What are perovskites and their applications

Named after 19ty century Russian mineralogist Lev Perovski, perovskite materials have a crystal-lattice structure of inorganic molecules like that of ceramics, along with organic molecules that are interlaced throughout.
Perovskites are a family of crystals that show promising properties for applications in nanotechnology, especially nanostructured solar cells. ‘Perovskite’ is a term used to describe a group of materials that have a distinctive crystal structure of cuboid and diamond shapes. They have long been of interest for their superconducting, electronic and ferroelectric properties.
Since it was discovered that they are also remarkably efficient at absorbing photons of light and that this can be converted into an electric current, research interest in perovskite solar cells has skyrocketed.
Crystals are configurations of atoms, molecules or ions, that are ordered in a structure that repeats itself in all directions. We encounter crystals all the time in everyday life - ordinary salt, diamond, snowflakes. What is perhaps less well-known is that certain crystals show very interesting properties when their size is reduced to the nanoscale. There, we enter the world of nanocrystals, structures that have shown to be extremely useful in constructing technological applications at tiny scales.
Quantum dots are manmade nanoparticles of semiconducting material comprising only a few thousand atoms. Exploring the fundamental physics of perovskite quantum dots, researchers have studied how electrons interact with light in these materials.
Nanomaterials of perovskite dispersed in hexane and irradiated by laser
Nanomaterials of perovskite dispersed in hexane and irradiated by laser. Light emission by these materials is intense thanks to resistance to surface defects. (Image: Luiz Gustavo Bonato)
Results like these are very important since knowledge of the optical properties of perovskites and how its electrons behave opens up opportunities for the development of new technologies in semiconductor optics and electronics.
For instance, the incorporation of perovskite is highly likely to be the most distinctive feature of the next generation of television sets.

Perovskite structures

Despite the simplicity of the original perovskite crystal structure, this family of compounds shows an enormous variety of structural modifications and variants.
The mineral perovskite (‘true perovskite’) is composed of calcium, titanium and oxygen in the form CaTiO3. Meanwhile, a perovskite structure is anything that has the generic form ABX3, with varying composition, and the same crystallographic structure as perovskite (the mineral).
A, B, and X can represent a combination of different organic and inorganic ions. A is an organic cation; B is a big inorganic cation; and X3 is a smaller halogen anion (frequently oxide) that bonds to both cations.
Depending on which atoms/molecules are used in the structure, perovskites can have an impressive array of interesting properties, including superconductivity, giant magnetoresistance, spin-dependent transport (spintronics) and catalytic properties. Perovskites therefore represent an exciting playground for physicists, chemists and material scientists.
In nature, perovskites are primarily found as oxides, with the majority being silicates (such as bridgmanite minerals), but they also exist as fluorides, chlorides, hydroxides, arsenides, and intermetallic compounds. While the number of natural perovskite minerals is limited, synthetic perovskites span across the whole periodic table in terms of elemental composition, and they can exist in many complex formulas such as metallic perovskites, hybrid organic–inorganic perovskites, metal-free perovskites, and even noble gas-based perovskites.
The majority of efficient perovskites are semiconductor lead metal halides with tunable bandgaps.
Standard depiction of the aristotype cubic perovskite
Standard depiction of the aristotype cubic perovskite. Shown in display styles evidencing either all the atoms (left) or only the BX6 octahedral network and A atoms (right). (Source: doi:10.1021/acsenergylett.0c00039)

Perovskite solar cells

A perovskite solar cell is a type of solar cell, which includes a perovskite structured compound, most commonly a hybrid organic-inorganic lead or tin halide-based material, as the light-harvesting active layer.
Perovskite materials are usually cheap to produce and relatively simple to manufacture. They possess intrinsic properties like broad absorption spectrum, fast charge separation, long transport distance of electrons and holes, long carrier separation lifetime, and more, that make them very promising materials for solid-state solar cells.
A defining moment came in 2012, when Professors Henry Snaith at the University of Oxford and Michael Graetzel at the Federal Institute of Technology Lausanne, building on the work of Tsotomu Mayasaka from Tokyo, found that solar cells with perovskite as the active component could be made with greater than 10% power conversion efficiencies for turning the sun’s rays into electrical energy. A mere two years later, Snaith increased this to 17%. For silicon-based solar cells, it’s taken 20 years of research to achieve this level.
Since then, the sunlight-to-electrical-power conversion efficiency of perovskite solar cells has skyrocketed, with the laboratory record standing at 25.6% (as of April 2021).
Efficiencies of solar cells.
Efficiencies of solar cells. Devices included in this chart of the current state of the art have efficiencies that are confirmed by independent, recognized test labs—e.g., NREL, AIST, JRC-ESTI, and Fraunhofer-ISE—and are reported on a standardized basis. Click here for details.
Researchers are also combining perovskite solar cells with conventional silicon solar cells. Nanotextured designs can achieve a power conversion efficiency potential above 29%, which simulations suggest could be further improved with better fabrication techniques and additional texturing.
Efficiencies of solar cells
With this rapid surge in efficiency, perovskite solar cells and perovskite tandem solar cells may soon become cheap, highly efficient alternatives to conventional silicon solar cells.
What’s holding their commercial success back is the fact that all existing perovskite processing methods have a significant deteriorating effect on the material’s optical qualities. Also, most perovskite solar cells are based on toxic lead elements, so making them mor environmentally benign has to be a key goal.
Mark Ziffer from the Ginger Lab at the University of Washington demonstrates how to construct a perovskite solar cell.

Perovskite applications

Perovskite lasers

Researchers have demonstrated that perovskite cells excel not just at absorbing light but also at emitting it.
More recently, researchers have realized a broadly and continuously tunable nanolaser by utilizing single perovskite alloy nanowires.

Perovskite x-ray detector

Using 3D aerosol jet-printing, researchers 3D-printed perovskite layers on a graphene substrate. The results were stunning. The method produced X-ray detectors with a record sensitivity and a four-fold improvement on the best-in-class medical imaging devices.
These highly efficient perovskite X-ray detectors that can be easily integrated into standard microelectronics to considerably improve the performance of medical imaging devices.
Efficiencies of solar cells
Example of the deposited perovskite pillars, defining a pixel for the creation of an image. (Image: L. Forró, EPFL)

Light-emitting perovskite nanoantennas

Researchers developed novel effective nanoscale light sources based on halide perovskite. The perovskite nanoparticles serve both as emitters and nanoantennas and allow enhancing light emission inherently without additional devices.
Moreover, perovskite enables tuning emission spectra throughout a visible range by varying the composition of the material. This makes the new nanoparticles a promising platform for creating compact optoelectronic devices such as optical chips, light-emitting diodes, or sensors.
light-emitting nanoantennas based on halide perovskites

3D-printed perovskite nanopixels

The three-dimensional (3D) geometry of 3D-printed nanopixels can increase the emission brightness of display pixels, varying with the height of the pixels, and can be used to fabricate super high-resolution devices. Researchers demonstrated 3D printing of perovskite nanopillars that can be used for creating nanoscale display pixels.
smiley face nanopixel matrix
Bright nanopixel color displays: design scheme of a 'smiley face' nanopixel matrix.