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Ultra-high efficiency, Perovskite tandem solar cells promise solving the world energy crisis and secure energy for military

Increasing energy demand, environmental issues and limited availability of fossil fuels are demanding the research on sustainable and renewable energy resources. The sun is ultimate source to accomplish clean energy demand and photovoltaics, also known as solar PV, have been growing exponentially to harness it. The global solar photovoltaics (PV) industry has entered a new era, as since 2018 the electricity generation from PV has become one of the cheapest, or in some cases, the cheapest, energy harvesting technology available to date. Many electricity generation scenarios are pointing towards a massive PV implementation in the coming decades, projecting PV to be the most important electricity generation technology in 2050.


The global PV Market has witnessed an annual growth rate of 24% between 2010 and 2017 . To fulfill the strategic climate goals the PV-markets both in Europe and the rest of the world will continue to grow between 10% and 30% up to 2030. . In order to enable the realization of this enormous PV deployment, improved, new and widely accepted PV technologies will be required besides the current existing PV technologies.


The, traditional solar cells, are bulky and expensive to manufacture, plus they are inflexible and cannot be made transparent, which can be useful for temperature-monitoring sensors placed on windows and car windshields. 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. One of the promising technology ofr efficient establishment of solar cell technology on a global scale is  organic-inorganic halide perovskites as solar cells because of it’s high power conversion efficiency (PCE) and the low-cost fabrication.


There are many existing applications where even disposable low-cost, high-efficiency solar cells could be attractive, such as use in disaster response, device charging and lighting in electricity-poor regions of the world. Perovskite solar cells also have the highest power to weight ratio amongst viable photovoltaic technologies.


In fact, labs have found promising success with perovskite when it works in tandem with other solar technologies. Perovskites absorb more of the light spectrum, so that layer is placed on top of a successfully stable material. Solar cells typically produce charge carriers by optimum absorption of the solar spectrum and ensure that charge carriers are efficiently captured and minimally re-combined on the way to the terminals. Single junction Perovskite solar cells already are at the edge of breaking the PCE (Power Conversion Efficiency) record of conventional silicon solar cell. The theoretical Shockley-Queisser (S-Q) threshold of 30% has bottled more leaps in its solar cell performance.


To improve the PCE beyond S-Q limit, carefully designing the integration of high-efficiency wide-bandgap top solar cells with low-bandgap bottom solar cell will form tandem solar cells. The 2T/4T tandem architectures have a calculated theoretical efficiency limit up to 43%, while the 3T predicts an achievable efficiency of 32%. Tandem solar cells with multiple layers of light absorption might however, achieve an efficiency up to 86%. This can be realised by a tandem of an infinite number of cells, with a smoothly varying series of bandgaps (from 0 to infinity), illuminated by heavily concentrated sunlight.


Tandem solar cells are hence looked at optimistically due to the existence of both high and low band gap material in a single device. This type of arrangement enables the device to be active in both long and short wavelength regions, where each wavelength region can effectively be converted to electric power leading to enhanced efficiency. Perovskite SCs (solar cells) are excellent choice for integration with silicon solar cell as they possess unique properties like high absorption coefficient, tuneable band-gap, high defect tolerance, ever increasing performance figures, high open circuit voltage, abundant availability of its constituent elements and easy processability.


Perovskite SCs can use the high energy blue and green light much more efficiently than silicon SCs. On the other hand, silicon solar cells respond to red and infra-red light


The silicon-perovskite tandem cells will be more expensive than the current silicon cells because extra process steps and material are needed compared to the standard cells. However, the gain in efficiency will compensate considerably for this extra cost, resulting in a lower cost/watt peak. Tandem solar cell modules will certainly win the race with current cheap silicon modules.


This type of cell will mainly be used in solar cell parks. For solar cell parks in less sunny areas—where the light intensity and clouds are more variable-bifacial silicon solar cells can also be used with a layer of perovskite on top. This allows you to also capture light that is reflected from the ground.


