**3.8. Electrical properties**

Electrical properties of **WK-1** and **WK-2** compounds are investigated in the regime of space charge limited current (SCLC) [52-54]. Similar sandwich type samples as used for photoelectrical measurements are prepared for this study as well. The thickness of the organic thin film is at least 500 nm.

The current-voltage characteristics of compounds **ZWK-1**, **ZWK-2**, **JWK-1**, **JWK-2** and **DWK-2** in thin solid films are shown in Fig.32.

The current-voltage characteristic of **DWK-1** films could not be measured due to unstable current. This may be due to formation of small crystallites (see Fig.21) around 1 m in size. Such aggregates are found throughout the sample and induce instability in the current. In all other cases the current-voltage characteristics have similar shapes with three regions. In the first region, 0-2 volts, the current is found to depend linearly on voltage. In the second range, 2 to 50 volts the current increases superlinearly with voltage, following Child's law. In the third region, > 50 V, the current depends on voltage to the power of at least ten, which may be attributable to charge trapping in the local trap states. More details of this aspect will be discussed further below.

Usually the work function of ITO should be near the ionisation energy level of the organic compound while that of aluminium (Al) should be around the middle of the energy gap. This provides efficient hole injection from ITO and electron injection from aluminium when a positive voltage is applied to ITO. Holes may also be injected from the aluminium when positive voltage applied to it. Electron injection may be more difficult in the second case due to the large difference between the ITO work function and electron affinity potential of the organic compound. This is confirmed by the current voltage characteristics shown in Fig.32. A similar current is observed at the lower voltage where only holes are injected either from ITO or aluminium when biased with a positive voltage. At higher voltage current is higher when ITO is positive in comparison with positive aluminium.

224 Organic Light Emitting Devices

**Photoconductivity threshold** 

**value (eV)**

the best fit with slope coefficient one.

1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95 2

**3.8. Electrical properties** 

be discussed further below.

organic thin film is at least 500 nm.

gap despite the various molecule structures.

**DWK-2** in thin solid films are shown in Fig.32.

**Figure 31.** Linear correlation between optical band gap and photoconductivity threshold value. Line is

1.8 1.85 1.9 1.95 2 2.05 2.1 2.15 2.2 2.25 2.3 **Optical band gap (eV)**

The energy of the photoconductivity threshold is defined as the difference between the conduction levels of holes and electrons [51]. The value of the intercept implies that the optical band gap is 0.28 eV larger than the difference between the conduction levels of holes and electrons. It shows a constant energy difference between optical band gap and adiabatic

Electrical properties of **WK-1** and **WK-2** compounds are investigated in the regime of space charge limited current (SCLC) [52-54]. Similar sandwich type samples as used for photoelectrical measurements are prepared for this study as well. The thickness of the

The current-voltage characteristics of compounds **ZWK-1**, **ZWK-2**, **JWK-1**, **JWK-2** and

The current-voltage characteristic of **DWK-1** films could not be measured due to unstable current. This may be due to formation of small crystallites (see Fig.21) around 1 m in size. Such aggregates are found throughout the sample and induce instability in the current. In all other cases the current-voltage characteristics have similar shapes with three regions. In the first region, 0-2 volts, the current is found to depend linearly on voltage. In the second range, 2 to 50 volts the current increases superlinearly with voltage, following Child's law. In the third region, > 50 V, the current depends on voltage to the power of at least ten, which may be attributable to charge trapping in the local trap states. More details of this aspect will

Usually the work function of ITO should be near the ionisation energy level of the organic compound while that of aluminium (Al) should be around the middle of the energy gap. This provides efficient hole injection from ITO and electron injection from aluminium when a positive voltage is applied to ITO. Holes may also be injected from the aluminium when positive voltage applied to it. Electron injection may be more difficult in the second case due to the large difference between the ITO work function and electron affinity potential of the

**Figure 32.** Current-voltage characteristics of pure thin films of **ZWK**, **JWK**, **DWK** compounds. Solid line – compounds with one electron donor group, dashed line - compounds with two electron donor group.

The temperature modulated space charge limit current (TM SCLC) method is used to analyse the charge carrier local trapping states in solid films [55]. The condition for using this method (TM SCLC) is monopolar injection, which is achieved in our case when a positive voltage is applied to the aluminium electrode. The measured activation energy is plotted as a function of the applied voltage for the investigated compounds as shown in Fig.33.

