BOX 3.2
How Light Is Emitted in OLEDs

Shown in Figure 3.2.1 is a pictorial view of the light-emitting layer in an organic light-emitting diode (OLED). This layer is typically sandwiched between electron and hole transporting layers. The blue background represents the thin film that is comprised of a molecular species that transports the charges injected from contacts at the boundaries of the OLED itself. The red dots are the dopant molecules that are interspersed at low density within the charge transporting matrix These dopants can either be fluorescent moleaJles or phosphorescent molewles Ftlosphorescent molecules can produce devices with the highest internal quantum efficiency. The inset on the lower left shows a typical phosphor molecule. It can be very inexpensive and is only used in trace amounts. Ultimately, it consists of carbon, nitrogen, and hydrogen atoms (open circles) that are bonded together (lines) along with a heavy metal atom (typically iridium) in its center (red dot). Light emission occurs when an electron injected from the cathode travels to the same molew Ie as the ho Ie (positive charge) injected from the anode, forming a IllJ bile excitation or “exciton.” Light is then generated when the electron and hole (or exciton) recombine on the edges of the dopant molecule. This emission process is depicted by the yellow burst around the dopant molecule in the emitting region. By varying the structure of the molecule, the entire visible and near-infrared spectra can be accessed.

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FIGURE 321 Pictorial view of the light--emitting layer of an OLEO

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FIGURE 3.10 Illustration of the optical pathways taken by a photon following emission from a luminescent molecule (shown as yellow star).

be discussed further in the section on necessary technology developments.

Finally, the excited state ratio is X = 0.25 for fluorescent emitting molecules, and X = 1 for phosphors, as will be discussed in the following section (Baldo et al., 1999b). Putting all of the efficiencies together, it is demonstrated that ƞEQE is 20 to 60 percent in the very best cases. Even with these limitations, the power efficiency of phosphorescent white organic light-emitting devices can exceed 150 lm/W, making them especially attractive for use as efficient lighting sources.

CONTROLLING THE COLOR OUTPUT OF THE OLED

For OLEDs, changing the composition of the molecular components of the material influences the wavelength (color) of the light emitted. White light is generated by mixing red,



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