**3. The PL spectra of the films containing blends of zinc complex and of hole-transporting material**

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes 185

BTZ)2/Al:Ca (device D4). Figure 2 shows the EL spectra of devices D3 and D4 (Figure 2a, curve 3 and Figure 2b, curve 2, respectively). In both cases, the EL spectra contain no wide band around 560 nm and exhibit only one band in the blue region with the maximum at 471 nm (device D3) and 469 nm (device D4), which may be attributed mainly to the intrinsic

Similar to the devices based on Zn(PSA-BTZ)2, the exciplex band can be eliminated by introducing the intermediate layer of CBP between NPD and Zn(POPS-BTZ)2 or PTA and Zn(TSA-BTZ)2. The EL spectrum of the device ITO/PTA/NPD/CBP/Zn(POPS-BTZ)2/Al:Ca (device D9) is shown in Figure 4 (curve 2). The exciplex band in the region of 540 nm is absent, and only the intrinsic emission of Zn(POPS-BTZ)2 at *λ*max = 460 nm is observed. The EL spectrum of the device ITO/PTA/CBP/Zn(TSA-BTZ)2/Al:Ca (device D12) is shown in Figure 3b (curve 2). The exciplex band in the region of 580 nm is absent, and only the

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

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

lowest unoccupied molecular orbitals (LUMOs) of the constituent species [56].

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

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

intrinsic emission of Zn(TSA-BTZ)2 at *λ*max = 465 nm is observed.

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

emission of the Zn(PSA-BTZ)2 complex.

that of NPD (5.2 to 5.7 eV) [58-60].

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- 15,17,40,53].

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 contact interface.

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.
