**5. The relation between intrinsic and exciplex bands in the devises containing NPD/Zn(TSA-BTZ)2 interface**

Some of the EL devices based on amino-substituted zinc complexes demonstrate the EL spectra with only exciplex bands, other demonstrate the intrinsic EL bands either. The relation between intrinsic and exciplex bands can be affected by different factors: materials of contacting layers [22], thicknesses of layers [17,29,30,31,62], applied voltage and current [20,63-64]. The dependence of the relation between intrinsic and exciplex bands in the EL spectra of the devices based on amino-substituted zinc complexes on the thickness of holetransporting layer and on the applied bias voltage were studied for the devises based on Zn(TSA-BTZ)2 as an illustrative example.

**Figure 6.** Normalized EL spectra of ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca devices with different thicknesses of NPD layer: 8 nm (1), 15nm (2), 30 nm (3) and 45 nm (4). Spectra are measured at the same current 7.4 mA/cm2 for all devices.

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

186 Organic Light Emitting Devices

formation of 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

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].

**5. The relation between intrinsic and exciplex bands in the devises** 

**Figure 6.** Normalized EL spectra of ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca devices with different thicknesses of NPD layer: 8 nm (1), 15nm (2), 30 nm (3) and 45 nm (4). Spectra are measured at the same

1

400 500 600 700 800

 (1) 8 nm (2) 15 nm (3) 30 nm (4) 45 nm

, nm

Some of the EL devices based on amino-substituted zinc complexes demonstrate the EL spectra with only exciplex bands, other demonstrate the intrinsic EL bands either. The relation between intrinsic and exciplex bands can be affected by different factors: materials of contacting layers [22], thicknesses of layers [17,29,30,31,62], applied voltage and current [20,63-64]. The dependence of the relation between intrinsic and exciplex bands in the EL spectra of the devices based on amino-substituted zinc complexes on the thickness of holetransporting layer and on the applied bias voltage were studied for the devises based on

**containing NPD/Zn(TSA-BTZ)2 interface** 

Zn(TSA-BTZ)2 as an illustrative example.

I, a.u.

0,0

0,2

0,4

0,6

0,8

1,0 <sup>4</sup> <sup>3</sup>

2

current 7.4 mA/cm2 for all devices.

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 nm in addition to exciplex bahds.

**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.

Figure 7 shows the dependence of the EL spectra of Zn(TSA-BTZ)2 based devices with different thicknesses of hole-transporting NPD layer on the applied bias voltages and corresponding currents through the device. With increasing voltage, maximum of the exciplex band shifts to lower wavelength by 5-8 nm, maximum of the intrinsic band practically does not change. The increasing voltage leads also to appearance of additional exciplex peak at 540 nm in the EL spectra of the devices with the NPD layer thicknesses of 15 and 30 nm (figure 7b,c).

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes 189

current. The intensity of the intrinsic band relative to that of the exciplex band increases with the increase in current and saturates at large currents. The saturation may be due to

**6. White OLEDs based on zinc complexes with amino-substituted ligands** 

The combination of narrow intrinsic band and wide exciplex band gives a very wide emission spread over the whole visible spectrum, which is a way to obtain white light

For the novel zinc-chelate complexes of sulphanilamino-substituted ligands, the combination of narrow intrinsic band in blue region and wide exciplex band in yellow region can be observed for some devices with NPD as hole-transporting layer. For example, the device 1 based on Zn(PSA-BTZ)2, gives the EL emission (Figure 2a, curve1) with the CIE chromaticity coordinates *x* = 0.31 and *y* = 0.34 which is close to that of the white light (*x* = 0.33, *y* = 0.33). Corresponding point on CIE color diagam (Figure 9) is marked by open circle.

**Figure 9.** The emission of the devices ITO/PTA/NPD/Zn(PSA-BTZ)2/Al:Ca (open circle) and

ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca with the NPD layer thickness of 30 nm (filled circles) on the CIE

For the devices based on Zn(TSA-BTZ)2, the presence of both intrinsic and exciplex emission bands in the EL spectra can be observed for the devices with appropriate NPD layer thickness. The EL spectra of the device ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca with the NPD layer thickness of 30 nm exhibit both intrinsic and exciplex emission bands with the relation depending on bias voltages (figure 7c). The device emits nearly white light. The CIE color

extending the carrier recombination zone to the whole Zn(TSA-BTZ)2 layer.

emitting diodes [8,12,36,37].

color diagram.

