Nanoscale Perovskite Particles Self-Assemble to Produce Advanced LEDS

LED technologies have taken the lighting industry by storm in the last decade, offering numerous features such as efficiency, durability, and long life.

Recently, Princeton Engineering researchers have revealed another step forward for LED technologies. They tweaked the manufacturing process of light sources made using crystalline substances known as perovskites, a more efficient and potentially lower-cost substitute compared to materials used in LEDs commercially available in the market.

The researchers developed a method in which nanoscale perovskite particles self-assemble to produce more efficient, durable, and stable perovskite-based LEDs. This progress was reported in January 16 in Nature Photonics, and could accelerate the use of perovskite technologies in commercial applications such as lasers, lighting, and TV and computer screens.

The performance of perovskites in solar cells has really taken off in recent years, and they have properties that give them a lot of promise for LEDs, but the inability to create uniform and bright nanoparticle perovskite films has limited their potential. Our new technique allows these nanoparticles to self-assemble to create ultra-fine grained films, an advance in fabrication that makes perovskite LEDs look more like a viable alternative to existing technologies.

Barry Rand Assistant Professor, Princeton

When voltage is applied across the LED, LEDs discharge light. When the light is switched on, electrical current pushes electrons from the diode’s negative side to its positive side. This discharges energy in the form of light. LEDs work very well when this current can be rigorously controlled. The thin nanoparticle-based films in Rand's devices allowed just that.

LEDs offer several advantages when compared with incandescent bulbs, including longer life, durability, energy efficiency, smaller size, and low-heat. Although they are still quite expensive compared to fluorescent lights for room illumination, they light up faster, provide better energy efficiency, and present less environmental concerns linked with disposal.

Rand's team and other researchers are analyzing perovskites as a possible lower-cost substitute to gallium nitride (GaN) and other materials used in LED manufacturing. Lower-cost LEDs would help to accelerate the acceptance of the bulbs, minimizing environmental impacts and energy use.

Discovered in the mid-1800s in Russia, the mineral perovskite was named in honor of the Russian mineralogist Lev Perovski. The term "perovskite" covers a group of compounds that have the same crystalline structure of Perovski's mineral, a distinct combination of diamond and cuboid shapes.

Perovskites display several intriguing properties, for instance they can be semiconductive or superconductive, according to their structure, which make them potential materials for application in electrical devices. In the last few years, they have been publicized as a prospective replacement for silicon in solar panels: costing less to manufacture while offering equal efficiency as certain silicon-based solar cells.

By dissolving perovskite precursors in a solution that contains an organic ammonium halide and a metal halide, hybrid organic-inorganic perovskite layers can be fabricated. It is a fairly cheap and simple process that could offer a low-cost alternative to LEDs based on silicon and other materials.

However, while the resulting semiconductor films could produce light in bright colors, the crystals creating the molecular structure of the films were very large, which rendered them unstable and inefficient.

In their new paper, Rand and his team explain that the use of a supplementary type of organic ammonium halide, and in particular a long-chain ammonium halide, to the perovskite solution during fabrication greatly inhibited the development of crystals in the film. The resulting crystallites were a lot smaller (about 5 - 10 nm across) than those produced using earlier techniques and the halide perovskite films were much thinner and smoother.

This resulted in improved external quantum efficiency, meaning the LEDs discharged more photons per number of electrons passing into the device. The films were also more stable than those fabricated using other techniques.

Russell Holmes, a professor of materials science and engineering at the University of Minnesota, said the Princeton research moves perovskite-based LEDs nearer to commercialization.

Their ability to control the processing of the perovskite generated ultra-flat, nano-crystalline thin films suitable for high efficiency devices. This elegant and general processing scheme will likely have broad application to other perovskite active materials and device platforms.

Russell Holmes, Professor, University of Minnesota