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

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.

RELATED: How OMRON reduces solar module PID potential through its inverter topology 

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

NREL researchers prove perovskite solar cells more stable than previously thought

perovskite solar cells

Over the past decade, perovskites have rapidly evolved into a promising technology, now with the ability to convert about 23 percent of sunlight into electricity, but work is still needed to make the devices durable enough for long-term use. Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) created an environmentally stable, high-efficiency perovskite solar cell, bringing the emerging technology a step closer to commercial deployment.

NREL’s unencapsulated solar cell -— a cell used for testing that doesn’t have a protective barrier like glass between the cell’s conductive parts and the elements -— held onto 94 percent of its starting efficiency after 1,000 hours of continuous use under ambient conditions, according to research published in Nature Energy.

“During testing, we intentionally stress the cells somewhat harder than real-world applications in an effort to speed up the aging,” said Joseph Luther, who along with Joseph Berry directed the work titled “Tailored Interfaces of Unencapsulated Perovskite Solar Cells for >1000 Hours of Operational Stability.” “A solar cell in the field only operates when the sun is out, typically. In this case, even after 1,000 straight hours of testing the cell was able to generate power the whole time.”

While more testing is needed to prove the cells could survive for 20 years, or more, in the field (the typical lifetime of solar panels) this study represents an important benchmark for determining that perovskite solar cells are more stable than previously thought.

The typical design of a perovskite solar cell sandwiches the perovskite between a hole transport material, a thin film of an organic molecule called spiro-OMeTAD that’s doped with lithium ions and an electron transport layer made of titanium dioxide, or TiO2. This type of solar cell experiences an almost immediate 20 percent drop in efficiency and then steadily declines as it became more unstable.

“What we are trying to do is eliminate the weakest links in the solar cell,” Luther said. The researchers theorized that replacing the layer of spiro-OMeTAD could stop the initial drop in efficiency in the cell. The lithium ions within the spiro-OMeTAD film move uncontrollably throughout the device and absorb water. The free movement of the ions and the presence of water causes the cells to degrade.

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

A new molecule, nicknamed EH44 and developed by Alan Sellinger at the Colorado School of Mines, was incorporated as a replacement to spiro-OMeTAD because it repels water and doesn’t contain lithium. “Those two benefits led us to believe this material would be a better replacement,” Luther said.

The use of EH44 as the top layer resolved the later more gradual degradation but did not solve the initial fast decreases that were seen in the cell’s efficiency. The researchers tried another approach, this time swapping the cell’s bottom layer of TiO2 for one with tin oxide (SnO2). With both EH44 and SnO2 in place, as well as stable replacements to the perovskite material and metal electrodes, the solar cell efficiency remained steady. The experiment found that the new SnO2 layer resolved the chemical makeup issues seen in the perovskite layer when deposited onto the original TiO2 film.

“This study reveals how to make the devices far more stable,” Luther said. “It shows us that each of the layers in the cell can play an important role in degradation, not just the active perovskite layer.”

Other co-authors of the paper are Jeffrey Christians, Philip Schulz, Steven Harvey, and Bertrand Tremolet de Villers from NREL; and Jonathan Tinkham, Tracy Schloemer, and Alan Sellinger, who work jointly between NREL and Colorado School of Mines.

— Solar Builder magazine

Perovskite breakthrough: NREL gains new insight into how the cells degrade

Perovskite solar cells are the most tantalizing research category in the solar industry because of their efficiency and versatility, but thus far haven’t budged outside a lab setting. A microscopic analysis conducted by the Department of Energy’s National Renewable Energy Laboratory has revealed new insight into how the devices degrade— huge information for moving the technology closer to commercialization.

NREL perovskite solar cell

Published in Nature Communications, the “Impact of Grain Boundaries on Efficiency and Stability of Organic-Inorganic Trihalide Perovskites,” outlines the first quantitative nanoscale photoconductivity imaging of two perovskite thin films with different power conversion efficiencies.

Highly efficient at converting sunlight to electricity, perovskite solar cells have emerged as a revolutionary new technology with the potential to be more easily manufactured and at a lower cost than silicon solar cells. Ongoing research, including at NREL, focuses on moving perovskites beyond a laboratory setting.

