Stable Perovskite LEDs One Step Closer

Particularly in the post-Corona period, countries' approaches and practices regarding energy and environment will closely influence the balance between renewable and fossil fuels. As a result of increased renewable energy use, greater reductions in carbon emissions can be observed [1]. This situation will increase demand for renewable energy over time. When we think of renewable energy, what is the first example that comes to mind? Solar energy and solar panels... Solar cells used in solar panels have recently been equipped with perovskite technology. Hundreds of studies have been conducted and continue to be conducted on this application. Now that we have mentioned "renewable energy" and "solar cells," why not embark on a journey towards perovskites in light technology and newly emerging successes? Let us start with perovskite... Perovskite is a calcium titanium oxide mineral with the chemical formula CaTiO3. The mineral was first discovered in 1839 by Gustav Rose in the Ural Mountains and took its name from Russian mineralogist Lev Perovski (1792-1856). The terms perovskite and perovskite structure are often used interchangeably. However, while true perovskite (mineral) consists of calcium, titanium and oxygen in the form of CaTiO3, perovskite structure can be any molecule that has the same crystallographic structure as perovskite mineral and has the general form ABX3. The simplest way to describe a perovskite structure is a cubic unit cell with titanium atoms (pink) at the corners, oxygen atoms (yellow) at the midpoints of the edges, and a calcium atom (blue) in the center.

Depending on which atoms/molecules are used in the structure, perovskites have an impressive array of interesting properties, including "giant magnetoresistance."

For example, their electrical resistance changes when placed in a magnetic field (which could be useful for microelectronics). Some perovskites are superconductors, meaning they can conduct electricity without any resistance. Ferroelectricity, charge ordering, spin-dependent transport, high thermopower, and the interaction of structural, magnetic and transport properties are characteristics commonly seen in this family. Perovskite materials have exciting application areas for physicists, chemists and materials scientists [2]. Perovskite photovoltaics have a wide band energy range. This property offers the opportunity to pair them with low band gap photovoltaic technology. The new technology obtained enables improved energy efficiency. Thus, perovskites will occupy a very important place in a highly competitive market where system costs depend on efficiencies.

What is LED?

A light emitting diode (LED) is an electronic component with two terminals that exhibits semiconductor properties and can also be characterized as a light source. When voltage is applied to the system, it is a p-n type junction diode that can emit light. With the applied voltage, electrons combine with electron vacancies within the device and release energy as photons. This effect is called electroluminescence and the color of the light is determined by the band energy range of the selected semiconductor. Advantages of LEDs over incandescent light sources include lower energy consumption, longer service life, improved physical durability, smaller size and faster switching. LEDs are found in various applications in the aviation and automotive industries, as well as in advertising, traffic signals, camera flashes and much more.

What Can Perovskites Do for LEDs?

Current high-quality LEDs are direct band gap semiconductors. However, building these devices is not an easy task, as they need to be processed at high temperatures and in vacuum. This makes large-scale LED production quite expensive. One of the direct band gap semiconductors is perovskites. These materials are a real alternative to other types of direct band gap materials for applications such as color displays. Perovskites are inexpensive and easy to process. Researchers have reported that organometal halide-based perovskites (a combination of lead, organic and halogen incorporated into perovskite crystal structure in solid form) could be suitable for developing optoelectronic devices because they can be processed in solution and do not need to be heated to high temperatures. This is an advantage and means that the materials can be deposited on large-area flexible or rigid surfaces, thereby obtaining the desired films. Another property of perovskites is that they have an optical band gap that can be tuned in the visible and infrared regions, and this property shows them as promising materials for a range of optoelectronic applications. The strong light emission of perovskites makes them very suitable for use in LED applications. The fact that the color of light emitted by perovskites can be easily adjusted makes them ideal molecules for color displays, lighting and optical communication applications.

However, a major obstacle that perovskites must overcome for use in LED-type devices is that electrons and holes are only weakly bound in perovskite thin films.

This means that excitons are spontaneously separated into free carriers at the bulk recombination layer, resulting in low photoluminescence quantum efficiency, high leakage current and low light efficiency. This clearly impairs the ability of perovskites to create high-performance LEDs, and to make perovskites comparable in light emission impact, slow radiative recombination kinetics must be overcome. To put it simply, researchers will need to find ways to effectively confine electrons and holes in perovskites, so that this pair (perovskite and LED) can recombine to provide light emission. Already significant progress is being made in this area and perovskites are seen to open the door to a low-cost, color-tunable approach to advancing LED technology. Researchers at Linköping University, working with colleagues in England, China and the Czech Republic, developed a perovskite light emitting diode (LED) with both high efficiency and long operational stability. The results were published in Nature Communications. Professor Feng Gao, Head of Research at the Department of Biomolecular and Organic Electronics at Linköping University, says, "Perovskite-based light emitting diodes are still not stable enough for practical use, but we have brought them one step closer." Most progress has been made in research related to the use of perovskites in solar cells, but it is also suitable for LED production. The efficiency of LEDs, which measures the fraction of charge carriers entering the light-emitting material that is emitted as light, has increased significantly in recent years and will soon compete with many technologies. However, due to their instability, they have so far not been used in practice.

Xiao-Ke LiU, Research Assistant at the Department of Biomolecular and Organic Electronics, says, "There is much to be done. So far, most perovskite LEDs have had either low efficiency or poor device stability." Liu and Gao are the lead authors of the paper.

Many research groups have worked on this dilemma without particular success. Now, researchers at LiU, working with colleagues in England, China and the Czech Republic, have found a different path. They used a perovskite made of lead, iodine and formamidinium, an organic substance. They then embedded the perovskite in an organic molecule matrix to form a thin composite film. Heyong Wang, a PhD student at the Department of Biomolecular and Organic Electronics, says, "The two amino groups at the ends of the molecule help form a high-quality crystal structure that stabilizes the crystal with other substances. This is a characteristic feature of these perovskites." The new composite thin film enabled the research group to develop LEDs with approximately 100% long-lived efficiency of 17.3%. According to this new study, perovskites containing lead, halogen and iodine have the best light emitting properties. "We want to get rid of lead. We haven't found a good way to do it yet, but we are working hard on it," says Feng Gao. The next steps in this research will be to test combinations of new perovskites and organic molecules and to understand in detail how nucleation and crystallization processes occur [3-4-5]. Obtaining white light in LED and OLED technology is very valuable. In recent years, physicists, chemists and materials scientists working in light technology have conducted and continue to conduct many studies to obtain white light. The ability of perovskites to emit light at different wavelengths is a long-term but worthwhile goal for obtaining white light LEDs.

References

1. https://www.iklimhaber.org/ 2. https://www.perovskite-info.com/ 3. https://phys.org/ Abdelkader M. M. and Gamal W. M., "Some aspects of dimensionality and phase transitions of organic–inorganic hybrid perovskite (n-C14H29NH3)2ZnCl4", Appl. Phys. A, 2017, 123,153 4. Abdelkader M. M. and Gamal W. M., "Some aspects of dimensionality and phase transitions of organic–inorganic hybrid perovskite (n-C14H29NH3)2ZnCl4", Appl. Phys. A, 2017, 123,153 5. Liu X.-K., Gao F. and colleagues, "Perovskite-molecule composite thin films for efficient and stable light-emitting diodes", Nature Communications, 2020, 11, 891 * Photos and images were obtained from https://www.ensonhaber.com/, Reference 2 and Science in HD on Unsplash.

  Dr. Assistant Professor Sultan Funda Görkem Eskişehir Technical University Faculty of Science Department of Chemistry