Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-19 GTBL042-Callister-v2 October 5, 2007 17:40

2nd Revise Pages

Materials of Importance • 777

We noted in Section 12.17 that some polymeric

materials may be semiconductors (bothn- and

p-type). As a consequence, light-emitting diodes

made of polymers are possible, of which there

are two types: (1)organic light-emitting diodes

(orOLEDs), which have relatively low molecular

weights; and (2) the high molecular-weightpoly-

mer light-emitting diodes(orPLEDs). For these

LED types, amorphous polymers are used in the

form of thin layers that are sandwiched together

with electrical contacts (anodes and cathodes). In

order for the light to be emitted from the LED, one

of the contacts must be transparent. Figure 19.12 is

a schematic illustration that shows the components

and configuration of an OLED. A wide variety of

colors is possible using OLEDs and PLEDs, and,

in fact, more than a single color may be produced

from each device (such is not possible with semi-

conductor LEDs)—thus, combining colors makes

it possible to generate white light.

Although the semiconductor LEDs currently

have longer lifetimes than these organic emitters,

OLEDs/PLEDs have distinct advantages. In addi-

tion to generating multiple colors, they are easier

to fabricate (by “printing” onto their substrates

with an ink jet printer), are relatively inexpen-

sive, have slimmer profiles, and can be patterned to

give high-resolution and full-color images. OLED

Metal cathode

Organic

emitters

Electron

transport

(n-type)

layer

Hole injection

and transport

(p-type) layer

Light output

Anode

Glass

substrate

2–10V DC

Figure 19.12 Schematic diagram that shows the

components and configuration of an organic

light-emitting diode (OLED). (Reproduced by

arrangement withSilicon Chipmagazine.)

displays are currently being marketed for use on

digital cameras, cell phones, and car audio com-

ponents. Potential applications include larger dis-

plays for televisions, computers, and bill boards. In

addition, using the right combination of materials,

these displays can also be flexible. Can you imag-

ine having a computer monitor or television that

can be rolled up a like projection screen, or a light-

ing fixture that is wrapped around an architectural

column or is mounted on a room wall to make ever-

changing wallpaper?

Photograph showing a very large light-emitting diode

video display located at the corner of Broadway and

43rd Street in New York City. (©cStephen

Chemin/Getty Images news as Sports Services.)