Perovskite Solar Cells Are Greener Than Silicon
It’s no secret that solar panel manufacturing is a dirty business, largely due to the intense heat that’s required to purify silicon. The amount of CO 2 emitted during that process is more than negated by the fact that once operational, the panels will generate lots of carbon-free electricity. Nonetheless, scientists and engineers are still looking for ways to reduce the carbon footprint of photovoltaic (PV) panels.
One promising silicon alternative is perovskite, whose PV properties have been improving so rapidly that the material has been the subject of thousands of research papers over the past decade. The U.S. Department of Energy (DoE) also thinks the mineral has potential, as it recently earmarked 20 million to advance perovskite stability, efficiency, manufacturing and testing. (Interested in applying for a DoE grant? The agency is hosting a webinar on August 21 to discuss funding opportunities.)
A recent life-cycle analysis of various PV technologies found that manufacturing multilayer perovskite cells has a lower carbon footprint than fabricating silicon cells or perovskite-on-silicon tandem cells. Even better, perovskite panels are less expensive to manufacture and easier to recycle. Oh, and they’re slightly more efficient than their silicon counterparts. But there are trade-offs—most notably, their lack of stability.
Perovskite solar cells have three enemies: air, moisture and heat. The first two are easily defeated by encapsulation, but it’s difficult to keep something cool when it sits in direct sunlight for hours on end. As a result, even the most stable perovskite cells have only survived about 4,000 hours of continuous light in laboratory testing. (That’s roughly equivalent to two years at five peak-sun-hours per day.) But given that the material’s photovoltaic efficiency has increased from less than 4 percent to over 25 percent in a mere decade, and that stackable tandem cells can inexpensively improve that number, it’s easy to see why researchers are focusing their efforts on making this technology practical.
Life Cycle Analysis
In Life Cycle Energy Use and Environmental Implications of High-Performance Perovskite Tandem Solar Cells. published in the July 2020 issue of Science Advances. researchers examined three types of solar panels—state-of-the-art silicon, perovskite-silicon tandems (silicon cells coated with a perovskite layer), and perovskite-perovskite tandems—to compare their carbon footprints and energy payback periods. They also considered the environmental impact of the additional materials needed to make a panel along with manufacturing scalability.
Given the number of unknowns regarding perovskite’s field performance, the scientists made certain assumptions. First, they conceded that the first generation of commercial perovskite panels may only last 15 years, compared to 30 years for silicon. Second, they didn’t include the additional carbon footprint associated with replacing the perovskite panels in the field.
Carbon Footprint
What most of us call “carbon footprint” is scientifically known as the “greenhouse gas emission factor” (GGEF), which represents the CO 2 emitted per unit of energy generated. The lower the number, the “greener” the technology. In the electrical world, the kilowatt-hour (kWh) is the standard unit of energy, so the GGEF is measured in grams of CO 2.equivalent per kilowatt-hour (g CO 2.eq/kWh). For renewable technologies like solar and wind, the only emissions are those related to mining, transporting, manufacturing, and end-of-life, so that number gets spread out over the lifetime of the device. For fossil fuels, the GGEF takes into account drilling/mining, transporting, refining, and burning.
In this study, the researchers calculated that the GGEF of state-of-the-art silicon panels—the kind used on most utility-scale PV farms—is 24.6 g CO 2.eq/kWh. Perovskite-on-perovskite tandem cells came in at 10.7 g CO 2.eq/kWh, and perovskite-on-silicon tandems fared worst of all at 46.8 g CO 2.eq/kWh.
(Side note: How green are renewable energy sources compared to fossil fuels? Silicon solar panels have a GGEF of ~25 g CO 2.eq/kWh, wind power is 10, whereas natural gas and coal tip the scales at 488 and 1,000, respectively. In other words, today’s solar panels are nearly 20x more eco-friendly than natural gas and 40x greener than coal.)
Pros and cons of perovskites
Scientists have been researching perovskite technology for years, but recent advancements are raising excitement about the potential for tandem modules at commercial scale.
Earlier this month, researchers in the Netherlands announced a breakthrough with a perovskite-silicon tandem cell that reached 30 percent efficiency, which they called “ a big step in accelerating the energy transition.” Other research laboratories have been announcing milestones as well.
Perovskites have some promising traits:
- The rate of Hero-experiment efficiency gains for perovskite materials and tandem structures has been phenomenal, outpacing other technologies.
