All-plastic lights for a lightweight glow.
At the heart of the novel, almost entirely plastic device is a material called poly(p-phenylenevinylene), or PPV, which emits green light when stimulated with electricity (SN: 8/24/96, p. 119). Other polymers could, in principle, be incorporated into LEDs that glow in different colors and could be arrayed into a plastic television or computer screen.
To create the LED, the researchers sandwiched PPV between two mirrorlike layers: a thin metallic film and a layered structure called a distributed Bragg reflector. Like a butterfly's iridescent wings, this reflector bounces back a single wavelength of light while it absorbs the others (SN: 12/13/97, p. 375). Light emitted by the PPV layer reflects back and forth between the two surfaces until a single wavelength emerges from one side of the device.
The new LED, developed by Peter K.H. Ho, D. Stephen Thomas, Richard H. Friend, and Nir Tessler, emits light at a wavelength of 530 nanometers. The researchers describe their device in the July 9 SCIENCE.
A distributed Bragg reflector consists of several alternating layers of two materials with different optical properties. Light that enters the reflector bounces back and forth between the layers. Interference between the shuttling signals amplifies one wavelength but cancels out others. By tailoring the thickness and properties of the layers, the researchers can select a particular wavelength of light for the LED.
Most Bragg reflectors use two different inorganic materials. The Cambridge group used PPV not only as the light-emitting film but also in the reflector. The team blended PPV with tiny silica beads just 5 nm in diameter and then alternated layers of the composite and pure PPV.
The same coating process can lay down most of the critical components of the device. Moreover, PPV's electrical conductivity makes integrating the different components of the device much easier. The researchers have more flexibility in where they place the electrodes because electricity can travel through the reflector, Friend notes.
Organic materials have several advantages over the inorganic ones used in displays today. "It's much easier to mix and match functionalities with polymers," says Friend. Experimenters can easily tune polymers' optical and electrical properties by changing their chemical composition.
"Although further studies are clearly needed to make practical devices, this work shows much potential," William L. Barnes of the University of Exeter and Ifor D.W. Samuel of the University Durham, both in England, comment in the July 9 SCIENCE. The goal is a polymer display that is cheaper and lighter than today's products.
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|Title Annotation:||poly(p-phenylenevinylene) used to make light-emitting diode|
|Article Type:||Brief Article|
|Date:||Jul 10, 1999|
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