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

What to see at Panasonic’s booth at Solar Power Northeast in Boston, Feb 5-6

Panasonic solar solutions

Are you headed to Boston for Solar Power Northeast next week? Panasonic Eco Solutions North America is and will be showcasing its high performance HIT solar panels and energy storage products at booth 208.

The HIT solar panel products that will be on display include the all-black 40 mm N320K panels that feature an industry-leading temperature coefficient of -0.258%. The Panasonic booth will also be fitted with the high-efficiency 40 mm N330 and N325 solar panels that offer 19.7% higher power output and greater energy yields, as well as a lower 0.26% annual degradation when compared to conventional panels.

The Harbor Plus smart battery, co-developed by Panasonic and Pika Energy, will also be available for viewing in the booth. The Harbor Plus is a scalable and flexible smart battery unit that offers between 10.6 kWh and 15.9 kWh of usable energy, making it simple for system owners to accommodate their changing energy needs. Alongside the Harbor Plus battery will be Panasonic’s lithium-ion rechargeable battery that can be used across a wide range of portable electronic applications.

Representing Panasonic at Solar Power Northeast will be Mukesh Sethi, Group Manager for the Solar and Energy Storage Products Division of Panasonic Eco Solutions North America, and Chris Brown, Business Development and Sales Manager. Mukesh and Chris will be available to discuss the current lineup of Panasonic products, as well as what the company has in store for 2018.

— Solar Builder magazine

Ask a Distributor: We ask distributors for their purchasing advice, products to watch in 2018

solar distributors

Solar is now the No. 1 new source of capacity being added to the grid, according to the U.S. Energy Information Administration, and solar installer is the fastest growing job in the country according to the Bureau of Labor Statistics. And what’s the reason for the rise of this new era? The impending doom of the ice caps melting? Hardly. The advancements in technology? Getting warmer.

Really, things just got cheaper.

This is to say that purchasing plays a large, perhaps outsized, role in the value proposition of the industry and your business. Given that reality, we wanted to kick off the year by polling a handful of solar distributors for their purchasing advice.

You’ll be hearing from:

purchasing

Here’s the No. 1 way to avoid purchasing problems

What are the common problems distributors see when it comes to purchasing systems to install? There are a few quick, solvable issues — stuff like reviewing the details of an order before signing off on it or avoiding last minute purchases.

“Plan ahead and inspect shipments to avoid/mitigate mix ups and ensure fast and timely solutions when needed,” McShea says. “Waiting three weeks until install to say something is missing and you need it now is not effective.”

But all of that feeds into the advice that resonated with us the most: Seeing the distributor relationship as a two-way street that adds value to your business. According to the distributors we talked to, the more often you communicate, the more the distributor knows about your business, the easier it is for them to help.

“Valuing price over loyalty and the quality of the relationship with a distributor, I think, is a mistake,” Schoder from Civic notes.

A simple step up in communication will not only solve a bunch of issues but could create additional value for your business.

“Good communication and transparency are always the best remedies for doing good business and mitigating potential errors,” Dufrenne says. “Everyone is busy and going to make mistakes, including distributors and shipping companies. When all parties pay attention to detail and plan accordingly, deliveries and installations seem to go more smoothly.”

And it’s not just an exercise in mistake avoidance. Keeping everyone on the same page, forecast-wise, can only make the ordering process smoother for everyone.

“At least one call a week to plan upcoming jobs would help avoid supply chain issues,” Kyler says. This ensures all orders and forecasting align with expectations.

There’s also the training aspect. “Manufacturers are constantly offering training, quite often through their distributors,” Bailey notes. “Yet we still see many installers deal with headaches for hours which they could have learned to avoid by going through some quick training on the product features and installation.”

Think about kits

Ordering from multiple suppliers for a given residential job can leave more room for error, such as damage during shipping, incorrect parts arriving or a delayed delivery, all of which lengthens install time and increases costs.

“The best solution I’ve seen yet for lowering soft costs is to have one to two standardized kits which the sales team and installation crew are both very familiar with,” says Leone with Civic. “This mitigates both sales and labor costs. We recommend having multiple kits to protect against upstream challenges such as availability.”

