A new type of LED is made with crystalline substances known as perovskites. CREDIT Sameer A. Khan/Fotobuddy
Princeton engineers have refined the manufacturing of light sources made with crystalline substances, perovskites, a moreefficient and potentially lower-cost alternative to materials used in LEDs found on store shelves. The researchers developed a technique in which nanoscale perovskite particles self-assemble to produce more efficient, stable and durable perovskite-based LEDs. The advance, reported January 16 in Nature Photonics, could speed the use of perovskite technologies in commercial applications such as lighting, lasers and television and computer screens.
LEDs emit light when voltage is applied across the LED. When the light is turned on, electrical current forces electrons from the negative side of the diode to the positive side. This releases energy in the form of light. LEDs operate best when this current can be strictly controlled. In Rand’s devices, the thin nanoparticle-based films allowed just that. LEDs have many advantages over incandescent bulbs, including durability, longer life, smaller size, energy efficiency and low-heat. While they are still more expensive than fluorescent lights for room illumination, they are more energy efficient, light up faster and present fewer environmental concerns related to disposal.
Rand’s team and others researchers are exploring perovskites as a potential lower-cost alternative to gallium nitride (GaN) and other materials used in LED manufacturing. Lower-cost LEDs would speed the acceptance of the bulbs, reducing energy use and environmental impacts.
Perovskite is a mineral originally discovered in the mid-1800s in Russia and named in honor of the Russian mineralogist Lev Perovski. The term “perovskite” extends to a class of compounds that share the crystalline structure of Perovski’s mineral, a distinct combination of cuboid and diamond shapes.Perovskites can be superconductive or semiconductive, depending on their structure – that make them promising materials for use in electrical devices. In recent years, they have been touted as a potential replacement for silicon in solar panels: cheaper to manufacture while offering equal efficiency as some silicon-based solar cells.
Hybrid organic-inorganic perovskite layers are fabricated by dissolving perovskite precursors in a solution containing a metal halide and an organic ammonium halide. It is a relatively cheap and simple process that could offer an inexpensive alternative to LEDs based on silicon and other materials. However, while the resulting semiconductor films could emit light in vivid colors, the crystals forming the molecular structure of the films were too large, which made them inefficient and unstable.
In their new paper, Rand and his team used an additional type of organic ammonium halide, and in particular a long-chain ammonium halide, to the perovskite solution during production dramatically constrained the formation of crystals in the film. The resulting crystallites were much smaller (~5-10 nm across) than those generated with previous methods, and the halide perovskite films were far thinner and smoother.
This led to better external quantum efficiency, ie LEDs emitted more photons/ number of electrons entering the device. The films were also more stable that those produced by other methods.
Prof. Russell Holmes, University of Minnesota, said the Princeton research brings perovskite-based LEDs closer to commercialization. “Their ability to control the processing of the perovskite generated ultra-flat, nano-crystalline thin films suitable for high efficiency devices,” said Holmes, who was not involved in the research. “This elegant and general processing scheme will likely have broad application to other perovskite active materials and device platforms.” http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2016.269.html
https://www.eurekalert.org/pub_releases/2017-01/pues-spb011317.php
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