What are perovskites?
Perovskite is a calcium titanium oxide mineral, with the chemical formula CaTiO3, discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and named after Russian mineralogist Lev Perovski (1792–1856).
Perovskites are a class of materials with a similar structure that are easily synthesized and relatively low-cost. Perovskites are considered the future of solar cells and are also predicted to play a significant role in next-gen electric vehicle batteries, displays, sensors, lasers and much more.
Perovskites can have an impressive collection of interesting properties including “colossal magnetoresistance” - their electrical resistance changes when they are put in a magnetic field (which can be useful for microelectronics). Some Perovskites are superconductors, which means they can conduct electricity with no resistance at all. Perovskite materials exhibit many other interesting and intriguing properties. Ferroelectricity, charge ordering, spin dependent transport, high thermopower and the interplay of structural, magnetic and transport properties are commonly observed features in this family. Perovskites therefore hold exciting opportunities for physicists, chemists and material scientists.
What are LEDs?
A light-emitting diode (LED) is an electronic component that is essentially a two-lead semiconductor light source. It is a p–n junction diode that emits light upon activation by a voltage applied to the leads, which makes electrons recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light is determined by the energy band gap of the chosen semiconductor.
LEDs’ advantages over incandescent light sources include lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes have become ubiquitous and are found in diverse applications in the aerospace and automotive industries, as well as in advertising, traffic signals, camera flashes and much more.
LEDs meant for general room lighting currently remain more expensive than fluorescent or incandescent sources of similar output, but are significantly more energy efficient.
What can perovskites do for LEDs?
Current high-quality LEDs are based on direct bandgap semiconductors, but making these devices is no easy task because they need to be processed at high temperatures and in vacuum, which makes them rather expensive to produce in large quantities. Perovskites that are direct-bandgap semiconductors could be real alternatives to other types of direct-bandgap materials for applications like color displays, since they are cheap and easy to make and can be easily tuned to emit light of a variety of colours.
Researchers have found that organometal halide-based perovskites (a combination of lead, organics and halogens that arrange into perovskite crystal structure in the solid state) could be very suitable for making optoelectronics devices, since they can be processed in solution and do not need to be heated to high temperatures. This means that large-area films of the these materials can be deposited onto a wide range of flexible or rigid substrates. The perovskites also have an optical bandgap that can be tuned in the visible to infrared regions, which makes them very promising for a range of optoelectronics applications. These materials also emit light very strongly, which makes them very suitable for making LEDs. The light emitted by the perovskites can be easily tuned, which could make them ideal for color displays and lighting, and in optical communication applications.
However, a major obstacle that perovskites will have to overcome in order to be used in LED-type devices is that electrons and holes only weakly bind in perovskite thin films. This means that excitons (electron-hole pairs) spontaneously dissociate into free carriers in the bulk recombination layer, leading to low photoluminescence quantum efficiency (PLQE), high leakage current and low luminous efficiency. This obviously impairs perovskites’ ability to create high-performance LEDs, and for perovskite materials to make a comparable impact in light emission, it is necessary to overcome their slow radiative recombination kinetics. Simply put, researchers will have to find ways of effectively confining electrons and holes in the perovskite so that they can “recombine” to emit light. Major progress is already being made in this field, and it seems that perovskites will indeed open the door to a low-cost, color-tunable approach to LED development.
Recent work in the field of perovskite-based LEDs
In July 2016, researchers at Nanyang Technological University in Singapore have fabricated high-performance green light-emitting diodes based on colloidal organometal perovskite nanoparticles. The devices have a maximum luminous efficiency of 11.49 cd/A, a power efficiency of 7.84 lm/W and an external quantum efficiency of 3.8%. This value is said to be about 3.5 times higher than that of the best colloidal perovskite quantum-dot-based LEDs previously made.
In March 2016, researchers at the University of Toronto in Canada and ShangaiTech University in China have succeeded in using colloidal quantum dots in a high-mobility perovskite matrix to make a near-infrared (NIR) light-emitting diode (LED) with a record electroluminescence power conversion efficiency of nearly 5% for this type of device. The NIR LED could find use in applications such as night vision devices, biomedical imaging, optical communications and computing.
In February 2016, researchers from the Universitat Jaume I and the Universitat de València have studied the interaction of two materials, halide perovskite and quantum dots, revealing significant potential for the development of advanced LEDs and more efficient solar cells. The researchers quantified the "exciplex state" resulting from the coupling of halide perovskites and colloidal quantum dots, both known separately for their optoelectronic properties, but when combined, these materials yield longer wavelengths than can be achieved by either material alone, plus easy tuning properties that together have the potential to introduce important changes in LED and solar technologies.
