Page 587 - Polymer-based Nanocomposites for Energy and Environmental Applications

540 Polymer-based Nanocomposites for Energy and Environmental Applications

The performance of OLEDs can be expressed by their external quantum efficiency η EL
defined as the number of photons emitted and collected in the external medium per
injected electron [144]. It depends on the quality of the emitter material, its optical
properties, and its electric properties. Typically, η EL varies between 0.1% and
26% [145].

Use of OLEDs for lighting
2
For lighting, a brightness of 3000 cd/m is required for OLEDs [146]. Such a high-
value impacts on the stability of the device. However, the brightness and the active
area can be adjusted for obtaining the desired luminous flux (expressed in lumen).
To obtain a desired luminous flux, a diode with a large active area can operate with
a reduced luminance. This approach increases however the production costs and there-
fore the costs per lumen.
In electroluminescent diodes, the wavelength of the emitted light depends on the
semiconductor bandgap. When using conjugated polymers, the bandgap can be tuned
to the desired value by chemical modification of the polymer backbone. It is possible
therefore to obtain any light color with OLEDs. Furthermore, these diodes can be
deposited on flexible substrates with a large emitting area providing multiple possi-
bilities of applications for display and lighting. It should be noticed however that for
commercialization of devices, some conditions on their technological impact are
required: (i) devices should be stable for a reasonable duration of use. Conventionally,
a lifetime of 10,000 h is required for OLEDs; (ii) for practical purpose, white light is
preferred than other colors.
The stability of OLEDs is dependent on the nature of the materials used for the
emitting layer. As conjugated polymers are not thermally stable at relatively high tem-
perature (<100°C), many degradation mechanisms can occur when the diodes are
operating in high-current or high-voltage conditions. In particular, oxidation, recrys-
tallization, metal atom migration from electrodes, and damage of the interfacial layer
are currently observed in OLEDs. For an OLED in use, one consequence of the deg-
radation is a decrease in light intensity under constant applied voltage of the diode. In
order to keep a constant light emission, the applied voltage should be increased, and
the power drawn from the OLEDs increases. To prevent diodes from degradation, sev-
eral solutions can improve their lifetime: (i) use of an encapsulation or a barrier thin
film to slow down the air diffusion to the active layer, (ii) use of polymers with high
Tg, and (iii) use of hybrid polymer composites.
White-light emission from conjugated polymers is difficult to be obtained since
none of the known polymers has a photoluminescence spectrum that covers the whole
visible light range. A strategy using a single copolymer with different chromophores
has been proposed for obtaining a white-light emission [147]. The host polymer is a
blue-emitting poly(fluorene-co-benzene), and two kinds of dopants (green and red)
are introduced in the side chain of its backbone. The electroluminescent spectra of
the doped copolymer show three simultaneous blue, green, and red emissions and pro-
vide a high-brightness white light. One other approach for generating white-light
emission consists of using a multilayer emitter comprising three layers emitting the