Thin Films of Electroluminescent Polymers

This chapter is organized as follows in Section 11.2 the relevant properties of electroluminescent polymer films are summarized in Section 11.3 the electronic properties of metal/polymer jimcfions are described in Section 11.4 single layer polymer LEDs are discussed in Section 11.5 multi-layer polymer LEDs are considered and Section 11.6 summarizes the conclusions. 

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Thin Films of Electroluminescent Polymers 335 Electronic Eneigy Structure 336 Optical Properties 336 Electrical Transport Properties 338... 

In thin films of electroluminescent conjugated polymers, Kanner et al. [12] and Yan et al. [13] explained the quenching of PL by invoking an exciton migration mechanism to nonradiative traps based on the Balagurov-Vaks model [14], The survival probability of an exciton on a 1-D lattice (fi- = 1/3) is characterized by a diffusion time t(i. For times t t,i. the PL intensity follows... 

Interest surged in studies of fluorescence after the discovery of electroluminescence from PPV (Burroughes et al., 1990), because of its potential for practical application in light emitting devices (LEDs). Electroluminescence is fluorescent emission produced by the recombination of electrons and holes injected into a thin film of conjugated polymer, and will be discussed in the next section. If photoluminescent emission from a polymer is weak, then the electroluminescence is unlikely to be of practical significance, and consequently studies of photoluminescence and photoluminescent quantum efficiency have been used as a means of selecting polymers likely to be useful in LEDs. 

Poly(p-phenylenevinylene) derivatives are promising candidates for the active layer of polymer light-emitting diodes. Thus, various types of substituted poly(p-phenylenevinylene)s have been synthesized. Whereas these derivatives are soluble in organic solvents, a high-quality thin film of these polymers can be formed. Die optical properties of these polymers have been reviewed previously [8, 169]. The electroluminescence spectrum of the device fabricated with a conjugated pofymer is almost the same as that of the photoluminescence spectrum of the thin film of the polymer. The peaks in photoluminescence spectra are compiled in Table XVII [170, 171]. 

A thin film ( 100 nm) of a PPV derivative is sandwiched between two electrodes supported on a solid substrate, such as glass, a flexible plastic substrate, such as poly(propylene), or even a thin film of aluminium, see Figure 6.1. The device substrate provides the mechanical support to the anode, electroluminescent polymer layer and the cathode. The anode is transparent in... 

Conjugated polymers like poly(l,4-phenylene-vinylene), PPV, or more generally Poly(arylene-vinylenes), PAVs have evoked considerable interest as electrically conductive and nonlinear optical materials. More recently, electroluminescence properties of PPV have attracted substantial attention, since it was first reported in 1990. Direct synthesis of PPV has been limited by its insolubility. Hence, the most commonly used routes are based on soluble polymer precursors or soluble conjugated precursors. The latter process is also commonly referred to as the sulfonium-based polyelectrolyte precursor route. PPV thin films from these solution-based routes, however, have problems related to contamination by solvents and oxidative defects in the polymer. C VP is an alternate method for the deposition of high quality thin films of PPV. Reported first by Iwatsuki et al., it was investigated for electroluminescence applications by Staring et al. ... 

Devices based on composites of nanocrystals with polymers or polyelectrolytes have been fabricated to exploit electroluminescent and related properties of nanocrystalline media. These devices utilize thin films of composites obtained by layer-by-layer deposition using self-assembly, drop-casting, or spin-casting methods. 

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