How Perovskite Minerals Have Inspired Promising, new Semiconductors

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 our silicon world? To answer that question, we’ll assess perovskites’ inherent benefits and growing popularity amongst today’s manufacturers. 

Making an Important Distinction

It’s noteworthy that the term ‘perovskite’ refers to two different things: both a distinct oxide mineral derived from calcium titanate (CaTiO3), and a category of compounds which share calcium titanate’s crystalline structure. We typically refer to such related compounds in the plural form—perovskites. 

A sample of naturally-occurring perovskite. Image courtesy of Britannica.

Though perovskite itself is naturally occurring, ‘perovskites’ are conversely manmade. Each of these facts is uniquely critical. 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. That’s where the magic lies. 

Industry Needs and Material Properties

That may seem like a lot of scientific effort, considering how capable modern silicon components are. We do know, however, that a number of new electronics are hitting shelves every day. These products depend on silicon-based components (AKA microelectronics) like CPUs, GPUs, memory units, capacitors, and sensors. That last need carries particular weight as smart homes, businesses, and cities spring about. 

Should designers continue placing every egg in the silicon basket, we’ll eventually risk widespread material shortages. In fact, this is already happening—due to a combination of COVID production challenges and skyrocketing demand. It’s clear that alternative compounds are needed. This is crucial for combatting supply-chain issues, lowering costs, or overcoming silicon’s deficiencies within certain applications. What, then, specifically sets perovskites apart? 

Special Characteristics

The flexibility of perovskite compounds works highly in their favor. The electrical, physical, and optical characteristics may be readily matched to a given application—ensuring that internal components are performant and stable. We also attribute the following benefits to perovskites:

•  High absorption coefficient – less material can absorb more light

• Long-range, ambipolar charge transport – dynamic conduction within FETs and CMOSs

• 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

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. 

An orthorhombic perovskite’s molecular structure. Image courtesy of ResearchGate.

Naturally-occurring perovskite shares many benefits shared above. The mineral readily absorbs light. Positive and negative charges can travel easily throughout. Accordingly, natural perovskite is also considered a semiconductor. Its manmade derivatives share that honor.

Promising Applications

Perhaps the best use case for perovskite(s) has been solar energy. The panels currently used are comprised of photovoltaic cells—taking sunlight and converting it into usable electrical energy. Those cells must naturally be absorbent. While silicon-based cells do this reasonably well, perovskite alternatives outshine them in key ways. 

Perovskite solar cells are also renowned for their flexibility. Image courtesy of the U.S. Department of Energy.

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. The perovskite used in this case is considered hybrid organic-inorganic perovskite (HOIP). It’s even believed that these HOIP cells can achieve 30% efficiency—thoroughly outperforming ‘previous-gen’ solar cells.

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 are now finding ways to stave off heat and moisture damage. Specialized coatings and encapsulation may help boost longevity.

Image courtesy of the U.S. Department of Energy.

More work is needed to also reduce defects. Though perovskite isn’t defect prone, the active conduction layer is a weak point. 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.

Multi-Purpose Semiconductors and Memory

We know that natural perovskite is a semiconductor, and thus has been adapted for a number of related applications. Apart from solar cells, low-dimensional perovskites are also ideal for optoelectronic devices. These transducers “guide, detect, modulate…and generate optical signals.” LEDs, optical fibers, and photoresistors are just a few examples of optoelectronic devices.

Perovskite devices of this type are highly tunable. They’re flexible. Their quantum electrical effects also make them suitable for such applications. Even lasing applications may benefit. However, it’s understood that extracting every ounce of reliability and performance will take additional work. 

Perovskites are generally ideal semiconductor materials due to their accommodating nature. The compounds readily accept layering when separated by an elemental substrate. Extracting favorable electrical behavior from these layers is relatively easy. 

Accordingly, there’s been much conversation around how we 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. These layers are unified via molecular bonding. 

Memory

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.  

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.

The Future of Electronics?

There’s no denying that perovskites have come into prominence the past handful of years. The mineral and its related compounds are viable alternatives to longstanding technologies. They bring unique conductive benefits to the table. 

While there are undoubtably some kinks to work out, the maturation process should bring about new enhancements. Further research will help perovskite proofs on concept evolve into powerful solutions. 

The largely-optimistic tone throughout the scientific community corroborates this. We’ve seen plenty of perovskite news come and go in the past year alone. The question remains, however: will silicon’s dominance slowly start waning as a result?