- Perovskites can be produced from earth-abundant materials.
- Perovskites have the potential to be recycled.
- Perovskite cells can be manufactured using lower-cost and more forgiving ink-based printing processes rather than complex semiconductor equipment.
- The low-temperature processes used in manufacturing perovskites mean that energy payback times are shorter than for state-of-the-art PV technologies, according to Graybeal.
But questions loom about perovskite lifetimes, degradation and stability.
Demonstrating the long-term reliability of a tandem cell will take years, as will gaining a good understanding of perovskites’ failure modes and degradation paths. Perovskites will have a “ slightly higher degradation rate than silicon” in the material’s early days in solar tandem modules, according to Graybeal.
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“ To be competitive in the marketplace, perovskite’s long-term durability must be tested and verified,” the U.S. Department of Energy said in 2020 when it announced 20 million in funding to advance perovskite solar photovoltaic technologies. Earlier this year, pv magazine noted that effective testing of perovskites is not yet on track: “ Much testing is required, yet today’s tests are geared toward commercialized PV technologies (Si, CdTe, etc.) and are unlikely to capture all the failure modes relevant to perovskite modules in the field.”
Working with, not against, silicon
Competing against silicon is an uphill battle. than 200 gigawatts of solar power capacity will be deployed around the world this year, the vast majority of it in the form of silicon PV panels. Silicon’s long-term behavior and failure modes are already well understood, allowing module producers to offer 25.year guarantees.
So for now, most perovskite solar companies are focused on producing tandem products that include silicon, not on head-to-head competition.
While Graybeal does envision a future that includes pure perovskite modules, he acknowledges that the only way to deploy this technology at scale “ is by combining perovskites with silicon solar” and complementing the existing PV manufacturing infrastructure.
“ We consign glass [from customers] that they’ve already spec’d and qualified for their product. [We] put our proprietary material stack on that glass and then move it along to their module manufacturing operations,” Graybeal explained. He said the perovskite factory has to be close to a major logistics node and close to a customer’s module manufacturer in order for this to work, “ and that’s been part of our plan. As much as the perovskite is part of our product portfolio, the ability to ramp and scale factories is also a competency that we’re developing.”
Caelux investor Reliance New Energy, a subsidiary of Reliance Industries, India’s largest private-sector company, is entering a strategic partnership with the startup. Reliance New Energy is building a global-scale solar factory in Jamnagar, Gujarat, where it plans to incorporate perovskites into some of the modules it produces. According to Graybeal, the partnership means that “ Reliance has essentially sold out our factory for the first year and a half of its operation — and that’s more important than the cash investment.”
“ If you’re going to get any product into the market, you have to start producing in volume. We can’t all sit around and play with our two-inch-by-two-inch devices — it will take us 10 years to get to the next technological leap forward,” said the CEO. He added, “ Tier 1 manufacturers are going to put us through our paces before they’re going to put their name on a module with our technology on it.”
With typical entrepreneurial fervor, the CEO said he expects Caelux’s tandem module, along with a 25.year warranty, to launch into the marketplace in the second half of 2024.
Perovskite contenders
Caelux is not the only company in this space. Canary reported last year on CubicPV, formed from the merger of U.S. wafer maker 1366 Technologies and Hunt Perovskite Technologies, which is also working on tandem modules. That article listed other startups that are developing perovskite and tandem solar materials using a range of device architectures and manufacturing processes.
- BlueDot Photonics uses“continuous flash sublimation production” techniques to improve the efficiency of perovskite photovoltaics.
- Energy Materials Corp. is developing a roll-to-roll perovskite deposited on a flexible substrate.
- Microquanta Semiconductor is building panels from glass-packaged perovskites.
- Oxford PV is developing perovskite-on-silicon tandem solar cells and modules.
- Saule Technologies is developing an inkjet printing technique for manufacturing perovskite solar cells packaged on bendable plastic.
- Swift Solar stacks perovskite solar cells to make tandem cells and can deposit its perovskite layers on flexible substrates and foils.
- Tandem PV aims to monolithically print thin-film perovskites on glass panels and mechanically stack them on top of silicon cells.
- Wuxi UtmoLight claims to have achieved a Japan Electrical Safety Environment Technology Laboratories–certified 18. 2 percent efficiency for its large-area 756 cm² perovskite solar module. The Chinese company says that it is conducting trial runs on its 150.megawatt module production line, which it claims is the “ world’s largest” for perovskite solar.
