**4.2. The role of amino groups in exciplex formation**

184 Organic Light Emitting Devices

15,17,40,53].

contact interface.

**hole-transporting material** 

**3. The PL spectra of the films containing blends of zinc complex and of** 

One of the main evidences of the exciplex nature of long-wave bands in the EL spectra is the presence of such bands in the PL spectra of blends of donor and acceptor materials [12-

For the zinc-chelate complexes with sulphanilamino-substituted ligands, the exciplex longwave bands were observed in the PL spectra of their blends with hole-transporting materials by Kaplunov et al. [44-45]. It was shown previously that the PL spectra taken from the layered structure exhibiting the exciplex EL OLEDs do not contain long-wave bands but only the intrinsic bands of components [46]. This is due to the extremely small thickness of the contacting interface of the two layers, which is responsible for EL. To observe the longwave bands in PL, films containing blends of zinc complex and hole-transporting material were prepared by casting from toluene solutions containing both components in appropriate concentrations. In such films, contacts between the two kinds of molecules take place in the whole volume of the film, unlike the bilayer OLED structure with very thin

Kaplunov et al. [44] studied the PL spectra of the films containing blends of Zn(DFP-SAMQ)2 with PTA. The spectra contain no intrinsic luminescence of zinc complex or PTA and exhibit only the exciplex band with maximum at 555 nm (figure 5, curve 4). Kaplunov et al. [45] studied the PL spectra of the films containing PTA, Zn(DFP-SAMQ)2 and their blends in different ratios. For the films with a relatively low fraction of PTA where PTA:Zn(DFP-SAMQ)2 = 0.5:1 and 1:1 (mass), the PL bands are close to that of Zn(DFP-SAMQ)2 with λmax = 490 nm (intrinsic emission). For the films with a higher PTA fraction where PTA:Zn(DFP-SAMQ)2 = 2.6:1 and 4:1 (mass), the exciplex PL band with λmax in the region of 560 nm is observed. This result shows that the exciplex PL can be observed for donor-acceptor blends with proper relation between components, which guarantees large

amount and good quality of intermolecular donor-acceptor contacts.

**complexes with amino-substituted ligands** 

**between the hole-transporting and the emitting layers** 

devices in which the long-wave EL bands are eliminated.

**4. Elimination of exciplex emission for the devices based on zinc** 

**4.1. Elimination of exciplex emission by introducing an intermediate layer** 

To prove the exciplex origin of the long-wavelength EL, we have fabricated several control

One of the methods for preventing exciplex emission is the insertion of an additional layer between the hole-transporting and electron-transporting materials [22,25-27]. CBP is considered as one of the materials appropriate for such layers [27,54,55]. We have fabricated two control devices with Zn(PSA-BTZ)2 as emitting layer and CBP as the intermediate layer: ITO/PTA/NPD/CBP/Zn(PSA-BTZ)2/Al:Ca (device D3) and ITO/PTA/CBP/Zn(PSA-

Exciplex can be formed at the solid interface between a hole-transporting layer and an electron-transporting layer, in case when there is a significant spatial overlap between the lowest unoccupied molecular orbitals (LUMOs) of the constituent species [56].

It should be noted that both NPD and PTA, as well as many other materials usually used to form the hole-transporting layer, are the derivatives of triarylamines. One may suppose that the interaction of the nitrogen atoms in the amino groups of the hole-transporting molecules and the amino groups of the zinc complexes (due to their spatial overlap) determines the exciplex formation in the studied systems. Evidence in favor of this supposition comes from our results on using other materials different from triarylamine derivatives for holetransporting layers. Figure 2a, curve 4 shows the EL spectrum of device ITO/PEDOT:PSS/Zn(PSA-BTZ)2/Al:Ca (device D5) where the hole-transporting layer is presented by PEDOT:PSS, a hole injecting and transporting material which does not contain nitrogen atoms at all. This spectrum does not contain a wide band around 560 nm and exhibits only one band with a maximum at 466 nm, which is close to the Zn(PSA-BTZ)2 powder PL band (450 nm) and may be attributed mainly to the intrinsic emission of Zn(PSA-BTZ)2 complex. One may suppose that the formation of exciplex in this case is suppressed by the absence of nitrogen atoms in the hole-transporting layers.

