Electroluminescence anisotropy

Figure 5-24. u) Polar represenlalion (Irom —90 io +90 ) lor lire angular dependence ol lliioreseenee anisotropy (ll-V) in Th films willi grain sizes of 1500 mil (open squares). 000 nm (filled circles), and 200 nm (open circles), b) The angular dependence of electroluminescence anisotropy (H-V) measured from the LED contacted pari of the same samples. 

Figure 8 (right) Polarised electroluminescence spectra from an ITOtruhhed PPVtPFOl Ca LED. The upper curve is for light emitted with polarisation parallel to the orientation direction and the lower curve for perpendicular polarisation. The anisotropy, IparaUei I perpendicular, hos a peak valuc of 25 1. 

The observed n-n transition is therefore the result of an optical excitation with the dipole moment parallel to the PPP chains. The above-described anisotropy of the optical absorption is also valid for photoluminescence emission and can be used to obtain polarized electroluminescence (EL) spectra from Langmuir-Blodgett... 

Even though the degree of the anisotropy reached for this double layer system is lower than that found for the single polymeric layers, it shows photoluminescence and electroluminescence emission with sensitive polarization depencences. 

The advantages of polynuclear aromatic materials are that because only relatively short hydrocarbon chains are required to produce good LB films, the electrical and optical properties of the materials are not diluted, while the films themselves still exhibit good stability properties - the aromatic ring is one of the most stable organic groups known. Useful electrical properties such as conductivity anisotropy and electroluminescence are exhibited by LB films based on the above anthracene material. Related materials such as naphthalene, pyrene and perylene derivatives are also interesting in this respect. 

The optical properties of materials are determined by the so-called dielectric function. This dielectric function was determined for PPP as a result of first principles band structure calculations. In Fig. 30.3 we depict one of the main results, namely the dependence of the imaginary part of the dielectric function (which is proportional to the optical absorption coefficient) on the orientation parallel (cc) and perpendicular ( , e ) to the chain axis. From comparison with the experiment one can see that the optical absorption in the visible and ultraviolet range is mainly determined by the dielectric function parallel to the polymer chain. This is shown in Fig. 30.4, where the calculated absorption coefficient along the chain is compared with experimental data. The observed n-n transition is therefore the result of optical excitation with the dipole moment parallel to the PPP chains. Another experimental proof for this are the absorbance properties of hexaphenyl single crystals [43]. The absorbance spectra for thin films with the hexaphenyl chains oriented perpendicular and parallel to the substrate plane are shown in Fig. 30.5 together with the photoluminescence (PL) and electroluminescence (EL) emission spectra. The above-described anisotropy of the optical absorption is also valid for photoluminescence emission and can be used to obtain polarized EL spectra from Langmuir-Blodgett films made from soluble PPPs [44]... 

In the same way as for photodetectors, it would be desirable to have polarized light-emitting diodes (LEDs) with a built-in polarization filter so that the emitted light may be completely linearly polarized. This application is particularly useful for flat-panel displays of laptops or notebooks. Gardner etal. [39] fabricated (In,Ga)N/GaN MQW LEDs on (1010) M-plane GaN films by low-pressure MOVPE on (1010) M-plane 4H-SiC substrates. The electroluminescence spectra showed a strong polarization anisotropy, with... 

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