NREL improves Perovskite thin-film solar cell production scalability

perovskite solar cell

Scientists at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) developed a new perovskite ink with a long processing window that allows the scalable production of perovskite thin films for high-efficiency solar cells.

Proven highly efficient at converting sunlight into electricity, perovskite solar cells have yet to move beyond the laboratory. The crystalline structure of perovskites must be carefully grown upon a substrate, which is normally done by laboratory-scale spin coating – a technology that can’t be scaled to large-scale manufacturing. The best devices fabricated using scalable deposition methods, which are suitable for future module production, still lag behind state-of-the-art spin-coated devices.

This research is supported by the DOE’s SunShot Initiative, a national effort to drive down the cost of solar electricity and support solar adoption.

Inside the breakthrough

To create a perovskite film, a coating of chemicals is deposited on a substrate and heated to fully crystalize the material. The various steps involved often overlap with each other and complicate the process. One extremely critical stage requires the addition of an antisolvent that extracts the precursor chemicals, and thus create crystals of good quality. The window for this step opens and closes within seconds, which is detrimental for manufacturing due to the precision required to make this time window.

NREL researchers were able to keep that window open as long as 8 minutes.

The formula for the precursor perovskite ink included a chlorine-containing methylammonium lead iodide precursor along with solvent tuning, coupled with an antisolvent, which could be deposited onto the substrate by either spin-coating or blade-coating methods. Both methods were tested and produced indistinguishable film morphology and device performance. Blade-coating is more attractive to manufacturers because it can easily be scaled up.

PV breakthrough: Thin-film solar cells can now be mapped in 3-D as they absorb photons

The researchers tested one precursor ink containing excess methylammonium iodide (MAI) and a second containing added methylammonium chloride (MACI). The MACI proved most effective in reducing the length of heat treatment the perovskites require, cutting the time to about a minute compared to 10 minutes for the MAI solution. The shorter time also should make the process more attractive to manufacturers.

Using blade-coated absorbers, NREL scientists made a four-cell perovskite module measuring about 12.6-square centimeters. Of that, 11.1-square centimeters were active in converting sunlight to energy and did so with a stabilized efficiency of 13.3 percent.

— Solar Builder magazine

Large solar installers offer higher PV system quotes than average, says NREL study

solar system pricing

The National Renewable Energy Laboratory (NREL) looked at the PV installer landscape of the country to glean information on how installer size affects solar system pricing. The results of the study (you can read the whole thing here) indicate that the larger installers (think Solar City) that take up most of the residential industry are keeping prices higher on average than smaller local installers.

From the Executive Summary:

The vast majority of U.S. residential solar photovoltaic (PV) installation companies are small local firms, yet a few large firms dominate the industry. In 2015, the largest 10 percent of installers accounted for nearly 90% of residential systems installed, and the largest 1 percent of installers accounted for more than 60% of systems. PV market literature has produced conflicting results on the effects of installer size on price behavior. In this report, we develop a novel approach using residential PV quote data, rather than installed system price data, to study the effects of installer size and market structure on price behavior. The methodological advantage of quote data is that customer and site characteristics remain constant across multiple quotes made to the same customer, allowing us to effectively control for otherwise unobservable customer- and site level differences.

Through a paired difference approach, we match quotes from large installers (those that installed more than 1,000 systems in any year from 2013 to 2015) with non-large installer quotes made to the same customers. We find that large installer quotes are $0.33/W (about 10%) higher, on average, than non-large installer quotes offered to the same customer. The difference falls to $0.21/W after controlling for systematic differences between large and nonlarge installer quotes, but it remains statistically significant. Large installers offered a higher quote price than a corresponding non-large installer in about 70% of pairings.

Our results suggest that low prices were not the primary value proposition of large installer systems, at least for quoted customer-owned systems during 2014–2016 in 27 states and Washington, D.C. These findings may support several hypotheses. Large installers may bid higher prices due to imperfect competition in the customer quote collection process. At the same time, customers may attribute a variety of additional values to large installers, such as higher quality and trustworthiness. Large installers may differentiate their services, such as through superior warranties and inverter replacement terms. Some customers may accept higher prices even if lower prices are available from smaller installers.