In December 2015, researchers at Pohang University in Korea are reportedly the first to develop a perovskite light emitting diode (PeLED) that could replace organic LED (OLED) and quantum dot LED (QDLED).
Organic/inorganic hybrid perovskite have much higher color-purity at a lower cost compared to organic emitters and inorganic QD emitters. However, LEDs based on perovskite had previously shown a limited luminous efficiency, mainly due to significant exciton (a complex of an electron and hole that can allow light emission when it is radiatively recombined) dissociation in perovskite layers. The research team overcame the efficiency limitations of PeLED and boosted its efficiency to a level similar to that of phosphorescent OLEDs. This increase was attributed to fine stoichiometric tuning that prevents exciton dissociation, and to nanograin engineering that reduces perovskite grain size, and concomitantly decreases exciton diffusion length. PeLED might be a game changer in the display and solid-state lighting industries, with significantly improved efficiency as well as advantages like excellent color gamut and low material cost.
In November 2015, Florida State researchers have developed a cheaper, more efficient LED, or light-emitting diode, using perovskites. The researchers spent months using synthetic chemistry to fine-tune the materials in the lab, creating a perovskite material capable of emitting a staggering 10,000 candelas per square meter when powered by 12 volts. The scientists say that such exceptional brightness owes, to a large extent, to the inherent high luminescent efficiency of this surface-treated, highly crystalline nanomaterial.
The latest perovskite LED news:
Researchers at Linköping University have developed efficient perovskite near-infrared (NIR) light-emitting diodes. The external quantum efficiency is a record 21.6%. The work was led by Linköping scientist Feng Gao, in close collaboration with colleagues in China, Italy, Singapore and Switzerland.
The external quantum efficiency (the ratio of charge carriers emitted as light over all of those fed into the materials) of light-emitting diodes based on perovskites has until now been limited by defects that arise in the material during manufacture. The defects act as traps for the charge carriers and thus cause energy losses.
A team of scientists from Washington University in St. Louis, Oak Ridge National Laboratory and University of Missouri studied the structure and properties of the commonly occurring planar defects at the atomic scale of lead halide perovskite.
When these materials are made, defects can occur where different crystals meet, known as grain boundaries. In conventional semiconductors, these defects can decrease their electrical conductivity and the solar energy-to-electricity conversion efficiency; however, in lead-halide perovskites, there are differing experimental reports on the activity of grain boundaries. In some cases, they are found to be harmful, while in other cases they either have no impact on performance or are even beneficial. But, to date, no one understood why. The research team in this work set out to discover these reasons.
A recent joint-research co-led by City University of Hong Kong (CityU) and Shanghai University has developed an efficient fabrication approach for all-inorganic perovskite films with better optical properties and stability, enabling the development of high color-purity and low-cost perovskite LEDs with a high operational lifetime.
The team has found that using cesium trifluoroacetate (TFA) as the cesium source in the one-step solution coating, instead of the commonly used cesium bromide (CsBr), enables fast crystallization of small-grained CsPbBr3 perovskite crystals, forming the smooth and pinhole-free perovskite films. This is because the interaction of TFA- anions with Pb2+ cations in the CsPbX3 precursor solution greatly improves the crystallization rate of perovskite films and suppresses surface defects.
Researchers from the Ulsan National Institute of Science and Technology (UNIST) have developed perovskite LEDs (PeLED) which are flexible enough to be folded. A transparent material was used in the electrode of the device as a replacement for metal to ensure translucency.
According to the team, PeLED is a kind of light emitting diode (LED) that emits light by injecting current into a compound. This device uses a perovskite material as an active layer that emits light by receiving electricity, and its advantages include high electron mobility, good color purity, and easy color control. However, conventional PeLEDs are low in flexibility and opaque due to limitations of metal electrodes.
Perovskite-Info is proud to present The Perovskite Handbook. This book is a comprehensive guide to perovskite materials, applications and industry. Perovskites are materials that share a similar structure, which display a myriad of exciting properties and are considered the future of solar cells, displays, sensors, lasers and more.
Reading this book, you'll learn all about:
- Different perovskite materials, their properties and structure
- How perovskites can be made, tuned and used
- What kinds of applications perovskites may be suitable for
- What the obstacles on the way to a perovskite revolution are
- Perovskite solar cells, their merits and challenges
- The state of the perovskite market, potential and future