Even if you have never heard of a perovskite, you rely on them every day. Practically every electronic device made today incorporates some kind of perovskite in its capacitors, sensors, LEDs, or other components. Perovskites are so ubiquitous because they offer a range of useful optical and electrical properties:
- Dielectric or semiconducting electrical properties
- Piezoelectric electromechanical properties
- Efficient light generation in LEDs
- Efficient power generation in photovoltaics
Perovskites, rather than being a single material, are actually a broad class of materials named after perovskite, CaTiO3, a naturally-occurring mineral.
In order to qualify as a “perovskite”, a material must have a chemical formula ABX3 as well as a cubic crystal structure. Inorganic perovskites, where “A” and “B” are metals and “X” is usually oxygen, include a range of semiconductor and insulator materials used in various devices and technologies. Before long, perovskite-based solar cells may also be powering those same devices.
The many uses of perovskites
Many perovskite materials are used in electronic devices that take advantage of their ability to sustain a strong electric field without conducting electricity.
Barium titanate, BaTiO3, and barium strontium titanate, [Ba1-xSrx]TiO3, (where either Ba or Sr take the “A” position in the crystal structure) are dielectric materials, insulators that can be polarized in an electric field. This makes them especially useful for high-voltage, high-capacitance ceramic capacitors.
Some perovskites, including BaTiO3 and [Ba1-xSrx]TiO3, are also piezoelectric materials that change shape when a voltage is applied and also create a voltage when they are strained.
This makes them useful for both actuators and sensors, which create and detect small mechanical motions with very high precision. Materials like Pb[ZrxTi1-x]O3, Pb[MgxNb1-x]O3, Pb[InxNb1-x]O3, LiNbO3, etc. are used in ultrasound transducers, like those used in prenatal and other medicine. Recent research has focused on developing lead-free piezoelectric materials like BaZrO3, which can be sourced through Matmatch. Piezoelectric sensors are also used as pickups in musical instruments as well as tilt sensors used in phones and other electronic devices.
Perovskites have useful optoelectronic properties, and they are particularly efficient at converting electrical energy to light and back again. Both hybrid organic-inorganic perovskites and inorganic perovskites can be used to create high-efficiency light-emitting diodes. However, the most exciting area where perovskites are being used may be in photovoltaics, where perovskites are close to making solar power cheaper and more efficient than ever before.
Remember that “perovskite” is a name for a class of materials with a particular chemical formula, ABX3, and crystal structure. “Traditional” inorganic perovskite materials like CaTiO3 have metal ions at the “A” and “B” locations in the crystal structure, but it is possible to create perovskites from a mix of organic and inorganic ions.
Hybrid organic-inorganic perovskites such as [CH3NH3]Pb[I3-xClx] where the organic molecule methylamine, CH3NH3, occupies the “A” site in the ABX3 crystal structure, are strong candidates to disrupt the solar power industry with low-cost, high-efficiency photovoltaic cells. Hybrid perovskites tend to have traits that make them useful in photovoltaic applications, such as:
Perovskite photovoltaic technology is maturing at a particularly exciting time because photovoltaics are close to making the leap from a specialized niche power source to a keystone of the world’s energy infrastructure.
A tipping point in the energy economy
Historically, solar power has been an expensive source of power that was mostly used in remote locations (like outer space) or an alternative energy source for environmentally-conscious people looking for sustainable energy.
However, in recent years, solar power technology has been approaching the point where it can compete with more traditional energy sources like coal and natural gas in terms of cost per Watt of power output. It is becoming not only more environmentally friendly than competing technologies, but also more profitable. As a result, solar power is poised to claim a large share of the power generation industry.
In order to become competitive in the energy marketplace, solar panels have to accomplish three key goals:
- efficient conversion of light to electricity
- reduced cost of each photovoltaic cell
- and improved service life.
Perovskite-based photovoltaic cells offer both excellent efficiency and low manufacturing cost, and maximising device life is a very active area of research.
Several researchers are working to bring perovskite-based photovoltaics to the market, and they may offer a low-cost, high-efficiency photovoltaic technology just as solar power becomes a key pillar of the world’s energy economy. But in order to understand why perovskite photovoltaics are so exciting, we must discuss how a photovoltaic cell is constructed.
How semiconductors power photovoltaics
What a photovoltaic accomplishes is easy enough to understand: they convert the energy in light (hence “photo”) to electrical power (“voltaic”). The key to how photovoltaics operate is the semiconductor absorber material at their heart. The semiconductor is such an important component of photovoltaics that most solar cell technologies are named after the type of semiconductor they use such as perovskite, silicon, CdTe, etc.
Photovoltaics are made from layers of different materials, and the semiconductor absorber layer is where light energy is absorbed by the material and used to lift electrons to a higher energy level.
These excited electrons are free to move around the material, but unless something drives them in a particular direction, they will wander in random directions and/or lose their energy by releasing light and re-combining with a nearby atom.
Solar cells are designed to drive the flow of electrons in one direction in order to convert those “excited,” high-energy electrons into electrical current. This is done by combining layers that have been “doped” or mixed with different impurities to make them more positive (“p-doped”) or negative (“n-doped”).
The junctions where n and p-doped semiconductors meet act like one-way streets for electrons, which converts what would otherwise be the random motion of excited electrons into useful electrical current.
The ideal solar cell converts as much of the light that strikes it into electrical energy, which is why perovskites’ excellent absorbance, high charge carrier mobility, and low cost make them such promising materials for efficient photovoltaic cells.
Competing solar technologies
The current photovoltaic market is dominated by silicon-based technologies, which constitute 70% of U.S. solar power capacity in 2017 and 92% of the worldwide market share in 2014. Thin-film photovoltaics constituting the largest minority technology. Silicon photovoltaic technology is very mature, dating back to the 1950s, which is a key part of why it has become established as the primary solar power technology.
Perovskite-based solar cells offer excellent light absorption, charge-carrier mobility, and low manufacturing costs, which makes them an excellent contender to offer low cost-per-watt solar power. Current research in perovskites is focused on improving their chemical stability in order to extend their service life.
Unlike silicon photovoltaics, which is processed from ingots of silicon, perovskite absorber layers can be printed or spin-coated, making them inexpensive to produce. Silicon is also highly sensitive to manufacturing defects, while perovskite-based cells are more defect-tolerant. Once perovskite photovoltaics achieve better long-term stability, they are expected to become the main alternative to silicon-based solar technology.
However, the best photovoltaic technology might not be the winner of the battle between silicon and perovskite photovoltaics. Instead, it could be the result of combining both technologies. “Tandem” solar cells consist of a perovskite absorber coated over the top of a silicon substrate. Each layer absorbs different wavelengths of light, resulting in even greater efficiency than offered by two separate cells.
Perovskites are already one of the most important material systems in modern technology. They are part of many of the capacitors, sensors, diodes, and other components of modern electronics. However, before long they may be powering those electronics. Just as solar power is becoming competitive in the energy marketplace, perovskite-based photovoltaics are on the verge of making solar power even more efficient and cost-effective.