Other very early-stage perovskite startups include Beyond Silicon, MujiElectric, SoFab Inks and Verde Technologies.
But for now, perovskites still have zero percent of the global solar market share.
Eric Wesoff is the editorial director at Canary Media.
Perovskite/Silicon Tandem Solar Cells: From Detailed Balance Limit Calculations to Photon Management
Energy conversion efficiency losses and limits of perovskite/silicon tandem solar cells are investigated by detailed balance calculations and photon management. An extended Shockley–Queisser model is used to identify fundamental loss mechanisms and link the losses to the optics of solar cells. Photon management is used to minimize losses and maximize the energy conversion efficiency. The influence of photon management on the solar cell parameters of a perovskite single-junction solar cell and a perovskite/silicon solar cell is discussed in greater details. An optimized solar cell design of a perovskite/silicon tandem solar cell is presented, which allows for the realization of solar cells with energy conversion efficiencies exceeding 32%.
Introduction
Photovoltaic is the fastest growing energy source in the electricity sector. The cost for production, installation, and maintenance of photovoltaic systems has decreased dramatically throughout the last 10 years. Nevertheless, the technology is not the most widely used primary electrical energy source due to the limited energy conversion efficiency and the system’s cost, which is still high compared to non-renewable energy sources [1,2,3]. Current commercial solar modules are predominately based on crystalline silicon single-junction solar cells. So far, laboratory solar cells with record energy conversion efficiencies of 26.3% have been demonstrated [4] while the upper theoretical energy conversion efficiency of a solar cell with a bandgap of 1.15 eV (e.g., silicon) is ~ 33.5% [5]. Different approaches have been proposed to increase the energy conversion efficiency of solar cells or to overcome the limits of conventional single-junction solar cells by applying novel physical principles. Based on the proposed approaches multi-junction solar cells have been the most promising approach [6,7,8,9,10,11,12]. Detailed balance calculations reveal that serial connected tandem solar cells can reach energy conversion efficiencies exceeding 40% if an ideal material combination is selected for the top and bottom solar cell [5, 6]. Energy conversion efficiencies higher than 40% can be reached if EG_top = 0.5 × EG_bot 1.15 eV, where EG_top and EG_bot are the bandgaps of the top and bottom diode absorbers. The relationship is valid if the bandgap of the bottom diode stays in a range from 0.85 to 1.2 eV. Hence a variety of material combinations can be selected. Crystalline silicon with a bandgap of 1.15 eV is well suited as a bottom solar cell. Hence a lot of research has been devoted to the development of tandem solar cells using a crystalline silicon bottom solar cell. In this case, the highest energy conversion efficiency can be reached if the bandgap of the top cell is equal to ~ 1.7 eV. Several aspects must be considered to combine the well-established crystalline silicon solar cell technology with other material systems or fabrication processes. Amorphous silicon exhibits an ideal bandgap, but the tailstates of the material prevent the realization of solar cells with high open-circuit voltages, which is a prerequisite for the realization of tandem solar cells with high energy conversion efficiencies [13,14,15,16,17]. Silicon oxide/crystalline silicon-based quantum dot and quantum well have been investigated as potential material of the top solar cell [18,19,20,21]. However, solar cells with high energy conversion efficiencies have not been realized using silicon-based quantum dots or quantum wells. Furthermore, compound semiconductors have been investigated as potential top solar cell absorber material. However, the high fabrication temperatures of compound semiconductors, the lattice mismatch between silicon and compound semiconductors, and the fabrication cost have so far prevented the successful realization. In recent years, the perovskite material system has been investigated as potential material for single-junction solar cells or as material for perovskite/silicon tandem solar cells [7, 8, 12, 22,23,24]. So far, the material exhibits very encouraging results [25,26,27,28,29,30,31,32]. High energy conversion efficiencies have been achieved for single-junction solar cells with open-circuit voltages close to the theoretical limit. Furthermore, the material system can be fabricated by a variety of deposition methods at low temperatures, which facilitates the integration of a perovskite top solar cell on a crystalline silicon bottom solar cell. Up to now, perovskite single-junction solar cells with energy conversion efficiencies exceeding 20% have been achieved [33,34,35,36,37]. Research on perovskite/silicon tandem solar cell is still a new research topic. The number of teams working on the realization of record perovskite/silicon tandem solar cells is still small. The realization of perovskite/silicon tandem solar cells with record efficiencies is only possible if the perovskite top solar cell and the silicon bottom solar cell operate very close to the theoretical