Single-layer LEDs from

FIGURE 26. Structure of single-layer LEDs. (Reprinted from Ref. 93.)... 

An accurate description of a single-layer LED should be obtained by using the injection and transport properties of electrons and holes, determined independently from the Schottky energy barrier and single-carrier device measurements, to describe the two carrier LED structure. To test this procedure consider structures fabricated from the conjugated oligomer 2-metooxy-5-(2 -etoylhexyloxy)-... 

The single-layer LEDs made from POBTPQ as an enussive material had a tum-on voltage of 9 V and a luminance of 53 cd/m at 17.5 V. The external EL quantum efficiency was 0.06 %. The P4HQ and P40Q LEDs had a tum-on voltage of 13-14 V and a luminance of 15-21 cd/m at 20 V. The external quantum efficiencies were 0.007% for P4HQ and 0.005% for P40Q. The poor electroluminescence efficiency of diese polyquinoline LEDs is due to their low solid state PL quantum yield (27). 

LEDs were fabricated with TA-PPP as the emissive layer. Single-layer devices of ITO/PEDOT/TA-PPP/Ca/Al were fabricated. PEDOT, poly(3,4-ethylenedioxythiophene), was used to enhance hole injection from the anode. Charge injections of the single layer LEDs were clearly hole dominant The barrier for electron injection, around 1.0 eV, is too high. Electron dominant materials such as DO-PF and 2-(4-t-butylphenyl)-5-biphenyloxadiazole (t-PBD) were used to enhance electron injection. The thin film of a TA-PPP and PF blend (95 5 weight ratio) was phase separated. Atomic force microscopy (AFM) showed PF spheres, close to 1 pm in diameter, dispersed in the TA-PPP matrix (Figure 6). This type of phase separation is common in blends of stiff and soft polymers. The PL emission of die blend film was characteristic of TA-PPP. However, once thermally treated, the spectrum shifted bathochromically much like PF. The EL spectrum from LEDs based on the blend thin film contained much emission from PF in the 500-700 nm regime. The device efficiency was about 0.43 cd/A. TA-PPP/PF double layer LEDs were also fobricated. But the efficiency was not improved because when PF was spin coated onto TA-PPP, the PF solution washed out most of the TA-PPP layer. 

Fig. I. Field emission dala from a mounted nanotube. An activated nanotube emits a higher current when heated by the laser than when the laser beam is bloeked (a). When aetivated by exposing the nanotube to oxygen while heating the tip, this behavior is reversed, and the emission current increases dramatically when the laser is blocked. The activated state can also be achieved by laser heating while maintaining a bias voltage of —75 V. Note that the scale of the two plots is different the activated current is always higher than the inactivated current. As discussed in the text, these dala led to the conclusion that the emitting feature is a chain of carbon atoms pulled from a single layer of the nanotube —an atomic wire.

EL experiments showed that the yellow-emitting LEDs prepared from LPPP 12 exhibit quite remarkable characteristics (single layer construction ITO/LPPP 12/Ca quantum efficiency ca. 1.0%, applied voltage 4-6 V 135]). These figures are in the range of the best values described hitherto for polymeric emitters in a single layer arrangement, for example, poly(pcira-phenylenevinylene) PPV and PPV derivatives. 

K. Hutchison, J. Gao, G. Schick, Y. Rubin, F. Wudl, Bucky Light Bulbs White Light Electroluminescence from a Fluorescent C60 Adduct-Single Layer Organic LED , J. Am Chem. Soc. 1999,121,5611-5612. 

Bilayered polysilane LEDs have been obtained by inserting a SiOx thin layer between the cathode and a Wurtz synthesized PMPS emitter film.94 The SiOx layers were prepared by 02 plasma treatment of the PMPS film surfaces. It was found that the external quantum efficiency was significantly enhanced by this treatment. This enhancement has been attributed to an increased electron injection via tunneling, resulting in a reduced hole current caused by the blocking effect of the thin SiOx layer. The weak visible emission observed from single-layer polysilane LEDs is almost completely eliminated. It was concluded that the visible emission is caused by the erosion of the PMPS surfaces due to the collision with hot metal particles during the vacuum deposition of the cathode, and this erosion process is avoided by the SiOx layer. 

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