NREL argues for value of ‘watts per kilogram’ in emerging thin-film, flexible solar technology

NREL lightweight CIGS

This lightweight CIGS photovoltaic cell, on flexible stainless steel, was made by Matthew Reese and his team at NREL. Photo by Dennis Schroeder / NREL

Rigid silicon solar panels dominate the utility and residential markets, but opportunity exists for thin-film photovoltaic and emerging technologies notable for being lightweight and flexible, according to scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).

Thin films such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS), along with perovskites and other new technologies, could be ideal for generating the electricity needed for unmanned drones, portable chargers, and building facades. The opportunities and challenges inherent in widespread adoption of these ideas appear in the new Nature Energy paper, “Increasing Markets and Decreasing Package Weight for High Specific Power Photovoltaics.

“We explore the limits behind power-to-weight ratios and how this can generate value for emerging players in photovoltaics to enable them to reach gigawatt scale without having to directly compete with silicon solar panels,” said Matthew Reese, an NREL researcher and lead author of the paper. The paper was co-authored by Stephen Glynn, Michael Kempe, Deborah McGott, Matthew Dabney, Teresa Barnes, Samuel Booth, David Feldman, and Nancy Haegel, all from NREL.

The market opportunity

Silicon panels constitute 95 percent of the global solar market, generating electricity for utilities, residences, and businesses, but the researchers identified applications that must consider value propositions beyond the standard value triad of cost, efficiency, and reliability used for conventional photovoltaic (PV) panels. Flexibility and portability will be important factors, with the performance of the technology quantified in terms of watts per kilogram.

The researchers identified three high-value markets, each with a potential to cumulatively generate a gigawatt (GW) of electricity—at a price above $1 a watt—over the next 10 years:

Aerospace and unmanned aerial vehicles – Powering satellites is driven by extremely high launch costs; whereas, there is an increasing desire to keep drones aloft for very long periods. For both of these applications, limited space makes efficiency and weight critical and cost secondary. A key player in this market is III-V PV, but while highly efficient it’s also too expensive for many applications.

Portable charging – Making it easy for one person to install or move a portable charger is driving the need for PV technology that’s efficient and flexible. Finding the correct balance between those requirements and cost could put millions of units into service by the military, disaster relief workers, and recreational users.

Ground transportation – The integration of PV in electric vehicles will compete with electricity coming from the grid, but the addition could extend the driving range. The PV would have to use smaller panels and be flexible enough to conform the contours of the roof.

The researchers identified these markets as smaller but significant and ones that will pay a premium for the added value of the technology being lightweight to support initial, low-scale production. As production increases, lower costs will follow.

The NREL team determined the lower limit for a lightweight PV device is between 300 and 500 grams per square meter. Below that would reduce reliability, durability, and safety. A lightweight module on the lower side of that range could generate more than a kilowatt of electricity from something that weighs as little as a six pack of soda. Conventional modules, even without the additional weight from the mounting equipment, might require 150–200 pounds to generate this much power.

— Solar Builder magazine

Solar-Tectic patents perovskite, crystalline silicon thin-film tandem solar cell

solar tectic perovskite

Perovskite materials are always on the horizon for the solar industry, holding promise as a future solution to the long-standing problem of solar cell efficiency, which is of primary importance in today’s solar panel market. And while there have been numerous reports of perovskite/silicon (wafer) tandem solar cells, remarkably there has been none on a perovskite/crystalline silicon thin-film tandem solar cell, until now.

The US Patent and Trademark Office (USPTO) awarded Solar-Tectic LLC  two patent applications for perovskite thin film solar cells, one of which covers all kinds of perovskites. The inventors are Ashok Chaudhari, Founding Manager of Solar-Tectic, and the late Dr. Praveen Chaudhari, renowned materials physicist.

Tandem cells explaned

Wafer-sized bottom poly- and monocrystalline silicon layers in PERC, PERL, HIT, HJ, or perovskite/silicon tandem cells are typically 200-280 microns thick, whereas Solar-Tectic’s thin-film crystalline inorganic bottom layers can be as thin as 20-30 microns with the same or similar efficiency; moreover, they can be processed at much lower temperatures thereby lowering costs of production significantly. The top perovskite layer is less than only 1 micron – an ultra-thin film — and a thin film crystalline silicon (CSiTF) bottom layer decouples the need for a silicon wafer. If the price of polysilicon rises less silicon material use will be an additional cost savings.

RELATED: Modules and integration: Four reasons why AC, smart modules are on the rise

Tandem perovskite solar cells are capable in theory of 45 percent efficiency, though Solar-Tectic has set a more realistic 30 percent efficiency goal, higher than the best silicon wafer technologies such as PERC, PERL, HIT, HJ cells with 25-26.6 percent efficiencies. The efficiencies of today’s solar cells on the market in general range from 14 – 25 percent. A cost effective 30 percent efficient solar cell with a simple design would revolutionize the solar energy industry by dramatically reducing the balance of system (BoS) costs, thereby lessening the need for fossil fuel generated electricity significantly. Silicon wafer technology based on polycrystalline or monocrystalline silicon, which is 90 percent of today’s market, would become obsolete.

Non-toxic

Importantly, the entire Solar-Tectic process is environmentally friendly since non-toxic Sn (tin) or Au (gold) is used to deposit the crystalline silicon thin-film material for the bottom layer in the tandem/heterojunction configuration as well as in the top, perovskite, layer. The more commonly used toxic Pb (lead) is not used in the perovskite here. The manufacturing methods used in this technology – sputtering or electron beam evaporation — are conventional and similar to those used in today’s thin-film solar cell industry, and importantly also in the display industry with which there is much overlap and potential for synergy.

The breakthrough patents correspond to a “Tandem Series” of solar cell technologies which has been launched by Solar-Tectic, and that includes a variety of different proven semiconductor photovoltaic materials (i.e. III-V, CZTS, a-Si, etc) for the top layer on silicon (or germanium) bottom layer, on various substrates such as cheap soda-lime glass. A paper reporting a successful step in this approach was recently published. Last year, Solar-Tectic announced the first patent ever granted for this perovskite/silicon thin-film tandem approach.

A patent for a copper oxide thin-film tandem solar cell was also granted to ST (US 9,997,661) this month thereby expanding the IP portfolio of the tandem series.

— Solar Builder magazine

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 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