limit. Nevertheless, perovskite/silicon tandem solar cells with certified energy conversion efficiencies exceeding 27% have been demonstrated [38]. The realization of solar cells with higher energy conversion efficiencies approaching or even exceeding 30% can be expected soon. A thorough investigation of the losses of a solar cell is required to close the gap between theoretical energy conversion efficiency limits and the performance of real solar cells. In this study, we review different thermodynamic and detailed balance approaches used to calculate the upper energy conversion efficiency limit of solar cells. We present several theoretical approaches to determine fundamental energy conversion efficiency limits, starting with the fundamental Carnot process to the well-established Shockley–Queisser limit. However, the Shockley–Queisser limit is still a model making several idealistic assumptions, e.g., the absorber of the solar cell is only described by the bandgap and electrical and optical properties of real materials are not considered. Furthermore, only radiative recombination is considered in the calculation of the upper energy conversion efficiency limit. In Sects. 3.5 and 3.6, we review approaches published by different teams on more generalized detailed balance approaches. Models will be described, which take charge transport processes into account (Sect. 3.5), while optical losses and limits of the absorption of a solar cell, commonly called Yablonovitch limit, are introduced in Sect. 3.6. Section 3 ends with a review of the detailed balance calculations for tandem solar cells. In Sect. 4, we describe how optics and nanophotonics can be used to optimize not only the short-circuit current density of a solar cell, but also all solar cell parameters.
Researchers push four-terminal perovskite-silicon tandem cells to 30% efficiency
Researchers have claimed a record 30.1% conversion efficiency for four-terminal perovskite-silicon PV tandem cells.
Achieved by combining a perovskite solar cell with conventional silicon solar cell technologies, the result was presented during the 8th World Conference on Photovoltaic Energy Conversion in Milan, Italy last week.
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In four-terminal tandem devices, the top and bottom cells operate independently of each other, making it possible to apply different bottom cells, according to a press release.
The researchers improved the efficiency of the semi-transparent perovskite cells up to 19.7%, certified by standardisation organisation ESTI.
“This type of solar cell features a highly transparent back contact that allows over 93% of the near infrared light to reach the bottom device,” said Yifeng Zhao, PhD student at Delft University of Technology.
The silicon device – a heterojunction solar cell featuring optimised surface passivation – is optically stacked under the perovskite, contributing with 10.4% efficiency.
“Combined, 30.1% is the conversion efficiency of this non-area matched [four-terminal] tandem devices operating independently. This world’s best efficiency is measured according to generally accepted procedures,” Zhao said.
The result was achieved through a collaboration between teams at Delft University of Technology, research organisation the Netherlands Organisation for Applied Scientific Research (TNO), Eindhoven University of Technology and research and innovation centre Imec.
According to the scientists, the four-terminal architecture makes it straightforward to implement bifacial tandems to further boost the energy yield.
“Now we know the ingredients and are able to control the layers that are needed to reach over 30% efficiency,” said Gianluca Coletti, programme manager of tandem PV technology and application at TNO.
“Once combined with the scalability expertise and knowledge gathered in the past years to bring material and processes to large area, we can FOCUS with our industrial partners to bring this technology with efficiencies beyond 30% into mass production.”
Publication of the results comes after researchers in the US recently constructed a perovskite solar cell with a certified stabilised efficiency of 24% that they said is both highly efficient and stable.
Revolutionizing Solar Energy: The Rise of Perovskite-Silicon Tandem Solar Cells. Researcher Level
Perovskite silicon tandem solar cells could be the next viable step in the evolution of mass adoption of solar technology. 2023 is reportedly the year when we will see several pilot manufacturing plants burst into life in China and Europe.
Can we say that the next generation of PV cells has reached a critical point for mass production and application?
This is the inspiration for our latest Fluxim’s Science Shorts video: Perovskite Silicon Tandem Solar Cells and How to Simulate Them.
Watch Dr. Antonio Cabas Vidani explore the advantages of multijunction solar cells, how they work, and why perovskite-silicon tandem cells are a game-changer for solar energy.
You will also learn how the advanced optics module in Setfos can:
- Compute the reflection and transmission of coherent thin-film components
- Use a ray-optical approach to evaluate the (angular) scattering properties of the textured interfaces
- Deploy a net-radiation algorithm that uses this information to quantify the light propagation in the entire layer stack.
