Materials Spotlight: How Perovskite Was a Hotbed of 2020 Research
The topic of perovskites never seems far from research outlets, and last year was no different.
While silicon is still king (and for good reason) in semiconductor technology, alternative compounds are needed to combat supply-chain issues, lower costs, or overcome silicon’s deficiencies in certain applications. Perovskites are one such material gaining traction, especially last year.
While perovskites first gained notoriety in 1839, researchers have adapted their unique crystalline structure for diverse applications in recent years. Just how can perovskite components carve their own niche in a silicon world? To answer that question, we’ll assess perovskites’ inherent benefits and growing popularity amongst today’s manufacturers as new research rolled out in 2020.
First, How Do We Define Perovskites?
The term perovskite may refer to two different things: a distinct oxide mineral derived from calcium titanate (CaTiO3) or a category of compounds that share calcium titanate’s crystalline structure. We thus typically refer to such related compounds in the plural form: perovskites.
Basic structure of a perovskite crystal. Image used courtesy of Wikimedia Commons
Though perovskite itself is naturally occurring, "perovskites" are conversely human-made. Because perovskite is mined in multiple (albeit somewhat limited) global locations, it’s possible to create perovskite-based electrical components.
However, researchers have developed numerous synthetic, chemical compounds with that same crystalline structure. These distinct compounds (like GdFeO3 or NaMgF3, for example) have beneficial properties of their own. Manufacturers must create perovskites bearing that natural structure while incorporating conductivity into the mix.
Distinct Properties of Perovskites
The flexibility of perovskite compounds works highly in their favor. Some of their key electrical, physical, and optical characteristics include:
- High absorption coefficient: Less material can absorb more light
- Long-range, ambipolar charge transport: Dynamic conduction within FETs and CMOS technology
- Low-exciton binding energy: Energy needed to make an electron conductive
- High dielectric constant: Allowing sustained electrical charges across a surface
- Ferroelectric properties: Electrical polarization may be flipped by external electric fields
Perovskite solar cell. Image used courtesy of Dennis Schroeder/National Renewable Energy Laboratory and Cornell University
Perovskites may also be layered or applied as a thin film for certain uses. They’re also thermodynamically stable. This lack of electrical spontaneity is useful within semiconductors where predictability is favorable as electrical currents travel across wafers. Lastly, the orthorhombic (prismatic) structure of perovskites, paired with their ferroelectric nature, opens the door for superconductivity under certain conditions.
Naturally-occurring perovskite shares many benefits of the human-made material. The mineral readily absorbs light. Positive and negative charges can travel easily throughout. Accordingly, natural perovskite, like lab-made perovskite, is also considered a semiconductor.
Why Perovskites Take the Lead in Solar Energy
Perhaps the most prevalent use case for perovskite(s) has been solar energy. Since the basic premise of a solar panel (comprised of photovoltaic cells) is to convert sunlight into usable electrical energy, those cells must naturally be absorbent. Silicon-based cells do this reasonably well, but perovskite alternatives outshine them in key ways.
Diagram of a perovskite solar cell. Image used courtesy of the Clean Energy Institute
Number one is efficiency. While silicon cells convert anywhere from 18% to 21% of the sun’s energy into electricity, solar cells combining silicon and perovskite can achieve 27% efficiency, according to a recent study from CU Boulder researchers.
The perovskite used in this case is considered hybrid organic-inorganic perovskite (HOIP). It’s even believed that all-perovskite cells can achieve 30% efficiency—thoroughly outperforming "previous-gen" solar cells.
A silicon-perovskite solar cell. Image used courtesy of Dennis Schroeder/NREL
Perovskites also offer the following benefits:
- Panels are easier to produce with less waste and inefficiency.
- Manufacturers can leverage modern additive techniques, like 3D printing, to craft cells.
- Perovskite panels may be tweaked to optimally absorb (or react to) sunlight.
It’s believed that perovskite’s electron excitability will help create more electrical current. Additionally, these new thin-film panels absorb more sunlight per surface area than their predecessors. Researchers state that perovskite cells are lightweight, bendable, and produced at much cooler temperatures.
Longevity has been a question mark—at least early on. While older perovskite panels failed to challenge silicon’s 20-to-30-year lifespan, scientists, like those at the US Department of Energy’s National Renewable Energy Laboratory, are now finding ways to stave off heat and moisture damage. Specialized coatings and encapsulation may help boost longevity.
A Note on Defects
More work is needed to also reduce defects. Though perovskite isn’t defect-prone, the active conduction layer is a weak point, according to researchers at North Carolina State University. This is one area where silicon semiconductors have a sizeable head start. However, the cost efficiency and performance of perovskite vaults it into consideration for farm use.
Beyond Solar Panels: Optoelectronics and Memory
Apart from solar cells, low-dimensional perovskites are also useful for optoelectronic devices, like LEDs, optical fibers, and photoresistors. Perovskite optoelectronic devices are highly tunable and flexible. Their quantum electrical effects also make them suitable for such applications.
Halide perovskites—mainly comprised of cesium lead or methylammonium lead—show promise within resistive random-access memory (ReRAM) units. The switching behaviors and electrical properties of these materials might make them suitable. Halide compounds have displayed high ON/OFF current retention. ON/OFF ratios for 2D compounds were also higher compared to those of 3D variants.
A team at Pohang University of Science and Technology recently used halide perovskite material for a "next-gen" memory device. Image used courtesy of POSTECH
Some nagging issues have presented hurdles. Environmental factors can influence halide perovskite performance more readily. Air exposure, humidity, and ambience all factor into longevity. Thankfully, 2D ReRAM constructions have improved upon these weaknesses. Stacked ReRAM devices with platinum-coated switching layers have performed admirably. Bipolar resistive switching behavior and ultralow operating voltages are achievable—as shared in a Nature study.
Smoothing Out Scaled Perovskites Production
There’s been much conversation around how suppliers might facilitate scaled perovskite fabrication. Peidong Yang’s team at UC Berkeley drove this production process a step forward in early 2020—debuting single-layer crystallization. Each successive layer is incorporated into a live circuit, unified via molecular bonding.
More recently, a team at Stanford developed a perovskite manufacturing method known as "rapid-spray plasma processing," in which two robotic nozzles create thin films of perovskite—one spraying perovskite chemical precursor liquid onto a pane of glass while the other blasts plasma.
Solar panel produced using Stanford researchers' patented rapid-spray plasma processing. Image used courtesy of Nick Rolston and Stanford University
The result was 40 feet of perovskite film produced per minute. This outpaces the time it takes to manufacture a silicon cell by four times.
While there are undoubtedly some kinks to work out, the maturation process should bring about new enhancements, especially as perovskites continue to be a hotbed of research.