Aug 12, 2014
Perovskites make bright LEDs of many colours
Lead halide perovskites can be used as light-emitting semiconductors in an LED structure, according to new experiments by researchers at the universities of Cambridge and Oxford in the UK and at the Ludwig-Maximilians-University in Munich, Germany. The light emitted by the perovskites can be easily tuned – something that could make the devices ideal for colour displays and lighting, and in optical communication applications.
The best solid-state LEDs available today 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. Now, a team of researchers led by Richard Friend of Cambridge University’s Cavendish Laboratory has made the first bright LEDs from semiconducting materials known as organometal halide perovskites. The devices can be made to emit light in the infrared, green and red parts of the electromagnetic spectrum by simply tuning the composition of the halide in the perovskite.
For example, to make an IR-emitting device, the team sandwiched a thin, 15 nm layer of CH3NH3PbI3-xClx perovskite emitter between large-bandgap titanium dioxide and poly(9,9’-dioctylfluorene) (F8) layers. This structure effectively confines electrons and holes in the perovskite so that they can “recombine” to emit light.
The perovskite device emits light with an infrared radiance of 13.2 W/s/r/m2 at current densities of 363 mA/cm2 and boasts the highest external and internal quantum efficiencies of 0.76% and 3.4% respectively.
The green LED made by Friend and colleagues contains an ITO/PEDOT/PSS/CH3NH3PbBr3/F8/Ca/Ag structure and has a luminance of 364 cd/m2 at a current density of 123 mA/cm2, and external and internal quantum efficiencies of 0.1% and 0.4% respectively.
Strong light emitters
Organometal halide-based perovskites could be ideal for making optoelectronics devices, thanks to the fact that they can be processed in solution and do not need to be heated to high temperatures. This means that large-area films of the materials can be deposited easily 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. They also strongly emit light and thus are ideal for making LEDs.
“These organometal halide perovskites are remarkable semiconductors,” says Zhi-Kuang Tan, a PhD student in the Cavendish Lab and the lead author of the paper describing the research. “We have designed the diode structure to confine electrical charges into a very thin layer of the perovskite, which sets up the conditions for electrons and holes to be captured and produce light.”
Good for flat-panel displays
“The big surprise for the semiconductor community was to find that such simple process methods can still produce very clean semiconductor properties, without the need for the complex purification procedures required for traditional semiconductors such as silicon,” Friend tells nanotechweb.org.
“It is also remarkable that perovkites can be easily tuned to emit light of a variety of colours, which makes these materials extremely useful for colour displays, lighting and optical communication applications,” adds Tan. “This technology could certainly benefit the ever expanding flat-panel display industry.”
The team says that it is now busy looking into the photophysical properties of these materials and hopes to increase the efficiencies of the LED devices it has made. “We also plan to make diode lasers using these highly emissive materials,” says Tan.
The current work is detailed in Nature Nanotechnology.
About the author
Belle Dumé is contributing editor at nanotechweb.org