Single-Layer LEDs

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

For the LEDs based on m-LPPP PPDB, a maximum external EL quantum efficiency of 1.6% is obtained for doping concentrations of only 0.2 wt% PPDB in m-LPPP (see Fig. 8.19). This value is higher than that of single-layer LEDs based on pure m-LPPP (maximum t]EL under cw operation around 1% see Sec. 8.3.1). 

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)-... 

Figure 12-31. Absorption spectrum of the MSA cation prepared by oxidation of a chloroform solution 1.07x10 mol/L MSA (dotted) compared to the room temperature spectrum of injected charge carriers in a MSA single-layer LED containing 20% MSA in PSu (solid) (Ref. [96]).

Table 13.10. Quanhim efficiency of anT-based single layer LEDs...

While polyphenylsilane (1) and poly(l-methyl-2-phenyldisilane) (2) rather deteriorate the diode behavior polyaminophenylsilane (3) strongly reduces the onset voltage of the devices and more than doubles the power efficiency and brightness compared to the single-layer LEDs. (Figs. 2, 3) These results compare favorably to the data Kido et al. reported for double-layer devices using Alqs as emissive layer and polymethylphenylsilane (PMPS) as HTL [2]. In this case a luminescence of 115 cd/m was measured at a current of 10 mA. For the device Al/Alqs/polyaminophenylsilane/lTO, 130 cd/m were obtained at the same current density. 

Figure 5.4 Typical I-V and EL-V curves for a single layer LED based on poly(3-alkylthiophene). The onset voltage (V- ) is shown.

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. 

Figure 11.8. Schematic set-up of a single-layer LED and operating mode of the electroluminescence.

Schulz et al. [497] described a new series of poly(oxadiazole) derivatives of the structures shown in Fie. 12-21 with promising properties for use in single layer LEDs. Some of their results are shown in Fig. 12-22. 

A bulk heterojunction (BHJ) organic solar cell (Figure 3) has essentially the same stmcture as a single-layer LED, except that the active layer is a mixmre of an electron acceptor and an electron donor. By contrast in a bilayer device, the electron donor and acceptor are deposited as separate layers. In both cases, the devices produce electricity by separation of an excited state (exciton) at the interface between the donor and acceptor, followed by transport of the charges so formed to the electrodes. 

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