not only bring down the overall cost of the module itself, but also enhance the performance and profitability of solar power systems.”

“Seraphim is thrilled to reach such collaboration agreement with Solaria, which will further strengthen Seraphim’s leadership position for high-efficiency modules globally,” said Polaris Li, CEO of Jiangsu Seraphim Solar. “This agreement is a strong commitment from both parties to bring shingling technology to full mass production and make it a mainstream technology for the solar industry.”

Solaria’s PowerXT module series and its solar shingling products utilizes advanced cell interconnect and module production processes to create a new standard in PV module efficiency and reduction in system costs. Providing labor savings on racking and system components, Solaria’s PowerXT module series and its solar shingling products significantly boost power generation while eliminating reliability challenges that can reduce conventional PV modules’ long-term performance. This ensures that solar installers maximize power deployment on customer roofs – enabling them to install attractive, cost-effective distributed power plants that accelerate payback period and profitability.

— Solar Builder magazine

NREL promotes new solar tool to accurately calculate PV module degradation rates

How long a product can be expected to perform at a high level is a fundamental indication of quality and durability. In the solar industry, accurately predicting the longevity of photovoltaic (PV) panels is essential to increase energy production, lower costs, and raise investor and consumer confidence. A new software package developed by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and industry partners SunPower and kWh Analytics is making the measurement of PV system expected lifetime performance more reliable, consistent and accurate.

RdTools

RdTools results show time-series data along with a year-on-year degradation distribution. The same system is analyzed with the clear-sky method (a), and sensor-based method with a poorly maintained sensor (b). In this case, high reported degradation is likely caused by sensor drift, rather than a degrading PV module

RdTools combines best practices with years of NREL degradation research to deliver new methodologies that change how solar field production data is evaluated. The software package makes it possible to accurately evaluate PV systems faster, despite common challenges with performance data.

“There’s a high level of interest in this software because it provides user-friendly, accurate, and objective assessments that can help owners make sense of their data,” said Dirk Jordan, engineer and solar PV researcher at NREL. “We spent years building consensus in the industry around a common set of analytical rules. Now PV stakeholders can learn much more about the performance of their technology and improve decision-making on multiple fronts.”

How it works

PV module and system degradation have been historically difficult to assess in the PV industry. Field performance can be impacted by many confounding variables including ambient weather conditions, seasonal changes, sensor drift, and soiling, to name a few. Extracting system degradation rates previously required years of production data, high accuracy instrumentation, and the presence of staff scientists to conduct the evaluation.

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The RdTools software package solves these problems by providing a robust and validated software toolkit for calculating and analyzing PV system performance and degradation over time. The tool can deliver valuable insights for manufacturers, engineers, investors and owners who have a stake in system performance, such as identifying under-performing sub-arrays, and quantifying system performance relative to neighboring systems.

For co-developer SunPower, the results of its own data analysis were compelling. “The RdTools method was used to analyze energy generation from 264 PV systems at locations across the globe, revealing that degradation rates were slower than expected,” said Greg Kimball, a senior performance engineer at SunPower. “The result prompted improvements to and extension of our warranty coverage to customers.”

According to Adam Shinn, a data scientist for co-developer kWh Analytics, RdTools is valuable because of the information it provides to the solar investors with whom they work. “As more and more solar is deployed, there is an ever-increasing amount of PV performance data available to analyze,” Shinn said. “For solar investors who seek to understand the long-term financial risks of their energy-producing assets, analysis RdTools will help them quantify PV durability.”

RdTools was led by a NREL team of researchers: Michael Deceglie, Chris Deline, Dirk Jordan, and Ambarish Nag and funded by the U.S. Department of Energy Solar Energy Technologies Office. The software is actively being developed as a set of open-source Python scripts and usage examples on GitHub and is publicly available to interested users who can access, download, and customize the software.

— Solar Builder magazine