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.

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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.”

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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

The ‘Carportunity’: How our electric vehicle future means big things for solar carports

California’s Franchise Tax Board complex

Electric vehicles taking over the road is no longer a question. Sales of plug-in hybrid electric vehicles and all-electric vehicles have surged recently. So now the question is where are all of these things going to get their juice?

A new study from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) quantifies how much charging infrastructure would be needed in the United States to support various market growth scenarios for plug-in electric vehicles (PEVs). NREL notes that most PEV charging occurs at home, but widespread PEV adoption would require the development of a national network of non-residential charging stations. Strategically installing these stations early would maximize their economic viability while enabling efficient network growth as the PEV market matures. NREL says about 8,000 fast-charging stations would be needed to provide a minimum level of urban and rural coverage nationwide.

No one asked us, but we think carport developments have a big opportunity (a carportunity!) to lead the way. The segment is seeing notable reductions in system costs and installation timelines that only make more projects viable.

Quest Renewables

The Value of Expertise

There is enough institutional knowledge among the chief carport construction companies now to give developers and larger investors confidence. Feast your eyes on California’s Franchise Tax Board complex, for example (pictured above). Developed by DGS-Building Property Management and installed by Ecoplexus at one of the largest business campuses in northern California, it is the state’s largest carport installation (10,400 PV panels), covering 1,276 employee parking spaces, spanning over 622,000 sq ft and generating 3.6 MW.

The project was made possible because of Baja Carport’s specialization in pre-engineered, pre-fabricated high-tensile, light gauge steel structures. And in chatting with its team at SPI this year, we’ve learned the company has been able to further streamline the costs of its system.

Then there is 4 S.T.E.L. and its standardized processes. Carport projects involve a ton of engineering and civil approval. 4 S.T.E.L.’s staff of engineers, project managers and drafters can design and erect a carport in their sleep at this point, but the big value comes in swift preapproval of its designs with the California Division of State Architects among other strict jurisdictions and building departments. Design preapproval can literally shave months off certain project timelines.

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Carports are certainly spreading beyond California too. At Michigan State University (MSU), Inovateus Solar is nearing completion of a 14-MW solar carport project spanning five parking lots and 700 sq ft on the East Lansing campus (pictured below). Using Schletter’s Park@Sol concept, the design is a maintenance-free, lightweight aluminum system with canopies standing 14-ft tall at the lowest point to provide enough room for recreational vehicles to park during football season. The carport install is expected to generate 15,000 MWh of electricity annually for MSU with projections showing a savings of $10 million in electricity costs over the next 25 years.


Disruptive Designs

Key to the Schletter approach is its Micropile foundation, a hollow metal rod installed deep into the ground (pictured to the right), that requires less concrete material to accomodate even high wind and snow loads.

“The technology innovation of using Schletter micropiles as foundations and precast concrete pads, in addition to the engineering design, cut the construction schedule in half and minimized the risk factors in a rainy environment like Florida,” said Javier Latre Gorbe, VP of Technical Operations for ESA Renewables.

A newer entrant into the carport system space, Quest Renewables, has an especially exciting concept. Hatched as project at Georgia Tech Research Institute in 2011, the design received a work grant from the DOE’s SunShot Initiative and was commercialized in 2014. The hook here is a triangular support structure that requires less steel and allows for most of it to be assembled on the ground (pictured above).

Solar carports will spread across the country as costs decline

A vehicle auction company in Elkridge, Md., put in a 304-kW system and selected the Quest Renewables QuadPod to reduce foundation counts by 50 percent (using 50 percent less steel) to mitigate the poor soil conditions. From site survey to powering up, the system was completed in 45 days with minimal interruption to the parking lot. Another project in Portland, Maine, needed to minimize disruption of the work area. The 90 percent ground-level construction allowed it to be built in just eight days from start to finish. This first parking garage canopy install in Maine will sustain 112 mph winds and 50 psf of snow.

There’s a long way to go to fill in that void NREL is talking about, but it’s a start.

— Solar Builder magazine

NREL labs demo a high-efficiency solar window that tints

NREL solar window

Thermochromic windows capable of converting sunlight into electricity at a high efficiency have been developed by scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).

What the? How it works

Relying on such advanced materials as perovskites and single-walled carbon nanotubes, the new technology responds to heat by transforming from transparent to tinted. As the window darkens, it generates electricity. The color change is driven by molecules (methylamine) that are reversibly absorbed into the device. When solar energy heats up the device, the molecules are driven out, and the device is darkened. When the sun is not shining, the device is cooled back down, and the molecules re-absorb into the window device, which then appears transparent.

Demo results

The NREL-developed demonstration device allows an average of 68 percent of light in the visible portion of the solar spectrum to pass through when it’s in a transparent, or bleached, state. When the window changes color—a process that took about 3 minutes of illumination during testing—only 3 percent is allowed through the window. Existing solar window technologies are static, which means they are designed to harness a fraction of the sunlight without sacrificing too much visible light transmission needed for viewing or the comfort of building occupants.

“There is a fundamental tradeoff between a good window and a good solar cell,” said Lance Wheeler, a scientist at NREL. “This technology bypasses that. We have a good solar cell when there’s lots of sunshine and we have a good window when there’s not.”

The proof-of-concept paper published in Nature Communications established a solar power conversion efficiency of 11.3 percent.

“There are thermochromic technologies out there but nothing that actually converts that energy into electricity,” Wheeler said. He is the lead author of the paper, “Switchable Photovoltaic Windows Enabled by Reversible Photothermal Complex Dissociation from Methylammonium Lead Iodide.”

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The path to commercialization?

In testing under 1-sun illumination, the 1-square-centimeter demonstration device cycled through repeated transparent-tinted cycles, but the performance declined over the course of 20 cycles due to restructuring of the switchable layer. Ongoing research is focused on improving cycle stability.

The path to commercialization of the technology was explored last year during a two-month program called Energy I-Corps. Teams of researchers are paired with industry mentors to learn what customers want of the technology and develop viable ways to reach the marketplace. Lance Wheeler and Robert Tenent, the program lead for window technology at NREL and co-author on the paper, teamed up to develop a market strategy for a product they called SwitchGlaze. The effort was funded by the Emerging Technologies program within the Department of Energy’s Building Technologies Office.

Wheeler said the technology could be integrated into vehicles, buildings and beyond. The electricity generated by the solar cell window could charge batteries to power smartphones or on-board electronics such as fans, rain sensors, and motors that would open or close the windows as programmed.

— Solar Builder magazine