Single-layer OLED

Single layer OLEDs have been fabricated with a variety of emitter molecules and conjugated polymers such as poly(phenylene vinylene) (PPV). 

Figure 13-4. Encigy level diagnim of a single-layer OLED, where the organic malerial is depicted as a fully depleted semiconductor. The valence band Ey corresponds to the HOMO and the conduction band Ec corresponds to the LUMO. Tile Fermi levels of the two metal electrodes are marked as Et-. Upon contact a built-in potential is established and needs to be compensated for, before the device will begin to operating.

Recently, Mailiaras et al. [ 1511 have shown that for the analysis of the current-voltage characteristics of single layer OLEDs, it is of fundamental importance to properly account for the built-in potential. The electrical characteristics of MEH-... 

FIGURE 3.7 Energy-level diagrams of (a) a single-layer OLED and (b) a two-layer OLED based on a p-type emitter and an ETM. (From Kulkarni, A.P., Tonzola, C.J., Babel, A., and Jenekhe, S.A., Chem. Mater., 16, 4556, 2004. With permission.)... 

BTZA2 containing a benzo[l,2,5]thiadiazole core and peripheral diarylamines groups exhibits strong red fluorescence in the solid state. Single-layer OLEDs fabricated using this compound demonstrated a maximum brightness of over 5000 cd/m2 [220]. 

Fowler-Nordheim tunneling of, 22 258 in HBTs, 22 167-168 Moore s law and device scaling and, 22 254 in RTDs, 22 170-171 in semiconducting silicon, 22 485-486 in semiconductors, 22 233, 237-239 in SETs, 22 171-172 in single layer OLEDs, 22 215-216 in spinel ferrites, 11 60-61 in the superconducting state, 23 804 Electron spectrometer system, components of, 24 100-101... 

Single-layer OLEDs, 22 215-216 Single-layer photovoltaic devices,... 

Doping of ionic electroluminescent films of [Ru(bpy)3]2+ with [Os(phen)3]2+ produced single layer OLEDs with luminescence emerging predominantly from the Os MLCT excited state, but the emission energy can be tuned to some extent by varying the concentration of the dopant. The devices prove to have better stability than devices prepared from either of the pure complexes [132],... 

A single-layer OLED with [Er(acac)3phen] doped into a 80-nm thick film of PVK (see fig. 117) prepared by spin-coating and deposited on an ITO electrode, and with a 100-nm lithium-doped (0.1%) aluminum cathode has also been tested and shows an onset voltage of about 12 V for electroluminescence (Sun et al., 2000). [Er(dbm)3bath] has a photoluminescence quantum yield of 0.007% in dmso-7fl at 1 mM concentration the OLED based on this compound and similar to the one described above for Ndm has a NIR external electroluminescence efficiency of 1 x 10-6 (Kawamura et al., 2001). 

Figure 6 Phosphole-derived conjugated systems used to prepare single-layer OLED devices.

Figure 6. Schematic illustration of organic light-emitting diode (OLED) operation (upper left) a single-layer OLED at zero bias, (upper right) a single-layer OLED at forward bias, and (bottom) a two-layer OLED at forward bias.

Finally, the anthracene-based [2] rotaxanes were studied in single layer OLED architecture [73, 74], The measurements revealed new features in electroluminescence spectra comparing to photoluminescence both in solid state and in solution. However, the origin of the phenomena is not still well understood and far to be exploited in applications. 

When in the stationary state equal numbers of holes and electrons are injected per unit hme, but e.g. the hole mobility is much higher than the electron mobility, then in a single-layer OLED, the recombination occurs very near the cathode. This leads as a rule to an increase in non-radiative recombination and thus to a reduction of the Hght yield. If different numbers of holes and electrons are injected and transported per unit time, then that part of the charge carriers which is in excess, i.e. a part of the current, cannot contribute to the production of Ught This too reduces the efficiency of the OLED. 

Nevertheless, even such simple single-layer OLEDs function well. Figure 11.3 shows a single-layer OLED made with the polymer PPV (see Fig. 11.5) in operation with an appUed voltage V = 4.5 V. Their luminescence can be seen from the last row of a large auditorium. However, they are inefficient and have a limited operating life. 

Fig. 11.3 A 16-segment display with a single-layer OLED. The active organic layer is made of PPV. The glass plate serves as substrate and is coated with iTO the black dots are the contacts for the Al cathodes. The OLED segments which are supplied with a voltage of 4.5 V emit light, the others remain dark. (Prepared by J. Gmeiner, University of Bayreuth, 1992.) Compare the coloured plate in the Appendix.

Single-Layer OLEDs with Perylene Doping... 

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