Watch it all here
Of course, it would be remiss of us not to acknowledge the paper on which we based our latest Science Shorts. We would like to extend our thanks to our colleague, Dr. Urs Aeberhard, for his paper:
Analysis and optimization of perovskite-silicon tandem solar cells by full opto-electronic simulation
U. Aeberhard, R. Häusermann, A. Schiller, B. Blülle and B. Ruhstaller, 10.1109/NUSOD49422.2020.9217773.
About this paper: We present a comprehensive opto-electronic simulation framework for the computational analysis and optimization of perovskite-silicon tandem solar cells, consisting of a combination of a multiscale optical model for the simultaneous consideration of interference in thin coatings and scattering at textured interfaces with a mixed electronic-ionic drift-diffusion transport model that captures the peculiarities of the geometries and materials used in the tandem architecture.
Tandem Solar Cell Papers Enabled by Setfos
Impact of mixed perovskite composition-based silicon tandem PV devices on Efficiency limits and global Performance
Ahmer A.B. Baloch, Omar Albadwawi, Badreyya AlShehhi, Vivian Alberts,
Energy Reports, Volume 8, Supplement 16, 2022, ISSN 2352-4847,
What is the worldwide performance of silicon/perovskite solar cells compared to the respective single junctions?
That is the question that the group of Vivian Alberts from the Research and Development Center of the Dubai Electricity and Water Authority answered by simulating the energy yield and cell temperature depending on the geographical location.
Considering a perovskite solar cell with a bandgap of 1.7eV, tandem cells generate on average 26.7% more energy than silicon solar cells, while dissipating less heat thanks to the higher electrical efficiency. The optimal latitude for tandem performance is in the 45◦N to 45◦S range.
With the simulation software Setfos they could estimate the theoretical efficiency limit of the tandem device under standard testing conditions (STC) using detailed balance analysis.
Perovskite–organic tandem solar cells with indium oxide interconnect.
Brinkmann, K.O., Becker, T., Zimmermann, F. et al.
In this Nature paper, the research team reached a new outstanding certified efficiency record of 23.1% with a two-terminal perovskite/organic solar cell.
Thanks to an ALD-deposited InOx interconnection layer, the current between the two subcells is matched at 14.1 mA/cm2. The high Voc of 2.15 V indicates an almost ideal interconnection between the two subcells. These devices use an organic absorber for the narrow-gap subcell, which doesn’t need the high-temperature processing of silicon and is more stable than the commonly used narrow-bandgap perovskites based on Sn.
With the software Setfos, they performed optical simulations to identify the wide bandgap perovskite that gives current matching with the organic subcell.
Tandem Solar Cell Research Project
To finish this month’s newsletter on Tandem Solar Cells we’d like to highlight a very exciting RD Project we are collaborating on.
SuPerTandem is a 3-year project financed with the sources from Horizon Europe research aid program and SERI, the Swiss state secretariat for education, research, and innovation, and is designed to help accelerate the European transition to clean energy by developing a scalable, low CO2 footprint photovoltaic technology for highly-efficient (30%) two-terminal tandem cells and modules based on complementary metal-halide perovskite absorbers.
SuPerTandem uses and develops sustainable and earth-abundant perovskite absorber materials, ancillary materials, and scalable large-area manufacturing processes to create a novel low-cost environmental friendly photovoltaic (PV) technology at an affordable 20 Euro per square meter.
Fluxim is part of a 15-strong consortium of leading European labs, industrial equipment makers, and flexible PV module-producing companies – namely represented by: TNO – Netherlands Organisation for Applied Scientific Research, HZB Helmholtz Zentrum Berlin, Fraunhofer ISE, Saule Technologies and Saule Research Institute, TuE – Eindhoven University of Technology, CEA – Alternative Energies and Atomic Energy Commission, 3D Micromac, FOM Technologies, SALD – Spatial Ald Innovators, Tecnalia, Amires, EMPA, Flisom, and Fluxim.
SuPerTandem began in Oct 2022 and is set to complete in September 2025. We would like to acknowledge our partner’s contribution so far and we look forward to working with them over the coming years.
Want to share your thoughts on the latest devlopments in Tandem Solar Cell research? (KAUST (King Abdullah University of Science and Technology)
Do you think there will be a significant development in terms of commercialization this year? Let us know below.