“Ordering from one supplier and having the experts kit the equipment per job before shipping to the jobsite or warehouse can reduce most mistakes that cause delays and additional labor/mobilization costs,” Dufrenne says.

Sticking with brands also avoids the hassle of resubmitting permits with different equipment types or brands.

4 soft cost reduction tips

Schoder: “Utilizing online software programs like Helioscope and Energy Toolbase to drastically reduce the amount of time and effort that goes into a respectable customer proposal.”

Bailey: “Smaller installers like to use microinverters for [lowering soft costs]. In many cases they don’t have the in-house expertise to properly design a string inverter system which can maximize production. Microinverters are flexible, and the installers can design in the field by adding modules as they see fit.”

Dufrenne: “Good operations management [from lead generation to final completion]. Use software, like ENACT.”

Kyler: “Installers should keep at least one administrator dedicated to handling all incoming and outgoing paperwork such as permits, SREC registrations and contracts. This would help reduce soft costs and avoid any confusion or dilemmas later on.”

Truck these rolls

For starters, it helps to have a mini-inventory of small accessories (L-feet, clamps, wire), but stocking extra parts for the rest of the system is always a good idea.

“A common purchasing mistake I see installers make is purchasing just enough for their project instead of calculating for inevitable adjustments when they get on the roof,” Kyler says. “Mostly when it comes to racking, I recommend keeping spare parts in each truck.”
Dufrenne laid it out like this: “Not ordering extra parts for racking and attachments ends up costing much more than you’d think, once you have to roll another truck and pay for overnight shipping charges for parts that are usually less than $10.”

When it comes to the inverter (and we will get into this more on page 28) MLPE or string inverters could each offer a route to reduced truck rolls, if handled correctly. But for now, we just note what Kyler recommends: “Select products that allow remote updates or choose manufacturers that provide service programs.”

“Carry spare parts and get your system up and communicating with your gateway and online account before leaving the site,” Bailey says. “Installers who use gateways can track performance as soon as the system lights up. They can coordinate with technical support and determine within a few minutes that all modules and inverters are performing to spec.”

Head to page 2 for advice on system purchasing

— 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

Jinergy’s high-efficiency modules first to achieve new IEC certification via TUV Rheinland

JINERGY solar modules

The ultra-high efficiency heterojunction (HJT) module from Jinneng Clean Energy Technology Limited (JINERGY) passed the new IEC standards in a test conducted by TÜV Rheinland, an independent third-party testing, inspection and certification organization. JINERGY became the first photovoltaic enterprise to receive the certification.

The new PV module performance and safety standards (IEC 61215:2016 and IEC 61730:2016) issued by International Electrotechnical Commission (IEC) has raised the requirements on design and production of PV modules and set up entrance standards to regulate the PV market.

JINERGY, a leading PV cell and module manufacturer headquartered in Shanxi Province, China, follows a technology iteration strategy and has deployed three generations of cutting-edge technology, polycrystalline, Passivated Emitter and Rear Cell (PERC) monocrystalline and HJT. Among them, HJT is considered to be a bridge for solar cell efficiency to reach 25%.

Compared with other ultra-high efficiency technologies, HJT is manufactured using fewer procedures and JINERGY’s bifacial HJT module features excellent performance in weak light, leading temperature coefficient, and ultra-low degradation with N-type silicon wafer. Moreover, with a bifacial design, power output of JINERGY’s HJT module can be increased by 8%-20% in different application scenarios such as grasslands, cement floors and snow covered fields. The combination of dual glass and HJT eliminates PID fundamentally. JINERGY’S HJT modules help boost 44% overall power generation compared with regular polycrystalline modules.

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

“Bifacial ultra-high HJT technology is an ideal solution to reduce levelized cost of electricity (LCOE), the widely used standard to evaluate investment for PV power stations. So far, mass production power of 72-cell bifacial HJT module has reached 400W and mass production efficiency of HJT cells has exceeded 22.5%,” said Dr. Liyou YANG, general manager of JINERGY. Dr. YANG also predicted that overall cost gap between HJT products and regular polycrystalline products would narrow down to 30% and the gap could further shrink to 20% in the future. After mass production of HJT modules, cost reduction and efficiency improvement will speed up solar power to reach grid parity.

— Solar Builder magazine