Shift of exciplex bands maxima with change of the layer thicknesses and of the applied voltage may be due to plurality of excited states in the excited donor-acceptor complexes [28,51].

The relation of intensities of intrinsic and exciplex bands depends on applied bias voltage and corresponding currents. With increasing voltage, the intensity of the intrinsic band relative to the exciplex band increases. For the device with 30nm NPD layer thickness, the intrinsic band becomes more intense than the exciplex band at some voltages. Growth of the intrinsic band can be attributed to the shift of the carrier recombination zone from the NPD/Zn(TSA-BTZ)2 interface to the bulk of the emitter layer and increasing number of holes injected into the emitter layer due to increasing electric field [29,63,65].

It should be noted that the intensity of electroluminescence depends on the number of recombinating electrons and holes that is on current through the device. So the dependence of the EL spectra on the applied bias voltage should more properly be considered as the dependence on the current.

**Figure 8.** Plot of relative intensities of the intrinsic EL band and that of exciplex band as a function of the current for the devices ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca with NPD layer thicknesses 30 nm (1, squares) and 45 nm (2, circles). Solid curves represent only guide to an eye.

Figure 8 shows the relation of the intensity of the intrinsic EL band (Lintr) to that of exciplex band (Lexc) for the devices with thicknesses of NPD layer 30 and 45 nm depending on current. The intensity of the intrinsic band relative to that of the exciplex band increases with the increase in current and saturates at large currents. The saturation may be due to extending the carrier recombination zone to the whole Zn(TSA-BTZ)2 layer.

188 Organic Light Emitting Devices

15 and 30 nm (figure 7b,c).

dependence on the current.

Lintr / Lexc

[28,51].

Figure 7 shows the dependence of the EL spectra of Zn(TSA-BTZ)2 based devices with different thicknesses of hole-transporting NPD layer on the applied bias voltages and corresponding currents through the device. With increasing voltage, maximum of the exciplex band shifts to lower wavelength by 5-8 nm, maximum of the intrinsic band practically does not change. The increasing voltage leads also to appearance of additional exciplex peak at 540 nm in the EL spectra of the devices with the NPD layer thicknesses of

Shift of exciplex bands maxima with change of the layer thicknesses and of the applied voltage may be due to plurality of excited states in the excited donor-acceptor complexes

The relation of intensities of intrinsic and exciplex bands depends on applied bias voltage and corresponding currents. With increasing voltage, the intensity of the intrinsic band relative to the exciplex band increases. For the device with 30nm NPD layer thickness, the intrinsic band becomes more intense than the exciplex band at some voltages. Growth of the intrinsic band can be attributed to the shift of the carrier recombination zone from the NPD/Zn(TSA-BTZ)2 interface to the bulk of the emitter layer and increasing number of holes

It should be noted that the intensity of electroluminescence depends on the number of recombinating electrons and holes that is on current through the device. So the dependence of the EL spectra on the applied bias voltage should more properly be considered as the

**Figure 8.** Plot of relative intensities of the intrinsic EL band and that of exciplex band as a function of the current for the devices ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca with NPD layer thicknesses 30 nm (1,

0 2 4 6 8 10 12 14

J, mA/cm<sup>2</sup>

2

1

Figure 8 shows the relation of the intensity of the intrinsic EL band (Lintr) to that of exciplex band (Lexc) for the devices with thicknesses of NPD layer 30 and 45 nm depending on

squares) and 45 nm (2, circles). Solid curves represent only guide to an eye.

0,0

0,2

0,4

0,6

0,8

1,0

1,2

injected into the emitter layer due to increasing electric field [29,63,65].

#### **6. White OLEDs based on zinc complexes with amino-substituted ligands**

The combination of narrow intrinsic band and wide exciplex band gives a very wide emission spread over the whole visible spectrum, which is a way to obtain white light emitting diodes [8,12,36,37].