Military applications

Military is also interested in using Pervovskites. Solar power has been and will continue to be instrumental in the DoD’s efforts to procure more secure and independent energy. On-site solar generation allows the military to be less reliant on aging transmission infrastructure and remote power plants. A solar energy system, coupled with a battery backup, diesel generator or thermal energy storage, can operate in island mode.” This allows the solar project to continue to provide power independent from the grid, which provides an extra layer of redundancy and reduces the risk posed by blackouts and potential cyber-attack,” writes Dr. Richard Smardzewski is currently the Principal Scientist at SciTech Services, Inc. in Havre de Grace.


As the military shifts to solar power and away from traditional generation sources, its energy supply will be less dependent on fossil fuels and less susceptible to global supply and price disruptions. Solar power provides the military with locally generated energy insulated from natural or man-made instabilities that could put missions at risk. Solar power will continue to be a vital piece of the Army, Navy and Air Force’s efforts to meet their renewables targets moving forward. More specifically, solar PV power accounts for 58 percent of the 1.9 GW (gigawatt) of identified DoD renewable energy capacity additions from 2012 to 2017.

NREL Achieves Breakthrough In Perovskite tandem Solar Cells

Most of those research efforts have centered on lead-based perovskites, which have a wide bandgap. High-efficiency, low-bandgap perovskites would enable the fabrication of very high-efficiency all-perovskite tandem solar cells, where each layer absorbs only a part of the solar spectrum and is optimally configured to convert this light into electrical energy. However, low-bandgap perovskites have long suffered from large energy losses and instability, limiting their use in tandems.


The efforts NREL scientists made to narrow the bandgap by replacing part of the lead atoms in the perovskite structure brought the newly refined low-bandgap perovskite solar cell to about 20.5% efficiency. Their results are detailed in a new paper, “Carrier lifetimes of >1μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells,” which appears in Science.“This is going to be an active research area in the coming years,” says Kai Zhu, a senior scientist at NREL and a corresponding author of the paper.


Replacing lead (Pb) in perovskite solar cells can narrow the bandgap. Adding tin (Sn), however, creates other problems. The rapid crystallization and oxidation of tin creates pinholes and other defects in Sn-based perovskite thin films. A tandem solar cell utilizing layers of perovskites holds the theoretical maximum efficiency of more than 30%. To reach that, the low bandgap absorber layer by itself must be between 21% and 23% efficient. Solar cells based on a lead-tin mix have reported efficiencies of about 19%, compared to between 21% and 24% for their pure-lead counterparts.


Tandem solar cells combining a wide-bandgap perovskite top cell and a low-bandgap bottom cell based on mixed tin (Sn)-lead (Pb) perovskite or a dissimilar material such as silicon (Si) or copper indium gallium selenide (CIGS) offer an extraordinary opportunity to achieve PCEs higher than Shockley-Queisser (SQ) radiative efficiency limits (~33%) for single-junction cells.


To offset the effects of tin in the mix, NREL scientists introduced the chemical compound guanidinium thiocyanate (GuaSCN). After discovering how 7% GuaSCN was the optimal amount to reduce defects considerably, they validated these findings to make the solar cell more efficient in another key way. Solar cells generate electricity by using light to “excite” electrons. The longer the electrons stay excited, the more electricity is generated. The new low-bandgap material after the chemical modification enabled the electrons to stay excited for more than 1 microsecond, or about five times longer than was previously reported.


The improved low-bandgap, single-junction solar cell with its 20.5% efficiency was then coupled with a conventional wide-bandgap perovskite cell. The researchers achieved a 25% efficient four-terminal and a 23.1% efficient two-terminal perovskite thin-film tandem cell.


Zhu’s co-authors from NREL are Jinhui Tong, Dong Hoe Kim, Xihan Chen, Axel Palmstrom, Paul Ndione, Matthew Reese, Sean Dunfield, Obadiah Reid, Jun Liu, Fei Zhang, Steven Harvey, Zhen Li, Steven Christensen, Glenn Teeter, Mowafak Al-Jassim, Maikel van Hest, Matthew Beard and Joseph Berry. Some researchers are affiliated with the University of Toledo and the University of Colorado at Boulder. Funding for the research at NREL came from the DOE’s SunShot Initiative, the Solar Energy Technologies Office, and the Center for Hybrid Organic Inorganic Semiconductors for Energy.