The researchers took a close look at two organic-inorganic hybrid perovskite thin films made of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3). Perovskite solar cells possess a polycrystalline structure with individual crystals grains. These grains are adjacent to other crystals and the area where the crystals touch is known as a grain boundary.

“The general assumption is that degradation starts with grain boundaries,” said Kai Zhu, a senior scientist in NREL’s Chemistry & Nanoscience Department and co-author of the paper. “We were able to show that degradation is not really starting from the visible boundaries between grains. It’s coming from the grain surface.” As a result, this implies that the surface of a perovskite solar cell should be targeted for improving device performance.

The two thin films examined varied slightly. The first, with smaller grains, had a power conversion efficiency (PCE) of 15 percent. The second, with larger grains, had a PCE of 18 percent. Each film was protected by a layer of the plastic polymethyl methacrylate (PMMA); earlier research showed unprotected films tended to degrade within several hours under ambient conditions. The solar cells, illuminated by a focused laser beam from below, were examined by a novel instrument, termed light-stimulated microwave impedance microscopy (MIM). This allowed researchers to map the nanoscale photoconductivity of the samples.

“With the MIM technique, for the first time we were able to visualize the intrinsic nanoscale photo-response, which is of fundamental importance to solar cell performance,” said Keji Lai, an assistant professor of physics at the University of Texas at Austin, “Grain boundaries are usually the weak links in functional materials.” Lai worked with his colleague, associate professor Xiaoqin Li, graduate student Zhaodong Chu, and postdoc researcher Di Wu.

The analysis showed the photoconductivity of the 18 percent sample, which contained a better crystallinity, was five to six times higher than that of the other thin film. The perovskite thin films were tested over the course of a week in an area that was 74 degrees Fahrenheit and had 35 percent relative humidity. Little change in photoconductivity was observed the first few days, but by the third day the measure began to drop as water molecules moved through the PMMA coating. The drop in the photoconductivity emerged from the disintegration of the grains and not from the grain boundaries, the research found. In this instance, the scientists noted, the grain boundaries “are relatively benign” and determined perovskite films with better crystallinity should be a direction of future research for improving perovskite solar cell performance and durability.

— Solar Builder magazine

Eaton, NREL team up on new power management research

NREL Eaton

The Energy Systems Integration Facility (ESIF) at the National Renewable Energy Laboratory in Golden, CO. (Photo by Dennis Schroeder / NREL)

To expedite research and commercialization of new energy-related technologies, power management company Eaton and the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) have entered into a cooperative agreement to co-locate approximately 15 members of Eaton’s Corporate Research and Technology team at NREL’s Energy Systems Integration Facility (ESIF) in Golden, Colorado. NREL is the DOE’s primary national laboratory for renewable energy and energy efficiency research and development.

“This first-of-its-kind agreement for Eaton and NREL is an exciting next step in our long relationship,” said Ramanath Ramakrishnan, executive vice president and chief technology officer, Eaton. “By having Eaton engineers on-site every day, we will be able to substantially accelerate the innovation process by more closely leveraging NREL’s energy integration infrastructure. This infrastructure, combined with Eaton’s ability to mitigate the risks associated with early-stage technologies, will help us more efficiently translate ideas into next generation solutions.”

On the Scene: We went to the Eaton Experience Center to see the grid’s future

For more than a decade, Eaton and NREL have collaborated on a comprehensive portfolio of joint programs that includes optimizing energy systems for microgrids, buildings and communities, and developing a predictive battery management system for hybrid electric vehicles. This new agreement augments this relationship by enabling both organizations to collaborate closely on the evolving state of energy solutions such as microgrids, energy storage systems and grid intelligence.

“NREL’s industry partnerships are integral to the advanced energy research revolutionizing the global energy landscape,” said Dr. Martin Keller, NREL’s director. “This on-site, direct collaboration allows our fully-integrated teams to expand knowledge related to grid integration and power management.”

Igor Stamenkovic, director, global technology, will lead the team on behalf of Eaton.

Eaton is a power management company with 2016 sales of $19.7 billion. We provide energy-efficient solutions that help our customers effectively manage electrical, hydraulic and mechanical power more efficiently, safely and sustainably. Eaton is dedicated to improving the quality of life and the environment through the use of power management technologies and services. Eaton has approximately 96,000 employees and sells products to customers in more than 175 countries.

— Solar Builder magazine