So what’s this mean?

These findings suggest that increased price transparency would promote further PV price reductions. The results suggest that some residential PV customers may forego lower prices for the opportunity to work with a large installer, but also that customers could benefit from obtaining more quotes before deciding to install a system.

Further reading: Four steps for converting more solar sales 

— Solar Builder magazine

SMA system selected for pilot storage program at NREL facility

Renewable Energy Systems (RES) will utilize SMA technology for a 1 MW battery pilot project at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) National Wind Technology Center (NWTC), the nation’s premier wind energy technology research facility.

SMA Sunny Central Storage_sm

Using the proven RESolve energy storage system, the project will test various components of energy storage and educate the public on how energy storage can create a more stable, secure U.S. electric grid and accommodate increasing amounts of clean, renewable energy.

The test site utilizes a Sunny Central Storage unit, SMA’s power conversion system for large-scale battery storage systems. These systems enable the integration of large amounts of intermittent renewable energy into the utility grid while maintaining grid stability. The Sunny Central Storage is compatible with different types of battery technologies.

Upon completion, research staff will use various applications of the system to test certain modes ranging from frequency regulation to renewable energy integration. The NWTC is located south of Boulder, Colo., and construction for the test system is estimated to be completed this month.

RELATED: 2017 Solar Inverter Buyer’s Guide 

— Solar Builder magazine

PV breakthrough: Thin-film solar cells can now be mapped in 3-D as they absorb photons

Next-generation solar cells made of super-thin films of semiconducting material hold promise because they’re relatively inexpensive and flexible enough to be applied just about anywhere.

Researchers are working to dramatically increase the efficiency at which thin-film solar cells convert sunlight to electricity. But it’s a tough challenge, partly because a solar cell’s subsurface realm — where much of the energy-conversion action happens — is inaccessible to real-time, nondestructive imaging. It’s difficult to improve processes you can’t see.

Now, scientists from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a way to use optical microscopy to map thin-film solar cells in 3-D as they absorb photons.

What the?

The method, reported Nov. 15 in the journal Advanced Materials, was developed at the Molecular Foundry, a DOE Office of Science user facility located at Berkeley Lab. It images optoelectronic dynamics in materials at the micron scale, or much thinner than the diameter of a human hair. This is small enough to see individual grain boundaries, substrate interfaces, and other internal obstacles that can trap excited electrons and prevent them from reaching an electrode, which saps a solar cell’s efficiency.

So far, scientists have used the technique to better understand why adding a specific chemical to solar cells made of cadmium telluride (CdTe)—the most common thin-film material—improves the solar cells’ performance.

“To make big gains in photovoltaic efficiency, we need to see what’s happening throughout a working photovoltaic material at the micron scale, both on the surface and below, and our new approach allows us to do that,” says Edward Barnard, a principal scientific engineering associate at the Molecular Foundry. He led the effort with James Schuck, the director of the Imaging and Manipulation of Nanostructures facility at the Molecular Foundry.

The imaging method is born out of a collaboration between Molecular Foundry scientists and Foundry users from PLANT PV Inc., an Alameda, California-based company. While fabricating new solar cell materials at the Molecular Foundry, the team found that standard optical techniques couldn’t image the inner-workings of the materials, so they developed the new technique to obtain this view. Next, scientists from the National Renewable Energy Laboratory came to the Molecular Foundry and used the new method to study CdTe solar cells.

Inside the new method

To develop the approach, the scientists modified a technique called two-photon microscopy (which is used by biologists to see inside thick samples such as living tissue) so that it can be applied to bulk semiconductor materials.

The method uses a highly focused laser beam of infrared photons that penetrate inside the photovoltaic material. When two low-energy photons converge at the same pinpoint, there’s enough energy to excite electrons. These electrons can be tracked to see how long they last in their excited state, with long-lifetime electrons appearing as bright spots in microscopy images. In a solar cell, long-lifetime electrons are more likely to reach an electrode.