U.S. Department of Energy’s National Renewable Energy Lab and the University of Colorado developing tandem

The University of Toledo physicist working in collaboration with the U.S. Department of Energy’s National Renewable Energy Lab and the University of Colorado, Dr. Yanfa Yan, UToledo professor of physics, envisions the ultra-high efficiency material called a tandem perovskite solar cell that will be ready to debut in full-sized solar panels in the consumer market in the near future.


The new research paper, which is published in the journal Science, outlines how the photovoltaics team is fine-tuning a mix of lead and tin to advance the technology closer to its maximum efficiency. Efforts have currently brought the efficiency of the new solar cell to about 23 percent. In comparison, silicon solar panels on the market today have around an 18 percent efficiency rating. Scientists used a chemical compound called guanidinium thiocyanate to dramatically improve the structural and optoelectronic properties of the lead-tin mixed perovskite films.


“Our UToledo research is ongoing to make cheaper and more efficient solar cells that could rival and even outperform the prevailing silicon photovoltaic technology,” said Dr. Zhaoning Song, research assistant professor in the UToledo Department of Physics and Astronomy and co-author on the study. “Our tandem solar cells with two layers of perovskites deliver high power conversion efficiency and have the potential to bring down production costs of solar panels, which is an important advance in photovoltaics.”


While Yan’s team has improved the quality of the materials and the process to manufacture them at a low cost, more progress needs to be made. “The material cost is low and the fabrication cost is low, but the lifetime of the material is still an unknown,” Song said. “We need to continue to increase efficiency and stability.” “Also, lead is considered a toxic substance,” Yan said. “I am determined to work with the solar industry to ensure solar panels made of this material can be recycled so they don’t cause harm to the environment.”


Dutch Researchers Have Made A Huge Breakthrough

A team of Dutch and Belgian researchers from Eindhoven University of Technology, TU Delft, imec, and TNO has broken the 30% barrier for solar panels, reported in Sep 2022. The modules have a conversion efficiency of 30.1 percent, which is a first in the solar panel sector, an organizational press release said.

The key is in the design of such a “tandem solar panel.” Instead of using traditional solar cell construction, the researchers merged ‘the new perovskite solar cell technology with standard silicon solar cell technology, stacking the two solar panel technologies on top of each other to collect a broader spectrum of sunlight.

More specifically, the lowest layer, which mainly transforms near-infrared light into energy, has a conversion efficiency of 10.4 percent when employing silicon solar cell technology. The researchers then added a layer of perovskite solar cells on top. This layer effectively transmits near-infrared light (93 percent of that light) but transforms ultraviolet and visible light into energy with a 19.7 percent efficiency.

According to the researchers, nearly 30% of the sunlight that shines on this module is transformed into power, a feat that might significantly aid the energy transition.

For the time being, however, it remains a prototype; it is unknown when the tandem solar cells with perovskite and silicon panels will be offered to people.


Photovoltaic-Powered Sensors For The ‘Internet Of Things’

By 2025, experts estimate the number of “internet of things” devices — including sensors that gather real-time data about infrastructure and the environment — could rise to 75 billion worldwide. As it stands, however, those sensors require batteries that must be replaced frequently, which can be problematic for long-term monitoring.


MIT researchers have designed photovoltaic-powered sensors that could potentially transmit data for years before they need to be replaced. To do so, they mounted thin-film perovskite cells — known for their potential low cost, flexibility, and relative ease of fabrication — as energy-harvesters on inexpensive radio-frequency identification (RFID) tags.


In a pair of papers published in the journals Advanced Functional Materials and IEEE Sensors, MIT Auto-ID Laboratory and MIT Photovoltaics Research Laboratory researchers describe using the sensors to continuously monitor indoor and outdoor temperatures over several days. The sensors transmitted data continuously at distances five times greater than traditional RFID tags — with no batteries required. Longer data-transmission ranges mean, among other things, that one reader can be used to collect data from multiple sensors simultaneously.