Commonly, the reason for preventing the exciplex emission by changing the holetransporting material is argued to be the relation between the energy levels of the donor and acceptor molecules. Materials like CBP with low highest occupied molecular orbital (HOMO) energy level are considered as appropriate ones [22,25-27]. Really, the HOMO level of CBP is 6.1 to 6.3 eV below vacuum level [47,57,58], which is appreciably lower than that of NPD (5.2 to 5.7 eV) [58-60].

On the other hand, the highest occupied energy level of PEDOT:PSS is 5.2 eV below vacuum level [61], which does not differ from that of NPD. So, the fact that NPD produces exciplexes with the studied complexes and CBP and PEDOT:PSS do not may be explained not only by positions of energy levels but also by other reasons. Good spatial overlap of donor and acceptor molecular orbitals seems to be one of the most important factors promoting the formation of exciplexes.

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes 187

400 500 600 700 800 900

, nm

6

5

400 500 600 700 800 900

, nm

4

3

2 1 1. 3.5 V, 3.6 mA/cm2 2. 4.0 V, 4.7 mA/cm2 3. 5.0 V, 6.4 mA/cm2 4. 5.5 V, 9.7 mA/cm2

1. 5 V, 0.2 mA/cm2 2. 6 V, 0.4 mA/cm2 3. 7 V, 0.9 mA/cm2 4. 8 V, 2.1 mA/cm2 5. 9 V, 4.6 mA/cm2 6. 10 V, 7.4 mA/cm2

Figures 6 and 7 show the EL spectra of the devices ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca (device D6) with different thicknesses of NPD layer 0, 8, 15, 30 and 45 nm. Thicknesses of other organic layers are constant: about 100 nm for PTA and about 30 nm for Zn(TSA-BTZ)2.

The shape of the EL band strongly depends on the NPD layer thickness (Fig.6). For thicknesses of 8, 15 and 30 nm, the exciplex band with maximum in the region of 590 nm is observed. Further increase in thickness leads to some shift of the exciplex band maximum position. For the thickness of 45 nm, the exciplex band in the region of 540 nm is observed. Devices with 8 nm NPD layer thickness exhibit no intrinsic band in the EL spectra. Devices with 15, 30 and 45 nm NPD layer thickness exhibit intrinsic EL band in the region of 450-460

I, a.u.

0

50

100

I, a.u.

150

**Figure 7.** EL spectra of the devices ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca with different thicknesses of NPD layer: 8 nm (a), 15 nm (b), 30 nm (c) and 45 nm (d). The applied bias voltages and the currents

through the device are given along with curve numbers.

400 500 600 700 800 900

, nm

(c) (d)

(a) (b) 400 500 600 700 800 900

, nm

Spectra are measured at different bias voltages from 3.5 V to 10 V.

 1. 5 V, 1.4 mA/cm<sup>2</sup> 2. 6 V, 4.2 mA/cm<sup>2</sup> 3. 7 V, 8.2 mA/cm<sup>2</sup>

> 1. 3.5 V, 1.4 mA/cm<sup>2</sup> 2. 4.0 V, 2.8 mA/cm<sup>2</sup> 3. 5.0 V, 5.8 mA/cm<sup>2</sup> 4. 5.5 V, 7.8 mA/cm<sup>2</sup>

nm in addition to exciplex bahds.

3

2

1

4

3

2 1

0

20

40

60

80

I, a.u.

100

I, a.u.

From this point of view, molecules with amino groups are most appropriate for exciplex formation because of high electron density at nitrogen atoms. Zinc complexes studied in the present work contain amino groups bonded to metal atom and produce exciplexes in pair with triarylamine molecules NPD and PTA. Note that the analogs of our complexes containing oxygen atom bonded to metal such as Mq3, Znq2, Zn(BTZ)2 do not exhibit exciplexes in their EL spectra when triarylamine hole-transporting materials like NPD or TPD are used [1,3-6,47]. At the same time, the derivatives of Alq3 containing amino groups bonded to quinoline species exhibit EL exciplex bands for the devices with NPD [27].
