**4. Conclusions**

226 Organic Light Emitting Devices

additional charge carrier traps.

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

**Figure 33.** Activation energy dependence on applied voltage of the investigated compounds in solid

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

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

films. Positive voltage was applied to aluminium electrode.

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

interaction of molecules and injected charges.

encapsulated.

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.

Emission from the **IWK** film is too weak to detect, which may be attributed to the higher photoluminescence quenching in **IWK** than in glassy pyranylidene films. However, using the doping approach, the compounds we have introduced enable up to 3 times higher doping concentration without losing optical properties compared to other already known red-emitters.

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 229

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Four investigated compounds - **ZWK-1**, **JWK-2**, **DWK-1** and **DWK-2** show amplified spontaneous emission from pure solid films. Obtained threshold values are larger than those previously reported, but it should be mentioned that for pyranylidene type compounds, amplified spontaneous emission has been observed only in the doped systems until now.

Electrical properties are found to be better in compounds with one electron donor group due to absence of local trap states in their thin films. In the case of molecules with two electron donor groups shallow hole trap states have been observed, which may decrease efficiency of electroluminescence and should therefore be avoided in fabricating high efficiency light emitting diodes.

Even though we are able to prevent pyranylidene and isophorene type red-emitters from self crystallization in the solid state, their concentration in the emission layer would still be limited due to photoluminescence quenching caused by the short distance between molecules. Nevertheless, the glass materials can still be used not only as dopants for OLED applications, but also for lasing applications. Good thermal properties present a possibility of using them also for nonlinear optical (NLO) property studies.