Depending on certain factors in their environment, such as moisture and heat, the sensors can be left inside or outside for months or, potentially, years at a time before they degrade enough to require replacement. That can be valuable for any application requiring long-term sensing, indoors and outdoors, including tracking cargo in supply chains, monitoring soil, and monitoring the energy used by equipment in buildings and homes.


The cells could power the sensors in both bright sunlight and dimmer indoor conditions. Moreover, the team found the solar power actually gives the sensors a major power boost that enables greater data-transmission distances and the ability to integrate multiple sensors onto a single RFID tag.


The idea, then, was combining a low-cost power source with low-cost RFID tags, which are battery-free stickers used to monitor billions of products worldwide. The stickers are equipped with tiny, ultra-high-frequency antennas that each cost around three to five cents to make.


RFID tags rely on a communication technique called “backscatter,” that transmits data by reflecting modulated wireless signals off the tag and back to a reader. A wireless device called a reader — basically similar to a Wi-Fi router — pings the tag, which powers up and backscatters a unique signal containing information about the product it’s stuck to.


Traditionally, the tags harvest a little of the radio-frequency energy sent by the reader to power up a little chip inside that stores data, and uses the remaining energy to modulate the returning signal. But that amounts to only a few microwatts of power, which limits their communication range to less than a meter.


The researchers’ sensor consists of an RFID tag built on a plastic substrate. Directly connected to an integrated circuit on the tag is an array of perovskite solar cells. As with traditional systems, a reader sweeps the room, and each tag responds. But instead of using energy from the reader, it draws harvested energy from the perovskite cell to power up its circuit and send data by backscattering RF signals.


The key innovations are in the customized cells. They’re fabricated in layers, with perovskite material sandwiched between an electrode, cathode, and special electron-transport layer materials. This achieved about 10 percent efficiency, which is fairly high for still-experimental perovskite cells. This layering structure also enabled the researchers to tune each cell for its optimal “bandgap,” which is an electron-moving property that dictates a cell’s performance in different lighting conditions. They then combined the cells into modules of four cells.


In the Advanced Functional Materials paper, the modules generated 4.3 volts of electricity under one sun illumination, which is a standard measurement for how much voltage solar cells produce under sunlight. That’s enough to power up a circuit — about 1.5 volts — and send data around 5 meters every few seconds. The modules had similar performances in indoor lighting. The IEEE Sensors paper primarily demonstrated wide‐bandgap perovskite cells for indoor applications that achieved between 18.5 percent and 21. 4 percent efficiencies under indoor fluorescent lighting, depending on how much voltage they generate. Essentially, about 45 minutes of any light source will power the sensors indoors and outdoors for about three hours.


The RFID circuit was prototyped to only monitor temperature. Next, the researchers aim to scale up and add more environmental-monitoring sensors to the mix, such as humidity, pressure, vibration, and pollution. Deployed at scale, the sensors could especially aid in long-term data-collection indoors to help build, say, algorithms that help make smart buildings more energy efficient.


“The perovskite materials we use have incredible potential as effective indoor-light harvesters. Our next step is to integrate these same technologies using printed electronics methods, potentially enabling extremely low-cost manufacturing of wireless sensors,” Mathews says.


Conclusions and perspectives

Perovskite silicon tandem solar cell perhaps has a very high potential to reach low level cost of electricity.

Designing reliable and efficient tandem perovskite-silicon requires a multilevel approach that includes optimizing the performance of individual layers in each cell separately, when the cells are linked to each other and finally together with the substrate. Primarily, the key for high device efficiency is in the judicious choice of the bandgap of top perovskite cell in the stack. Parasitic absorption losses faced by tandem devices due to inefficient intermediate reflecting layers and inefficient absorption in the top cell have to be addressed for further research to tackle the optical losses. Low absorption coefficient of silicon bottom cell reduces the light absorption and reflects in performance of all tandem configuration. Efficient light management is required for further advancement in the device performance.


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