**1. Introduction**

176 Organic Light Emitting Devices

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> In typical organic light emitting devices (OLEDs), light originates from radiative recombination of molecular excited states formed by electrons and holes injected from electrodes and localized on individual molecular sites. That is, the results are interpreted as due to Frenkel exciton generation and recombination [1,2]. In particular, this is applied to the bilayer OLEDs composed of metal 8-hydroxyquinolates Mq3 (M = Al, Ga, In, or Sc) as an electron-transporting and emitting layer and amines like triphenylamine derivative (TPD) as a hole-transporting layer. The electroluminescence (EL) spectra of these devices are close to the photoluminescence (PL) spectra of corresponding Mq3 molecules [1-3]. The similarity of the EL and PL spectra was also observed for zinc complexes with hydroxy-substituted quinolines, benzothiazoles and related ligands [4-6].

> In some bilayer devices, interactions of donor and acceptor molecules at the organic/organic interface can lead to formation of an exciplex state. Exciplex is a kind of excited state complex formed between donor and acceptor, with one in the excited state and the other in the ground state. Exciplex usually leads to the red shifted emission and broadened spectrum relative to the emissions of the individual acceptor or donor [7-10].

> Exciplex formation at the solid interface between Alq3 and the electron-rich multiple triarylamine hole-transporting materials m-MTDATA and t-Bu-TBATA was observed in a study by Itano et al., [11]. Exciplexes can also be observed in the PL spectra of donoracceptor blends [10,12-15].

> Sometimes, another sort of bimolecular excited complex called electroplex can be generated around heterojunction. Unlike exciplex emission which can be observed under both photoexcitation and electric field excitation, electroplex emission can not be typically observed under photo-excitation and can be formed only in the presence of high electric field in some OLEDs. [16-19].

© 2012 Kaplunov et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Kaplunov et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The intrinsic luminescence of the emitting layer is quenched by the formation of exciplexes [20- 24]. So, for pure monochromatic OLEDs, exciplexes should be avoided [25-27]. On the other hand, exciplexes were proposed to tune the OLED emission color [28-30] and to design white OLEDs [12,18,31-35]. The use of exciplex emission simplifies the structure of white OLEDs. Efficient white electroluminescence from a double-layer device based on a boron complex was demonstrated by Liu et al. [36]. High-efficiency nondoped white organic light-emitting device based on the triarylamine derivative was demonstrated by Tong et al. [37] and Lai et al. [38]. For some OLEDs, pure exciplex emission was obtained by Wang et al. [39] and Nayak et al. [40].

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes 179

**Figure 1.** Structures of zinc complexes and of materials for hole-transporting layers.

PEDOT:PSS: poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate).

(4-methylphenylsulphanylamino)phenyl]benzothiazolate}zinc; Zn(POPS-BTZ)2: bis{2-[2-(4 penthadecyloxyphenylsulphanylamino)phenyl]-benzothiazolate}zinc; Zn(DFP-SAMQ)2: bis[8-(3,5 difluorophenylsulphanylamino)-quinolato]zinc; NPD: N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4' diamine; PTA: oligo(4,4'-(4''-methyl)triphenylamine); CBP: 4,4′-bis(N-carbazolyl)-1,1′-biphenyl;

Zn(PSA-BTZ)2: bis{2-[2-(phenylsulphanylamino)phenyl]benzothiazolate}zinc; Zn(TSA-BTZ)2: bis{2-[2-

Methods of preparing the devices and measuring their properties are described elsewhere [42- 44]. Materials of hole-transporting layer were triaryl derivatives: PTA, olygomer of triphenylamine with high glass-transition temperature [48] and a well-known N,N'-bis(1 naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPD). The carbazol derivative 4,4′-bis(N-carbazolyl)- 1,1′-biphenyl (CBP) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) were also used for forming the hole-transporting layer. The structures of these compounds are also shown in Figure 1. In some devices, both PTA and NPD deposited in succession were used as materials for hole-transporting layers. In any case, the EL spectrum of

One of the problems in utilizing the exciplex effects in devices is finding systems with high exciplex EL efficiency, so design of new materials and investigation of the active factors for efficient exciplex emission are a subject of significance.

Recently, spectral properties of the electroluminescent devices based on the novel zincchelate complexes of sulphanilamino-substituted quinolines and benzothiazoles were investigated and some exciplex phenomena were found [41-46]. The structures of zinc complexes are shown in Figure 1.

Most presently known metal complexes used for OLEDs contain the chelate cycles including the C-O-M-N chains [2-4,6,22,27,47]. In the amino-substituted complexes, the oxygen atom in the chelate cycles is replaced by a nitrogen atom of the sulphanylamino groups forming the C-N-M-N chains. The presence of a spatially extended, electron-rich amine segment in the zinc complex molecule can enhance its ability of intermolecular interactions with the molecules of the hole-transporting layer and hence magnify the possibility of exciplex forming. This chapter presents a review of electroluminescent properties of sulphanylamino-substituted zinc complexes.