For the novel zinc-chelate complexes of sulphanilamino-substituted ligands, the combination of narrow intrinsic band in blue region and wide exciplex band in yellow region can be observed for some devices with NPD as hole-transporting layer. For example, the device 1 based on Zn(PSA-BTZ)2, gives the EL emission (Figure 2a, curve1) with the CIE chromaticity coordinates *x* = 0.31 and *y* = 0.34 which is close to that of the white light (*x* = 0.33, *y* = 0.33). Corresponding point on CIE color diagam (Figure 9) is marked by open circle.

**Figure 9.** The emission of the devices ITO/PTA/NPD/Zn(PSA-BTZ)2/Al:Ca (open circle) and ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca with the NPD layer thickness of 30 nm (filled circles) on the CIE color diagram.

For the devices based on Zn(TSA-BTZ)2, the presence of both intrinsic and exciplex emission bands in the EL spectra can be observed for the devices with appropriate NPD layer thickness. The EL spectra of the device ITO/PTA/NPD/Zn(TSA-BTZ)2/Al:Ca with the NPD layer thickness of 30 nm exhibit both intrinsic and exciplex emission bands with the relation depending on bias voltages (figure 7c). The device emits nearly white light. The CIE color

coordinates (x, y) corresponding to the emission spectra of the device at bias voltages 3.5, 4.0, 5.0 and 5.5 V are (0.40, 0.38), (0.37, 0.36), (0.34, 0.33) and (0.33, 0.33) respectively. Corresponding points on CIE color diagram (figure 9, filled circles) are close to the black body emission line between color temperatures 3500 and 6000 K.

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes 191

triarylamine derivatives. The relation between intrinsic and exciplex EL bands for the EL devices depends not only on the material of hole-transporting layer but also on the applied voltage. This may be due to to the shift of the carrier recombination zone from the interface

[1] Burrows PE, Sapochak LS, McCatty DM, Forrest SR, Thompson ME. Metal ion dependent luminescence effects in metal tris-quinolate organic heterojunction light

[2] Burrows PE, Shen Z, Bulovic V, McCarty DM, Forrest SR, Cronin JA, Thompson ME. Relationship between electroluminescence and current transport in organic

[3] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl. Phys. Lett. 1987;

[4] Hamada Y, Sano T, Fujita M, Fujii T, Nishio Y, Shibata K. Organic electroluminescent devices with 8-hydroxyquinoline derivative-metal complexes as an emitter. Jpn. J. Appl.

[5] Hamada Y, Sano T, Fujii H, Nishio Y, Takahashi H, Shihata K. White light emitting material for organic electroluminescent devices. Jpn J Appl Phys 1996; 35(10b), L1339-

[6] Tanaka H, Tokito S, Taga Y, Okada A. Novel metal–chelate emitting materials based on polycyclic aromatic ligands for electroluminescent devices. J Mater Chem 1998; 8(9)

[7] Cocchi M, Virgili D, Giro G, Fattori V, Marco PD, Kalinowski J, Shirtota Y. Efficient exciplex emitting organic electroluminescent devices. Appl Phys Lett 2002; 80(13) 2401-

[8] Chao C-I, Chen S-A. White light emission from exciplex in a bilayer device with two

[9] Bolognesi A, Botta C, Cecchinato L, Fattori V, Cocchi M Poly(3 pentylmethoxythiophene)/Alq3 heterostructure light emitting diodes. Synthetic Metals

[10] Tian WJ, Wu F, Zhang LQ, Zhang BW, Cao Y. Light emission from exciplex of organic

[11] Itano K, Ogawa H, Shirota Y. Exciplex formation at the organic solid-state interface: yellow emission in organic light-emitting diodes using green-fluorescent tris(8-

blue light-emitting polymers. Appl Phys Lett 1998; 73(4) 426-428.

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emitting devices. Appl Phys Lett 1994; 64(20) 2718-2720.

M.G.Kaplunov, S.N. Nikitenko and S.S. Krasnikova

**Author details** 

**8. References** 

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Phys. 1993; 32, L514-L515.

As an example, Fig. 10 shows a photograph of a light-emitting diode based on the electroluminescent structure ITO/PTA/Zn(TSA-BTZ)2/Al:Ca. The area of the luminescent surface is 6×6 mm2 and the operation voltage is 6–8 V. The efficiency of such light-emitting diodes reaches 5–6 lm/W.

**Figure 10.** Photograph of an organic light-emitting diode based on an ITO/PTA/Zn(TSA-BTZ)2/AlCa electroluminescent structure.