No charge carrier local trap states are found in films of compounds with one electron donor group due to only one plateau which reaches zero. All compounds with two electron donor groups are found to have charge carrier trap states. The additional plateau of activation energy, which can be clearly seen from Fig.33 means that the thin films contain local trap states. The hole shallow trap depths are found to be 0.1, 0.24 and 0.3 eV in **ZWK-2 JWK-2** and **DWK-2,** respectively. Such trap states decrease the efficiency of electroluminescence and should be avoided in fabricating high efficiency light emitting diodes. The activation energy increases at lower voltage for compounds **JWK-1**, **JWK-2** and **DWK-2**. This is indicative of a contact problem where the electrode – organic interface also works as additional charge carrier traps.

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 227

**Figure 34.** a) Electroluminescence spectrum and b) light intensity dependence on voltage of **ZWK-1**

The light emission is observed at 6 V in the electroluminescent device with **ZWK-1** molecules and 9 V in with **ZWK-2** molecules. The light intensity is one order less in **ZWK-2** molecules compared to that in **ZWK-1**. This may be due to the lower PLQY and shallow

The absorption and emission bands of the synthesized pyranylidene type compounds **ZWK-1**, **DWK-1**, **JWK-1** are comparable with those of other already known one electron donor fragment **DCM** and benzopyran type derivatives of pyranylidene within the spectral region studied here. Similar conclusions can be drawn about **ZWK-2**, **DWK-2**, **JWK-2,** which have similar properties to **IWK** and two other already known electron donnor group containing derivatives of pyranylidene. These properties are also similar to those of one electron donor fragment chromene red-emitters. However, incorporation of bulky trityloxy groups in such molecules not only enchances glass transition temperatures by 5° to 20°C compared to previously published pyranylidene type compounds containing one and two electron donor groups, but also enables the formation of a glassy structure in the solid state from volatile organic solvents. In addition, no glass transition values have been observed so far for low molecular mass isophorene type compounds. The photoluminescence quantum yield of investigated molecules in solution is up to 0.54 and is also comparable with the quantum yield of pyranylidene and isophorene derivatives already reported. Most of the thin solid films obtained from **WK-1**, **WK-2** have almost no crystals in the sample. Newertheless the photoluminescence quantum yield is reduced by one order of magnitude due to the closer intermolecular distance between molecules, resulting in strong excitonic interaction.

(line) and **ZWK-2** compounds (doted line)

charge carrier trap states in **ZWK-2**.

**4. Conclusions** 

**Figure 33.** Activation energy dependence on applied voltage of the investigated compounds in solid films. Positive voltage was applied to aluminium electrode.

#### **3.9. Electroluminescence of ZWK-1 and ZWK-2**

A multilayer structure is used for electroluminescence (EL) measurements. Polyethylenedioxythiophenne:polystyrenesulfonate (PEDOT:PSS) (from H.C. Starck) is used as the hole injection layer and LiF as electron injection layer. PEDOT:PSS and organic compounds are sequentially spin coated on ITO glass. Then LiF and Al are thermally evaporated in vacuum. The final structure of the device has a structure of ITO/PEDOT:PSS(40nm)/ZWK1 or ZWK-2(~90nm)/LiF(1nm)/Al(100nm) and is not encapsulated.

The EL spectrum of the device is estimated in International Commission on Illumination (CIE) coordinates: *x*=0.65 and *y*=0.34 for **ZWK-1** and *x*=0.64 and *y*=0.36 for **ZWK-2**. The spectral maximum peak is observed at 667 nm and 705 nm in **ZWK-1** and **ZWK-2**, respectively, as shown in Fig.34. These peaks are slightly red shifted compared with those of PL spectrum of **ZWK-1** and **ZWK-2** thin films. This red shift may be attributed to the interaction of molecules and injected charges.

**Figure 34.** a) Electroluminescence spectrum and b) light intensity dependence on voltage of **ZWK-1** (line) and **ZWK-2** compounds (doted line)

The light emission is observed at 6 V in the electroluminescent device with **ZWK-1** molecules and 9 V in with **ZWK-2** molecules. The light intensity is one order less in **ZWK-2** molecules compared to that in **ZWK-1**. This may be due to the lower PLQY and shallow charge carrier trap states in **ZWK-2**.