In addition, the laser beam can be systematically repositioned throughout a test-sized solar cell, creating a 3-D map of a solar cell’s entire optoelectronic dynamics.

What have we learned so far?

The method has already shed light on the benefits of treating CdTe solar cells with cadmium chloride, which is often added during the fabrication process.

Scientists know cadmium chloride improves the efficiency of CdTe solar cells, but its effect on excited electrons at the micron scale is not well understood. Studies have shown that the chlorine ions tend to pile up at grain boundaries, but how this changes the lifetime of excited electrons is unclear.

Thanks to the new imaging technique, the researchers discovered the cadmium chloride treatment increases the lifetime of excited electrons at the grain boundaries, as well as within the grains themselves. This is easily seen in 3-D images of CdTe solar cells with and without the treatment. The treated solar cell “lights up” much more uniformly throughout the material, both in the grains and the spaces in between.

“Scientists have known that cadmium chloride passivation improves the lifetime of electrons in CdTe cells, but now we’ve mapped at the micron scale where this improvement occurs,” says Barnard.

The new imaging technique could help scientists make more informed decisions about improving a host of thin-film solar cell materials in addition to CdTe, such as perovskite and organic compounds.

“Researchers trying to push photovoltaic efficiency could use our technique to see if their strategies are working at the microscale, which will help them design better test-scale solar cells—and eventually full-sized solar cells for rooftops and other real-world applications,” he says.

The research was supported by the Department of Energy’s Office of Science and by a SunShot Initiative award from the Office of Energy Efficiency and Renewable Energy.

— Solar Builder magazine

Solar system costs continue to fall in U.S. in 2016

PV solar costs decline

The way forward for the solar industry is still through proving its economic value, a big part of which is driving down costs. And for as many costs that have already been cut out of a system install the last few years, a new report shows that the downward trend continues.

The modeled costs to install solar photovoltaic (PV) systems continued to decline in the first quarter of 2016 in the U.S. residential, commercial, and utility-scale sectors, according to updated benchmarks from the Energy Department’s National Renewable Energy Laboratory (NREL). NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. Driving the cost reductions were lower module and inverter prices, increased competition, lower installer and developer overheads, improved labor productivity and optimized system configurations.

“The continuing total cost decline of solar PV systems demonstrates the sustained economic competitiveness of solar PV for the industry across all three sectors,” said NREL Senior Analyst and Project Lead Ran Fu.

RELATED: SEPA: How to achieve low LCOE utility-scale solar without cutting costs 

The modeled costs for the first quarter of 2016 were down from the fourth quarter of 2015 by 6 percent, 4 percent and 20 percent in the residential, commercial, and utility-scale sectors, respectively. The costs fell to $2.93 per watt of direct current for residential systems, $2.13 per watt of direct current for commercial systems, and $1.42 per watt of direct current (Wdc) for residential systems for fixed-tilt utility-scale systems, and $1.49 Wdc for one-axis-tracking utility-scale systems.

“Such accurate cost benchmarks are critical for tracking the progress of PV systems toward cost-reduction goals. Because our cost model categorizes hardware and non-hardware costs with a high degree of resolution, the results can also be used to identify specific cost-reduction investment opportunities and assess regional levelized costs of energy,” Fu said.

The new results also highlight the importance of non-hardware, or “soft,” costs. As the pace of cost reductions for modules and inverters has slowed in recent years, the proportion from soft costs-such as labor, overhead and permitting costs has grown. In the first quarter of 2016, soft costs accounted for 58 percent of residential system costs, 49 percent of commercial system costs, and 34 percent of utility-scale system costs.

NREL uses a “bottom-up” modeling method to construct total capital costs by quantifying the typical cost of each individual system and project-development component, largely through dialogues and interviews with solar industry collaborators. The results represent total installed system costs from the perspective of the PV project developer or installer, including net profit in the cost of the hardware. The benchmarks are national averages weighted by state installed PV capacities.


Read the report for yourself here.

— Solar Builder magazine