**2. EL spectra of OLEDs based on sulphanilamino-substituted zinc-chelate complexes**

We have prepared and measured the EL spectra of the following OLED devices based on zinc complexes with sulphanilamino-substituted ligands.


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].

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

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

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

**2. EL spectra of OLEDs based on sulphanilamino-substituted zinc-chelate** 

We have prepared and measured the EL spectra of the following OLED devices based on

efficient exciplex emission are a subject of significance.

complexes are shown in Figure 1.

**complexes** 

sulphanylamino-substituted zinc complexes.

zinc complexes with sulphanilamino-substituted ligands.

 device D3: ITO/PTA/NPD/CBP/Zn(PSA-BTZ)2/Al:Ca device D4: ITO/PTA/CBP/Zn(PSA-BTZ)2/Al:Ca device D5: ITO/PEDOT:PSS/Zn(PSA-BTZ)2/Al:Ca device D6: ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca device D7: ITO/PTA/Zn(TSA-BTZ)2/Al:Ca device D8: ITO/PTA/NPD/Zn(POPS-BTZ)2/Al:Ca device D9: ITO/PTA/NPD/CBP/Zn(POPS-BTZ)2/Al:Ca device D10: ITO/PTA/NPD/Zn(DFP-SAMQ)2/Al:Ca device D11: ITO/PTA/Zn(DFP-SAMQ)2/Al:Ca device D12: ITO/PTA/CBP/Zn(TSA-BTZ)2/Al:Ca

 device D1: ITO/PTA/NPD/Zn(PSA-BTZ)2/Al:Ca device D2: ITO/PTA/Zn(PSA-BTZ)2/Al:Ca

**Figure 1.** Structures of zinc complexes and of materials for hole-transporting layers. Zn(PSA-BTZ)2: bis{2-[2-(phenylsulphanylamino)phenyl]benzothiazolate}zinc; Zn(TSA-BTZ)2: bis{2-[2- (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; PEDOT:PSS: poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate).

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 the device is determined by the hole-transporting material, which is in contact with the zinc complex. The devices are typically characterized by bias voltages of light appearance about 2.5 to 3 V and brightness of 103 cd/m2 at 10 V.

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes 181

The EL spectrum of device D1 contains two bands with maxima at 460 and 560 nm. Maximum of the first band is close to that of the PL peak of Zn(PSA-BTZ)2 powder at 450 nm and may be attributed to the intrinsic luminescence of Zn(PSA-BTZ)2. The second peak may be probably due to exciplex formation between NPD and Zn(PSA-BTZ)2. For device D2, the EL spectrum exhibits only wide band with a maximum at 553 nm, which may be attributed to exciplex formation between PTA and Zn(PSA-BTZ)2. Exciplex can be formed between the ground state of a donor molecule and the excited state of an acceptor molecule [12]. In our case, the donor molecule is presented by NPD or PTA, and the acceptor molecule by Zn(PSA-BTZ)2 complex. Exciplex band corresponds to the transition from the excited state of the acceptor and the ground state of the donor and has lower transition energy compared to the intrinsic emission band corresponding to the transition between the

Figure 3 shows the EL spectra of Zn(TSA-BTZ)2 in the electroluminescence devices: (a) device D6, ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca, (b) device D7, ITO/PTA/Zn(TSA-BTZ)2/AlCa and device D12, ITO/PTA/CBP/Zn(TSA-BTZ)2/Al:Ca. The PL spectrum of

For the devices D6 and D7, intensive exciplex EL bands are observed in the yellow region with the maxima around 585 nm. Only a weak shoulder in the region of the intrinsic Zn(TSA-BTZ)2 emission at about 460 nm is observed. For device D7, the EL spectra are shown for different bias voltages from 3.5 to 6.0 V. The spectra are normalized to obtain equal intensities of exciplex bands for all voltages. A small continuous growth of intrinsic emission relative intensity is observed. A small blue shift of exciplex band maximum from 585 nm at 3.5 V to 575 nm at 6.0 V is also observed. This is in contrast with previously reported strong dependence of EL bands positions on bias voltages [11,49,50] where field induced shift of EL band of about 50 nm could be attributed to the overlap of the emission from different excited states. As showed Kalinowski et al. [51], the field dependence of EL spectrum in such systems is a result of electric field mediated interplay among localized (monomolecular) excitons, exciplexes, and electroplexes in conjunction with their specific environment. For the device D12, no exciplex band is observed which is discussed in

Figure 4 shows the EL spectra of Zn(POPS-BTZ)2 in the devices D8 ITO/PTA/NPD/Zn(POPS-BTZ)2/Al:Ca (curves 1) and D9 ITO/PTA/NPD/CBP/Zn(POPS-BTZ)2/Al:Ca (curve 3). The PL spectrum of Zn(POPS-BTZ)2 powder (curve 2) is shown for comparison. Strong exciplex band in the green region with the maximum at about 540 nm and shoulder at about 460 nm due to intrinsic emission of Zn(POPS-BTZ)2 is observed in the

excited and ground state of the acceptor molecule [7-10,12].

Zn(TSA-BTZ)2 powder is shown for comparison (Figure 3a, curve 2).

**2.2. EL spectra of OLEDs based on Zn(TSA-BTZ)2**

**2.3. EL spectra of OLEDs based on Zn(POPS-BTZ)2**

section 4.
